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Compression Woes

  • Thread starter Thread starter dwvoss
  • Start date Start date
D

dwvoss

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I took my 850 into the shop to get a couple of snapped exhaust bolts removed from the engine. Following some advice from my snapped bolt thread, I decided to have the shop check the compression on the cylinders so that in the event that the cylinder head had to be pulled to remove the bolts, I could determine whether I needed to address any compression problems.

I was expecting to get less than perfect results, since I haven't checked the tappet clearances, and have been noticing some decreased power.

I wasn't quite prepared for the news that I had no compression on #2 and almost no compression on #3. :(

I'm guessing that this is something more serious than tappet clearances. Anyone got any words of hope for me? The bikes only got about 33K on it.
 
If one of the valves is super-tight, it will give you NO compression. :shock:

However, if the valve has been tight enough to not close (the reason for no compression), it might have gotten burned.

My advice, since you asked for it, is to adjust the valves and check the compression again. Yeah, you might be out a few bucks for new shims, but it just might save a tear-down of the top end, too.


.
 
Dan,
If you've got valves staying open due to clearance problems, that'll cause low compression. Also, listening to your bike on the Fennimore rally, I think you might even have a head gasket leak. Pull the head, have a valve job done, and unless you've run it seriously low on oil, I think it should solve the problem. (hope, hope, pray.......[-o<):-s
 
Dan,

The good news is that you'll be pleasantly surprised at how strong the bike will run once you get it straightened out!

Joe
 
I've finally had some time to devote to the 850 after loading up on all the shims and parts that I thought I would need. Now I need some advice.

I've got all my valve clearances adjusted except my #2 and #3 Exhaust. These are the two cylinders that also have big compression problems. The #2 exhaust had a 2.65 shim in place. I've taken it all the way down to a 2.30 (the smallest I have), and still haven't gotten enough gap. I'm in a similar situation for #3 exhaust, except it started at 2.40, and still not an acceptable gap with 2.30.

Here's my question: Do I get smaller shims, hope that I get enough gap, and then test compression -- or do I presume valve damage at this point on these cylinders and pull the head to inspect/replace valves?

Thanks!
 
I think that I'de try the shims first. If it saves you from pulling the head, you're time and money ahead. (no pun intended) :-D
 
Sounds like valve damage to me. How long has it been since your last valve check? No disrespect, but taking care of the valves are a pay me now, or pay me a great deal more later kind of thing.
 
Dan,

I agree with Dan the Man that you may have a blown head gasket. The popping noises your engine makes are a good sign of a blown gasket. I would definitely pull the head and replace the head gasket. It would be a good idea to replace the base gasket at the same time. Rather than doing a valve job you might want to consider finding a used head and installing it instead. That could save you a lot of $$$. Bwringer and Dpep both have a stash of GS850 parts and may be able to help you out. Paulyboy recently got a truckload of GS850 parts from an eBay auction and may also be able to help. You could post a request in the Parts Wanted forum to see if anyone else can help.

Thanks,
Joe
 
It does sound quite likely that the #2 and #3 exhaust valves are burned. Even if this is the case, you will probably only need new valves.

On the original cylinder head for my bike, these valves ended up badly burned (sad story involving intake o-rings -- it was long ago before I knew about these things). However, the valve seats were in fine shape, believe it or not. (That cylinder head vanished when the cycle shop owner skipped town to escape drug dealing charges... another sad story. And one reason I do ALL my own work now.)

I think it sounds like it's time to pull the cylinder head and see what's going on. Burned valves are pretty obvious -- the'll have little canyons eroded into the edges.

In my experience, even valves that are pretty tight -- requiring two or three shim sizes smaller -- don't lose all that much compression. If you have a severe lack of compression, chances are good that something more is wrong.
 
Might be stating the obvious but did you do the compression test properly, with the carbs full open?

If you did then I would concur with bwringer - even with tight valves you should get some compression so there is a chance something else has gone AWOL.

No harm in just re-shimming first though - if you're luck's in that might solve the problem and save a bit of work.

Good luck.

Wally
 
I'm amazed at the ammount of shim size reduction your having to do.

I would suggest that something is not right there with how the valves are seating or a couple of weakening valve spring's.

You may be to the point of taking the head off.

I'd check the compression again (yourself doing it if possible)

Suzuki mad.
 
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