Splashy idle troubleshooting. Possible pre vapourlock?

[Heuer]

Are the needles the correct ones (i.e. UM)? As the mixture is weak at tick-over (hence splashy exhaust) it could be the 1st and 2nd sizes, which control idle mixture, are wrong.


So if the idling dimensions are larger (weaker mixture) the rest of the the needle will give richer results. Conversely if the idling dimensions are smaller (richer) the rest of the needle will give weaker results. This is due to the different jet position setting caused by the changed idling dimensions, if that makes sense. I do know that the needles in my car showed signs of wear after 20,000 miles - they had the appearance of being sandblasted.

One other thought - have you changed the air inlet or filter at all? Again my testing of different air filters showed a marked effect on AFR (the car will run lean with no filter attached).

[Dennismo]

I did put brand new UMs in it and also tried UE needles and problem is still there.

[Dennismo]

The problem is there both with and without aircleaner....

[christopher storey]

Dennis : I doubt that the springs have anything to do with your problem . The primary function of the spring is to govern the full throttle/full rpm position of the piston by ensuring that it only reaches the top of its travel at that point, and not before . Thus with the piston at a low position the spring , which obeys Hooke's law, has virtually no influence on piston position . The only thing it is likely to influence at that end of the spectrum is ( marginally, the damper doing the Lion's share) the rate at which the piston rises if the throttle is opened

With all due respect to your method of checking the ignition timing , although it is a time honoured idea, it is not very accurate . I still suggest a thorough check of the ignition, particularly the mechanical advance side of things , using the book figures and with an accurate TDC datum

Also, it sounds as though your SU rebuild did not include the cast in bronze bushes . These are a swine to replace and need a special stepped reamer to do properly. New seals will not eliminate air leaks if the bushes are worn

[Dennismo]

I have replaced all the ignition components and took the distributor out looking to see if the bearing was worn which would cause varying timing. I did this even though the ignition timing was unwavering at the idle when looking at the timing mark.

I did discover that the distributor sliding plate (the has the points and condensor screwed to it) had worn so much on the base plate that it was not parallel to the fixed plate causing the points plastic knee to not run flat against the segmented rotor. Regretfully fixing that did not fix the problem.

It did check the timing mark against the camshaft timing using the cam timing tool and it is perfect. When I rebuilt the engine, I did set up the timing scale used for ignition timing by using a dial gauge on the number one piston when at TDC before I put the cylinder head back on/

The carbs have the teflon bushings and I have used rubber o-rings instead of the cork seals that degenerate quickly - at least they do with the petrol we buy in Canada - apparently because of the alcohol in the petrol. With the o-rings there is no play, they are a little stiff, and I have checked for leaks by blowing propane at the spindles when engine is idling to see if the engine speeds up thus showing a vacuum leak.

I appreciate you taking the time to mull over this problem that has perplexed me!

Dennis Vancouver

[Heuer]

It was the bronze bushes that caused my SU problems. As Christopher says there is no fix by using either cork or rubber grommets - they have to be replaced.

[christopher storey]

Dennis : something you have not told us is your idle speed . I am beginning to wonder whether you really have a problem at all, and whether in fact your idle is set too low. Most 4.2s need at least 700 rpm to produce even running, and below this the symptoms you describe may arise. The volume of the inlet tract on a 4.2 is very large indeed and the gas speeds at very low speeds are low with a consequent lack of turbulence and the possibility ( I am surmising here ) of fuel drop out onto the walls of the manifold which would account for your varying mixture colours

Another thought : have you tried taking the idle screws out and gently applying compressed air downwards ( i.e inwards) to the bleed passages in case there are obstructions

[Dennismo]

I have tried idle speeds between 800 and 1000 rpm and the problem is still there.
The mixture also varies when one goes above idle as I have had both a colourtune and an air fuel ratio meter on the engine.
At idle the mixture will vary from about 11 to 1 up to 14 to 1 whilt while driving it varies, but less.
I have had the carbs out three times andchecked and blown out all passages like from float bowl to jet, idle passages and the drill part of the dashpot that allows vacuum to reach the reservoir oil area.

I felt that mybest theory was that that float bowl was not venting causing pressure buil up that was forcing gas up through the jet in addition to being sucked, but the vent is fine.
I thought my second best theory was that the wrong float bowl covers had been put on resulting in the fuel level being set too high when set according to the book. So I dropped the fuel levels until they were well below the jets, when the jets were pulled down by choking.

Dennis

[christopher storey]

Just a moment !! have you got the slotted washers on top of your float chamber lids ? The correct sequence is, from the top downwards , securing bolt , aluminium washer, identification tag , overflow pipe, vented washer , chamber lid . If the slotted vented washer is not in the correct place, ( or has been replaced by an unslotted one) you get partial vacuum inside the chamber with very unpredictable results