So I finally got a borescope and checked out the piston - it is busted. There is a chunk missing off the exhaust side. About .25 inch of the side right above the first ring grove - about an inch long. This is what was beating the crap out of my plugs. The borescope clearly showed impressions on the top of the piston that looked like the size and shape of the broken piece. So it is time for a top-end rebuild.

Parts are on the way. . . should be "fun?" It just strikes me as funny that a "new" JE piston would break like that. I wonder what happened? if it was an installation error, or maybe the piston was faulty? I guess I'll never know.

I just hope there is not a bunch of other problems when I get in there. Cylinder walls looked very good in the scope, but of course I could not see the valves or the valve seats with the scope, so those are still a mystery. The engine will still start, although I have not run it since the trouble started to pan out.

 

i had a vent tube pinched it was about 3 feet long and routed all over the bike i cut it shorter and havent had a problem but was doing the same thing my baja though was dieing when i was going sideways on a step hill and it was the float which i still havent been able to get adjusted right it is either to much or not enough if anybody has a idea on how to adjust it let me know (i know how to adjust just cant get it right on the money lol)

 

To tell you the truth, ever since I had the premonition something was in my cylinder jumping around on the top of my piston (smashing the plugs and scoring the top of the piston) I have not ridden or even started the bike. So if there is still a problem with fuel delivery, I have not been able to test the fixes I have already tried. . .

My vent tube was already about 14" long and routed prefectly. I went through the carb and set the float level according to Service Manual specs, cleaned the entire fuel system from the gas cap to the manifold, and even double checked the jets. I cleaned and recharged the K&N filter, cleaned all carb pathways, dumped and flushed the tank, and went through the petcock.

However, the piston is in the mail - now I am just waiting for the top-end gaskets (on backorder). As soon as those items get here, I will be breaking the engine down to replace the piston.

 

What type of head gasket is on your Baja? I installed a new J.E 11.5 piston and cometic MLS gasket not realizing that this gasket being thinner raised compression another full point. Not running race gas I believe detonation did my piston in just as you've described also trashing the cylinder. Personally I don't like the J.E piston design for stock bore as they cut valve reliefs to a paper thickness of the ring groove. I believe the C.T. piston had more meat on the shoulder but I nver had one in my hands to physically inspect it.

 

That is interesting. I just finished removing the JE piston and finally got a good look at it. It really looked bad, but the bike still ran (see picture).

The piston looks like the top compression ring hung up in the cylinder and broke out the top. There was a piece of the ring missing and the piece of the piston was also gone. The cylinder wall, however, didn't even have one blemish, so I am not sure what that means.

When looking at the piston, a couple of scenarios came to mind. #1 maybe there was insufficient ring-gap when the piston was installed and upon expansion the ring got too tight and broke out the top. or #2 the piston's wrist pin was very tight (I had to smack it out with a hammer and punch) so maybe the piston angled so much in the cylinder at high speed that it caused the ring to hang up and bust out of the top.

The trouble is that I was told JE pistons were very good and so I replaced the broken one with one of the same. And yes, I used Cometic top-end gaskets as well. However, I torqued all bolts to spec, which the previous installers had not done. When I took the machine apart, the head bolts were very, very tight. It took a breaker bar to remove them. I am hoping correct torque gives more head-room above the piston and alleviates some of the issues.

I always run 91 octane gas in these bikes, but I guess I could go higher if I need to. Although it sure is convenient to use pump-gas.

 

"The cylinder wall, however, didn't even have one blemish, so I am not sure what that means." That means you got lucky!
Stock the DS compression is around 9 to 1. With a J.E. 11.5 and new gasket it should be 12.5 to 1. I suggest running a blend of race gas and super. I bought stock parts off eBay and a new Cometic MLS gasket so I guess I'm running around 10 to 1 and using super. It runs great.

I went through the same scenario as you but I new if anything I set my ring gap to loose. My ring broke and a small piece turned sideways eventually destroying some of the Nikasil coating of the cylinder. I am sure detonation caused this. I heard it at sea level and after that it started having more blow by every ride.