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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 478
Registered: 4-2015
Posted on Monday, 02 January, 2017 - 02:25 pm:   Edit PostDelete PostView Post/Check IP

If you had a 1979 carbeurated Shadow (US delivery with emission controls) that only provided 5 mpg on motorway and 4 mpg in town driving, to what could you attribute such poor performance?
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Jim Walters
Frequent User
Username: jim_walters

Post Number: 92
Registered: 1-2014
Posted on Monday, 02 January, 2017 - 05:14 pm:   Edit PostDelete PostView Post/Check IP

Probably choke not coming off, but could also be sunk floats in the HIF carbs. I've seen both. Either way it's running way too rich, rectify it soon or the excess fuel will wash the oil off the cylinder walls and wear out the piston rings prematurely.
Check the choke stove pipes for leaks, I'd lay odds on the problem originating with them.

SRC18015 SRE22493 NAC-05310
www.bristolmotors.com
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 479
Registered: 4-2015
Posted on Monday, 02 January, 2017 - 06:28 pm:   Edit PostDelete PostView Post/Check IP

Jim...
Thanks for the reply which baits the followup question as this is a newly acquired case: Since I have no idea how long it has already been operating this way and given that there are no obvious external symptoms like black smokey exhaust, how can I tell if the damage has already been done? Compression test? Monitor oil consumption? Other?

.
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Jeff Cheng
Frequent User
Username: makeshift

Post Number: 59
Registered: 2-2016
Posted on Monday, 02 January, 2017 - 10:19 pm:   Edit PostDelete PostView Post/Check IP

Another item which is easy to check is the air filter. Mine had a lot of fine silt clogging it, most of which could be shaken/blown out.
The result was a 1 mpg improvement in fuel consumption (hey, that's like a 10% improvement when you normally get 9mpg).
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ross kowalski
Prolific User
Username: cdfpw

Post Number: 246
Registered: 11-2015
Posted on Monday, 02 January, 2017 - 10:56 pm:   Edit PostDelete PostView Post/Check IP

Christian,

I get more than 4mpg, though I never really checked.

Check the plugs, if the engine is rich, they should be black and dry. First thing a decent garage would do is shove a wide band o2 sensor in the exhaust and decide if it was running rich.

If the plugs aren't black it's either a leak, gas being pumped out the bowl overflow line from a stickey needle valve in a bowl, or the car actually using a lot of gas from a tranny gear issue or extra mechanical resistance from stuck brakes or what have you.

If it is running rich though, the su hd8 ( if that's what it has ) can leak at the diaghprams a nd cause a rich condition. If that were the case you'd expect a little gas smell in the engine bay.

Good luck.
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Robert Noel Reddington
Grand Master
Username: bob_uk

Post Number: 1246
Registered: 5-2015
Posted on Tuesday, 03 January, 2017 - 01:04 am:   Edit PostDelete PostView Post/Check IP

damage from running rich is unlikely.
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Jim Walters
Frequent User
Username: jim_walters

Post Number: 93
Registered: 1-2014
Posted on Tuesday, 03 January, 2017 - 05:28 am:   Edit PostDelete PostView Post/Check IP

Slightly rich is probably not going to ruin an engine, but I have seen many ruined by extremely rich running like in Christian's post. (If in fact a rich fuel mixture is causing the extremely poor mileage)
First thing I would look at is the choke butterfly lever. It should be pointing downwards at a 45 angle when cold, and moving to point upwards at about the same angle when warm. If it's level or pointing down when warm then the choke operation is the issue. I would doubt it being a choke issue though if as you say there is no black smoke out the exhaust.
HIF carbs as fitted to Shadow II's don't have jet diaphragms.
If no gas smell or black smoke I would suspect something else like ignition timing being way off or dragging brakes. Easy to check for dragging brakes, just put a hand on each wheel cover after a run of a few miles. They will be warm but shouldn't be too hot to touch. That test will quickly determine whether brakes are dragging or not.
If the rings have been damaged you would likely see blue smoke out the exhaust from oil leakage past the rings. You would need to do a leakdown test to confirm. I wouldn't go there though until you have checked out other possible causes of the poor mileage. Choke lever inspection, smell of gas around the engine, heavy black exhaust when engine revved, gas stains on ground under the car, heavy coating of dry soot in the exhaust pipe outlet, check for hot wheel covers after short drive. After those quick checks in that order, look towards ignition timing. As a shop owner one always tries to diagnose in a logical, quick order to find the cause of a fault keeping in mind past experience with a particular model. With those inspections I can usually pinpoint where the problem is within a few minutes.

SRC18015 SRE22493 NAC-05370
www.bristolmotors.com
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Jim Walters
Frequent User
Username: jim_walters

Post Number: 94
Registered: 1-2014
Posted on Tuesday, 03 January, 2017 - 05:38 am:   Edit PostDelete PostView Post/Check IP

And also don't overlook the obvious, someone unfamiliar with SU HIF carbs adjusting them too rich. More than once I have seen "mechanics" tighten up screws they thought were loose not realizing they were adjusting the mixture.

SRC18015 SRE22493 NAC-05370
www.bristolmotors.com
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 480
Registered: 4-2015
Posted on Tuesday, 03 January, 2017 - 09:55 am:   Edit PostDelete PostView Post/Check IP

Ross...
Believe me, even without checking, if you only get 4mpg, YOU WILL KNOW IT!!

Otherwise thanks to all for the ideas. For reasons I am not presently at liberty to reveal, I can go no further on the topic at the moment. I can say however that the car came from a shop that has the experience and expertise to not make any of the cited or inexperienced mistakes, so I have to give them credit and assume that it is something else...
I will keep you updated in future if possible. I was simply incredulous when told that there was nothing wrong with the car.

.
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John Beech
Prolific User
Username: jbeech

Post Number: 150
Registered: 10-2016
Posted on Tuesday, 03 January, 2017 - 11:22 am:   Edit PostDelete PostView Post/Check IP

4 MPG is extraordinary. Something is wrong. Jim Walters put you in the ballpark with his excellent advice (notwithstanding what you say about the car having come from an experienced shop, my money is on what Jim said). Anyway, tuning the SU carbs is no trickier than adjusting a lawn mower's carburetor. Anyway, just for giggles and grins, use a common screwdriver and run the fuel needle screw of each carburetor in until it bottoms noting how many turns, then return it to where it was. Do the other carb. Now do the same for the air screws, then report back, please. Going off memory, I seem to recall Tootsie's fuel needle is at 2-1/8 turns and ~3 turns on the air needle and she runs superbly and gets about 9 MPG, which is about right for 2-1/2 carburated-tons with a tallish gear ratio.
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Patrick Ryan
Grand Master
Username: patrick_r

Post Number: 859
Registered: 4-2016
Posted on Tuesday, 03 January, 2017 - 11:33 am:   Edit PostDelete PostView Post/Check IP

Holy crap Christian.
That 5MPG = 47 Litres/100km!

I thought my car was bad at 28Litres /100km = 8.5mpg.
However this was on a particularly bad route, very hilly and a fully loaded car.

I need to check it on a better drive. But I knew it wasn't going to be good before I bought the car.

FYI,
my 25 Chev regularly uses 20 Litres/100km (12MPG) does not matter how I drive it or where I go or who or what is in it.

At Volvo Group, all of our trucks now have a factory determined fuel consumption done on a test bed.
Its called Brake Specific Fuel Consumption or BSFC.
Councils, government agencies and large fleets require this figure as part of the tender or quoting process.

Gents,
What sort of Miles/Gallon or Litres/100km are you getting from your Rolls Royce or Bentley?
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Patrick Ryan
Grand Master
Username: patrick_r

Post Number: 863
Registered: 4-2016
Posted on Tuesday, 03 January, 2017 - 11:46 am:   Edit PostDelete PostView Post/Check IP

As some had mentioned the choke,

I thought I would share this.

I fitted a light tension spring to my choke (no mods needed) to ensure when hot it is guaranteed to be off! (image shows a warm engine)



I did the following image for some club members so they could easily see if there choke (hot or cold) was operating correctly.

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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 481
Registered: 4-2015
Posted on Tuesday, 03 January, 2017 - 11:56 am:   Edit PostDelete PostView Post/Check IP

Yes. At California highway speeds, you would need to stop for a fill-up every hour! Vehicle is at another location where I cannot fuss with it and was instructed by seller to touch nothing while we determine a resolution.
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Patrick Ryan
Grand Master
Username: patrick_r

Post Number: 864
Registered: 4-2016
Posted on Tuesday, 03 January, 2017 - 12:08 pm:   Edit PostDelete PostView Post/Check IP

Ok,
Sounds interesting indeed.
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ross kowalski
Prolific User
Username: cdfpw

Post Number: 250
Registered: 11-2015
Posted on Tuesday, 03 January, 2017 - 02:45 pm:   Edit PostDelete PostView Post/Check IP

Patrick,

It does sound like quite the episode in the Christian Hanson Show.

Jim,

I had to google HIF carbs, any idea why they switched? I also like the people tightening down the mixture screws becsuse they were loose. A friends father was a mechanic and had woman with a drivability issue he could not diagnose. He took the car several times and finally went with her for a drive whereupon she immediatly pulled out the choke and hung her handbag on it.

Christian,

I had a 6.5l 69 Pontiac that could actually use enough gas to get to 5 mpg, but there was no question where that gas was going when it was drinking like that. I just remembered something else about that car the gas tank was behind the plate facing back so when you accellerated hard it would slosh out physically as well.

If you trust the shop and the mixture is fine, then at least you know it's external .
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Patrick Ryan
Grand Master
Username: patrick_r

Post Number: 865
Registered: 4-2016
Posted on Tuesday, 03 January, 2017 - 04:39 pm:   Edit PostDelete PostView Post/Check IP

Like my wife in the Chev?

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gordon le feuvre
Prolific User
Username: triumph

Post Number: 148
Registered: 7-2012
Posted on Tuesday, 03 January, 2017 - 07:14 pm:   Edit PostDelete PostView Post/Check IP

I live on small island in English Channel quaintly called a "tax haven". It is only 40 sq miles with an average speed of 17 mph(as recorded on modern car). On average I get 9-10 mpg from my '73 Corniche fixed head. If you are only getting 4mpg and NO black smoke from exhaust-must be fuel leakage? Even V12 Jaguars managed 6-7 on our roads!
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Alan Dibley
Experienced User
Username: alsdibley

Post Number: 35
Registered: 10-2009
Posted on Wednesday, 04 January, 2017 - 12:15 am:   Edit PostDelete PostView Post/Check IP

I am a bit wary to report that on the two occasions I checked my 'T' it did 14 mpg (UK gallons). When I bought it the ignition timing was 20 degrees after TDC and so the performance was s**t. After some fettling to get ignition and carbs set up it spins the rear wheels on a smart take-off.

Alan D.
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Jim Walters
Frequent User
Username: jim_walters

Post Number: 95
Registered: 1-2014
Posted on Wednesday, 04 January, 2017 - 05:21 am:   Edit PostDelete PostView Post/Check IP

Every year our region (RROC British Columbia Region) holds a spring meet incorporating a fuel economy run. This will be mostly secondary road driving of about 100 miles. This past year I managed 18.2 miles per imperial gallon in my 75 Shadow. Now of course one is driving with a very light foot for the test, but the figure attained during this run is only off my regular mileage by about 2 MPG. There is no reason why any Shadow should not get 15 - 16 MPG in regular driving. Carbs adjusted properly, timing set correctly, weakener system functioning as it should, no brake drag, slightly higher pressure in tires than factory recommendations. I also installed iridium spark plugs in my Shadow and that one thing appeared to give me an extra 1 MPG. It certainly starts and runs better with them, and they never misfire or foul. My car has 125,000 miles on it without the engine ever being opened up so it does burn a little oil.

SRC18015 SRE22493 NAC-05370
www.bristolmotors.com
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John Beech
Prolific User
Username: jbeech

Post Number: 157
Registered: 10-2016
Posted on Wednesday, 04 January, 2017 - 01:42 pm:   Edit PostDelete PostView Post/Check IP

Jim, I recently fitted Iridium plugs and despite my better judgement left the supplied gap alone and Tootsie starts and runs fine with them (though to be frank, in the back of my mind was the fact I had also purchased the Pertronix electronic conversion and intend to switch it over soon so the greater gap will be fine). Anyway, I'm wondering what gap you're running on the iridium plugs, e.g. as they were supplied, or did you reduce the gap for points? Speaking of which, does your car have points, or the Lucas electronic ignition?
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 1541
Registered: 5-2012
Posted on Wednesday, 04 January, 2017 - 03:28 pm:   Edit PostDelete PostView Post/Check IP

I guess I'm fishing for comments here because I am breaking all the rules when it comes to the ignition system on my car, but everything seems ok.

I'm running iridium plugs, out of the box (30 thou gap as I recall), carbon trace HT leads and a high output 12v coil. But here's the thing - I'm using points and condenser with no ballast resister i.e. 12v continuous. This means I should be destroying my points real quick. However, I've done about 2 thousand miles on them without the slightest misfire. Everything is running fine. I expected the engine to start running rough after a few hundred miles at which point I was going to drop in a powerspark electronic module. This is still on the shelf as I am still waiting for the points to deteriorate.

My theory is running this system will halve the life of the points, however since points can be expected to run for 20,000 miles, my mileage over the past year is nowhere near this figure. It is around 2000 miles. So I can probably expect 5 years before I need to change the points.

I'm just wondering if the standard advice about running a ballasted 9v coil to protect the points does not really apply to cars that do such a low annual mileage.

I'd also question whether electronic ignition is that great an improvement on these low revving engines. One of the big advantages of running points over electronic ignition is they give plenty of warning before failing. An electronic module can just give up with no warning at all.

John - my guess is iridium spark plugs out of the box would need the gap reducing. Your car will be running a 9 volt coil. The voltage at the plugs will be around 20k volts. A modern high output coil, which the iridium plugs expect to have, delivers twice that amount. I guess the real risk is overloading your coil.

Geoff
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John Kilkenny
Prolific User
Username: john_kilkenny

Post Number: 251
Registered: 6-2005
Posted on Wednesday, 04 January, 2017 - 05:07 pm:   Edit PostDelete PostView Post/Check IP

Geoff,
As this forum has previously covered, the purpose of a ballast resistor is to enable (in combination with a lower voltage coil) an increased voltage to be applied to the coil when cranking, by bypassing the ballast resistor. During normal running the ballast resistor is in series with the coil to give the required coil voltage.
Using a 12 volt coil without a ballast resistor is quite OK, except starting could be affected if your battery is weak.
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 1542
Registered: 5-2012
Posted on Thursday, 05 January, 2017 - 12:10 am:   Edit PostDelete PostView Post/Check IP

Hi John

Looks like I was under a misapprehension here. Good news indeed (in terms of the expected life of my points.)

I've never had a problem starting my car. Almost always first turn of the engine. I guess since my Rolls is not my daily driver I never have to try starting it in adverse conditions (e.g. sub-zero temperatures etc) so don't need the added oomph of a ballasted system.

Thanks for the clarification.

Geoff

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