GP1200 NPV Won't Start After Rebuild

402 Views 3 Replies 2 Participants Last post by wmazz, 1 mo ago

twerth

Discussion Starter · 2 mo ago

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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wmazz

· #2 · 1 mo ago

twerth said:
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

This engine has a hard starting problem caused by a mixture of normal
vacuum leaks and choke plates allowed too much manifold vacuum to
escape (Yamaha Service Bulletin).

I would bet there is a fuel primer installed and the pop-off lowered.
Possibly someones internet fix too.

twerth said:
got a good pressure test first try. I got the engine back in and buttoned up yesterday.

A pressure test does a poor job of determining engine condition unless the
inside spring (on the seal) has been damaged or fallen off. Hard starting and
other carb related problems are discovered by performing a vacuum test.

Did you test the pop-off? The carbs could be leaking fuel.

twerth said:
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 did get a backfire out the exhaust one time when I probably primed too much, but otherwise no fire at all.

Be careful, a backfire can cause the waterbox to explode!!
Do not ever use starting fluid.

It is very possible you have too much fuel. Ignition occurs within
a very narrow range of rich to lean.

Try this if the needle and seats are not leaking.
New spark plugs. Turn off the fuel supply (or pinch it off), hold the
throttle wide open until it lights off, then feather the throttle and
turn on the fuel.

After the engine starts, it is still possible that there will be a hard
starting problem after it sits for a week.

twerth said:
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.
Everything went together smoothly, and I got a good pressure test first try. I got the engine back in and buttoned up yesterday.

A completely worn out main seal is not enough to seize an engine.
Was there rust damage inside the mag cover?

twerth said:
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.

Obviously it was running on only 2 cylinders. A common mistake on
Yamaha triples.

Your engine seized several times 5 to 10 times or more.

Bill M.

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wmazz

· #4 · 1 mo ago

Back in the mid to late 80's we had a huge problem with Kawasaki 650's seizing
on a short wot run. Many of us thought fuel starvation by small needle and seats,
fuel pumps were part of the problem. But most of the problems were actually
caused by too much load (impellers), poor designed aftermarket product and
poor engine combinations.

Since then I have put little stock in the idea of a lean seizure caused by fuel
starvation. I avoid larger fuel pumps, eliminating fuel petcocks, using multiple
fuel tank pick-ups, etc.

When a rear cylinder fails, often a contributing factor is crankshaft twist because
it advances the ignition timing.

Cylinder seizures are often caused by more than one single problem that causes
the failure. In the case of the rear cylinder seizure engine load by impeller or tow
toys may contribute.

But I am not certain about Yamaha triple front cylinder seizures because under
a load (even with worn out main seals) the front cylinder has perfect ignition
timing. So there is a possibility that the fuel supply to the front cylinders fuel
pump is restricted by the rear and center cylinders.

A main seal that is completely trash will normally only cause common diaphragm
carburetor problems. Pressure and vacuum leaks do not cause immediate engine
failures like many believe.

Good Luck

Bill M.

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