Runs strong when cold... as it warms up, I lose the top end badly.

Mitchg

New Member
Sep 25, 2019
1
0
1
53
HI guys - thanks for reading. When my 80cc w/ the red "speed"/NT carb is cold, it starts right up, and I can hit 35mph no problem on a 41 tooth gear. After about 10minutes, the motor starts to bog down at full throttle, speed drops, and it certainly doesn't have the same RPMS. Hit about 29, and thats it.

I've replaced the plug, the CDI, and the magnet. The top end has all new gaskets and ALL is tight. I did pop the cap off the jug, and the cylinder wall is kind of rubbery smooth, but NOT wet at all. Doesn't feel a bit oily. Strange? The plug is not wet also.

Running a 67 jet as I thought it ran better than the stock 70. Maybe I'll test again w/ the 70 back in, but wanted to get your thoughts. The dry cylinder is very strange. Running 24:1 full synthetic oil, as motor is still new.

Thanks.
 

xXNightRiderXx

Active Member
Jan 12, 2017
415
62
28
Boise
HI guys - thanks for reading. When my 80cc w/ the red "speed"/NT carb is cold, it starts right up, and I can hit 35mph no problem on a 41 tooth gear. After about 10minutes, the motor starts to bog down at full throttle, speed drops, and it certainly doesn't have the same RPMS. Hit about 29, and thats it.

I've replaced the plug, the CDI, and the magnet. The top end has all new gaskets and ALL is tight. I did pop the cap off the jug, and the cylinder wall is kind of rubbery smooth, but NOT wet at all. Doesn't feel a bit oily. Strange? The plug is not wet also.

Running a 67 jet as I thought it ran better than the stock 70. Maybe I'll test again w/ the 70 back in, but wanted to get your thoughts. The dry cylinder is very strange. Running 24:1 full synthetic oil, as motor is still new.

Thanks.
Run it at 16:1. That should bring up the oil content enough to properly lubricate the upper cylinder. This is also the standard break in ratio recommended by most manufacturers. Trust them.
 

Greg58

Well-Known Member
May 1, 2011
4,142
1,033
113
62
Newnan,Georgia U.S.A.
This sounds like a air leak not a mixture problem, it could be the head gasket leaks when the engine gets to full temperature. I would remove the head and sand it on fine sandpaper taped to a known flat surface, after you get the head true torque the head nuts to 12 foot pounds, a torque wrench is a must because low grade aluminum distorts easy.
 

xXNightRiderXx

Active Member
Jan 12, 2017
415
62
28
Boise
This sounds like a air leak not a mixture problem, it could be the head gasket leaks when the engine gets to full temperature. I would remove the head and sand it on fine sandpaper taped to a known flat surface, after you get the head true torque the head nuts to 12 foot pounds, a torque wrench is a must because low grade aluminum distorts easy.
I didn't think of that Greg! I used a piece of 1/4" acrylic on a flat sheet of plywood on my 79cc head, and it worked beautifully, and the best part is: ITS CHEAP! A glass table will work even better though, because glass doesn't warp as easily as wood, if at all.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
235
63
up north now
Don't run it at 16:1 especially with synthetic.
Mix some fuel at 32 or 40 to 1 and then figure out your jetting.

At 16:1 you have half the fuel in your fuel to air mixture as you would at 32:1.
Too much oil craps up the plug, rings, and carbons up everything. It's as bad as not enough oil, just in a different way.
 
  • Like
Reactions: Tyler6357 and FOG

Tyler6357

Well-Known Member
Mar 15, 2012
1,120
163
63
Santa Barbara, CA
This sounds like a air leak not a mixture problem, it could be the head gasket leaks when the engine gets to full temperature. I would remove the head and sand it on fine sandpaper taped to a known flat surface, after you get the head true torque the head nuts to 12 foot pounds, a torque wrench is a must because low grade aluminum distorts easy.
Yep, I've seen cheap stock heads actually warp from the heat
 
  • Like
Reactions: Greg58