Usual thing. Haven't pointed a spanner at the M20 for months, so I've forgotten everything I thought I once knew.

Two problems, possibly related.

1. Left side petrol tap was blocked. Tank off. Flush. Refit. Works.

2. The bike, however, needs the choke to start, and it runs lean without it. Took off the carb. Cleaned what was cleanable. Main jet okay. Can't see that there's anything wrong with the pilot jet screw. No blockage. Refitted. No obvious air leak around the carb flange (tufnol spacer fitted). Flange not too tight, not too loose. Bike runs okay otherwise, but the air screw does little or nothing. It needs to be almost all the way in. No air filter fitted, by the way. So what did I miss? Can anyone shake some dust from between my ears. Might have asked this before, but you know how it goes ...

FOOTNOTE: I'm wondering whether this is connected to an exhaust valve leak. I had an exhaust valve problem about 500 miles ago, and that was premature for as engine with only 3000 miles from new. I'm wondering if I haven't fixed this properly, and whether air is being drawn in this way (via the exhaust) weakening the mixture. Can that be possible?

email (option): dannydefazio@sumpmagazine.com

There could easily be a blockage in the pilot jet air way. If you take the pilot jet screw out you will find they the flexible tube from your tyre pump will screw into the pilot screw hole. You can then use this like a straw and blow down the pump,tube, which sometimes clears the blockage. Otherwise I would say the slide may be worn. Cheers John T

John, thanks for that (and so promptly). Stupidly, it never occured to me to question the throttle slide. That's what happens when you stop messing around with bikes for a while.

Is there a simple test aside from dismantling?

email (option): dannydefazio@sumpmagazine.com

Hi Danny...I would suspect a blockage in the pilot airways which are very small drillings through the carb body and jet block...

If these were blocked the screw would indeed have no affect at all as it would not be regulating the passage of air through the pilot system..

If it were the slide, that may weaken the mixture but the range of pilot adjustment from fully closed to fully open (assuming it were functioning) would still be likely to have a noticeable effect...

It doesn't really matter how worn the slide is..you would be able to readjust the mixture to compensate...It would just mean that you would consume more fuel (well the bike would)

A complete carb strip down and clean out is probably the answer to your woes..
Ian

email (option): ian@wright52.plus.com

Hi Ian

Firstly thanks for your comments about Sump. Spotted the post just yesterday.

Next, your dirty carb suggestion makes sense. Haven't used the bike for a while. Hence possibly gummed up a little possibly. Also, there was a blockage in the petcock. So, dirty fuel is already in the mix.

I did have a problem some time ago that was traced to a small hole somewhere on the carb. Easy to miss. Will look for that.

I can't see that the throttle slide is worn at all. And I doubt my exhaust valve theory has much meat on the bone. Now I'm suspecting the plug. I've got an old Lodge CSN. Ted Bloomfield sold me a box years ago. This begs another question; does a failing plug generally demand more or less fuel? Do you know? Or is it more complicated than that?

Will get a new plug first and rule that out. And I'll check and clean the carb again. Ta very much.

email (option): dannydefazio@sumpmagazine.com

Danny, that little hole is at the bottom of the bore between the slide and cylinder flange. You need to remove the jet block and poke it out with a bristle from a wire brush, also the corresponding hole in the jet block. Ron

email (option): ronpier@talk21.com

Ron, thanks for reminding me. I had this problem some time ago. Will get onto it tomorrow, weather permitting.

email (option): dannydefazio@sumpmagazine.com