Weber Carb (ICH34) Idle Problem Solution

Hi Everyone,

Rather than putting up another question for you guys to throw yourselves at, I thought I would give a solution of sorts.

My 2a has been running ok since the rebuild (not engine) but slowly the tick over idle has started to get worse. On saturday I had to turn the idle screw right up (several turns) to get any sort of tick over.
During the last issue with the carb, I had bought a Rebuild kit from fastroadcars.co.uk (good service and advice) but never fitted. So dropped the carb off and stripped down. Lots of sediment in the bowl and the internal filter was also not so good (a bit restricted). Blew everything through/cleaned and fitted new idle, mixture screw, filter and o-rings and popped back on with a new external filter and..........all is perfect! well almost.

She has been developing (before the carb rebuild) a mild misfire.popping on overrun/downhill - as I live in the mountains I get it a lot- I'm debating whether the 50 idle jet is a bit small hence running too lean??? thoughts anyone??? Will double check for air leaks etc etc. Performance up hill is great (obviously that's a relative great)

For the sediment, it may be worth checking the mesh screens on the tank pick up pipe and in the fuel lift pump as well as the small filter on the carb inlet which you've cleaned out.

For the popping on over-run, it's worth checking the exhaust for leaks - also, perhaps, check the exhaust manifold for cracks where the downpipe connects.

The 50 jet is standard and should cope with hilly conditions but something else is causing it to get dirty. Backfiring is caused by other things too. Check ingition and points etc. Setting the idle requires establishing a baseline first. The carburettor kit should have the instructions for that. If not try googling there's loads of PDF downloads. Basically you start by screwing it all the way in and then turn out a full turn. Start engine and listen to the idle speed. If it's still to low turn it in half turns until you achieve an even idle speed.

if its got the black plastic float ,look out ,theyre full of sponge...........................hairline crack where brass arm enters and the sponge soaks up fuel and sinks....ask me how i know

check your exhaust manifold joins, i.e to the head, to the inlet manifold hotspot, to the downpipe. air leaks in the exhaust can cause secondary combustion (with unused fuel) which causes the popping.

Thanks for all your posts and replies. However things have developed!!!!
I thought all was ok with the carb and went for a longer test drive and to my disappointment she stopped ticking over again - could not believe it so please ignore my first post. really went through the possibilities and took the carb off and had a look at the adapter plate. It has a take off for a servo which I have blanked off but that looked ok. looked under the adapter plate and the plastic packing piece would not move and when attempting to lift if broke into a dozen pieces. As we have our only show this year on Sat, decided to try and make a new one. A quick visit to the local Rubber suppliers (yes we have them here) and he made a couple of spacers from 6mm oils resistant rubber, stuck these together with a few gaskets to increase the spacing. Must add also cleaned the carb again whilst off but looked ok.

To my amazement not only was the tickover OK again, but the backfire was solved - at this stage assuming is was an air leak at the manifold caused by failing packing piece which when hot expanded and leaked. Maybe this made a very lean mixture that would not ignite but did so in the Exhaust manifold.

Hope to receive the new packing piece next week but in the meantime she is running wonderfully.

No great innovation or invention but I was very surprised that the backfire seems to have been resolved and identified as a leaking inlet and not exhaust!!!!

Hope this could help someone.

Thanks for reporting back on your findings, always nice to have an end to a story

Unlikely one that, not sure many would go looking there

OK Carb latest - the misfire and the stopping ticking over is back - ahhhhhhhhhh. This time took the Dissy to bits but all looked ok re diaphragm and advance mech etc etc so back to the carb. Took the top cover off and there was sediment again in it!!! Took the sender out the tank and there is quite a lot of debris in the tank, Looking at the new inline filter shows, what looks like flakes in it. Cleaned carb again. Also tested the blank for the servo take off. High tech test by taking the hose off which is blanked by a m8 bolt and jubliee clip, and sucking on it, revealed not too well sealed.

Discovered that the servo take off is the perfect size for an M6 tap, so tapped and with some sealant on a m6 screw inserted.

Restarted and ticked over and the backfire gone again!!! - but for how long??
Will run the tank down and clean it out, new filter and try again. Hopefully towards the end of this one!!!

Well back again, The tank was drained down, cleaned out, carb cleaned and new Mann filter installed and all was fine.....until about a week later when the problem of not ticking over came back ahhhh. Took the carb off again and yes there was debris in the float bowl again. Can these filters be really this bad after only a few hours of use?? Well took the Carb to bits again and noticed something on the float. A kind forum reader sold me a new float which he had 'belt and braces' sealed with super glue around the seam. It appears this is now coming off!!! This could be the source of the Debris in the bowl. Swapped the floats and cleaned back on and runs fine (as expected). Never thought the enemy could be within the carb and on a new bit!!!!

Will report if happens again - Incidentally when running ok the exhaust pops are not present or vastly reduced. I am guessing to a richer mixture on tickover as idle jets doing their job.

Gripping story- I hope it has a happy ending!

OK, 8 Months later and a few developments. The above did sort the problem but it came back. Of course the dirt and crud in the fuel cannot of helped and was at least partly to blame. Also noticed that the idle jet was getting gunged up with oil!!!. Seem to be blowing a fair bit of oil so looking like an engine build a some point soon. but apart from throwing a bit of oil over the rocker cover running well. Hope to do the rebuild in the summer!!!! Will post when I do.