| Author | Message | ||
     John Rowney Experienced User Username: johnrowney Post Number: 162 Registered: 02-2015 |
My 1938 Wraith WXA68 is running nicely with the revs humming along. However at low revs there is somewhat of a miss, no backfiring or anything untoward like that. Once revs are in a good place, like doing 50 to 60kph in 4th gear, it runs as smooth as silk. This has happened before, but after taking plugs out etc, things have reverted to normal. I looked at the resistance of the plug leads recently to see if there was a high resistance and got the following results: The numbers are for cylinder number then resistance in k ohm 1- 19.6 2- 12.1 3- 13.8 4- 29.7 5- 18.3 6- 18.8 It looks like leads for cylinders no 1 and 4 are higher than expected. Does anyone have any thoughts that this might be a problem consistent with the poor running at lower revs? The resistance should be increasing with the number of the cylinder due to the increasing length of the leads. | ||
| Philip Sproston New User Username: phil2025 Post Number: 50 Registered: 07-2006 |
John put copper leads on And non resistance spark plugs. | ||
| David Hughes Experienced User Username: wedcar Post Number: 159 Registered: 07-2004 |
And turn the radio off.  David | ||
| John Rowney Experienced User Username: johnrowney Post Number: 163 Registered: 02-2015 |
Thanks, Philip and David. I brought the Wraith home from my shed today so I can drive it to the 20-Ghost Club Christmas party tomorrow, and guess what? It ran as smooth as anything from the start. Geez, I hate problems that are intermittent. I will let you know after my trip (about 100 miles there and back) tomorrow. Cheers John | ||
| NormanGeeson Unregistered guest Posted From: 81.98.117.162 |
John I know zero about pre war Wraiths. However, I know from reading and being distracted while in the R-R archive that the company changed the ignition distributer during production. They did so because they could not cure an idling and speed problem as you describe. In short, the problem was erratic. They did however pin down the issue to the gap between the rotor arm and distributer cap. I would pay attention to that gap and the rotor. (Message approved by david_gore) | ||
| David Hughes Experienced User Username: wedcar Post Number: 160 Registered: 07-2004 |
John More seriously - answers that you are looking for, resistance leads are very good when new and keep down any radio interference, they should have a resistance of approximately 1000 Ohms per foot (300mm), any more resistance than that they should be replaced. Along with Norman's comment, a worn distributor shaft / bush will give excessive clearance, allowing the points to give varying points gap, this can be checked with the rotor removed using a dial indicator and pushing/pulling the distibutor shaft, but is best detected using an ignition oscilloscope with the engine running. Trust this helps Regards David | ||
| John Rowney Experienced User Username: johnrowney Post Number: 164 Registered: 02-2015 |
Norman - many thanks for your comments. I know that at some stage RR changed the distributor from a Delco-Remy version to a RR version. The Delco-Remy was then reintroduced to the Bentley Mk VI after the war, with some slight differences, but still with twin points. The major difference is that the Mk VI distributor rotates in the opposite direction to the Wraith. Parts for the Delco-Remy are listed in Fiennes Parts as being for the Wraith and Mk VI. I wonder if the problem was cured after the war since the Mk VI distributor can be used in the Wraith. David - My resistance is much higher than the 1000 ohm per ft. I replaced all the leads a couple of years ago, so I don't think I should have experienced a significant change over the short time - or should I? I can report that I did about 90 miles yesterday, and the car ran beautifully. Before running the Wraith last Friday, I pushed and prodded the spark plug leads at the spark plugs and at the coil and distributor. Maybe this sorted a bad connection or two. I can recall a couple of years ago doing the same thing when I had a similar problem. Over the next few days I will look at David's suggestions on the excessive clearances. Many thanks for all comments. I will let you know the outcome, although I will lose the Wraith in a couple of days to get some fine tuning of repaired rear doors done and associated painting. This may take a couple of weeks before I get it back. | ||
| David Hughes Experienced User Username: wedcar Post Number: 161 Registered: 07-2004 |
John My apologies - typo mistake, the resistance of the Silicone type leads is 4000 Ohms per foot. Regards David | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 2586 Registered: 09-2004 |
Forget using a dial gauge and oscilloscope unless you would do the superimposed with engine running. Can all be carried out with the cheap automotive meter, point gap = DWELL angle and of course the variation of a distributor shaft. Dwell on a six cylinder in most cases is 30-35 degrees, any deviation of more than 2-3 degrees indicates a change in contacts gap that can point to dist shaft or bushing wear or in rare cases dist plate movement due to ware or looseness. Hope this helps but real prob IMO may be some plugs sooting up due to short startups with the ethanol petrol if used. C/O and hydrocarbons test would confirm. BTW the setting is showing dc volts!  | ||
| John Rowney Experienced User Username: johnrowney Post Number: 170 Registered: 02-2015 |
Patrick I finally got the Wraith back from the door restorer and did a dwell check. I got 45 degrees dwell, but the car has been running beautifully. I am dead ignorant about what I should do, if anything, now that the car is running so smoothly. (If it ain't broke, don't fix it?????) Many thanks | ||
| John Rowney Experienced User Username: johnrowney Post Number: 171 Registered: 02-2015 |
I looked at some older postings for dwell in Bentley Mk VIs, and the dwell quoted was 35 to 37 deg (Mark Taxis), 36 to 40 deg (Geoff Wootton), and 38 to 40 deg (Patrick Lockyer). I came across an article which had a marked up page from a Mk VI service book which showed the dwell of 42 deg for a points gap .019" to .021". The Mk VI Delco Remy distributor is basically the same as the early Wraith's Delco Remy distributor. When I set the points on the Wraith, I followed the advice of Steve Sparks from NBS Services, to set the gaps closer to .019". My understanding is that the smaller the gap, the higher the dwell. It might be that my dwell is in the right sort of ballpark. Am I deluding myself, and just not wanting to do any more fiddling? | ||
| John Rowney Experienced User Username: johnrowney Post Number: 172 Registered: 02-2015 |
The marked up page from the Mk VI service book was one I found on the RROCA website for early Mk VIs. I then looked at my article I wrote in Praeclarum, February 2021 and I quoted Tom Clarke's Wraith points gap as .018". This had been previously been quoted as .019" to .021". I must have used a .018" gap, which probably explains the dwell of about 45 deg. |
