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DaveySprocket

Looks like ring/piston time. Other good mods?

16 posts in this topic

My 94 600R was running fine, though I went to ride it yesterday and could not get it to start. I could feel when it was on the compression stroke, but there was not much resistance there. I pulled the compression release and checked the valves, but all of that was fine. Compression test showed around 10psi. Bummer, looks like time to pull the engine apart. I would actually be more excited about this if I did not already have one of my bike engines in pieces :thumbsup:

While I am in there, should I consider some performance mods? Is there any good aditional power to be had with these motors without making them terribly unreliable?

Thanks!

Dave

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My 94 600R was running fine, though I went to ride it yesterday and could not get it to start. I could feel when it was on the compression stroke, but there was not much resistance there. I pulled the compression release and checked the valves, but all of that was fine. Compression test showed around 10psi. Bummer, looks like time to pull the engine apart. I would actually be more excited about this if I did not already have one of my bike engines in pieces :thumbsup:

While I am in there, should I consider some performance mods? Is there any good aditional power to be had with these motors without making them terribly unreliable?

Thanks!

Dave

10 psi? I'd have to say that a running bike that suddenly showed 10 psi would tell me that it is not a wear issue, you broke something;-(

I'd be more inclined with a reading like that to thing valve problem, either not seating or...? If it is a piston, I'd guess part of the skirt broke off, or the ring broke or is stuck.

Seems to me it would be a good time for a big bore kit!

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Well yeah, that is the mystery that will be solved once I rip it's head off :thumbsup: It did run fine the last time I rode it, which was just a couple of weeks ago. I did not find any metal bits in the oil, though it was very dark for only having about 500 miles on it (though that could also be clutch). The valves do seem properly slacked, though it is possible I have a damaged seat or something.

Any thoughts on a big bore kit? What sizes will still keep it easy to start?

Thanks!

Dave

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My first thought is broken valve spring, but you already said your clearances where good. At least it won't take long to pull the motor and remove the valve cover to start checking. Things get more complicated after that....

Let us know what you find.

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I was just going to drop the motor down off the mounts but leave it in the frame. Will it be less of a headache to just pull it out now? Is there a good way to stabilize the motor on the bench? That was my main reason for leaving it in the frame for now.

Thanks!

Dave

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10 psi? I'd have to say that a running bike that suddenly showed 10 psi would tell me that it is not a wear issue, you broke something;-(

I'd be more inclined with a reading like that to thing valve problem, either not seating or...? If it is a piston, I'd guess part of the skirt broke off, or the ring broke or is stuck.

Seems to me it would be a good time for a big bore kit!

Yeah, but he still has to pull her apart. I would look into a boar kit, cam, and a nice porting/polishing.

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I just rebuild my 2001 XR650R engine last month. It was smoking quite a bit. I pulled the top end out and found that the exhaust valves were not seating properly because of all the carbon deposits. My buddy has the valve seat grinding tool so we ground all the seats"they needed it. The exhaust valves had to be replaced. Take a flashlight and shine it into the exhaust port, turn the lights off and see how much light shines through the valves.

For a little extra HP's I added a Hot Cams stage 1, for a little overrev protection I added Kibblewhite valve springs and Ti retainers. I also installed an Edelbrock pumper carb. It's winter here so I still have yet to ride the bike.

I also replaced the crank, crank bearings, piston, rings and all the seals while I was in there.

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Ok, I pulled it apart and unfortunately (fortunately?) did not find any major problems. Here is a page with the pics:

http://www.daveysprocket.com/projects/xr_rebuild.html

Thoughts?

At 10 psi a piston ring failure would be obvious. If the valve and seats look good, suspect a hairline crack in the head.

Your head gasket looked good right? No burn marks, scoring, or cutting?

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Did you check the timing on the cam chain before you tore it apart? They have been known to skip on occasion...

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Dumb question: could you decompression cable just be a little sticky? Maybe its just stuck open a little bit?

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Do a ring end gap check. Remove the top ring an put it in the bore. Use the piston to get it square in the bore. Measure the gap between the ends of the rings. I think the limit is .020. I took my engine apart last month. The end gap was huge, like .150.

Another possiility is oil breakdown. Perhaps it all drained off the piston and there was no oil to seal the rings.

Everything looks good from your photos. Kind of a mystery...

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The first thing I did when I notiched that there was not much compression resistance was disconnect the compression release. When I checked the valves, I also made sure that it was out of the way and not affecting anything.

Ok, some more pics up at:

http://www.daveysprocket.com/projects/xr_rebuild.html

Three of the valves are leaking, though it seems rather minor. There were two spots in the valve recesses on the piston that were shiny, as though there may have been some slight valve contact. However, it does not really match up to the valves that are leaking.

Unfortunately I did not check timing before I tore the motor apart. Jumping a tooth on timing does seem to be the most likely scenario right now, though the timing chain and sprockets are all in great condition. I also had a heck of a time rolling the timing chain off the cam sprocket after removing the cam chain tensioner. I did finally get it, but there was barely enough slack to work the chain off. This just makes it hard for me to believe that it was possible for the chain to have skipped a tooth.

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Take a quick look at your carb. The choke flapper, and the whole choke plate has been known to have fatigue issues and get sucked into the engine. I took my bike apart last month for some new rings. My flapper was hanging by a thread and the pot metal choke plate had a fracture half way through. I'm now running with no choke at all. I have a new cold weather starting ritual. Tip the bike over to cause it to "flood."

I sort of doubt the above since your engine either has new parts or is low time. Both the bore and the clutch look very good. Way better than what I saw when I opened up my engine.

The tensioners are known to wear out. Last year I replaced the tensioner and the chain. The engine was making all kind of noise when hot from the chain slapping around. Caused it to run poorly as well. I never slipped a tooth though. The tensioner looked good and worked when the engine was cold. Only had issues when running. Those marks on the piston seem to say valve timing issues...

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