I'm having some problems with the persistent acceleration stumble between 2000- 3000rpm and thinking that although they're clear and the carbs have been rebuilt in the last couple of years, the original jets might have gummed up a bit over 20 odd years. I've read here that because the jets are on the limit of what's required due to emissions control, even a small restriction can cause a problem. I've also seen here that a few folks have had success going to a slightly larger jet.

I'm on DHLA 45s and (from DeRoure) it looks like standard idle jets for an N/A car are 56. Is it worth going up a size or two (the Turbo uses 58) or just getting new 56s?

Edited June 2, 20187 yr by eeyoreish

I'd have thought you'd be running on the main jets once the RPM rises over 1,500.

Are the carbs perfectly balanced & the idle mixtures correctly set wih something like a Colortune? 

I think they're a touch out of balance just now but the stumble is still there when they've been balanced. Having driven it to Goodwood today the stumble is probably a bit lower than I guestimated when I was posting. If I keep the revs up above 2000 between changes it's fine so I'm pretty sure it's on the changeover from idle to mains.

What about the idle mixture?

Not checked that. Colourtune has been on the shelf for 3 years waiting 😳

I followed the manual recently to get the mixture better, closed the screws down gently then 6 half turns open again. I kept a note of how many turns each screw took to close just in case I needed to put it back as it was.

I have the same engine setup as yourself and I feel it made a difference once I'd set the revs at idle to just below 1.

I'm 6 half turns out now but the air flows do need balancing again. It seems like there's a knife edge of getting it right and being even slightly off the perfect settings results in this stumble. Hence I was wondering if slightly larger jets would help.

The method in the manual is just a starting point for carbs & engine in perfect condition.

If you're not using something, like a Colortune, to correctly set the mixture then you're just guessing.

Good luck. 😕

Better renew all jets, even it looks little suspiciously . You can also change idle jets to 58. not really necessary but better for cold starts.

What are you running?

iddle jets 56?

iddle holders?

mains jets?

air correctors?

 

You can easily whip the jets out and have a quick look to see if they are cruddy. Mine were, and I cleaned them by running a load of carb cleaner through them and into the jet housings - It did make a difference (mines a turbo)... I had a pretty fierce hesitation/stumble from 3000+ rpm