frustrated with cub

[riderboy]

I just got back from the dunes and I'm still having problem with spark plugs coming loose. I have the timing plate set at 4 and the dyna ignition at stock setting, 20cc domes on 68mm cub cylinder 145psi compression, 33mm carbs, cpi pipes, and running 100 0ctane gas. Seems ok if I just put around but when I start drag racing it only take 5 or 6 passes before one of the spark plugs either vibrates loose or starts to seperate. It runs pretty descent but not as fast as I think it should. Does anybody have any ideas. I am getting very frustrated with messing with this damn thing. I'm almost ready to put on some stock cylinders and selling the dang thing. I really don't want it to blow up on me, but I don't know what else to do.

[dawarriorman]

Check your squish band thickness. If its too tight, it could cause detonation. Stick a piece of lead solder (no flux core) in through the spark plug hole until it hits the side of the cylinder. Then crank the motor over. Then measure the thickness of the solder. Let us know what you get.

[dynodon]

Check your squish band thickness. If its too tight, it could cause detonation. Stick a piece of lead solder (no flux core) in through the spark plug hole until it hits the side of the cylinder. Then crank the motor over. Then measure the thickness of the solder. Let us know what you get.

Yeah, I agree !!

[dajogejr]

I agree...squish is CRITICAL on a cub. .040 to .050 is what I like to run, but I like lower compression, higher timing advance.

 

I'm surprised your compression is at 145...mine was a lot higher, on my 4 mil with 20cc domes...

 

What do your plugs look like? White? Tan? What jets are you running in them?

[jeepman128]

I have the exact same problem, my compression is the exact same, except I am there w/21cc domes. But mine come loose all the time as well, one side or the other, it is guarnteed to happen, At least once a day. My squish was at .38 on both sides. I have just learned to live with the problem, and just tighten them before every ride

[riderboy]

I finally got a chance to check the squish on my banshee. That's the problem. Where the dome is angled at the end by the cyl. wall, I have .045 and at the first part of the angle I have about .010 squish. Looks like the piston was even grazing the dome. Must be the wrong angle on the chamfered part of the dome. Might have been domes for a blaster piston instead of banshee pistons.