Do I need new rings?

[DWade]

Long story short. I bought an 81 750 a month ago. 18k miles, 100% stock. Bike ran fair, but wouldn't rev past 5500 and idled a little erratic. New plugs, valve check (.004 to .006, all within spec), adjusted timing chains, rebuilt carbs, bench sync.

Bike ran noticeably better, but popped out left exhaust at idle and surging at cruising rpms. Synced carbs on bike, symptoms got worse.

Pulled the plugs. Number one is very very lean, number 2 is very fat, and 3 and 4 look acceptable.

Did a compression test and found this:
Cyl 1: 75
Cyl 2: 25
Cyl 3: 50
Cyl 4: 50

Brand new compression tester. How could it even run with numbers that low?

 

[amc49]

Compression tester if one that simply pushes against the hole is worthless, any one under $50 is too. Most of them out there are junk. If you are not spinning motor fast enough then numbers will be down too. I use a car battery but car NOT RUNNING, the alt will kill you. ALWAYS suspect the tester tool and even more if a rental. If some blockhead has changed the valve in it that looks like a tire tube valve, tool will NOT read right, a compression valve stem has maybe 1/4 the tension a tire tube one has.

All 4 plugs yanked, throttle wide open and spin each cylinder enough so that tester quits bumping up.

Your plugs look like crap, all of them. Can't really see the last two due to lighting though, but black everywhere except plug tip is still too dark.

Unless you know of positive damage from something (oil not changed, overheated in some way) the rings usually are good at that low a mileage, the flaws are almost always in the valve sealing.