Engine Trouble engine help?

[wan37]

I have a 80cc BFG engine, my problem is kinda strange. I have a problem with sheering
key ways on the little bevel gear on the crank. I have also tried a centrifugal clutch on it
broke the gear on the centrifugal clutch. I'm also have a pull start on the other side it is
really hard to pull. I'm wondering if it's because something wrong with the big clutch
gear or inside the clutch, but I really don't know. The clutch seems to work great
when you use the lever? Hope someone has some idea's to help me. Oh I have also glued the
key ways in the slot with super glue ,and still sheers them. Hope someone has a answer for
me on this.

 

[crassius]

that gear sits on a tapered shaft - if it is on straight & tight, then the taper alone should hold it

that said, these are chinese POS shafts that may not be tapered straight or with enough angle, just be sure to drive it on tight and use an impact driver on the screw that holds it on

 

[Deleted member 12676]

take off the primary gear and the drive chain, then spin the clutch and see if there is much resistance to turning. could be a bad bearing. If you don't tighten the primary gear down tight enough then that would allow the keyway to shear off. It has to be very tight. But that is impossible if the taper of the shaft and the gear don't match well. To improve that you can put valve grinding paste on it and then turn it so that the abrasive paste will wear down where the two surfaces prematurely touch. (raised areas)

 

[wan37]

Okay how do you tighten the big screw the whole crank shaft wants too move. I would also want to know where to get the impact driver from, harbor freight looks to cheep made?

 

[crassius]

stick a rag between teeth of big gear/small gear to hold it

impact drivers aren't a high tech item, so a cheap one should be OK

 

[Purple Haze]

I think Jaguar is on to something; the bearings in these motors are crappy. Take the cover off the mag and put a wrench on the nut, with the clutch released it should turn easily. If not, replace the crank bearings with SK or other quality bearings. Most of these are sealed; remove the inside seal for lubrication, the outer seal will help seal the crankcase. Sometimes there is a bearing misalignment that causes binding of the crank, but replace those bearings anyway, you'll be glad you did.

 

[Deleted member 12676]

yeah and check the alignment of the flywheels.
put a straight edge on them at 4 locations 90 degrees apart from each other. check that it is flatly mating to the surface of both.
then cradle it in something so that you can spin the crank and watch the flywheels to see if they wobble. Tap them with a hammer to make corrections.
I just recently did that and then rev'd to 9000 rpm with smooth sailing. It isn't a job only for professionals.