Rough running

[connerdsny]

I just put together another 2 stroke bike after the first one cracked the bike frame. I used the same engine, but replaced several of the bolts and gaskets, and smoothed the head and cylinder in the hopes that my engine would run better. I noticed a bogging problem before my bike frame cracked, and it's only gotten worse since I put my new bike together. Since I'm running out of things to diagnose, I figured I'd post something here and let someone more knowledgeable guide me. On my previous bike, the engine would run great for 10 seconds at top speed, then suddenly bog down and decelerate. Now, the engine is a constant bog but will suddenly pick up after riding for 5 minutes. Ive replaced way too many parts on this engine, and I don't know where to look. Any help is appreciated!
New carb, new spark plug, new gaskets, fixed wiring, new fuel feed.

 

[mikedabomb]

Check out on google tuning the fuel and air ratio of a two stroke. Sputtering, not accelerating, blue smoke/oil out of exhaust - rich air/fuel. High idle, holds rpms after quick rev, wants to take off, no visible smoke/oil from exhaust - lean air/fuel.

 

[connerdsny]

Thanks, Mike! Although I've spent way too many hours trying to tune the carb with little resolve- I'm beginning to doubt thats the problem. It's had a very consistant decline in performance since I started riding... the intermittent bogging is what's confusing me- it seems to have no pattern other than occurring more often the longer I ride. could a leak in the engine case cause this?

 

[crassius]

I'd suspect that your motor mounts are allowing some shake in the motor at certain revs which is then shaking the carb and foaming your fuel in the bowl - slows down like ign missing, then picks up for a bit, then slows down again and so forth.

 

[Timbone]

If there is a weakness in the bike frame, these little two strokes will bring it out. I think the best approach is to consider a frame (unenhanced) to have a working life of one year. Always keep an eye out for a new frame.

Bogging out on the top end (with predictable 4 stroking) is a classic symptom of too rich mixture. The solution is simple: change to a smaller jet in the carb. That leads to another problem: these 2 strokes start best with rich mixture. It may be that the perfect jet size for top end riding can make the engine hard to start.

 

[connerdsny]

Thanks for the help! I need to make new mounts anyway, so I'll get on that and see if that helps. I'll also order some more jets, as this new carb I got didn't come with any.

 

[Deleted member 12676]

I'd suspect a leak at the crank seals.
the right side of the crankshaft has a lip that the seal can catch on, so even a new engine can have that seal leaking due to not seating correctly.
does it take a while for the idle to settle down to its normal putt-putt after twisting the throttle and then letting it go? that would indicate a leak at the seals or at the intake manifold (usually where the carb connects to it)

 

[connerdsny]

Yes, it does take a while to return to idle. I've replaced the intake gasket, and the carb/intake seal was leaking at one point but I think it is fixed. How would I check the crank seals?

 

[Deleted member 12676]

take the magneto and stator coil off
take the spark plug off
spray the seal area with soapy water (using dishwashing soap)
turn the engine over and watch for bubbles
then on the right side take the primary gear off and do the same
bubbles indicate a seal leak

 

[connerdsny]

Okay, thanks! I'll try that tomorrow when I get a chance.
For now, here are some pictures since I never posted any.