QuestionBill, Its great to see your responses covering a whole range of issues with older bikes. I have recently rebuilt a 1976 Honda XL 175 for the purpose of racing vintage enduros. The engine is freshly bored with new pistons and rings and the head, valves and cam reconditioned, the cam has a mild grind. I am also running a larger free flow exhaust. Carb and air filter is standard.
The bike starts fine, operates well at low revs but has no mid to high range at all - it will not rev out at all. I have tried various jetting, (mainly going richer,110 standard, now running 118 and lifted the needle), advancing and retarding spark via the points. It also back fires on de acceleration. Im beginning to run out of ideas.
This afternoon I check the spark advance unit - I noticed the springs are sloppy, ie if you open it up the weights do not return; there little in the way of spring tension. Is this likely to be cause and if so would I be safe to tweak the springs a little to get some tension back.
I would appreciate your thoughts and any other ideas you might have.
Thanks Andrew
AnswerAndrew, yes, absolutely clean, lube and adjust the return springs on the spark advancer.
Make sure all the basics are solid first, especially the spark timing. It sounds like your initial timing was coming in too fast or the weak springs may have been causing inaccurate timing. The backfiring on acceleration is probably spark timing issues and/or lean condition.
Check your compression readings. Running a mild cam and less restricted exhaust with stock carb may be causing some infighting between the parts. If you left the stock porting, the cam may be more agressive than the flow of the head can match.
Once you get the ignition timing dialed in, keep working with jetting/needle positions. Start RICH and work down, especially with a fresh piston/ring install.
Bill Silver
