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bad compression

  • Thread starter Thread starter suzukizone
  • Start date Start date
S

suzukizone

Guest
Just when I saw the 81 GS850G almost hitting the road,.........

Took a compression check, never did it before on this bike, bad news

1= 110 2= 88 3= 57 4=133

This is after the reallly bad initial readings & after I squirted a bit of oil in the cylinders to bring readings up. Engine is cold though. Not sure if warm engine would close the gap even if it did up the readings.

Well even tho I recently set valve clearances (approx 2000 miles ago) I will recheck as this is easiest. Doubt they are off as I am a stickler about not fudging the settings & have free access to all shims at the shop. Also I recheck all settings twice after I get them perfect. But gotta eliminate this possibility. Bye bye riding season I think

So I am thinkin after that it is valves or rings. Only has 32700 miles, last 12000 were mine & I'm a pretty gentle rider. Been keeping up on the valves since I got it. Twice between 20k & 30k

Always suspected the bike was a bit weak but it met my needs & only other big bike I've ridden was a GT750, so couldn't really judge.

Well guess I'll be having lots of speaks with the engine gurus here. Hope there's a few of you who have been down this road.

Haynes sez I don't have to pull the engine to get all the way down to the pistons.

Cheers,

DH :-(
 
Last edited:
Make sure the engine is warm and if your're using a screw-in gage make sure the 0-ring is new and screwed in tight, also keep the throttle wide open when cranking.
 
bad compression

Thanks racing jake,

Well engine was cold so I will retry with warm engine. All else was as you stated. The variation was consistent wet or dry tho wet the reading ran about 5-6 lbs higher. Perhaps before the weekend I can get engine running & then retest.

While I hope for best, I did feel, as stated, that I felt bike was underpowered based on specs & roadtests I had read. But since I am a slug as rider, it didn't worry me too much.

Thanks again,

DH
 
Since you have free access to shims, adjust your valves to the outer edge of the tolerance window.
This does several things:
It assures you that the valves are getting closed.
It gets the valves on their seats quicker so they have more time to cool down.
It lengthens the time the cumbustion has to act on the piston.
Closing the intake valve a couple degrees sooner drops the torque peak a few rpm, bringing it closer to modest-operation level.

Try to to excede the upper limit of valve clearance or things will get noisy, and the valves will be touching in a different part of the let-down curve on the cam, possibly slamming them into place.


.
 
I believe that if your readings are quite close between wet and dry compressions tests it indicates a valve problem (sever timing (jumped cam chain), valve clearence or worn valves or seats). The oil will create a better seal on the rings but will not affect the valves, there is another tool you can use to pin point the vavle problems (it injects air into the cylinder) but i can't remember the name of the tool (its pretty expensive, but maybe your shop has one you could "test").
 
suzukizone said:
Well guess I'll be having lots of speaks with the engine gurus here. Hope there's a few of you who have been down this road.

Haynes sez I don't have to pull the engine to get all the way down to the pistons.

Cheers,

DH :-(
Click to expand...

I'm no engine guru but I'm in the middle of the top end rebuild road right now. Kinda fun really. I worked on my bike for a year getting the carbs and all correct and it blew the head gasket on my first big ride, so I know what its like to miss a years riding.

If it really is a low compression issue it could also be your head gasket.
 
bad compression

Thanks for your feedback nightmare & Mark. Guess you're refering to a leak-down tester. Just watched one being used on a Suz SV 650 on Sat in fact.

Well I hear the thought on oil not changing the readings much if valves are problem. Will be looking at them before I take a chainsaw to it. Also will be doing a comp test after warm up just to have another reference but I bet it is much more than that.

Ahhhhh, a new hobby to get involved in.

Thanks again, will keep you & group posted as things move on.

DH
 
Leakdown test is the only way! I would bet it is the rings, center cylinders run hotter ! Regardless you need to test before you tear it apart
 
An 850 that needs rings at only 32K miles? That ain't right at all. There's no WAY the rings could be worn out.

I wonder if the rings are carboned up or stuck? It might be worth trying the Marvel Mystery Oil trick or something similar to try and loosen the carbon and varnish and clean things up in situ.

However, if they're really icky, a top end teardown could be the only way to set things right. Fortunately, it's quite easy on an 850. The hardest part by far is removing the ancient gasket residue. You can take it apart in an hour, and reassemble almost as quickly, but you'll be picking and scraping for days in between.

I don't see how you would need a re-bore at this low mileage -- at most you'll probably just need to clean up the ring grooves in the pistons, install new rings, and button it up again.

http://z1enterprises.com has the best prices I've seen on high-quality gasket kits. The Vesrah kits are very high quality. There's another brand out there, sometimes called "Athena", that's not all that great.

You might also try pressurizing the cylinders lightly (the hose from my compression tester has a standard air fitting on the end) and listening for the air leaks with a piece of tubing or a stethoscope to see if you can sort out where the compression is going.
 
bad compression

So it gets more interesting,

Original test, wet with cold engine was (for reference)
1-110
2- 88
3- 57
4- 133

Tonite warmed up engine real good, first numbers as taken soon as plugs were pulled

1- 124
2- 110
3- 111
4- 98

(I double checked that 98, read exactly both times It had been 133 at 2 readings)

odd???

Then I squirted some oil into plug holes & restested

1- 137
2- 125
3- 125
4- 108

odder??? Next day or two I will remeasure valve clearance & poss see if I can borrow a leakdown tester.

Life gozes on

DH :-s
 
And these numbers are with the throttle held wide open during testing?
 
bad compression

Hi, chief 1366

Yes throttle wide open BUT, when I ran this past the mechanics at the shop today they pointed out that because I had CV carbs, the throttle may have been open BUT the slides were still sitting in the gate waiting for the race to start. I will pull airbox & rig a simple device to hold up the slides & then retry it all.

I understand the Vegas bookies are now taking bets on what the readings will be, Hopefully somewhere this weekend.

will post further info as it occurrs.

DH \\:D/
 
I have CV carbs and read 180 across the board. Top end rebuild though with an 1166 kit.
 
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