Carby Jetting help on HP Carburettor

lukecanna

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Sep 25, 2017
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79
Hi,
I have a new build which is an ebay high performance motor, cdh66 pipe, HP carb and 36 sprocket. Im 31m above sea level. An it is currently summer so i am running a 25:1 ratio
I need to know what jet size i should be looking at as my bike is only topping out at 35kmh and is 4 stroking at WOT. Should i be going larger or smaller then the stock main jet?
 
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mark20

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Sep 26, 2018
Messages
526
i would actually try running a more fuel based mixture ( i run 32:1 on my engine, all stock)
this will mitigate the 4 stroking effect at WOT, if that doesn't help then i would go 1-2 sizes smaller on the jet size as the HP carbs run much richer than the speed/nt ones.
good luck man!
 

Tyler6357

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Sep 28, 2011
Messages
729
Each individual engine will need a different jet size, there is no standard for these little engines, I have 2 of the exact same engine, one runs best with a #60 jet, the other runs best with a #65 jet. The best jet for you is the biggest one you can run without compromising performance. The stock jet is a #70 and it's often times too rich. If you are experiencing bogging or 4 stroking, you are running too rich. It's best to start high and work your way down until you find the correct one that eliminates bogging and 4 stroking. When testing, it's best to get your engine hot before you make an analysis as these engines tend to run slightly leaner after they are warmed up. Remember that it's always better to be on the rich side than the lean side. If it's running too rich it will only bog and 4 stroke and affect performance, however if it's running too lean it will not get enough oil and cause excessive heat which will be followed by parts failure and a broken engine. Also, I agree with mark, 32:1 is better as long as it's a high quality synthetic 2 cycle oil, excess oil will only clog up the exhaust pipe.
 

lukecanna

Member
Joined
Sep 25, 2017
Messages
79
Each individual engine will need a different jet size, there is no standard for these little engines, I have 2 of the exact same engine, one runs best with a #60 jet, the other runs best with a #65 jet. The best jet for you is the biggest one you can run without compromising performance. The stock jet is a #70 and it's often times too rich. If you are experiencing bogging or 4 stroking, you are running too rich. It's best to start high and work your way down until you find the correct one that eliminates bogging and 4 stroking. When testing, it's best to get your engine hot before you make an analysis as these engines tend to run slightly leaner after they are warmed up. Remember that it's always better to be on the rich side than the lean side. If it's running too rich it will only bog and 4 stroke and affect performance, however if it's running too lean it will not get enough oil and cause excessive heat which will be followed by parts failure and a broken engine. Also, I agree with mark, 32:1 is better as long as it's a high quality synthetic 2 cycle oil, excess oil will only clog up the exhaust pipe.
Thanks mark i ordered some jets #50 - #72 and will be trying to 68 first. I tested the bike and it went 54kmh on the stock NT carb with the needle lowered one notch whilst on the HP carb it only went 34 major improvements.
 

Tyler6357

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Sep 28, 2011
Messages
729
Thanks mark i ordered some jets #50 - #72 and will be trying to 68 first. I tested the bike and it went 54kmh on the stock NT carb with the needle lowered one notch whilst on the HP carb it only went 34 major improvements.
That's understandable, assuming they are both using the same size jet (#70), lowing the needle leans it out and the HP don't have a needle clip to lean it out. However, I believe that the HP will make the engine run better than the stock NT once you find the right jet. It sounds like you need to lean it out some. Are you experiencing bogging or 4 stroking anywhere along the throttle range?
 

lukecanna

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Joined
Sep 25, 2017
Messages
79
That's understandable, assuming they are both using the same size jet (#70), lowing the needle leans it out and the HP don't have a needle clip to lean it out. However, I believe that the HP will make the engine run better than the stock NT once you find the right jet. It sounds like you need to lean it out some. Are you experiencing bogging or 4 stroking anywhere along the throttle range?
it was just 4 stroking a lot at wot but wasnt bogging (maybe because of exhaust) not really sure never delved into carbys and jetting before. When the jets come in ill swap the carbs out and see how i go
 

Tyler6357

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Sep 28, 2011
Messages
729
it was just 4 stroking a lot at wot but wasnt bogging (maybe because of exhaust) not really sure never delved into carbys and jetting before. When the jets come in ill swap the carbs out and see how i go
Yeah, mess around with the jets when them come, you certainly got enough to try just about everything so you should be able to dial it in. It can bog, even with that awesome pipe if it's rich enough, my guess is that you won't have to lean it out too much. Good luck and don't forget to come back and report your results.
 
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