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Chris Browne
Prolific User
Username: chrisb

Post Number: 136
Registered: 2-2010
Posted on Saturday, 26 July, 2014 - 03:42 am:   Edit PostDelete PostView Post/Check IP

Hello everyone,

I am hoping for help with an annoying fault which is occurring on a friends 1973 Shadow 1.

The car has always suffered from a slight misfire on tick over when the engine is thoroughly hot after about 25 minutes running. The misfire caused a momentary rocking of the engine and the ammeter gauge flicked into the minus zone very briefly but the engine continued to run. Over a period of months, we have tried to solve this problem and have done the following - renewed the spark plugs, HT leads and terminals both ends, fitted a new distributor cap, fitted Lumenition electronic ignition, replaced the coil and ballast resistor, replaced the stove pipes, set the timing, checked the operation of the fuel receiver and weakener solenoid, cleaned out the vacuum take off on top of the engine and renewed all the vacuum pipes and rebuilt and reset the carburettors to give the recommended CO emissions. Despite all this work, the problem has now become worse in that the misfire at high temperature is much more pronounced and in fact is causing the engine to stall on tick over. I should stress that the car starts on the first turn of the key, the automatic choke comes off quickly and the car drives beautifully until at about 25 minutes after starting, the misfire at tickover returns. If the car is driven, there is no evidence of a misfire but as soon as the revs drop to tick over, the misfire returns and the engine stops. It is clearly something which is temperature sensitive but we have run out of things to check, test or replace and it is spoiling my friends enjoyment of his car as it is no longer reliable. Any thoughts about the possible cause of this problem would be greatly appreciated. Thanks in anticipation.

Chris Browne
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 888
Registered: 9-2004
Posted on Saturday, 26 July, 2014 - 04:08 am:   Edit PostDelete PostView Post/Check IP

Rotor arm if all else is ok.
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Bob uk
Unregistered guest
Posted From: 94.197.122.93
Posted on Saturday, 26 July, 2014 - 06:38 am:   Edit PostDelete PostView Post/Check IP

Engine hot expansion vacuum leak

Spray WD40

Misfire goes leak found.


Check auto box vac module.

Check Cruise control.

A stab in the dark

(Message approved by david_gore)
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Mark Aldridge
Prolific User
Username: mark_aldridge

Post Number: 140
Registered: 10-2008
Posted on Saturday, 26 July, 2014 - 08:20 am:   Edit PostDelete PostView Post/Check IP

Try a hot compression test and if you have a local garage with an old Sun or Crypton engine analyser, these are invaluable.
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Tim North
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Posted From: 86.155.223.83
Posted on Saturday, 26 July, 2014 - 09:53 am:   Edit PostDelete PostView Post/Check IP

Following on from Pat's suggestion you could paint the plastic parts of the rota arm with nail varnish-shade not too important. This should cover/fill any cracks etc. If problem solved a new arm is the way to go. Not my idea-Bill Coburn and Steve Lovatt can take the credit.

(Message approved by david_gore)
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Chris Browne
Prolific User
Username: chrisb

Post Number: 137
Registered: 2-2010
Posted on Sunday, 27 July, 2014 - 07:05 am:   Edit PostDelete PostView Post/Check IP

Thank you all for your prompt replies and suggestions. I have printed them all off and when I am back with the owner (and car) on Tuesday, we will go through them together. I will report back if we have any success. Thanks again and kind regards,

Chris
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Bob uk
Unregistered guest
Posted From: 94.197.122.75
Posted on Sunday, 27 July, 2014 - 05:53 am:   Edit PostDelete PostView Post/Check IP

Great idea which I shall take note.

What colour is best

This is what I call a sensible bodge and low cost check I hate guessing and repairing by guess and buy.

(Message approved by david_gore)
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Bob Reynolds
Prolific User
Username: bobreynolds

Post Number: 129
Registered: 8-2012
Posted on Sunday, 27 July, 2014 - 07:38 am:   Edit PostDelete PostView Post/Check IP

Why would someone fit electronic ignition, replace the ignition coil, plugs, HT leads and distributor cap, but not replace the rotor arm? Doesn't make any sense. But the cheap Chinese rotor arms beak down when hot. This is a well-documented failure, previously discussed on here and on other classic car forums. You need a red rotor arm from Distributor Doctor.

Just search Google for 'red rotor arm'.

Even if this is not the cause, at least it will eliminate the possibility.
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Bob uk
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Posted From: 94.197.122.77
Posted on Sunday, 27 July, 2014 - 08:34 am:   Edit PostDelete PostView Post/Check IP

Bob R
The thing is the rotor arm is such a simple bit that one tends to take a quick look for cracks and tracking and if none found the rotor is good.

This is standard practice.
It catches loads of experts out because mysterious rotor faults are rare but they do happen.

Experts including myself like to strut their stuff and not do what amateurs do.
Bar stool mechanics fix misfires by replacing everything, without finding the exact fault.
This does work so money is.saved on Labour

Experts might be asked by the customer the exact cause and if the customer is someone like my wife then because she has a knowledge of ignition systems and even trembler coil then the expert will appear foolish. If the garage says the cap was tracking and we fitted a new rotor just in case and because the old one is ok keep it as a spare just in case. Then fine. Bit say I am not sure madam then a customer is lost.
I would have fitted a new rotor but wasn't there. I make mistakes often which I then put right after a cuppa. All in a day's work.

I forgot to fit a cam follower in a A plus rover 1300cc engine and fitted the engine and tried to adjust tappets.
My boss laughed like a drain for days and kept talking about tappets and tapping his fingers on the desk. He suggested I take tap dancing lessons.

(Message approved by david_gore)
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Bob Reynolds
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Username: bobreynolds

Post Number: 130
Registered: 8-2012
Posted on Sunday, 27 July, 2014 - 08:46 pm:   Edit PostDelete PostView Post/Check IP

The OP was replacing the entire HT circuit anyway. So it makes no sense to replace everything except the rotor arm, which is arguably the cheapest and most troublesome component in the HT system!

A visual inspection of the rotor arm may have worked in the old days, but not anymore. The modern rotor arms short out internally between the brass rivet and the spring clip, and only when they are hot. The only way you can be sure it's not faulty is to change it.

http://distributordoctor.com/rotor_arms.html
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Mark Aldridge
Prolific User
Username: mark_aldridge

Post Number: 141
Registered: 10-2008
Posted on Sunday, 27 July, 2014 - 08:46 pm:   Edit PostDelete PostView Post/Check IP

I always carry a spare Rotor arm in all my cars ,after my son gridlocked the centre of Northampton at 5.00 pm and made the local news , when the rotor on his mk1 Jag failed accross a major roundabout. The congestion was so bad that I had to park and walk to take him a spare.
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Chris Browne
Prolific User
Username: chrisb

Post Number: 138
Registered: 2-2010
Posted on Sunday, 27 July, 2014 - 10:37 pm:   Edit PostDelete PostView Post/Check IP

Hello all,

Thanks again for your continued input. Just for clarification, the Lumention system dispenses with the rotor arm entirely. An eight bladed "fan", for want of a better description, replaces the rotor arm and the blades of the fan interrupt an optical beam to make and break. Hope this puts your mind at rest, Bob.
Kind regards,
Chris
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 1235
Registered: 6-2006
Posted on Sunday, 27 July, 2014 - 11:42 pm:   Edit PostDelete PostView Post/Check IP

clever, how does it send the spark to the correct sparkplug?
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Chris Browne
Prolific User
Username: chrisb

Post Number: 139
Registered: 2-2010
Posted on Monday, 28 July, 2014 - 04:59 am:   Edit PostDelete PostView Post/Check IP

What an idiot I am! I must have been half asleep when I replied to the post. Of course it has a rotor arm. I'm afraid I must put that down to a "senior moment" and hope that at least some members of this message board can sympathise! Until I get back to the car and check the service records, I can't honestly remember if the rotor arm has been changed or not but I will certainly check. Thank you, Paul for highlighting my stupidity. I'm glad someone out there knows what they are talking about - I clearly don't!
Kind regards,

Chris
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Bob uk
Unregistered guest
Posted From: 94.197.122.88
Posted on Monday, 28 July, 2014 - 07:06 am:   Edit PostDelete PostView Post/Check IP

I wonder if the rover V8 dizzy would fit, because rip speed sell new electronic dizzy for 150

The distributor is called a dizzy because the Rotor arm spins and gets dizzy.

(Message approved by david_gore)
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Martin Taylor
New User
Username: martin_taylor

Post Number: 8
Registered: 7-2013
Posted on Monday, 28 July, 2014 - 10:48 pm:   Edit PostDelete PostView Post/Check IP

The rover unit spins in the opposite direction so would not advance if you could fit it
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Jim Walters
Experienced User
Username: jim_walters

Post Number: 22
Registered: 1-2014
Posted on Tuesday, 29 July, 2014 - 04:51 am:   Edit PostDelete PostView Post/Check IP

About 40 years ago an old guy (much younger than I am now!) showed me how to test the rotor arm. Remove the cap, remove the coil wire from the cap, hold the end of the coil wire 1/4 inch away from the centre of the rotor (where the cap carbon brush touches it). Crank over the engine if electronic ignition or make and break the points with a screwdriver to fire the coil. If the spark jumps to the rotor is is faulty, replace it. If it does not jump to the rotor it is OK. This is often the first test we will do on a client's car when diagnosing engine misfires as it only takes a minute or two to do. In a pinch nail polish usually works on cracked distributor caps to get one home, coat the cracked area with a couple of coats and let dry before firing it up. I doubt it would work on rotors because the short is through the plastic under the brass, it shorts to the top of the distributor shaft. Some form of insulation at that location may be effective enough to get one home.

SRE22493 NAC-05370
www.bristolmotors.com
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Chris Browne
Prolific User
Username: chrisb

Post Number: 140
Registered: 2-2010
Posted on Tuesday, 29 July, 2014 - 05:01 am:   Edit PostDelete PostView Post/Check IP

Hello Jim,
Many thanks for your advice. I will try your test on the rotor arm tomorrow.
Kind regards,
Chris
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Bob uk
Unregistered guest
Posted From: 94.197.122.78
Posted on Tuesday, 29 July, 2014 - 06:51 am:   Edit PostDelete PostView Post/Check IP

If the rover dizzy goes backwards then retime the cam rewire the starter turn the diff upside down job done and then you will have spark plug leads that don't fit that come with dizzy thus saving lots of money. Shame it doesn't fit but still cheap even for a points.
Rip.speed have knowledge of RR V8 so if tuning they are worth checking out..

(Message approved by david_gore)
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Bob Reynolds
Prolific User
Username: bobreynolds

Post Number: 131
Registered: 8-2012
Posted on Tuesday, 29 July, 2014 - 07:53 am:   Edit PostDelete PostView Post/Check IP

"Many thanks for your advice. I will try your test on the rotor arm tomorrow."

As it only fails on the return journey, don't forget to heat it up first.
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Chris Browne
Prolific User
Username: chrisb

Post Number: 141
Registered: 2-2010
Posted on Saturday, 02 August, 2014 - 07:17 pm:   Edit PostDelete PostView Post/Check IP

Hello all,

Just a quick update. I substituted the rotor arm on the car yesterday for a known good one off a running car and ran it for around twenty five minutes to get everything thoroughly warm - and the hot engine misfire is still there which rules out, I guess, the rotor arm. I also tried another new coil in case the first one was faulty but no difference. It looks like it is going to be a long process of elimination but VERY frustrating!

Kind regards,

Chris
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 892
Registered: 9-2004
Posted on Sunday, 03 August, 2014 - 03:28 am:   Edit PostDelete PostView Post/Check IP

Why was the original ignition system changed to electronic, IMO the contact system when set up correctly is bullet proof.
Could be the electronic ignition module breaking down when hot.

My Shadow 2 to had a simular prob that caused the car to fail to proceed however fitted a new original distributor,had to fit my old gear to the shaft though.
Has been ok since.

Have had plugs fail when hot if sooted up!
Not on my cars though.
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Bob uk
Unregistered guest
Posted From: 94.197.122.87
Posted on Monday, 04 August, 2014 - 06:48 am:   Edit PostDelete PostView Post/Check IP

I believe that the backward running rover distributor is the same as RR one
Apart from the body and drive arrangement.
The rover V8 with Lucas electronic ignition I think it was called Rita. Is not unknown to cause misfired when hot. I have changed a small few of the module and base plate fitted to SD1 cars. The part was on the shelf at all Rover places.
I thought about electronic dizzy and decided with advice from this site don't fix what isn't broke.
The original single points is the best for this car
It is simple robust reliable cheap parts and easy to repair. This dizzy will run fine for 10k minimum with just an odd dwell check. I have known a.set of points last 50k with filing and setting. Which every mechanic does to save money on their own car.

But none of the above fixes the problem.
Luminition are good reliable easy fit system that has no known problems. And will help slightly worn dizzys be more accurate due to no mechanical load and shafts like to spin and not wobble.

Try this warm engine then observe ignition system in the dark. Any blue hases or flashes in time with the misfire and the problem is found.

No2 on b bank on my car had a small spark going from plug cap to the head. So common on dizzy systems.

The most unreliable part is wiring connectors, points and the connection between the rotor cap. Everything else is bulletproof

Coils when they fail always seem to cause misfires first they never just stop dead. Which is a pain because misfires could be any thing.

To test coils other than by resistance is difficult
get an oscillscope and back probe the coil tower.

There is software and a dongle for laptops which are common place and you get a friendly chap who will lend you the software and dongle.

If the coil is faulty the spike from the coil will show a short one in time with the misfire.
Pat
I had a spark plug fall out on a 350 Bullet and spark my leg. Up until then it was running fine. Motor bikes always try to kill you.

(Message approved by david_gore)
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Hubert Kelly
Prolific User
Username: h_kelly

Post Number: 130
Registered: 3-2012
Posted on Tuesday, 05 August, 2014 - 06:11 am:   Edit PostDelete PostView Post/Check IP

Hi, any chance the main fuel filter needs replaced?
hk
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Bob uk
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Posted From: 94.197.122.80
Posted on Tuesday, 05 August, 2014 - 07:31 am:   Edit PostDelete PostView Post/Check IP

In general in short no. Because the car runs fine then misses and I assume than when driven apart from a misfire has lots of power, then the carbs have enough fuel. Block fuel filters show up as no top end power and very blocked a real pain to keep running and almost undriveable
Filters very rarely block up on petrol cars unless in a dirty fuel area.
Probably the shadow doesn't need the one next to tank in USA Europe Oz etc.

This misfire as described is a typical ignition fault. Usually quick and easy.

Try this. The leads run close to many other bits which are earthed. I think the sparks are leaking
Using another plug lead change ea ych lead for that lead and see if misfire goes. Just drape the lead over the engine.

I dislike the arrangement of the plug leads. I worry about crossfire (rare) and leaks to earth which I suspect is happening here. It does look neat though.
Mine are standard.

Fuel misfires are usually more like rough running and are often ok when engine is revved. Engine fault misfires tend to be constant like dead cylinders because engines tend not to repair itself then a few revs later unrepair.

However there are exceptions to the rule.
This sort of fault is a poser because the people on the job have done what I would done except the luminition which has sold millions and no where except the above has anybody had trouble
Further to that if one needs to convert look no further. The odds of buying new faulty stuff from reputable ignition makers is Nye zero.

(Message approved by david_gore)
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Chris Browne
Prolific User
Username: chrisb

Post Number: 142
Registered: 2-2010
Posted on Tuesday, 05 August, 2014 - 05:49 pm:   Edit PostDelete PostView Post/Check IP

Hello Hubert and Bob,
Hubert, all the fuel filters have been changed recently and I am inclined to agree with Bob that it "feels" like an ignition problem rather than fuel related. Bob, strangely enough, I was discussing the problem (yet again!!) with the owner on Friday, and we decided on exactly your course of action - to substitute a long plug lead for each HT lead in turn, under hot conditions - and see if that cures it.
Thanks again for all your input. I will return!
Kind regards,
Chris
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Jan Forrest
Grand Master
Username: got_one

Post Number: 599
Registered: 1-2008
Posted on Friday, 08 August, 2014 - 09:19 pm:   Edit PostDelete PostView Post/Check IP

Back in the late 80's I had a lot of running problems with my then XJS V12 which turned out to be a blocked fuel filter. I must point out that back then I was covering a lot of mileage in Europe where fuel quality could be quite variable.

Similarly; in the early 80's I had a new (81) Ford Escort III which I serviced before a 2,000 mile European trip. On the first high speed run to bring my wife home from my mother's 'Summer Home' one of the spark plugs just blew straight out of the head. When we returned home - on the back of a recovery truck - I stripped the engine down and found that the side electrodes of all the plugs had completely eroded away and that No. 2 piston had a 5mm hole in the crown!
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Bob uk
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Posted From: 94.197.122.76
Posted on Saturday, 09 August, 2014 - 05:07 am:   Edit PostDelete PostView Post/Check IP

Sounds like ignition timing caused the melt down. Trouble with temp sensors is they don't work on steam.

(Message approved by david_gore)
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Nick Adlam
New User
Username: crewes_control

Post Number: 2
Registered: 12-2015
Posted on Tuesday, 12 April, 2016 - 07:17 pm:   Edit PostDelete PostView Post/Check IP

Thought I'd resurrect this old but pertinent thread.

I'm also custodian of a 1973 Shadow and she suffered the same annoying niggle. Poor idling in gear seems to be a theme with plenty of these cars, understandable when they're 40+ years old.

The main issue is valve stem seals. Prolonged idling and decelerating to traffic lights creates a high vacuum which sucks oil past the inlet valve seals and creates a misfire, especially when the oil is hot after a decent run.

After replacing the whole ignition system (points, condenser, cap, rotor, leads, plugs, relays, etc etc etc) I was able to solve my idle woes by fitting NGK BP4EY plugs (1 step hotter, with V-Groove), and tweak the jet and volume adjusting screws to give a smooth but richer mixture. The plugs all appear somewhat brown, but def not black/sooty. Several roadside stops to tweak those screws and now I have an old '73 Shadow that idles nicely in heavy traffic and runs cool, even in sunny Qld heat. Fuel consumption is still good- around 15mpg highway. I found that a lean or even "normal" mixture will allow the misfire to happen- it needs to be on the rich side.

My 2c worth.
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Nick Adlam
New User
Username: crewes_control

Post Number: 3
Registered: 12-2015
Posted on Tuesday, 12 April, 2016 - 08:06 pm:   Edit PostDelete PostView Post/Check IP

Forgot to mention...

Plugs were gapped to the usual 0.6mm and jet adjusting screws turned in tiny increments- got down to 1/16ths before finally nailing the sweet spot.

I used an IR temp gun on the thermostat housing/radiator top tank and exhaust manifolds and an engine analyser/multimeter to monitor rpm changes, dwell, etc.

I simply checked the colour of the plugs to see how I was progressing, being careful with tightening of course!.
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richard george yeaman
Grand Master
Username: richyrich

Post Number: 486
Registered: 4-2012
Posted on Tuesday, 12 April, 2016 - 08:07 pm:   Edit PostDelete PostView Post/Check IP

Hi Nick my 1974 Shadow is running nice and sweet at the moment, I think my plugs are fives I will try fours at my next service thanks for the 2 cents.

Richard.
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Jean-Pierre 'JP' Hilbert
Prolific User
Username: jphilbert

Post Number: 134
Registered: 9-2013
Posted on Thursday, 14 April, 2016 - 06:19 am:   Edit PostDelete PostView Post/Check IP

Nick, me too, I'm tracing a rough idle, when warm, on my fuel injected Corniche, though the problem is much less pronounced now since I started to drive the car very often.
Still, your theory about the valve stay seals got my attention. Question: on your car, were the seals bad enough for the car to show a smoking exhaust?
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Nick Adlam
New User
Username: crewes_control

Post Number: 4
Registered: 12-2015
Posted on Thursday, 14 April, 2016 - 08:00 pm:   Edit PostDelete PostView Post/Check IP

Hi JP

If I let her sit for 30 mins after a hot run, there's a quick puff of smoke at the exhaust. Not real bad, but noticeable in the rear view mirror if the right breeze is blowing. Compression is good, thank goodness, and oil consumption is around 1 litre per 4000kms. The main problem with mine is that cylinders A1 and A4 show somewhat oily plugs. The rest are good. This is what I'd say caused the misfiring and I simply fitted the BP4EY plugs and tuned the carbs to suit the conditions.

It's possible your injectors are/were clogged and not spraying correctly?, and now that you're driving more often, they're cleaning out. What are your plugs showing?. That's how I arrived at my happy ending.

Hi Richard. Yes, I've found the 4's work well, especially in these older engines with less than perfect oil control. They certainly improved the idle quality on mine. I use modern suppressed carbon leads, so didn't require the resistor (R) series plugs. Definitely no pre-ignition or detonation. Funny- NGK USA actually recommend the 4's for our cars, but UK and Oz go for 5's. Climate or emissions differences?.
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Martin Taylor
Frequent User
Username: martin_taylor

Post Number: 62
Registered: 7-2013
Posted on Sunday, 22 January, 2017 - 09:40 am:   Edit PostDelete PostView Post/Check IP

I don't know if this problem was ever solved but I had a similar miss at idle on mine that was cured by retarding the timing to 5degrees btdc as suggested in the manual, this didn't give the correct high speed ignition advance which turned out to be sticking weights in the distributor.
I also found the fuel weakener system was not working correctly, this turned out to be caused by myself disconnecting the hot air intake, there is a bi-metal switch that cuts it off with cold air. These carbs are turned for a warm air mixture (which goes against the cold air gives more power theme we tune engines to these days), warm air allows for a leaner mixture, less oxygen means less fuel required. The system cuts off below 16 degrees C, I imagine at high speed the air is moving too fast to be heated as it flows past the manifold into the intake.
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Alan Dibley
Experienced User
Username: alsdibley

Post Number: 38
Registered: 10-2009
Posted on Sunday, 22 January, 2017 - 10:03 pm:   Edit PostDelete PostView Post/Check IP

If this fault is still outstanding (after six months???) here's a thought:-

The original posting in this thread said "The misfire caused a momentary rocking of the engine and the ammeter gauge flicked into the minus zone very briefly but the engine continued to run."

This ammeter behaviour suggests that the problem is an intermittent short to earth or broken connection somewhere. The ammeter should not respond like that if
A) the engine is still running at about the same speed, as it is, and
B) the load on the battery is about the same, or
C) there is not some sudden extra electrical load or
D) the feed from alternator to battery is not interrupted.

I would start by wriggling/tightening all main battery leads and checking all the toe-board connectors, and lots of things that would occur to you when you start fiddling. The ammeter behaviour does not suggest ignition problems unless there is a short in the ignition wiring.
Try wiggling the ignition switch, too.

Has the problem been solved? Am I talking rot? Someone tell me, please.

Alan D.
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David Gore
Moderator
Username: david_gore

Post Number: 2385
Registered: 4-2003
Posted on Monday, 23 January, 2017 - 09:01 am:   Edit PostDelete PostView Post/Check IP

Alan,

You are not "talking rot" - the cause of problems such as this is invariably hard to find and the fix usually [but fortunately not always] requires dealing with a very difficult to access region of the car [Murphy's Laws apply here].

My suspicions are an intermittent short somewhere in the electrical system rather than a broken wire. A broken wire should result in a terminal failure unless the wire has broken at a terminal and is floating free but still able to come into contact with a ground. If this is the case, I would still expect a permanent failure of part of the system fed from the broken wire to become obvious.

Finding where the fault is located will be difficult and require much patience and huge amounts of time or a terminal failure of part of the electrical system to identify the probable location of the fault.

Hopefully serendipity will intervene and expedite a cure.
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Alan Dibley
Experienced User
Username: alsdibley

Post Number: 40
Registered: 10-2009
Posted on Saturday, 28 January, 2017 - 12:45 am:   Edit PostDelete PostView Post/Check IP

.....but we still don't know if the fault in the original posting is fixed. Chris, are you there? I would like to know - it's always fascinating to discover more devious ways that cars can fail.

Alan D.
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Nick Adlam
Frequent User
Username: crewes_control

Post Number: 81
Registered: 12-2015
Posted on Saturday, 28 January, 2017 - 08:32 pm:   Edit PostDelete PostView Post/Check IP

Gents

Just thought I'd report back on this thread as my '73 Shadow suffered the EXACT same issue a year ago, right down to the flickering ammeter when the miss occurred.

I fitted NGK BP4EY plugs (1 step hotter, no resistor) and set the carbs SLIGHTLY rich and have had no idling issues since. The problem manifests mostly when decelerating then sitting at the traffic lights, especially when the oil has reached operating temp. The high manifold vacuum on decel causes oil to be sucked past the inlet valve stem seals on the induction stroke, resulting in the lumpy misfire. My engine is a tad oily on A1 and A4- compression good, valve stem seals not so good. I know, replace the seals!.

The slightly rich mixture promotes stable ignition, the hotter plugs burn off the oil/residue, maintaining the spark.

Remember- if you fit resistor spark plugs in a car with modern carbon/resistor leads, you're effectively weakening the spark. Our cars were originally fitted with old-school wire (non resist) leads, hence the original requirement for resistor spark plugs.

Just my 2c
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h_kelly
Prolific User
Username: h_kelly

Post Number: 218
Registered: 3-2012
Posted on Saturday, 15 July, 2017 - 07:37 pm:   Edit PostDelete PostView Post/Check IP

Hi Chris,just wondering if you ever solved the problem with the Rolls (poor performance when engine warm),my Shadow has developed similar symptoms .
Thanks
Hk
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Nick Adlam
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Username: crewes_control

Post Number: 105
Registered: 12-2015
Posted on Saturday, 15 July, 2017 - 09:08 pm:   Edit PostDelete PostView Post/Check IP

Hi HK

My 73 Shadow once suffered the exact same symptoms as what Chris's friend endured but I'm pleased to say that after more than a year and 12K Miles since my first posting on this thread, my old girl still idles great, even in stinking hot Qld weather sitting in Brisbane traffic with the AC running- the toughest test for any classic car. Basically, you need to adjust the carb mixtures to compensate for 40+ years of sundry wear & tear, vacuum leaks, valve stem seal wear, etc, etc. Plenty of trial and error with the small jet adjusting screws eventually gave me a good result- it took me well over a month of highway roadside stopping and tweaking, often to the amusement of onlookers. I also used the colour of the spark plug electrodes as a guide, as they definitely tell a story of what's really happening within. Our engines usually do not wear nice and evenly.
Unorthodox, yes, but it worked well in my case.
I also used NGK BP4EY spark plugs which run slightly hotter, to burn off the soot, and are compatible with modern resistor leads.
Hope it all works out.
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h_kelly
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Username: h_kelly

Post Number: 219
Registered: 3-2012
Posted on Sunday, 16 July, 2017 - 01:06 am:   Edit PostDelete PostView Post/Check IP

Hi Nick thanks for your help, I'll report back with results.
Hk
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h_kelly
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Username: h_kelly

Post Number: 220
Registered: 3-2012
Posted on Monday, 17 July, 2017 - 03:00 am:   Edit PostDelete PostView Post/Check IP

Hi Nick ,update ..took your advice and tuned the(carburettor) air and fuel mixture screws, the car drives like a dream. Thanks for your help!!.
Many thanks
Hk

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