Intermittent Fault - Engine "missing" and cut out

[david340]

Thanks Al

Much appreciated - can I confirm those readings are before TDC so for a 4.2 at tickover im looking for 7 to 9 degrees BTDC on the strobe ?

Going out to the garage soon to have a check round for any air leaks or hoses that have become dislodged - would be too simple but worth a check - ive been reading up that air leaks would likely cause increase in rpm.

Regards.

Dave

[piman]

Hello David,

Al's chart looks to me like the distributor specifications, and not for timing the engine. Note the refrerence on column one is headed 'distributor rpm' which is half of engine rpm. The static timing is in addition to those figures so your 20 degrees at 1700 rpm seems slightly low? I don't know what the static timing is for your engine but nowadays that is likely to be different anyway due to the fuel.
I would start with book figure and test run the car, if it pinks then retard slightly it until you don't experience any pinking, and vice versa.
Incidentally I had a DB5 once and the book timing was x degrees at 3,000 rpm (And no that wasn't a Haynes manual:-))


Alec

[david340]

Thanks Alec

Ive had a visual check around the engine bay to make sure nothing obvious like a hose off somewhere and all looked ok.

I did notice when looking at the distributor that it was turned all the way against the slotted hole on the clamp bolt ie furthest travel one way, this struck me as unusual. The unit doesnt seem loose though.

Also the front carb is not retaining oil level in the dashpot, ive topped it up a few times to correct level and it always drops.

Havent had chance to run the car or check timing with my strobe as yet.

Regards.

Dave

[J44EAG]

Dave,

Just a quick check list that immediately comes to mind.

Have you tightened all inlet manifold and carb flange nuts?

Rubber vac elbows on manifold and distributor advance capsules are notorious leakers given a bit of age hardening.

Is the distributor vac capsule able to move the advance mechanism plate OK and do bob weights have full and free movement if fitted?

Is vac unit able to retain vac if you suck on the pipe and "tonge it" to hold the vac?

Brake servo pipe and NRV OK?

Manifold vac take off nozzle not leaking?

Both throttle plates and venturi throats clear of crud and equally set as regards plate opening uniformity? Choke fully returning when engine hot?

Odd to loose dash pot oil rapidly. I can`t recall if there is a seal between the needle and the dash pot itself.

Is there full and free movement of the dash pot within the outer cover? Does the dash pot on each car fall with a nice clunk (dash pot rods and pistons removed) onto the carb body bridge indicating that nothing is fouling up. Bent needles are not unknown.

Just a few basic ideas.

Mike

[almcl]

can I confirm those readings are before TDC so for a 4.2 at tickover im looking for 7 to 9 degrees BTDC on the strobe ?
Dave

Yes, that's it David. The explanatory text actually says 8° BTDC static.
It also suggests a warm idle speed of 650 - 750 rpm.

Alec's point is interesting, but I don't think the table was meant to be read that way. The box at top left was meant to be read as distributor type to the right (the dizzys are different for the North American market) and the rpms are engine rpms reading downwards...

[piman]

Hello David,

just how high are you filling the dashpots, I think the book says 1/2" below the top of the piston tube? In practice it can be quite a bit lower without affecting the damper operation. The fact that one appears to use more may indicate that it is opening more than the other, i.e. the carburettors are not balanced? There is no where for the oil to go really, but if overfull, opening the throttle wide will push oil out of the cap.
Ideally your distributor should be mid way in the slot when the timing is correctly set and to reset yours means removing the distributor and the drive shaft and rotate it slightly to engage a different tooth on the crankshaft drive gear. This can be a bit fiddle as this shaft also engages, with a square dog into the oil pump so you'd need to rock the engine back and fro to re - engage the pump. However as you haven't yet established the timing it may well be that the distributor will need to be turned into the slot.
Do remember to disconnect the vacuum pipe from the distributor if using a strobe to set the timing?

Alec

[piman]

Hello Al,

I had to look again at your chart and it says quite clearly Distributor rpm, the other telling factor is that it says the readings should be at decreasing rpm which is the standard way of measuring distributor advance against distributor rpm.

Alec

[almcl]

I had to look again at your chart and it says quite clearly Distributor rpm, Alec

Yes, but like many other misprints in the SII ROM (and there are dozens), I don't think it means it. If it really was distributor rpm, the top figure of 2800 rpm would require the engine to be going round at 5600. SII 4.2 engines were red lined at 5000 and what would be the point of achieving maximum advance after the engine's rev limit had been reached?

BTW, it's not my chart, it's Jaguar's.

[piman]

Hello Al,

what you say sounds logical, but exactly the same situation is true of the distributor for a Morris Minor, its specification has the same thing. I know it is not logical but that's what my book says.
The other point is that the total advance at maximum rpm is small if it were engine advance, even adding about 8 degrees (?) static. I would have expected total advance to be about 30 degrees, perhaps a bit more?

I did realise that it was not your chart, It was just to identify you as the poster.

Alec

[CliveRothwell]

Me thinks the dashpot oil is the real issue here as it governs how the carb piston moves in the plenum chamber ie no oil damping = piston opens too quickly and stalls out the engine, hence why it can't move off the drive when cold. Stripping the plenum chamber, filling the piston "holes" next to the jet needle with blutack, and then counting how many seconds it takes for the piston to drop to the bottom of the plenum will tell you if the piston is worn in the plenum (and using the oil). Under 4 seconds is out of tolerance. Thicker oil is a worth while experiment.

[piman]

Hello Clive,

what you say is true but it is very unlikely that there is no oil in the piston tube, there's no where for it to go. If you think how long the damper rod is, the oil needs to cover the base of that and the damper will work. In practice pistons rarely wear out as long as the dashpot and piston assembly have been cleaned with solvent rather than polish or fine wet and dry.

Alec

[david340]

Thanks for all the advice and pointers.

I have someone coming over Thursday who is very knowledgable to help me out and get the carbs and timing correctly set up.

Without sounding thick, if the car is ticking over at say 750 rpm - what should I be aiming for with the strobe light - 8 degrees before TDC ?

Will report back with updates !!!

[almcl]

if the car is ticking over at say 750 rpm - what should I be aiming for with the strobe light - 8 degrees before TDC ?

Erm, qualified 'yes'.

You have to remove (and block) the vacuum advance line to the dizzy. This will probably affect the idle speed. So, as this is now not the 750 rpm that you had when everything was connected, be prepared to reduce the timing a little bit.

The other issue is that as the timing marks are on the opposite side of the engine from the dizzy, it can be quite fiddly twisting the body round in the slackened clamp, then moving round to see what effect this has had.

Another possibility is that moving the timing will affect the idle, small amounts won't make much difference but if it's after TDC (not unknown) then moving it to 8° Before will promote quite a healthy rise in rpm.

Probably all a bit easier if there are two of you, though.

[david340]

Cheers Al

Update to follow after todays efforts !!!!!

Fingers crossed !!

[david340]

ok the update.......

slightly disappointing really

carbs set up for mixture, one slightly rich.

unable to detect any air leaks but tickover still high, bit of a problem adjusting it down and now when you lift off the throttle the car settles to almost a stall before rising up to steady out at just under 1000 rpm - not sure why this is happening.

seems to be a slight misfire on the car

It has fairly recently had a top end rebuild, new electronic ignition, plug leads and carbs rebuilt twice for some reason - so im a little confused.

One of the idle speed screws was found to be broken so im going to try free it and replace it and also have a goood check round for any air leaks.

Timing was slightly retarded.

Drives actually quite well but the tickover is the problem now - coming up to a roundabout and lifting off the throttle the idle is dropping that low that when you go on the gas again it is hesitating which is rather disconcerting to drive !!!