Bored sp carb for 212 predator and cm cam

I was wondering if this carb I bored to around 690 would be too much for my predator 212 with cm cam, a little head porting on both intake and exhaust, 26lb springs and the rest of valve train is stock, billet rod and flywheel, 32 degrees timing, 3910x plug, .930 id big dog header, head shaved a little and .09 head gasket, I’m running 110 race fuel with about 5% of methanol mixed. The main is a 38 that I took out my Dover stock carb and the low speed I took out also but not sure the size. Would these jets be good for the bigger bore carb or should I go bigger on the jets and what e-tube should I run or size? I notice some of the tubes have more holes than others and the e tube out of the Dover carb has fewer holes than the stock sp carb I bored. The Dover carb the jets come out of is a 615 bore and the motor seems to run good on it but feel sluggish over 6,800 rpm all the way to around 7,500 (the cm cam is rated to 7,500) and the plug looks like it’s running a little lean after wide open running and pulling the plug boot off to do a plug check. Also what other modifications should I do to this carb? This is my first time boring one and not sure of what else needs to be done internally besides changing the stock jetting. Thanks for any/all help/advice.
 

Attachments

  • FF00ABEB-4873-46F4-82EF-A705820A6E17.jpeg
    FF00ABEB-4873-46F4-82EF-A705820A6E17.jpeg
    80 KB · Views: 64
  • BA61513D-4B56-48E2-BD8D-C67105115A07.jpeg
    BA61513D-4B56-48E2-BD8D-C67105115A07.jpeg
    88 KB · Views: 63
Glad to see you step on carb size but I noticed 20lb springs are recommended with the CM cam. More spring pressure causes added friction and residual hp lose. The springs should match the cam card most of the time, I think.
https://www.dynocams.com/item/cl-cm-pred/pred-mod/
I have 22lb springs also but decided to run the 26 ones just in case I turned more rpms, I know this specific cam is not made to make more power over 7500 but I’m still figuring out what gearing I need at the two tracks I just started running at so figured it couldn’t hurt anything if I turn a little more rpm if I get the gearing wrong. I’m eventually gonna go to the f275 without ratio rockers so I can get above 8,000 and hopefully can keep up with the other karts then! They are turning around 8,200-8,400. I know I’m gonna have to upgrade the valve train also to turn that much.
 
I have 22lb springs also but decided to run the 26 ones just in case I turned more rpms, I know this specific cam is not made to make more power over 7500 but I’m still figuring out what gearing I need at the two tracks I just started running at so figured it couldn’t hurt anything if I turn a little more rpm if I get the gearing wrong. I’m eventually gonna go to the f275 without ratio rockers so I can get above 8,000 and hopefully can keep up with the other karts then! They are turning around 8,200-8,400. I know I’m gonna have to upgrade the valve train also to turn that much.p
It sounds like they are going to be tough to beat but you're putting together no slouch. Just got to dial it in. RPM only means a little but definitely have to gear the track for the engines power curve. Sometimes, when restricted by carburetor, like a restrictor plate, you can do more with less
 
I keep forgetting to ask ya if you had a Micron or something that you can add an exhaust temp probe to adjust jetting the fuel ratio. The plug can lie with that pure of a fuel and cooled with methanol and dyno tunning with O2 sensor can be hard to find locally. I understand it as 2 jets. One for idle, one for wide open throttle. Everything else is done through mixing air into the emulsion tube to adjust the vacuum on the main fuel jet. By enlarging the center, it riches up the fuel top to bottom. The holes on the side let air in at different vacuum levels. The smaller or less holes, the richer it is. There is always some power to be gained there but need a way to see where you are at.
 
Last edited:
It sounds like they are going to be tough to beat but you're putting together no slouch. Just got to dial it in. RPM only means a little but definitely have to gear the track for the engines power curve. Sometimes, when restricted by carburetor, like a restrictor plate, you can do more with less
That’s what Jody form arc always says, you don’t have to turn the hell out of the engine to make great power, you want to gear it so that your hitting the peak or just past peak of the power curve before letting off gong into the turn. I’m hoping the bored carb is gonna help give me a little more power across the board.
 
I keep forgetting to ask ya if you had a Micron or something that you can add an exhaust temp probe to adjust jetting the fuel ratio. The plug can lie with that pure of a fuel and cooled with methanol and dyno tunning with O2 sensor can be hard to find locally. I understand it as 2 jets. One for idle, one for wide open throttle. Everything else is done through mixing air into the emulsion tube to adjust the vacuum on the main fuel jet. By enlarging the center, it riches up the fuel top to bottom. The holes on the side let air in at different vacuum levels. The smaller or less holes, the richer it is. There is always some power to be gained there but need a way to see where you are at.
Yes I have a mychron 4 but just have the rpm and temp add on that goes on the spark plug wire and bottom of spark plug. Yes that would be great to be able to see the air fuel ratio at different rpms.
 
This is something to consider , in depth .
Everything else is done through mixing air into the emulsion tube to adjust the vacuum on the main fuel jet. By enlarging the center, it riches up the fuel top to bottom. The holes on the side let air in at different vacuum levels. The smaller or less holes, the richer it is.
 
I put a reworked head on my Predator that had the CM cam and it didn't run as well as the stock head shaved .040 and the Tillotson PK1 carb. 6500-6700 rpm was pretty strong. That's not a lot of cam.
 
I put a reworked head on my Predator that had the CM cam and it didn't run as well as the stock head shaved .040 and the Tillotson PK1 carb. 6500-6700 rpm was pretty strong. That's not a lot of cam.
Yeah you can’t hog out the head with a cam like this unless using ratio rockers and even then not too much. I’m gonna get the f275 soon I hope, Just hate having to redo the whole valve train as I’m on tight budget. Jody in one of his arc videos had said to run the 1:2 ratio rockers with the cm cam and would really help the performance of it, which I guess ratio rockers will help most cams, not all but most.
 
Yeah you can’t hog out the head with a cam like this unless using ratio rockers and even then not too much. I’m gonna get the f275 soon I hope, Just hate having to redo the whole valve train as I’m on tight budget. Jody in one of his arc videos had said to run the 1:2 ratio rockers with the cm cam and would really help the performance of it, which I guess ratio rockers will help most cams, not all but most.
Some of the best heads don't offer any significant flow increase above .300" lift. The cm265 with 1.2 rockers is at .318". (Right there) The F275 has a boat load of duration that moves the power band higher but the crazy ramp angles and duration would tend to narrow up the power band to top only. I'd be almost willing to bet the 265 with 1.2's has more power to get off the corner with all else being equal.
 
Some of the best heads don't offer any significant flow increase above .300" lift. The cm265 with 1.2 rockers is at .318". (Right there) The F275 has a boat load of duration that moves the power band higher but the crazy ramp angles and duration would tend to narrow up the power band to top only. I'd be almost willing to bet the 265 with 1.2's has more power to get off the corner with all else being equal.
Do you know where I could get a cheap set of 1:2 rockers? Everywhere I look they are at least $100. Does anyone make oem style stamped 1:2 ratio rockers that would be cheaper than the roller ones I see online?
 
Back
Top