Rough Running

GoldenMotor.com

Trey

$50 Cruiser
Jan 17, 2013
1,432
5
0
Where cattle outnumber people 3 to 1.
I'll bite. As a guess, I'd say the raised center indented portion is off-center. Or is that just the pic? There may be a portion of a ring missing. (Chinese?) 20 teeth, made out of metal... I give, what is it? lol I'll wait around to see how I did. ;)
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
It looks like the gear isn't concentric with the center hole.
Give the man a cigar.

Yes this is the primary gear from a Chinese 2 stroke. The engine runs good, very powerful but it has always been noisy and has a strange pulsation and a roughness that wasn't consistant with crankshaft speed but much slower. The gear ratio is something just a little over 4:1, more like 4.3:1 so the pulsations I could feel were more like clutch RPM than crank speed.

Our weather has been so bad that I haven't had a chance to really run this bike much but during another snow storm today I was putzing in the garage and I pulled the gear cover off, pulled the plug out and using a cordless drill with a screwdriver bit, spun the crankshaft. "Holy wobbles, Batman. That little gear is way off center and it wobbles side to side."

Not only is the gear not concentric with the crankshaft but the bore is drilled at an angle. The gear would bind then loosen, bind and loosen as it meshed with the clutch gear and it wobbled. Close examination of the teeth clearly shows the abnormal wear pattern.

My point is that I wonder how many times owners have blamed an out of balance crankshaft on their rough running engines when in fact it might be this gear that is causing their problems.
I'm replacing this gear with a new one tomorrow and I'll report on the results.

Tom
 

ocho ninja

Member
Jan 14, 2012
564
2
16
San Jose, CA
I had a similar situation with my output sprocket way long back when i built my first two stroke.
Had a grubee heavy duty hub and with a new kings sprocket all centered with the hub... but my chain still had tight spots.
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Sooooooo much better. :)
When gears mesh instead of fighting each other things run so much smoother. I put the new primary gear on today and just got back from a cold test ride. It's only 41 degrees outside. The pulsating gear noise is gone as is the jerky throbbing I was feeling and hearing.

If you have a rough running engine you might want to check the gears on the right side of your engine instead of blaming an out of balance crankshaft.

You can see how they run by removing the spark plug and reattaching it to the boot, grounding it then disengaging the clutch and spinning the engine with a drill fitted with a large screwdriver bit in the primary gear center screw. Watch the way the gears run and look for any wobble in the small gear. It just might be that the gear was like mine, drilled off center and angled to the crankshaft.

Tom
 

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
Very nice discovery 2door! I could have easily chased that kind of thing for weeks without thinking of your find.
The quality control on the chinagirls never ceases to amaze.