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Dyno cam cm cam max rpms?

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Scarface5029

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Hey guys I have a predator 212 with a dover holey Moses box stock carb with a 38 main I’m not sure of the e tube, a billet flywheel and rod, some head porting on exhaust and intake, 26lb white stripe valve springs, head shaved but not sure how much and a .09 head gasket and a big dog .930 header pipe, my question is what can I do or is it even possible to get my dyno cam cm cam to turn over 8,000 rpm and make good power without doing too much more work to the motor? Or should I get another cam? I’m currently turning it a little over 7,500 and running a small dirt track not sure the size but running 12/63 gearing. A new dover carb that is made for this type of setup is in the cards when I get the extra money. Thanks for any/all help.
 
The CM cam is designed for a size bigger than stock carb, like a bored or maybe a GX390 carb. You can add some ratio rockers and stainless valves to increase HP at that RPM but to go with a higher RPM, you'll need a longer duration and different lobe separation on the cam (plus a carb that matches the intake valve size or just a couple mm smaller.)
 
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I suppose, if you're good, you can adjust the crank gear to move the cam timing and that will slide the power curve up a bit.
 
Ok thanks, I have a cheap 390 carb but the plastic piece that turns the butterfly is backwards and will not work with my throttle linkage since the 390 I have is suppose to be pulling instead of pushing the butterfly black piece on top, are they suppose to be like that and I have to order a special linkage to make it work correctly? You can see the difference in the black pieces on top, the 390 is on the left.
 

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Hook the throttle cable to the other side of the lever if its got a regular top plate
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Or bypass the lever completely via extended cable .
 
The linkage I have can only be ran one way and I have to run the stock gas tank for the class I’m in, was hoping I could make it work without buying anything else to make it work but looks like that’s not gonna happen. Thanks for the reply.
 
Make a new L bracket . Where the throttle cable end is longer .
Put the throttle rod where the cable attachment is and move the cable out too the end .
Even a straight lever would work , may have to source a different return spring .
All attachments for throttle on the same side .
 
IndyHarry said:
The CM cam is designed for a size bigger than stock carb, like a bored or maybe a GX390 carb. You can add some ratio rockers and stainless valves to increase HP at that RPM but to go with a higher RPM, you'll need a longer duration and different lobe separation on the cam (plus a carb that matches the intake valve size or just a couple mm smaller.)
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I prob wouldn’t recommend ratio rockers if he has cut the head and is usin a .009 head gasket .. if he doesnt know how to check the clearance between the valve and piston
 
IndyHarry said:
You'll still need a carb to block adapter and jet kit though so it's not a free upgrade.
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The butterfly will only work one way with the shaft unless I notch the other side of the shaft out. Yes I know I had to get an adapter with it but didn’t want to get one until I knew I could make the carb work. I bored out my spare so carb to about 690 so I’m gonna try that with the 38 main and low speed jet out the Dover carb just to see how it runs. Thanks for the help.
 
Curflo said:
I prob wouldn’t recommend ratio rockers if he has cut the head and is usin a .009 head gasket .. if he doesnt know how to check the clearance between the valve and piston
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I didn’t want to use ratio rockers but may in the future and I should be able to go to a .45 thick or bigger head gasket for clearance depending on cam, I really want to run a f275 without ratio rockers but not sure if it would make the power im looking for at the rpm range im trying to run at.
 
Scarface5029 said:
The butterfly will only work one way with the shaft unless I notch the other side of the shaft out. Yes I know I had to get an adapter with it but didn’t want to get one until I knew I could make the carb work. I bored out my spare so carb to about 690 so I’m gonna try that with the 38 main and low speed jet out the Dover carb just to see how it runs. Thanks for the help.
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That 390gx carb wasn't the one I was thinking of. This is the one I see people use.
https://www.arcracing.com/dj-3225-carburetor-honda-gx390-hp/
The pic is carb bored out to .900" Its pretty much straight through.
 

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Scarface5029 said:
I didn’t want to use ratio rockers but may in the future and I should be able to go to a .45 thick or bigger head gasket for clearance depending on cam, I really want to run a f275 without ratio rockers but not sure if it would make the power im looking for at the rpm range im trying to run at.
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If your really wanting to spin it up to say 9k, probably going to need at least a 315 cam minimum going by the cam specks. Every little piece on that engine would need to match up. I'm thinking even a billet side cover, billet crank, everything sized, machined to fit, and built to perfection. Basically keep the block and head core and replace literally everything else.
 
The class I’m running which is called the “pipe” class or stage 1 is basically a stock appearing class but you can run aftermarket pipe and filter but no billet sidecover or anything like that, technically we are suppose to be only running an aftermarket pipe and filter with stock carb and can use aftermarket jetting, stock lift etc but there is no tech guy and the front guys are basically running a mod class motor just not turning them as high rpms as a fully mod/open motor. And they all running a tillotson block, I just noticed that this past weekend.
 
IndyHarry said:
That 390gx carb wasn't the one I was thinking of. This is the one I see people use.
https://www.arcracing.com/dj-3225-carburetor-honda-gx390-hp/
The pic is carb bored out to .900" Its pretty much straight through.
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Yeah I have the el cheapo version lol, but the black piece looks like it’s the same orientation as mine, either way I’m gonna try my bored carb and see how it runs now I’m sure I’m gonna have to change the jetting but hopefully will give me a little more pep in the upper rpms.
 
Scarface5029 said:
Yeah I have the el cheapo version lol, but the black piece looks like it’s the same orientation as mine, either way I’m gonna try my bored carb and see how it runs now I’m sure I’m gonna have to change the jetting but hopefully will give me a little more pep in the upper rpms.
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The bored carb and cam setup should work great if you dial in the jetting. There isn't as much force sucking fuel out of the bowl so bigger jet. The methanol additive seems like it would make you go up another size as well. I'm not sure how big to go but it's going to make a difference. The rule is stock valves and keepers shouldn't have over 22lb springs because of excessive wear and it likes to pop the end off and swallow the valve. The CM cam is the largest to still use stock valve train because it stays under that limit on spring pressure. If you can find a different gx390 carb that you can get to .900", I would be thinking maybe a 308 cam but then you'll need the complete valve train package to keep it together. It's target rpm is 8200 to 8400. Your limiting factor is going to be how big of a carburator you can get by with then build from there. Your pretty close with bored stock carb. It should get you in the front of the field.
 
IndyHarry said:
The tillotson's come in all different displacements like 225 and 236 but nothing says full stop like one of their blocks with a 3" bore.

http://www.nrracing.com/product-p/76mmblock-3003.htm
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Tillotson make a 212 also and the block is more stout than the predator 212 block and doesn’t have the hole in the block for the wires for the oil sending unit it’s solid and has some built in bracing in the block casting in one or two other areas on the block which is why I assume they all run it. Thanks again for you help.
 
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