Hi I have a problem with my twin webers and its doing my head in!
I am running twin weber 40mm dcoe carbs on a standard l20b. The engine, timing, plugs are all fine and I know the problem is the carbs as its the second l20b I have bolted them to in the last week or so and they were doing the exact thing on the last engine.
The problem is that in low revs they miss fire and back fire through the carbys , they run OK up in the revs.
Also when number 3 lead is pulled there is no change in revs in the engine at all. All others make revs drop as it should.

I have taken the idle jets and main jets out and cleaned them and it made a little bit of difference but still number 3 lead off and no change. I have also tested for air leaks between carb and manifold but seems OK although my methods were crude.
Any advice would be appreciated as I'm ready to use the webers as a door stop.

You've pulled the jets and checked they're clean, but have you checked the passage ways thru the carbies ?

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Happiness is watching the race.... in your rearview mirror

No I didn't do that. Can it be done without taking them off? Getting sick of buying manifold gaskets.
Also the idle mixture screw on the barrel on the carb that runs into cylinder 3 does nothing when its turned?

run 17deg or more timing, missing thru carb means lean

Would timing effect one side of one carb?

Have you synchronized all the barrels. Put a small piece of tube in your ear and then in each throat. Adjust the synchronization until they all sound the same.

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Circuit Race 1600 in the build

Next time you need to pull them off or buy a gasket, coat it both sides in neverseize , or grease if that's all you've got. Fixes any leaks and makes the gasket reusable.
I have loctite nickel antiseize - expensive but lasts a lifetime.

Compressed air through the passagways. Or before you pull them off again, rev them up somewhat and stick your palm over the inlet of the bad one, then release. Rinse and repeat. Old trick, might suck out whatever blockage is in the. It's perfectly safe, I've never seen anyone lose an arm yet - but I did hear about this one guy...

Get a set of soft mounts, you won't need to replace them every time the carbs come off. It does sound like you have a blocked idle circuit on number 3 cylinder as stated. Dissemble entire carby and blow the crap out of everything with compressed air and carby cleaner.

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Chris Thomas

1971 1600, mild L18, 5 speed

All I want is less to do, more time to do it and higher pay for not getting it done.

Thanks for the tips and I think I am getting somewhere.
I took the jet covers off and idle and main jets all out. I then sprayed carb cleaner everywhere I could get it inside the carbs. When back together I started it up and put my hand over the ram tubes then released (thanks for this tip)

The carbs after a few trys of this entire process are running way better and cylinder 3 carb side seems good now.
When I drove it there was still backfiring through the carbs at cruising speed and had a sort of flat spot up high in the revs but off the line the car is great.
I think the next step is to take the tops off and use compressed air and more cleaner.Then adjust timing.
Cheers