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Lumpy idle and a racing engine.
Alex. - 27/4/18 at 09:15 AM

Hi Guys,

As a follow on from my head gasket mishap with the Pinto, I finally got it all fitted up and ready to start. After quite a while of cranking, fiddling, poking, cranking, checking, adjusting, cranking... she actually started! Sort of...

She started, but there is lots of vibration and lumpy running at idle, which is only possible with the application of a little throttle. The engine is not happy. Then if the revs are blipped a little higher, say above 1,500, then the engine races up to 5k or 6k revs and I cut the ignition pretty quick.

This behaviour had me stumped, then later on I thought it might be an air leak on the induction side.
The spacers and o-rings holding on the pair of twin 45's are new, so I have been told, but they are a good few years old and so might not be in the first flush of youth!

Am I right in thinking that this might be an air leak?
If so, would a thin coating of red rubber grease help the o-rings with a bit of suppleness and sealing?

Alex.

mackei23b - 27/4/18 at 10:03 AM

Sounds like a sticking throttle cable, I'd check that the 45s are quickly returning fully to the throttle stops if you've not already checked this.

nick205 - 27/4/18 at 10:03 AM

Guessing you've had the cam belt off?

If so are you sure the engine is timed correctly?

Alex. - 27/4/18 at 10:11 AM

quote:
Originally posted by mackei23b
Sounds like a sticking throttle cable, I'd check that the 45s are quickly returning fully to the throttle stops if you've not already checked this.


Cable is nice and free and the snap shut pdq.

Alex.

Alex. - 27/4/18 at 10:12 AM

quote:
Originally posted by nick205
Guessing you've had the cam belt off?

If so are you sure the engine is timed correctly?


Yep, belt had been off. Checked the timing marks numerous times yesterday, all good.

Alex.

[Edited on 27/4/18 by Alex.]

nick205 - 27/4/18 at 10:16 AM

quote:
Originally posted by Alex.
quote:
Originally posted by nick205
Guessing you've had the cam belt off?

If so are you sure the engine is timed correctly?


Yep, belt had been off. Checked the timing marks numerous times yesterday, alĺ good.

Alex.



Good - just a thought I had when reading your post.

coyoteboy - 27/4/18 at 11:38 AM

Hmm sounds a bit like it, or, conversely, too much fuel. I'm not a carby guy though, but it's pretty common if you're running way rich (either through fuel or ign timing) to run lumpy idle then a breath of throttle gives enough air to breath properly for a second.

owelly - 27/4/18 at 02:35 PM

To run, the engine must have air so with the butterflies shut, so the carbs will have a way to let a bit of air in to tickover. Anything more than this must be an air leak. Check all the manifold joints and then look at the advance/retard pipework and servo gubbins. Check by blanking everything off. Try wafting WD40 round the inlet to see if the revs change.

snapper - 27/4/18 at 05:52 PM

quote:
Originally posted by Alex.
quote:
Originally posted by nick205
Guessing you've had the cam belt off?

If so are you sure the engine is timed correctly?


Yep, belt had been off. Checked the timing marks numerous times yesterday, all good.

Alex.

[Edited on 27/4/18 by Alex.]


Are you confusing ignition timing and cam timing?

Air leak would not cause a run on to max revs, tends to lean out the mixture as a manifold air leak is unmetered air that is not seen by the carbs.

I think there maybe several issues and as always the only way is to check each piece in turn and rule it out.
Timing light on the ignition at tickover then at 3600 rpm this should show if the ignition curve is as it should be.
Check cam timing as you removed the belt.
WD40 or carb cleaner around each carb manifold runner on tickover should indicate if there is an air leak, revs will rise.
The only other thing I can think of is fuel pressure, Weber’s don’t need much could over pressure be defeating the float valve?

Alex. - 27/4/18 at 10:37 PM

Thanks for the suggestions guys.
There were probably a couple of issues, now fixed.
I took the carbs off and used a liberal smear (but not too much) of red rubber grease. While refitting the carbs, I noticed that the coupling for the throttle cable between the pair would make more sense in a slightly different configuration. This was probably the main factor in the uncommanded high revs.

Now she is running, I just need an MOT and some shakedown runs. When she is settled back in running order, I will take her to someone who knows wbat they are doing for a good setup session.

Alex.