Diagnostic 2nd opinion on a problem much appreciated

[KeithMeadmore]

I would very much appreciate a diagnostic 2nd opinion on the following. I am not a skilled mechanic but I think can do most things okay ........ except diagnosing. I have read the workshop manuals and studied the Forum and I think its a valve problem (maybe inlet valve staying open?). But before I start taking seat, tank and rocker boxes off and maybe head also etc I thought best get advice from the Forum just in case its something embarrassingly simple.

1960 G12CSR. Engine was professionally rebuilt maybe 3 or 4 years ago. Regularly used bike and a good first or second kick starter. All-of-a sudden difficult to start and when it does engine sounds like it is only running on one cylinder with lots of black exhaust smoke filling the garage (siamese pipes so cannot tell which cylinder). Needs half throttle to keep engine going. Adjusting the carb has no affect whatsoever but slightest obstruction to carb air intake and the engine stalls. Likewise if I give it choke. Petrol vapour blowing out of carb! Both plugs are very sooty. In-line HT tester confirms both are firing and a good spark.

New plugs fitted. HT leads changed. Checked pick-ups. Checked the mag earthing. Checked the points. Checked the Amal 389 carb and everything seems to be in order. Gauze carb filter cleaned. No sign of any air leaks at the manifold, but then that would cause symptoms of weakness rather than sooting. No visible evidence of head gasket blowing.

Advice very much appreciated

Keith

[dodger]

Test the compression on each cylinder, if there is a difference then it is likely a valve is stuck open.
If a compression tester is not available try taking out one plug and press down on the kick start, note/feel the effort required. Replace the plug and take out the other and press down on the kick start again. If you can detect a difference a stuck valve is the likely culprit.
Laurie

[Merlin]

When running just splash water on each pipe near relevant cylinder should tell you which side to start looking.

[clanger9]

From your description, I'd guess at a stuck/leaking float valve. It just sounds like it's running horribly rich, hence the "8 stroking", black plugs and black smoke. Those aren't the symptoms I'd associate with a stuck valve.

Here's a nice description of how to check the float height on a Monobloc: http://www.whfolk.co.uk/REOCHUMBER/Articles/monobloc.htm

Good luck, hopefully a simple/trivial fix...

[Aprillia750]

Keith, It happened to my model 31 fitted with a concentric carb. Exactly the same symptoms. The carburettor needle had slipped out of the needle clip. On closer inspection the copper clip had a tiny crack across it. I bought a new clip-problem solved. Just something to check before taking more drastic action.

Best of luck, John.

[KeithMeadmore]

Thanks to all, and especially to you John. Problem now solved and it was indeed embarrassingly simple.

Started off testing the compression as suggested by Laurie. I didn't have a tester that would fit without removing tank, so resorted to the finger method and everything seemed okay to me, so then moved on to the carb. I was pulling the top of the carb out in the process of taking carb off and blow me the needle fell out and dropped to the floor. But copper clip still in place! Pulled the clip out to ditto find this also had a bad crack in it and too wide to be able to retain the needle in place..........just as John had found. Rummaged around in a few boxes and luckily found another in good condition. While carb was off also checked the float and valve for sticking per clanger9 and everything there was okay, but I've kept a print of the attachment in the library just in case. All now back together and running perfectly.

Thank you all from stopping me from going down the wrong path.

Best regards, Keith

[clanger9]

Brilliant, glad it was such a simple fix!
I've not come across a cracked needle clip before, so that's definitely something I will check for in future...