74 250 HS backfiring

[Richard Carlson]

My bike was running fine than I took it for and ride and it started bogging down and then quit and I walked it home. Once I got it home I thought carb problem I pulled the air cleaner cover and it was drenched with gas Aha I thought stuck needle or float. Since then cleaned carb several times checked float level pulled drain on crankcase (got 1/2 tsp of gas) checked timing and still it will run a little then backfire out the carb and quit. Still seems boggy when it does start.

[Larry Perkins]

See if you did not shear the flywheel key.  That would put it out of time and if not firing at right time the gas will not burn completely and therefore the soaking.

Larry P

[Richard Carlson]

12 volt ignition no timing key plus checked timing seemed to be on

[KJDonovan]

Richard,

You may want to check and see if any critters or insects (mud wasps) took up residence in your exhaust.  Getting to be that time of year where mice and chipmunks are looking for cozy spots to spend the winter.

Thanks,

Kevin

[Richard Carlson]

Don't think so KJ got the cone tip and as for wasps started it every two or three days. I will check the timing a little closer

[Carl Hill]

I am with Larry P. I believe I would pull that flywheel off and double check the key.

[Richard Carlson]

Gentlemen I think you are correct with the timing issue. As for a key way I put a Power Dynamo ignition on it and when you do it eliminates the key way. I think when I retime  it I will put a couple more lbs. of torque on the nut

      Thank-you Sirs, Richard

[Daniel P. McEntee]

The MZB Power Dynamo systems can slip. Get some fine lapping compound and lap the flywheel to the crank shaft carefully and then thoroughly clean it. Then when you re-time and secure the flywheel, just a drop of lock tight on the shaft taper will help keep it from slipping again. Old Husqvarnas, especially when they converted to Motoplat ignitions, were known to shear keys, and that was because there was a very slight mismatch in the tapers. I got an email from Dane Leinbeck at Penton Imports with the specifics severakl years ago (Thanks Dane,we miss you!) A simple lapping and checking with hi spot blue or even a magic marker can tell you how well the fly wheel fits. There was a guy on the VINDURO list (Bob "Rocket"Rigby)that carried a kit with him to races that consisted of a cordless drill, lapping compound and special bits to attach to a flywheel to lap them in in the field. Once done correctly, you never have to worry about it again.
  Type at you later,
  Dan McEntee

[Rocket]

I would suggest using some lapping compound and lap the flywheel to the crankshaft.  If the flywheel is moving, you probably don't have good contact between the two tapers.
RCG

[ALB]

I originally had problems with the MZB that I installed on my Jackpiner.
Timing was spot on, it started right up, ride it out and as soon as the power band would hit, the bike would run with significant loss of power.


Problem - the flywheel would spin a little bit (no woodruff key is used)on the end of the shaft. There was a little film of oil or grease on the shaft.  [:0]
The Fix - Removed the flywheel, carefully clean the shaft and inside taper of the flywheel with a solvent. Reset the timing. Tap the flywheel to seat it onto the shaft, then tighten down the flywheel nut.

No more flywheel spin. Bike runs great. [8D]

Alan Buehner