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Carby issues...

Wondered if anyone could help me sort out an issue I’m having with my WDM20. I can start it with choke full on and it will tickover fine. If I open the throttle, still at full choke, it will hesitate slightly but rev up ok. If I open the choke halfway, the tickover will get rough and any throttle opening will cause it to stumble. Full open choke will cause it to die. The pilot air screw setting has no effect. I can close it entirely and the motor will continue to tickover. Any thoughts as to what is going on with the carby?

Re: Carby issues...

An often missed problem is this tiny drilling which can get blocked with verdigris. The jet block needs to come out and clear it with a thin piece of wire and the corresponding drilling in the jet block. Clean and blow out all other drillings and jets and make sure the flange is absolutely flat by rubbing it on W&D paper on a surface plate or sheet of plate glass. The flanges can bow quite easily from over tightening the nuts (just nip them up with a spring washer). Ron

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Re: Carby issues...

The initial symptoms that you listed were indicative enough, but the comment that the pilot air screw has no effect suggest that you've pretty well diagnosed the problem already.

The cause will be blockages in the pilot air drillings in the 'jet block ' and possibly the 'mixing chamber' also. The only way to clean these effectively is to dismantle.

If you haven't done it before, be careful. It is easy to damage the jet block. Best technique in my opinion is to drop the carb in a pan of water at boiling point. Hook it out and tap it upside down on a length of softwood dowel that fits loosely in the chamber. It should then tap out. Warm it up agin before re-fitting.

There are a number of tiny drillings in the jet block which allow both air and fuel through.

Re: Carby issues...

Hello Keenan,

It sounds as if you have a problem with the mixture being too lean. First thing to do is a visual check. Is fuel arriving freely at the carb? Then check that the carb is firmly attached to the intake, and that the face that attaches to the inlet is not bowed. This can happen with over-tightened bolts. An easy test is to smear some grease on the join and see it that makes the engine run better.

I'd advise also taking the carb apart for a good clean. Is the throttle slide very loose? You could be getting extra air that way. That said: the single most likely problem is that the jets are clogged. Dirt in the main jet will upset everything! The bike will still run on the choke, though, even if the main jet is clogged.

I hope this helps.


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Re: Carby issues...

Thanks all for the fast replies and advice! I thought that I had checked that tiny port for blockage before I mounted the carb, but maybe it needs another look. I will take down the carby for a good cleaning as recommended. Thanks again!

Re: Carby issues...

Without removing the brass jet block, it's simply not possible. Invented fifty years before ultrasonic cleaners were in common use, the Type 76 / 276 could have been developed precisely to be cleaner-proof ! :-)


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