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spark knock (I think) any ideas ?

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Howdy all, a few years back I went through a xl600 I had, it ran fine for awhile then noticed it had what seemed to be spark knock under load. It was suggested that it may in fact be a head gasket. Well, after trying many things to hopefully avoid pulling the head, I gave up and pulled it. The gasket was blown, I replaced it with a shiny new one and all seemed fine. The other day I was out for a spin and noticed the knock again, the gasket was replaced with new, no sealer and torqued to spec. I hate to think I have lost another gasket so I thought maybe someone had an idea what else to look for. Stock muffler, stock carbs with stock jets and stock air cleaner.

any help or ideas is much appreciated, thank you

Edited by thumpharder

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Do you see any gas or oil escaping,,Chances are it's possibly blown again. I'd be thinking one of the mating surfaces is slightly warped or has some sort of mark on it that's causing the thing to keep blowing..Both the top of the bore and corresponding part on the cylinder head need checking with a straightedge and feeler gauge..There's a section in the manuals that deal with doing that sort of thing,,either that or get it checked by an engine reco shop that know what they're doing if you don't feel confident with the procedure..

Personally I myself would get a shop to check the parts,,They're supposedly the experts so I'd rather rely on them to check the parts and fix the problem when it's found..

Edited by Horri
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Hello Horri, been quite awhile since I've been on. I was hoping for something else but I'm thinking your probably right.

I see no oil around the head/barrel but I didn't last time either. I will pull it and check it for flat, I have a machine shop in my backyard, I have a granite surface plate and drawers of indicators, I didn't check it last time.........guess I should have..dang it.

Thanks for the thoughts

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Would a compression test show if gasket was leaking? That noise, you describe, I wonder if that is what I have? On my 92 xr600, it has always had a deep, almost like rod, knock when accelerating, I always thought it was a loose rod that was worn & getting ready to let go. But it has done it a long time. If you do nothing , what would happen? Is it possible to suds up around parting joint of gasket & keep steady compression pressure on at TDC? (Kickstarter?) To look for foaming.

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Yes you can soap it up, but it may not be leaking to the outside, it could leak into the cam chain area. If it does that you won't see it, that's what mine did the first time. The low knock you I think you are hearing is not a problem, most a bikes will do that under load at low rpm's. I try to not let it do that by down shifting to keep that load off the rod bearings.

What I have is a lighter spark knock sound almost like what you get running crap gas in your car with the timing too far advanced. does it when I'm cruising along and then hit the gas, like say 30mph in 4th and hit it hard. Will also do it if I have to pull a long hill and try to maintain speed or accelerate going up.

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Is your jetting and spark plug heat range correct? Pressures from detonation can blow out the head gasket.

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Is your jetting and spark plug heat range correct? Pressures from detonation can blow out the head gasket.

yes, all jets are stock, plug is stock range, muffler and air box all stock. I put this bike back to a stock condition when I did the restore.The one ting I did not do was check head for flatness, I will check that this time and resurface if needed.

I appreciate the questions, but I think since all is stock my only recourse is to pull the head and go from there.

I even tried different cdi and coil just to rule them out. I have also gone to the coldest plug...running out of ideas for a magic pill, looks like gaskets are in order.

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That is the RPM at idle? Sounds to me that you are lugging the motor Too tall a gear and too low RPM for the load applied is my guess.

Edited by ohio rusty spokes

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You can look for detonation when you disassemble it. Look for hot spots near where the dome meets near the head gasket.

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You can look for detonation when you disassemble it. Look for hot spots near where the dome meets near the head gasket.

I'll check that out, will probably pull it apart this weekend.

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