Engine bogging

[steelfan41]

I have a 66cc HT with a CNS-V2 carb. I get engine bog from 12-16/17 mph. Lower or higher engine purrs like a kitten. I've just gotten used to going WOT through it to minimize the amount of time it's choppy. Other wise she wants to run like the proverbial raped ape, seems happiest at 25mph, which sucks cuz I'm in 20 mph AZ. Is this a symptom of a bigger problem or am I just being paranoid? This is my first MB so I'm erring on the side of caution until I get to know our lil HTs better. Any advice is welcome. I should mention this is a new issue since I changed to the V2 carb.

 

[motorpsycho]

sounds like you need to re-jet the carb and richen it up. 9 times out of 10 a bog in the upper rpms or at w.o.t. indicates a lean condition.
once you get to about 3/4 throttle, the jet needle is out of the equation and the main jet is wide open (needle totally out of the way).
if the jet is too small, the air-fuel ratio will become too lean. (more air than gas).

 

[steelfan41]

Thanks for the input. I'm getting an expansion chamber in a few weeks, I'll try to fix it then. Order some jets... Any suggestions on what ones to get? Also I do have an air/mixture screw on the carb. Should I screw it all the way in? (only about 1 1/2 turns) I'm on the middle setting with my c-clip...should I try to adjust it, or am I just chasing ghosts trying to use these methods?

 

[motorpsycho]

An expansion chamber will make it run a little leaner in case you weren't aware of that.
as far as what jets to get? it's a crapshoot because no 2 engines will run the same.
you have to do re-jetting by trial and error until you come up with a jet that works best for your engine.
adjusting the c-clip will only have an effect from idle to about 3/4 throttle so you shoudl adjust teh c-clip to get the best idle and mid range that you can.
again, that's all done by trial and error.
I have no idea what size the stock jet is that's in your carb, so you will have to take it out and then take a guess as to which jet to replace it with.
Just pick one, put it in and see what happens.
if you still get a high speed bog, then you know that you'll have to go richer.
if the engine starts 4 stroking at w.o.t. then you knwo that you have to go leaner.
also, check the spark plug electrode after every jet change...you are looking for a tan/brown color. clean the plug off after you check it each time.
if the plug is black, it's too rich.
if the plug is a silvery color, then it's too lean.
once you start playing with jets and all that, it comes pretty easy and it becomes pretty self explanitory. make sure that you look at the size of each jet that you install, and maybe take notes on how the engine runs according to the jet sizes.
this will help you remember what jet did what in case you have to back track.
you want to put the jet in and run it, then see what the results are.
as it starts to run better and better, keep going until it starts running bad again.
like if you start with a (for example) .68 jet and it runs crappy, go up to a .69 and keep going up until it starts to 4 stroke. so lets say that it runs great with a .72 jet, don't stop there. Keep going up in jet sizes until it starts 4 stroking. so lets say that it runs great with a .72, but starts to 4 stroke with a .74. go back to the .72 and try it again.
it's all trial and error, and you want to go from one extreme to the other (from too lean to too rich for example) until you get it running as best as possible.
when i re-jetted my carbs i think i went through 3-4 jet changes before i found the right jet.
This may not be the method that others use, but this is what worked for me.

 

[Big Red]

Jetting

An expansion chamber will make it run a little leaner in case you weren't aware of that.
as far as what jets to get? it's a crapshoot because no 2 engines will run the same.
you have to do re-jetting by trial and error until you come up with a jet that works best for your engine.
adjusting the c-clip will only have an effect from idle to about 3/4 throttle so you shoudl adjust teh c-clip to get the best idle and mid range that you can.
again, that's all done by trial and error.
I have no idea what size the stock jet is that's in your carb, so you will have to take it out and then take a guess as to which jet to replace it with.
Just pick one, put it in and see what happens.
if you still get a high speed bog, then you know that you'll have to go richer.
if the engine starts 4 stroking at w.o.t. then you knwo that you have to go leaner.
also, check the spark plug electrode after every jet change...you are looking for a tan/brown color. clean the plug off after you check it each time.
if the plug is black, it's too rich.
if the plug is a silvery color, then it's too lean.
once you start playing with jets and all that, it comes pretty easy and it becomes pretty self explanitory. make sure that you look at the size of each jet that you install, and maybe take notes on how the engine runs according to the jet sizes.
this will help you remember what jet did what in case you have to back track.
you want to put the jet in and run it, then see what the results are.
as it starts to run better and better, keep going until it starts running bad again.
like if you start with a (for example) .68 jet and it runs crappy, go up to a .69 and keep going up until it starts to 4 stroke. so lets say that it runs great with a .72 jet, don't stop there. Keep going up in jet sizes until it starts 4 stroking. so lets say that it runs great with a .72, but starts to 4 stroke with a .74. go back to the .72 and try it again.
it's all trial and error, and you want to go from one extreme to the other (from too lean to too rich for example) until you get it running as best as possible.
when i re-jetted my carbs i think i went through 3-4 jet changes before i found the right jet.
This may not be the method that others use, but this is what worked for me.

GREAT, Where can ya get different size jets?
Big Red.

 

[motorpsycho]

well, s.b.p. sells delorto jets and they will fit in the old style n.t. carbs.
the threads have a slightly different pitch so they screw in tight, but they will screw in.
I have no idea if these delorto jets will screw into the cns style carbs or not tho.