Diagnostic 2nd opinion on a problem much appreciated
Posted: Fri Aug 19, 2016 3:07 pm
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
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