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1979 GS550 Engine Randomly Seized Up

bill_face

Forum Apprentice
Hello all.

I was wondering if anyone could shed any light on my latest quandary:

I've recently rebuilt the engine in this 79'GS550 (oversized pistons/bored out cylinders (not by me) /serviced cam chain tensioner / reseated valves, adjusted shims), and it's been running well, but I've only done around 20-30 miles since rebuilding it.

One of the remaining problems was I had a spark plug that had gone in threaded and wouldn't come back out. So I asked in my local and very helpful bike shop, and he said the guy that did my cylinder boring could get the plug out without taking the engine apart. He's a machinist who works with bits of older car and motorcycle engines, and came recommended by a few people.

So I took the bike round to him, and he said he could get the plug out, so he had a go at it. Using a breaker bar the plug snapped. So he said he was going to drill it out. I was concerned about what would happen to the debris and he said that he would blow it out with his pressure blower. So he did that. And installed a new insert and blew it out and put a new plug in, whilst I went to get some lunch.

When I returned he got me to start it up. The bike started and ran, but there was a little tinking sort of noise that sounded a lot like a bit of debris in the chamber. So he stopped it and took the plug off and put an endoscope in there and couldn't see anything. He then started turning the engine manually by the kickstart (I'm not sure why) and after a few turns it got jammed. It wasn't completely jammed, it would go forward or back about a 1/4 turn at the big nut. The pistons all moved this amount, and he said 1 & 4 were turning from just before to just past TDC, which indicated they weren't hitting anything.

He couldn't figure out what was up with it and was proposing to get a mechanic in who deals more with whole vehicles in the morning.

I'm wondering if anyone here has any ideas about what might be causing the seizure?

Thanks in advance.
 
Do you know if the valves were closed when he drilled out the spark plug? It would have to be at TDC compression stroke for the valves to be closed. My guess is he didn't get all of the debris out and some got jammed in between the valve and the valve seat. I'd check your valve clearances to see if one of them has gotten really large. Basically something is holding the valve open and the valve is hitting the piston. Did the engine sound like it was running on all four?
 
He blew some air down the hole in the spark plug (it'd pulled right out and left a tiny hole where the wire goes through it to the bottom of the plug) before drilling, and summised that as he couldn't hear any air blowing down the exhaust or into the engine that valves were closed. He didn't check at the timing marks though...
 
The spark plug is steel and any thing that doesn't come out the hole after drilling gets stuck in the top of the relatively soft piston or the aluminum cylinder head as soon as you turn it over.
I had the same sort of issue, but it was on a Zamboni, a much larger engine- 2.3 litre. The side electrode of the old spark plug was embedded in the top of the piston. It didn't stop it from running but it sure made a racket at idle.
I ended up pulling the head, ... and then an easy fix, but...
...if I knew what to look for at the time I think I could have fixed it through the spark plug hole.
 
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There was a bit of spark plug wedge down the side of the piston. It's dented the cylinder : (

On the bright side least the guy who buggered it up is a machinist who is going to re-sleeve the cylinder, and put right whatever damage may have been done to pistons and valves and not charge me for drilling out the spark plug.

IMG_20181214_125401.jpg
 
Bad luck,
but that picture doesn't look like a rebuild with only 30 miles on it...
 
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