GS850G Novice Compression Questions

[Guest]

Hi folks

I recently bought an 850G. Had the Carbs cleaned and tuned, including cycleorings, gaskets and fuel valves replaced. Also changed the Carb and airbox boots and orings, also new air filter and also a new petcock.

And now the bike's in much better shape than previously. !!!
Starts pretty easily with little choke and no trottle, revs are a bit unsteady until it warms up. After that its pretty much ok,

but....

The mechanic (yeah, sorry, I can't do that work myself) says one of the cylinders is losing compression, about 40% loss. He won't know what is the cause until he opens the engine. Consequently he couldn't fully tune the carbs.

So being a total novice and not having really understood the posts I've read so far on the matter, I was wondering if people could help me understand (in simple terms) what are the possible/likely causes of compression loss on these bikes, and what symptoms I should look for.

More importantly however, I'm trying to get an idea of parts it's worth ordering in advance of the work. Anything affordable that is likely needing replaced anyway while the engine is open, given that the engine has not been well looked after. Any suggestions?

For example, I guess I should get hold of a full gasket set. The mechanic also suggested getting hold of the oil seals for the camshaft valves.

Any other suggestions?

Many thanks in advance
Assaf

[cowboyup3371]

I had a mechanic tell me my engine was shot because the compression ratings were very low even with a cold engine. Yet my bike is running great (when I get everything back on) with numbers about where they should be.

With that said, what are the different numbers for each cylinder? Was the bike already warmed up? Was the throttle held wide open during the tests?

[hampshirehog]

Are the valves adjusted? Could be one (or two) not closing 100%.

[Blastr2783]

rebuilding

I recently bought an old gs850 as well. And I am just refreshing everything except the primary engine components. I ordered a complete gasket set from oldbikebarn. It supposedly has all the gaskets needed for a rebuild.

http://www.oldbikebarn.com/GS850-G-79-81-Athena-Engine-Gasket-Set?sc=12&category=73748

[bbjumper]

Assaf,

Your about to get a bunch of great information any time now from BassCliff and others on this site, but usually compression loss, assuming your mechanic adjusted the valves as part of the tune up, comes from burnt valves or ring issues, worn, broken or stuck or possible hole in the piston.

I am in the middle of doing a top overhaul on my "850 G" model right now and I can share with you what I found. Mine had good compressions but was suffering from high oil consumption, it would have run forever probably but I just couldn't let it be.

When I pulled the head and cylinder I found allot of carbon, every o-ring hard, brittle and usually cracked, a leaking head, and cylinder base gasket so a complete OEM gasket and seal kit, at least for the cylinder and head would be a must. My cylinder bores were in good shape but were glazed, I honed them with a 320 grit ball hone. New rings obviously, my rings were in good shape but the oil control rings were stuck in the groove and would not expand. This and the hard as a rock valve stem seals is where my oil was going.

Carbon was completely blocking the oil return holes in the pistons and needed to be cleaned. I cleaned everything up and did all the measurements and surprising to me found very little if any wear. Valve guides were good, cylinders and pistons were good. I re-cut the valve seats but that was just because I was already in there and I don't know how many miles are on this engine. I'm replacing the piston pins, they were showing some bluing, evidence of heat, probably because they weren't getting enough oil and that's it.

Good luck with yours, I will tell you that I was really impressed with how rugged these engines are, I can see why they are still running strong after 30+ years. I am looking forward to many thousands of miles out of mine, Great bikes and the guys on this site are the greatest.

Cheers,

Brad

[tkent02]

Do NOT let this guy into your engine. He either knows nothing or he's lying to you. Compression loss can have a number of fixes, most are easy to repair without tearing the engine apart. Check the valve clearances first, they generally get tighter as they wear. If the valves get too tight, there's low compression, then none, then after a while the valves burn up. Adjust them before any damage is done and it's an easy fix... Sometimes the engine just needs to run a thousand miles or so to get the rings sealing again, but make sure the valves are adjusted properly first.

[Big T]

This thread (while containing good information) is useless without the compression readings

Here in the States, it's measured in psi - 170 lbs compression. 40% is meaningless

As stated, adjust the valves and see what the compression is

[tkent02]

And which cylinder is the low one?
Hopefully not #2.

[Assaf]

I'll try to get the details you've all asked for.

When he said 40%, he just meant that in layman's terms (ie 'my' terms) the reading was around 40% lower than it ought to be. Doesn't that make sense on its own, assuming he performed the measurements correctly?

My post may have been misleading. He didn't immediately propose to tear the whole engine apart. He said that the problem may be simple to fix or might be more complex. He may only need to take the head off, or may have to open it further. But he won't know till he starts, and doesn't want to start until I have a set of gaskets. And I may as well get a full set as they'll all be useful at some point (I have a lot of oil leaks).

So I then decided that since we may be opening my engine quite extensively, then it would be good to be prepared by having in my possession any affordable parts that are worth replacing once the engine is already open, particularly parts related to compression issues.

That's what I had meant to ask you guys. See, it took 2 months to gather the parts for the carb rebuild etc described above and so I'd like to avoid that in future if possible.

Sorry if I mislead and gave the impression that the mechanic just wanted to tear open the whole engine. Without going into details, I do think he's good. I did some research and went with referrals.

I'll certainly look into the valve adjustment as a priority. I've seen the tutorial and it looks like something I may even be able to do myself
but, need to get hold of some gaskets first.

Again, if you can recommend any parts worth getting hold of for while adjusting the valves or chasing compression issues, then it would be good to know.

Many thanks for your help and detailed responses
Assaf

[Assaf]

...every o-ring hard, brittle and usually cracked, a leaking head, and cylinder base gasket, so a complete OEM gasket and seal kit...would be a must....

...my rings were in good shape but the oil control rings were stuck.... hard as a rock valve stem seals...

.... Valve guides were good.... I'm replacing the piston pins....

Thanks for all the detail Brad
A few questions please:

Does the 'complete OEM gasket and seal kit' contain the o-rings you mention that were hard and brittle?
It looks from Blastr2783's link like it probably does.

What are the 'rings'? Are they different from the 'oil control rings'? Are both these parts something worth getting hold of and replacing at some point on an old bike? Or are they too expensive to buy if not specifically needed?

Same questions re the valve guides, piston pins and valve stem seals.
What are these and are they affordable parts I should buy, or better to wait and see if really necessary?

If it makes things simpler, maybe refer to a fiche and tell me the numbers.
eg: http://www.cmsnl.com/suzuki-gs850-19...498/partslist/


Many thanks

[bbjumper]

Assaf

If you do determine that a tear down is necessary,

1. The complete OEM gasket kit does, you will not need everything if your not doing a complete tear down though. For the cylinder and head I would take a look at the service manual available on BassCliff's site and start a list. I replaced all gaskets, seals, o-rings, everything from the head down, overhauled the cam chain tensioner, tach drive, replaced carb boots and o-rings, airbox boots, etc. Like yours, mine was leaking air and oil from everywhere.

2. As far as the rings go, they come in a set for each piston, so you will need four sets. Your mechanic, when the engine is apart, will measure everything and determine if you can go back in with a standard set, or if there was excessive wear or damage, he may need to bore the cylinder and in that case oversize pistons and rings will be needed. A four piston ring set alone will set you back around $ 100 US so take it step by step. At this point you really don't know how far you need to go.

3. I would suggest wait and see what is needed, make sure all of the basics on BassCliff's site have been covered first so you have a base line to start from. Then if need be disassemble, inspect and go from there.

I went overboard on mine and got all the engine parts for around $275. I don't know what shipping is like for you though.

Cheers,

Brad

[Nessism]

Assaf,
I strongly advise you to first adjust the valves, then ride the bike, and ride it hard, before you go off and spend a bunch of money on parts. It will take some miles before the bike starts to show you it's true condition so don't jump to conclusions.

If you do need to tear into the engine, the only gaskets worth having are the original Suzuki parts, which are not cheap. Thus my suggestion above.

Take your time and systematically go though all the bikes maintenance and then see how it's running. For example, the 850 is very sensitive to intake system sealing so make sure the internal foam in the airbox is serviceable otherwise replace it. Check voltage at the coils, check the spark plug caps, etc. Go though the service manual and dig around on Basscliff's site to gain knowledge on what needs to be done. 90% of the time all the bike needs is maintenance, not a tear down.

Good luck

[Assaf]

Ok folks, thanks for your advice, will do as u suggest.

Ed, just so I understand: first adjust valves, then I guess re-attempt carb tuning, then ride for a while and only after that evaluate whether to open her up. And meantime perform the maintenance you've described.
Is that right?

Thanks

[tkent02]

Even if you need to open up the engine to fix leaks, it is very worthwhile to get it running as well as possible first. A rebuilt engine needs to be hopped on and ridden hard right away, it does not need to spend it's first running hours on tuning, misfiring, running rich, getting hot synching carbs, and doing all of the crap that should have been taken care of before.

[Nessism]

Ok folks, thanks for your advice, will do as u suggest.

Ed, just so I understand: first adjust valves, then I guess re-attempt carb tuning, then ride for a while and only after that evaluate whether to open her up. And meantime perform the maintenance you've described.
Is that right?

Thanks

Perform the maintenance now Assaf. Only after you do all the maintenance (such as valve adjustment, airbox sealing, carb syncing, etc), and ride the bike for awhile, can you judge the condition of the engine properly.

Good luck