A year ago I purchased a 2002 Baja DS with crappy 720 kit, but nicely built head.
Head has Web cams, 1mm oversized valves, mild porting + full exhaust and big carb, and also has a Ron Woods CDI.

I purchased a used stock bore cylinder from ebay, a JE 11.5:1 piston and did the R and R with new gaskets, etc.

It runs like an SOB. It really has a lot of power. I've had many quads, and this thing is just a beast.

Now the not so good:

Since I put it together, its making a sound under 3/4 to full throttle, any RPM. Low RPM, all the way up to high RPM. It may go away at very high RPM, but I cant be sure.
I am fairly sure its ping/detonation. The bike is running 91 octane.
The sound is very metallic sounding, like piston slap type sound, or something like that. But it ONLY does it at 3/4 throttle or more. Otherwise, the engine sounds great.

Now, I took the bike riding and rode the bike pretty good, revved it out, played with it, etc.. with it making this sound. (I know.. I know)

Now I think I have a blown head gasket. It spits coolant out the overflow tank cap at high RPM under lots of throttle, indicating pressure in the cooling system.
Its not overheating. It seems like a bad radiator cap could cause this, but suddenly? After beating on it for a little while? Seems more likely its a head gasket prob.

Now the big question, was/is the noise PING? I also need to add that I ran some octane booster in the tank, but who knows what that crap does. (if anything)
Do I NEED to run race fuel in this think with the current setup?

And would/could ping cause the head gasket to go if it was bad enough?

Thanks for any help. Better start tearing into this thing once again.

 

headgasket.. did you surface the head when you r&r ???

 

Nope, I did not.

 

Your head gasket may just be leaking compression into the coolant under high RPM, might check the head bolt torque to make sure nothing has worked loose or stretched and see if it seals up, it evidently isn't leaking real bad if you're not noticing it at low RPM. As far as the ping goes, it could be coming from coolant in the combustion chamber taking up space and bumping the compression ratio if the head gasket is bad, it could be pulling it in on the intake stroke, but I would think you would be able to smell hot prestone in the exhaust and would probably be slobbering black crap from the pipe and on the guys in your roost. You might try running some race gas 100 octane or better and see if the ping goes away. Mine was pinging with the dual carbs on it running pretty rich, doesn't do it now since I went back to the stock carb, which is just a little on the lean side.

 

Thanks very much for the replies.

I am going to go out and check the torque of the head bolts first thing tomorrow. I already have the cams out, anticipating pulling the head... (and need to re-shim the exhaust valves anyway).

Thanks again and if anyone has any more ideas, please chime in.

 

On automobiles, sometimes you can tell you have a headgasket leak if you remove the radiator cap, start the engine and let it warm up enough for the thermostat to open. If you get bubbles that don't stop that it might be a head gasket. But, if you are not seeing symptoms until 3/4 throttle or more, this may not work.

NAPA also sells a block tester kit to "sniff" for combustion gases in the coolant. You basically add a chemical to a tube, and using a bulb on the other end, place the tube up against the rad cap ans suck some "coolant vapr" into the tube. If the chemical changes color, you have a bad head gasket.

 

I have to run some race gas with my JE 11.5 piston or it piings. It seems that the later 11.5 pistons are really more than 11.5 and will not run on 91.

I run a mild head with OMW 2 cams, TM 48, pipe and Big Gun CDI.

 

I really enjoyed my JE 11.5/1, that is until I melted it down and destroyed the cylinder.
Did you happen to use a stock fiber head gasket? I went with multi layer steel which happens to be thinner, enough to raise compression to 13 something.

 

check your cam chain tensioner and up the octane first with real gas not additive

 

funny because i just purchased a ds 650 the other day, noticed some coolant coming from the rad cap. replaced the cap and coolant came bursting out of the overflow tube, so i tied it off figuring "its just an overflow tube." and next thing i know im leaking oil from the head somewhere.... gonna tear it down tomorrow. head gasket????