This issue has me at a complete loss. Need help..

[Marlin710]

So I bought a 100cc motor kit to upgrade my 80cc bike when the engine went out. My 80cc kit would hit 32mph easy. When I installed the new 100cc kit it would only get up to about 20mph. Not exactly what I'd call an upgrade lol. I checked and rechecked everything twice, fuel ratio, carb, spark plug, fuel flow, cable tensions, only thing I could think was maybe compression. After a while I was so frustrated I ordered another 80cc engine by it self. Installed that and it also only gets to about 20mph. I know these bikes should have a little more gitty up than this.
My oil ratio is 35:1, Ngk spark plug, spark plug wire from a car (bought at AutoZone), heads torqued to spec. Carb, float and bowl are clean.
I just can't figure out why my power has been so restricted on these last 2 engines. Any help or ideas would be greatly appreciated.

 

[Sidewinder Jerry]

What were you using to determine the speed on all three engines?

 

[Marlin710]

What were you using to determine the speed on all three engines?

I have a speedometer headlight combo (calibrated the same) but I can also feel the difference. The last 2 engines are allot slower then the ones I've used before.

 

[Sidewinder Jerry]

I have a speedometer headlight combo (calibrated the same) but I can also feel the difference. The last 2 engines are allot slower then the ones I've used before.

Have you changed anything on the bike and/or gained a lot of body weight? Finally have you tried taking the carburetor off the first engine and tried it on the other engines?

 

[Marlin710]

No significant weight gain, and I've tried 3 different carbs, the one that came with the 100cc kit the one from the first engine and a HP carb. All run the same. I'll upload some pics of the bike in a while. Just for a visual reference.

 

[kaneto]

No significant weight gain, and I've tried 3 different carbs, the one that came with the 100cc kit the one from the first engine and a HP carb. All run the same. I'll upload some pics of the bike in a while. Just for a visual reference.

Have you tried the old engine again? Maybe something on the bike itself is causing extra friction (rubbing brakes, axle bearing starting to seize etc).

 

[Greg58]

Is the carburetor level or at a angle? Try this, while riding turn off the fuel to the carburetor, if the engine speeds up right before it run out of fuel your air to fuel ratio is too rich. With the carb at an angle sometimes the needle doesn't seat before fuel flows into the engine.

 

[Testname111]

I I'm having the same issues with my minarelli build, just jet-ed to a #70 and I can get my bike to start with the choke on but when I accelerate it gets choppy and stuttery .... I'm at a loss please help

 

[Testname111]

I've used a #75 jet in the past with no issues? Starting to suspect its this spark coil is bad idkkk ughhh,I fixed all my gaskets and new intake

 

[Testname111]

My spark plug is very wet

 

[MEASURE TWICE]

I just throw this out, check no restriction in muffler and pipe. The air filter is not restricted. As Click & Clack
on Car Talk Radio said nothing can go in if it ain't going out.
Don't know if kit includes the air filter.

When you first said the first 80cc kit went out. What did that mean? Was it also just going 20mph max or was there some other specific?

If you see what I might be thinking is that maybe the engine is not the cause?

I am not too familiar with these two strokes I have a Brigg 4 Stroke.

I wonder if it ok to test the engine with out a load and rev it some. I was thinking just to see if if bucks or seems fine. Tires spin freely and have enough pressure.

It is always good if you have two motor bikes at once. Then if one has a problem an the other does not, you have known good parts that sort of are a standard. Swapping part to evaluate what is wrong becomes easier.

Oops.... your last post maybe is the answer! There are spark tester and also OHM meter tests for the coil.

 

[Venice Motor Bikes]

I I'm having the same issues with my minarelli build, just jet-ed to a #70 and I can get my bike to start with the choke on but when I accelerate it gets choppy and stuttery .... I'm at a loss please help

try a new spark plug

 

[Testname111]

Starting to suspect its crank case gaskets that are just to old