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GP1200 NPV Won't Start After Rebuild

400 Views 3 Replies 2 Participants Last post by  wmazz
Hello. I have a pair of 1999 GP1200s that I bought about five years ago. When I got them, one had a cratered piston so I had to do a top end rebuildright off the bat. They've both been great runners since then with only what I consider normal maintenance type issues. But at the end of last year, the one my son rides took a dump. It would run, but it took some finessing to get it over ~30 mph. We were able to limp it to shore, and when I got it home I did a compression test. I found two cylinders with ~120 psig and one (the front) was completely dead. We were done for the year, so I winterized and parked them.

About a month and a half ago, I pulled the engine, tore it apart, and this is what I found:
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No smoking gun, but the front seal was a bit chunked up. I suspect it was drawing air causing the front cylinder to go lean. I was able to clean the cylinders up with a 1.0mm bore, and I got a top end kit from WSM. Everything went together smoothly, and I got a good pressure test first try. I got the engine back in and buttoned up yesterday.

But, here's where I need help. The engine spins freely on the starter, but it doesn't even try to fire. I'm priming through the flame arrester ports with fresh premix so I know I've got fuel. I also have spark at all three plugs. I had originally put the old plugs in since I was concerned about fouling with all the assembly oil and 50:1 break in mix in the tank and an operational oiling system. I went ahead and swapped new plugs in and checked compression (all ~125 psig) while I was at it. Still nothing. I did get a backfire out the exhaust one time when I probably primed too much, but otherwise no fire at all.

I have fuel, air, spark and compression, so the only thing left that I can think of is timing. The flywheel is keyed, so it's almost impossible to install that incorrectly. Is there something inside the mag cover that could have been damaged during removal or installation? I'm about out of ideas, so I'd like to hear your thoughts. Thanks in advance.
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Thanks Bill, lots of good info there. Almost too much to digest o_O. Since you responded, I'll go ahead and wrap this one up.

I tried one more time to get it to start and got another backfire, worse (louder) than the first one I mentioned. I was afraid I might have damaged a seal, so I tore it apart far enough to pressure test again. Pressure was still good, but since I had it that far apart I went ahead and pulled the engine so I could give it a good once over. Tested pressure again on the bench, and an overall inspection didn't reveal a smoking gun. I gave the magneto cover a good cleaning inside (something I didn't do the first time) and put it all back together.

I hit the button and it fired right up. I only ran it a couple of seconds since I didn't have the hose hooked up, but it sounded great. I have no idea what I did wrong the first time, and the only difference I'm aware of was cleaning the magneto. I've R&Red WaveRunner engines a couple of times over the years and never had an issue. Totally stumped, but she's running now.

Now I'm just waiting for warmer weather so I can take it to the lake and break it in.
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