Engine Trouble Sudden loss of power. No clue what caused it.

[jefuchs]

I went on three or four short rides yesterday, and everything was finally just right. I think I rode about 30 miles.

At the end of the day, I decided to go for one last spin, and a new problem developed. When I accelerate to about 19 or 20 mph, the motor loses power and sounds like it's going to die. The throttle seems to have no effect on it when it does this. It slows down to the point that I'm going 18 mph or less, then it's fine again.

As long as I don't approach 19-20 mph, you'd never know anything was wrong.

I'm finishing up my break-in period, and have burned over 1.5 gallons of gas. Still on standard blue oil, 30:1, if that's important.

I don't see any compression loss through the decompression valve anymore (thanks for your help with that) and even when it was leaking, i didn't have power issues.

I don't know if this is carby, timing, fuel, or what.

Any ideas?

 

[jefuchs]

Update: Before anyone asks, the air filter is clean, and I have the same problem with the filter taken off altogether.

I get the same problem under a no-load situation (throttle disengaged). It revs up, then dies down, then revs, then dies down, in a uniform sequence that lasts three or four seconds per cycle.

I do not have the copper tailpipe installed at this time. I had put it back on yesterday, and I suspected it was the cause of this problem, so I removed it, with no improvement.

 

[crassius]

sounds like a fuel problem - maybe a clogged breather hole in the gas cap, or fuel filter problem, or float problem

 

[jefuchs]

Tell me if this makes sense: After crassius' post, I took the fuel system apart and confirmed no clogs, etc. Still, the problem was there, and getting worse. Now it was doing it at 17 mph, instead of 19. I noticed increased vibration, too.

Then I spotted a potential problem. A mounting bolt had loosened, allowing the motor to vibrate more and more at lower and lower speeds. I tightened the bolt, and took it for a 4 mile run, and had no problems.

Could that have been the only problem?

I think the banner day for a motored bike owner isn't the day he builds it, or gets it running for the first time. It's when he can ride the bike a whole day without having to repair anything.

 

[jefuchs]

Correction... 16:1 oil ratio. 8 oz. per gallon.

 

[BigBlue]

Tell me if this makes sense: After crassius' post, I took the fuel system apart and confirmed no clogs, etc. Still, the problem was there, and getting worse. Now it was doing it at 17 mph, instead of 19. I noticed increased vibration, too.

Then I spotted a potential problem. A mounting bolt had loosened, allowing the motor to vibrate more and more at lower and lower speeds. I tightened the bolt, and took it for a 4 mile run, and had no problems.

Could that have been the only problem?

I think the banner day for a motored bike owner isn't the day he builds it, or gets it running for the first time. It's when he can ride the bike a whole day without having to repair anything.

Loose bolt and shaking could have caused foaming of the fuel.

http://www.motoredbikes.com/showthread.php?5259-WOT-Performance-top-end-bogging

Also might want to check your spark plug wire for loose connection at the CDI and other electrical connections. Might be the spark plug wire grounding out from a crack in the wire, bad plug or a loose ground connections.

Good Luck,

Chris
AKA: BigBlue

 

[crassius]

yeah, I've also seen fuel shaken to the point at which the bike acts like it is running out of gas