Zenith carb set up

[Gary71]

Just wanted to compare notes with anyone else that has done this!

I've spent the last few days trying to get decent running from my freshly rebuilt Zenith 40's.

Getting it going at the basic settings as per the book was a complete nightmare. The book settings of 2.5 turns out on the mixture screws was incredibly lean with almost continuous popping and banging at idle or light throttle.

Eventually I managed to get a fully air flow balanced 900rpm idle at 3.00% co after 2 hours spent in my local Sainsbury's car park with a syncrometer, and a Gunsons engine tune box and one whole lot of noise!!

Driving home it was still hesitating at very light throttle opening so I've wound out the mixture screws another half turn. Things seem to be better now, very little popping or banging, but response is still not quite right.

Next step I think is to get the accelerator pumps matched. According to the book I'm supposed the measure the injection volume with a pipette...back in the real world will just ensuring the screws from the control rods are set up the same suffice?

Any other ideas on the hesitation? Timing is checked and OK, dissy centrifugal advance seems to be working.

[Barry]

Gary, I would of thought you are on the right track just with the idle mixture adjustment: ignition is un-stressed here as trigger speed and combustion pressures are both so low.

I can't imagine that the accelerator pumps are going to have much effect either, as at these revs they are only operating during the actual throttle movement. Knowing you, you will already of double checked for inlet manifold / throttle bushing problems. Engine valve adjustment is not really going to effect things at these revs either.

I would be tempted to try the idle mixture screws out a bit more (I'm sure you were about to anyway) at least if they clear / make worse the problem, you know you are in the right area.

Sorry, not much help that lot I'm afraid.

[Gary71]

Thanks Barry,

Did a solid 250 miles cross country last night, luckily I was nowhere near the idle circuit most of the time! [apologies to residents of Worcestershire, Shropshire and Cheshire]

My first half turn on the mixture screws helped, so I'll just keep going until it stops getting better I probably end up about 9% co, but hey it's exempt from MOT emmisions!

The throttle bushings appear to be in good condition, there is very little play so hopefully it's not leaning out breathing through the gaps. As far as I know the inlet manifolds and spacers are all sealed off OK.

I think also I need to double check my blanking of the auxiliary idle system, maybe there is a leak somewhere there.

It's nearly there, hot running is fine, but this hesitation makes it difficult to start and catch on the throttle. I'll let you know how it goes.

[Barry]