Bike engine refuses to run!

[Pompey]

Thanks to all that have responded.

I took Peter Gibson up on his very kind offer off the loan of his carb which I fitted with my jets installed (his is a G80 mine is a G80TCS). Found no change in result until I screwed the pilot jet screw in with 1/8 turn left (1/8 out) and the engine ran! Put my carb back on with its jets and even with pilot screw "bottomed out" would not run but made promising attempts.

So proves the problem is with the carb and namely the pilot jet size. So as a 35 currently installed will now try a 40 and see what happens. The only thing that I can't comprehend is why a bike runs perfectly for 7K+ miles on a 35 jet then suddenly overnight decides it requires a 40 or more?

Nick

[1608]

Which I think only goes to prove that there is still a blockage, probably in the very small hole in the body immediately engine side of the slide. Short piece of guitar steel string is ideal for clearing that. Don't think jet size is the issue.

[g80csp11]

The carburetor that Nick borrowed is fine on my G80CS , and not blocked in any way

[SPRIDDLER]

......has been running fine until last year. When I went out to start it and other than a single bang would not run.

If it ran O.K. for 7k miles up to last year I can't see any reason at all why you should need to fit a larger Pilot jet now.
By closing up the Pilot screw you are reducing the volume of air to fuel being fed through the jet, thus richening the mixture.

Are you using fuel with Ethanol? Ethanol contains 30% less energy than petrol. Ethanol also absorbs moisture whereas petrol doesn't. Ethanol also evaporates more slowly than petrol so since there was no improvement when using Thrifty Peter's carb and the bike has been unused since last year could it be simply that stale (i.e. Ethanol rich) fuel is causing the problem?

[Groily]

If the jets are 'right', and it ran with them in the known-good carb but only with the pilot air almost completely shut, then the weakness may be remedied by upping the pilot jet - but that may just be compensating for something else.
Could it, for reasons not always easy to fathom, need a smaller cutaway on the slide for example?
Makers' specs don't always work best, or at least not round here sometimes! (And certainly that has been true with (some) replacement carbs.)

Or maybe there's an air leak at work here, despite all best efforts - carb to head, or on the carb lid, or jet block to body, or somewhere?

What happens if you try to start it with your hand over the carb intake?

[jackstringer]

Clean the carb in an Ultrasonic cleaner as that might remove some of the random crap in the holes.

Also, clean the jets in the ultrasonic as there maybe some crap in there. If in doubt buy some new jets.

Check the parts diagrams, Are you sure you have everything you need?

[SPRIDDLER]

Clean the carb in an Ultrasonic cleaner as that might remove some of the random crap in the holes.

Also, clean the jets in the ultrasonic as there maybe some crap in there. If in doubt buy some new jets.

Yebbut, (edit: not Neil) Nick said that it wasn't any better after fitting Thrifty Peter's carb which definitely didn't have any blockages.

(We seem to be going around in circles with this one).

[g80csp11]

Spriddler , has Neil got a problem as well as Nick . must be more common than i thought LOL
The other thing I would change is the insulated spacer between carb and head as a leak does still sound like its there somewhere
Im sure I have one
Nick , how thick is your spacer ?

Peter

[SPRIDDLER]

Spriddler , has Neil got a problem as well as Nick . must be more common than i thought LOL
The other thing I would change is the insulated spacer between carb and head as a leak does still sound like its there somewhere
Im sure I have one
Nick , how thick is your spacer ?

Peter

Senior moment Thrifty - now corrected to 'Nick'.

Yep, an air leak would be a common cause of the problem (need to close up the pilot screw, so forth) but in Nick's original post he said:

6. Carb face and heat spacer all flattened and checked on a surface plate so no air leaks there.

[g80csp11]

cracked spacer , porous spacer. its difficult to get stock carb setting as it would have had a GP carb ( I think ). I always fit a paper gasket between the head and spacer , then just carb O ring