Cm .275 Maxing out at 5k Revs

[Blmpkn]

Non hemi predator

.020 longer rod
Cm .275 cam
Billet flywheel
26# springs
22mm mikuni 20 idle/130 main
Inverted intake
8" header

30 series converter, geared 5-1, 14" tires.


I just finished the build and with the supplied jets I got with the mikuni I was real rich down low BUT running out of fuel around half throttle (anyone else find that Wierd?) Managed to do 40mph if I held the throttle in its sweet spot, meaning my revs were somewhere around 5k.

I significantly(?) bumped the size of the main jet from 100 to a 130, and while I now have fuel til WOT, I'm still holding to 40mph. I haven't done a plug chop yet, but I find it hard to believe I'm missing out on 2k more revs because my main jet is too small. However; like I said, with the 100 main I started bogging about half throttle, so perhaps a few sizes bigger is what I'm after.


Hopefully I'm out of work tomorrow with daylight to spare so I can take a plug reading.. But until then... What's everybody think?

 

[Joe-405]

What's the timing at ?

 

[Blmpkn]

What's the timing at ?

I'm relatively sure it's set at 8° btdc. I'm running the stock key and a fixed arc flywheel.

I wanted the adjustable one, but I was in a rush to get the kart running before snow arrived and at twice the price I opted out of it.

I've read that at the rpms this cam can provide it'll just shear off the offset keys? Any truth to that?

 

[Joe-405]

So my engine was stuck at 5k also. Then I turned the timing up to 36* total and it fixed the issue. My torque converter also wouldn't hold the power past that rpm either. So might outta leave it. On a heads up fyi note.

Which Is why my mini dragbike is no longer running also. The full build is in the buildoff section if you wanna go looksy. I talked about the rpm issue also.

 

[Blmpkn]

The timing built into my flywheel is actually 32° btdc, just checked on the arc website. Perhaps I'll get a 4° key and see what that does for me. If it's too much for the 30 series belt I'll just baby it until I get a 40 series or something. I really wanna hear 7500 rpms blasting out my header

 

[Bbqjoe]

Mr Blumpkin, did you try anything inbetween 100-130, or did you just jump all the way there?

Maybe you've overdone it.

 

[Joe-405]

The timing built into my flywheel is actually 32° btdc, just checked on the arc website. Perhaps I'll get a 4° key and see what that does for me. If it's too much for the 30 series belt I'll just baby it until I get a 40 series or something. I really wanna hear 7500 rpms blasting out my header

There isn't any torque converter out there that will hold 5k+ rpms unless you buy a $1000 jr. dragster setup. Which is what I was trying to tell you. I had the 44c Magnum and i destroyed the driver from the inside out !

 

[Blmpkn]

Mr Blumpkin, did you try anything inbetween 100-130, or did you just jump all the way there?

Maybe you've overdone it.

I jumped right to a 130. I found a recommendation on jet sizes for my setup online somewhere, said 17.5&130 so that's what I bought.

I tried the 17.5 pilot and it was too small. Would barely start. So I'm thinking the 130 may be a touch too small as well. A plug reading will tell me for sure. Fingers crossed I have the time tomorrow to take one.

---------- Post added at 10:57 PM ---------- Previous post was at 10:50 PM ----------

There isn't any torque converter out there that will hold 5k+ rpms unless you buy a $1000 jr. dragster setup. Which is what I was trying to tell you. I had the 44c Magnum and i destroyed the driver from the inside out !

If there's no way around that problem I suppose I'll just have to look for a cam that has a 5kish target rpm that THUMPS, cause there's no way I'm giving up the converter setup. It's more of an off-road kart so it's basically a necessity. I wish there were more dynosheets comparing cams online lol

 

[Bbqjoe]

If there's no way around that problem I suppose I'll just have to look for a cam that has a 5kish target rpm that THUMPS, cause there's no way I'm giving up the converter setup. It's more of an off-road kart so it's basically a necessity. I wish there were more dynosheets comparing cams online lol

I'm only throwing this out there because I had a similar experience as you're describing ie. not enough fuel, half throttle, etc.

I tried just about everything, including a handful of jets, and was pulling my hair out. Idle to mid throttle was almost fantastic. Anything much above 1/2 gave me a number of problems.
I came to find that my intake manifold had a crack on the underside that I couldn't see. Hopefully you didn't get yours from GKUSA.

 

[Blmpkn]

I'm only throwing this out there because I had a similar experience as you're describing ie. not enough fuel, half throttle, etc.

I tried just about everything, including a handful of jets, and was pulling my hair out. Idle to mid throttle was almost fantastic. Anything much above 1/2 gave me a number of problems.
I came to find that my intake manifold had a crack on the underside that I couldn't see. Hopefully you didn't get yours from GKUSA.

Got lucky and had the time to take a plug reading today, one side was "toasted marshmallow" and the other side was lighter, almost white. I'd say the main sizing is pretty darn close at least.

I did get both the carb and manifold from gkusa, but I know mines not cracked or anything. Your motor idled fine with an intake leak? Usually idle would jump all over with a leak in my experience but I believe it.

 

[bob58o]

There isn't any torque converter out there that will hold 5k+ rpms unless you buy a $1000 jr. dragster setup. Which is what I was trying to tell you. I had the 44c Magnum and i destroyed the driver from the inside out !

My 30 series TC used to spin 7000 RPM.
I changed gearing. It still spins 6300 RPM.
Speedometer and Tach showed no slipping, even at 7k.
Just saying.

It was probably under 15 ft lbs however, your 25+ ft lb 420cc engine might have had too much torque.

I think the smaller 30 series hold up better for higher RPMs I believe.

---------- Post added at 02:32 PM ---------- Previous post was at 02:25 PM ----------

I had to gear my minibike with similar sized tires 54:8 with a 30 series to get to 7000.

Change 5:1 to 6:75:1 and see if you hit 7000 RPM.
Or lose weight. The average human head weighs 8lbs.

 

[Blmpkn]

My 30 series TC used to spin 7000 RPM.
I changed gearing. It still spins 6300 RPM.
Speedometer and Tach showed no slipping, even at 7k.
Just saying.

It was probably under 15 ft lbs however, your 25+ ft lb 420cc engine might have had too much torque.

I think the smaller 30 series hold up better for higher RPMs I believe.

---------- Post added at 02:32 PM ---------- Previous post was at 02:25 PM ----------

I had to gear my minibike with similar sized tires 54:8 with a 30 series to get to 7000.

Change 5:1 to 6:75:1 and see if you hit 7000 RPM.
Or lose weight. The average human head weighs 8lbs.

I had figured the motor would be making enough power where it needed to to be able to "redline" itself on flats.. So re-gearing wasnt even an idea in my head.

BUT, it's a good idea. During/after a 1/4 mile climb up my road which has decent grade I noticed the engine struggling a bit at the pace I wanted to go.. And when I parked it I noticed some belt degradation starting to happen.

Sounds like I'll be ordering another sprocket then, thanks for the insight

 

[Joe-405]

My 30 series TC used to spin 7000 RPM.
I changed gearing. It still spins 6300 RPM.
Speedometer and Tach showed no slipping, even at 7k.
Just saying.

It was probably under 15 ft lbs however, your 25+ ft lb 420cc engine might have had too much torque.

I think the smaller 30 series hold up better for higher RPMs I believe.

---------- Post added at 02:32 PM ---------- Previous post was at 02:25 PM ----------

I had to gear my minibike with similar sized tires 54:8 with a 30 series to get to 7000.

Change 5:1 to 6:75:1 and see if you hit 7000 RPM.
Or lose weight. The average human head weighs 8lbs.

That's possible also. I was in direct contact with the original owner of comet industries before it was sold figuring out my issues. He said all their tavs aren't designed to run past 5k or so rpms. The internals of the drivers act like a moving backwards weight system when expanded and doesn't allow them to rev past that rpm usually. Which is why he said I grenaded mine from the inside out.

 

[Blmpkn]

Update:

Today I got a chance to work on the kart for a couple hours. Changed leaky valve cover gasket, changed oil, and installed a 75t sprocket.

With the 75t sprocket Im getting way further into the cam than I was with the 60t, I haven't hooked up my tach yet but it's an absolute night and day difference in Max revs.

Now we get to see what happens to my clutches! If they implode at the revs I'm now able to run I'll either put my 60t sprocket back on, OR replace the cam with one profiled for low-mid range power.

My 1" aluminum axles keyway is getting mashed up fierce from the sprocket/key as well. I'm sure it's my driving style, but I'll be replacing it with a steel one shortly.

 

[bob58o]

Update:

Today I got a chance to work on the kart for a couple hours. Changed leaky valve cover gasket, changed oil, and installed a 75t sprocket.

With the 75t sprocket Im getting way further into the cam than I was with the 60t, I haven't hooked up my tach yet but it's an absolute night and day difference in Max revs.

Now we get to see what happens to my clutches! If they implode at the revs I'm now able to run I'll either put my 60t sprocket back on, OR replace the cam with one profiled for low-mid range power.

My 1" aluminum axles keyway is getting mashed up fierce from the sprocket/key as well. I'm sure it's my driving style, but I'll be replacing it with a steel one shortly.

My answer is almost always more teeth on the axle. Lol

 

[Blmpkn]

My answer is almost always more teeth on the axle. Lol

In regards to my mashed key/keyway? Or my motors new found ability to get way up In the rpm?

Because again, it wasn't even an idea In my head.. But I suppose too tall of gearing could of been what mangled the key/way potentially.

 

[bob58o]

In regards to my mashed key/keyway? Or my motors new found ability to get way up In the rpm?

Because again, it wasn't even an idea In my head.. But I suppose too tall of gearing could of been what mangled the key/way potentially.

For your RPMs (I didn't even remember the keyway, sorry). For Minibikes with 19" Tires, For Heavy Buggies, For more Fun, For Daily Stop and Go Travel, For (Fill in the Blank).....
The answer is MORE TORQUE! LOL