Constant misfire ??????

[Bluebird7]

Cheers guys, thanks for the info.
next question really, before I consider a Rolling Road option. is , can anybody sell, lend, give ????
me a selection of needles to trial, thanks in advance

[John Cornforth]

Hi again

I use a GG needle in a 1 1/8 inch SU (Prewar weighted piston with damper type) and the car runs very well. Sooty plugs can be the result of a too-rich idling mixture even if the running mixture is OK. Adjusting the jet height predominantly changes idling mixture strength, has a small effect on light throttle cruising and almost no effect on the top end.

I can't help feeling that from your previous descriptions changing SU needles is barking up the wrong tree. An engine will run on a range of Air-Fuel ratios from about 10 to 1 up to 16 to 1. OK, it may not give best power or economy, but won't actually misfire assuming the ignition system including plugs is up to scratch. You really have to be very rich or very lean to give a noticeable misfire or surging and kangarooing.

To lean out the mixture enough to cause a misfire you would either need a considerable restriction in fuel flow (noticeable when sustained power is needed) or a pretty serious air leak which would make idling near impossible. Another cause of rough and erratic running is fuel boiling / vapourisation if the float chamber gets very hot for some reason.

I'd suggest setting the idling mixture for fastest idle and then weaken just slightly. Then try a fresh set of spark plugs in case the existing ones have become fouled (clean them and keep as spares).

[Bluebird7]

Hi again

I use a GG needle in a 1 1/8 inch SU (Prewar weighted piston with damper type) and the car runs very well. Sooty plugs can be the result of a too-rich idling mixture even if the running mixture is OK. Adjusting the jet height predominantly changes idling mixture strength, has a small effect on light throttle cruising and almost no effect on the top end.

I can't help feeling that from your previous descriptions changing SU needles is barking up the wrong tree. An engine will run on a range of Air-Fuel ratios from about 10 to 1 up to 16 to 1. OK, it may not give best power or economy, but won't actually misfire assuming the ignition system including plugs is up to scratch. You really have to be very rich or very lean to give a noticeable misfire or surging and kangarooing.

To lean out the mixture enough to cause a misfire you would either need a considerable restriction in fuel flow (noticeable when sustained power is needed) or a pretty serious air leak which would make idling near impossible. Another cause of rough and erratic running is fuel boiling / vapourisation if the float chamber gets very hot for some reason.

I'd suggest setting the idling mixture for fastest idle and then weaken just slightly. Then try a fresh set of spark plugs in case the existing ones have become fouled (clean them and keep as spares).

Hi John, having tried just about everything to get the engine to run properly this is my last resort,ref the needle changes. I'm at a point where I simply cannot throw any more time & money at what's been a very very expensive mistake in not rebuilding the engine myself !!!!!

[Ivor Hawkins]

Looking through the complete thread, just one more thing you could look at is compression.

Run the engine up to the point when the misfire starts, whip the plugs out right away and check the compressions on each cylinder. I don’t think it’s a sticking valve, because that usually happens when the engine is cold and they loosen up when the engine has warmed up, nevertheless, I think a valve isn’t closing properly.

It’s an easy check…don’t lose the faith, it is something simple and can be fixed!

[Bluebird7]

Looking through the complete thread, just one more thing you could look at is compression.

Run the engine up to the point when the misfire starts, whip the plugs out right away and check the compressions on each cylinder. I don’t think it’s a sticking valve, because that usually happens when the engine is cold and they loosen up when the engine has warmed up, nevertheless, I think a valve isn’t closing properly.

It’s an easy check…don’t lose the faith, it is something simple and can be fixed!

Good call, will try that too.

[Alan]

Yes, it’s possible a valve is either hanging up on the gasket or a squeezed out bit of gasket sealant. Try a compression test when it’s actually misfiring

[Ruairidh Dunford]

I had a misfire when the engine was hot, I eventually pinned it down to one valve spring that was 0.020" oversize on the diameter, as it heated up it touched the side of the block causing a miss. I got the bottom of it as I heard a squeak as it stopped, rotating the flywheel back and forth produced the squeak and it all became clear.

My own experience with needles that are too rich is that they can cause a spit a certain revs, is your car spitting or missing, hard to explain how they differ in words, but if I could hear it doing it...

[Bluebird7]

I had a misfire when the engine was hot, I eventually pinned it down to one valve spring that was 0.020" oversize on the diameter, as it heated up it touched the side of the block causing a miss. I got the bottom of it as I heard a squeak as it stopped, rotating the flywheel back and forth produced the squeak and it all became clear.

My own experience with needles that are too rich is that they can cause a spit a certain revs, is your car spitting or missing, hard to explain how they differ in words, but if I could hear it doing it...

Hi Ruairidh, just stripped carb and found a GC needle, this looks rich, as confirmed by 'mintylamb' !!! and sooty plugs. Found a BJ on fleabay that I just purchased, but will be looking at valvetrain by the w/end as want to use the car for a local meet, but spitting at higher revs seems to be the issue, just very hard to fathom out ???? and frustrating too !!!

[Ruairidh Dunford]

I can loan you a GG needle and also an AN - send me your address info@albaaustins.co.uk

[Ruairidh Dunford]

Apologies, I only have a GG... I must have used the AN and forgotten to replace it.