Porting with unexpected result

R

Rigz

Member
Finally got my new motor broken in (about 250 miles) and got carb and jetting pretty good so I decided to go ahead and do some porting for extra power. Now my goal was to increase top end power without losing to much low end. I wanted to raise the exhuast port to get more top end but I was afraid to go too far so I only raised it about .6mm and enlarged the rest of the port by about .4mm.
I also added a second bottom gasket to further raise the ports.Then added a super thin .2 cooper head gasket. I basically just filled out the intake(40mm) to make it round and polished it smooth. I also filed down the part of the piston that was covering the port at TDC.
The result was crazy bottom end and midrange power. If I open the throttle to much when I start it up it nearly throws me off the bike and accelerates hard to about 6k rpm where the power seems to steadily drop and I can reach 34mph at 7500rpm with a 26 inch wheel and 38 tooth sprocket.
But it takes a looooooooooong time to get from 30 to 34. So my question is where did I go wrong on the porting? I know that the result I got is ideal for cruising and reliability and the motor pulls strong and runs really smooth but I want more top end and I live in an area where all the land is flat and I don't mind pedaling to start.
 
If you've got really strong low and mid power, why don't you go to a smaller tooth rear sprocket? Sounds like the engine would pull it OK, and still keep a rather sane RPM.........

I run a 36 tooth on mine with minimal engine machining. The expansion chamber and carb tuning I run works rather well. At full 'zing', the GPS shows 43 to 44 mph. I also run 26X1.75 tires.
 
I would remove that extra base gasket, & then do as xseler suggested & install a smaller rear sprocket.
You should also look into getting a good exhaust pipe.
 
I never got any gains from adding base gaskets, so I agree with Venice.

Your intake shouldn't be polished. Needs to be rough to atomize the mixture.

I agree with a smaller sprocket, since you have the torque, it'll give you a faster top end.

Match the intake port to the engine, deck the head, get a good pipe...
 
That's exactly what I was thinking. They make sprockets in just about every size. I was thinking maybe a 34. 36 seems too big and 32 seems too small. I don't mind pedaling at ow speed but with too small a gear I will have a very high minimum speed that might be to fast for me to peddle. Right now the slowest speed I can go without the engine bucking is about 12mph at 2500rpm and the power band is super strong between about 3k and about 5500. I do have a DAX pipe a cns carb and chineese ripoff 5.5cc CR machine head. I'm thinking a lot of that low end is coming from that head. And man does that pipe sound awesome. I get compliments all the time.
 
Rigz said:
That's exactly what I was thinking. They make sprockets in just about every size. I was thinking maybe a 34. 36 seems too big and 32 seems too small. I don't mind pedaling at ow speed but with too small a gear I will have a very high minimum speed that might be to fast for me to peddle. Right now the slowest speed I can go without the engine bucking is about 12mph at 2500rpm and the power band is super strong between about 3k and about 5500. I do have a DAX pipe a cns carb and chineese ripoff 5.5cc CR machine head. I'm thinking a lot of that low end is coming from that head. And man does that pipe sound awesome. I get compliments all the time.
Click to expand...


I tweaked on a CNS carb for 3 years --- thought I had it dialed in, then it wasn't. After that, I tried 'em all.....RT, Dellorto clone, NT, etc. Surprisingly, the best results were from the 'lowly' NT!! I'm in the process of rigging a Stihl 041 carb to a manifold. Hear very good things with this, though it requires quite a bit of 'improvisation'. We'll see.......

With my 36 tooth, the bike will take off from a stop without any pedaling. Not what I normally do, but the bike has no issue doing it.

Best of luck in your continuing adventure!
 
Soooooooooo. ...I feel kinda dumb about this but it turns out the carberator needle was clogged. I took it on my normal 8 mile round trip and I noticed the motor had a lot less power and after 4 miles shut down and would not start again. I initially thought it was the plug or ignition and after verifiing I had spark i noticed the plug was dry.
I took the bowl off the carb and opened the valve to see if there was a fuel restriction in the line and the fuel flowed freely so I was stumped for a min. I was about to take the carb apart and look for something stuck in there when I decided to just take a q-tip and poke around the needle and low and behold I pulled up a grain of sand blocking the hole in the needle. The bike started right up and runs amazing....I'm thinking my high rpm power might have been reduced significantly from lack of fuel so I'll go on a run and try it
 
And yes iv read a lot of people going back to the nt carb after trying out and being disappointed with other carbs. I might just try it. Iv never used it with this motor. I didn't like it becuase I had a lot of problems with it leaking and to be honest I didn't get any noticeable power gains going to the cns carb.
 
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