I'm writing this post to document my recent experience with my uncle's 1979 r65. I was looking for advice online and could not find it. I hope this will help other people.
Years ago I parked the bike due to a lack of power at high RPMs (>3000). A year after parking the bike, I prep it to give it to my uncle. This included rebuilding the carbs and replacing the diaphragms. The diaphragms did it. I think the diaphragms were bad since I bought the bike as I felt more power at that point than ever before! Over 1200mi in two days from Portland to LA and my uncle received a nice gift.
Fast forward 7 months to November 2007, I call my uncle up to see if I could borrow the bike for a Baja trip the week after Christmas. He says yes, I smile. January 2nd, I'm off. 4 days and 1500mi later, I'm back home with many great experiences and stories.
THE PROBLEM: After about 1000mi, the bike was severely down on power at low RPMs (<2500). The exact OPPOSITE problem of a few years ago!!
Except for a slight increase in vibration, the bike ran fine above 2500. I thought it might be the 5gal of Jerry can super unleaded I bought in Catavina. After purging the gas, the problem persisted. By feeling the exhaust, I realized the right cylinder was severely missing at idle. Verification happened back in Long Beach when adjusting the right side idle screw and pulling on the right throttle cable caused no change in the engine speed. The cylinder was dead! But again, when I revved the engine above 2500 RPM, the engine sounded fine.
Solution #1: Since the problem occurred at lower engine speeds, I decided to rebuild the carbs and replace the, I thought, fouled idle jet. No go! ARGHH!!!!
Solution #2: I had a feeling compression might be the problem. I wasn't blowing out of the head gasket, and since I was burning about 1qt every 700mi, I thought it might be the rings. I knew I was not going to be able to do much about the rings. So I decided to check the last thing that may affect compression. Sure enough, the exhaust valve on the right cylinder had no clearance. The valve likely was not sealing against the seat completely causing a slow rate loss of compression. I adjusted all of the valves to 0.006 and 0.010, intake and exhaust respectively. BINGO!!!
I mucked with the carbs so much they need to be adjusted for mix and balance, but the bike started really easily. The idle adjustment screw actually changed the speed of the engine. So did pulling on the throttle cable. The bike will soon be ready to for delivery to my uncle.
The theory: At lower engine speeds, the fuel air mixture spends more time compressed than at higher engine speeds. Assuming the pressure inside the combustion chamber at TDC is the same at all engine speeds, of course it isn't but bear with me for arguments sake, the pressure inside the combustion chamber at the time of combustion would be lower at lower engine speeds than at higher engine speeds because of a slow leak. This is why the engine ran OK at higher engine speeds. At lower engine speeds, the loss of compression caused problems with combustion.
While I am glad my uncle is experiencing the thrill of riding again, I am missing this really cool motorcycle!
_Hugo
