Compression Results and a Question

[ccs]

Hello All;

I bought an 82 GS850G last May with 19,600 miles on it. Since then I've put 7,000 miles on it. It starts quite easily, runs good -- but not great, and the mileage is poor (25-30 mpg). I've always wanted more oomph, and better mileage, out of it as I thought an 850 should perform better.

Well... I did a compression test the other day at 26,541 miles.

Engine fully warmed, all plugs pulled, choke set to normal running position, cranked engine 10 seconds each cylinder.

#1 -- 105
#2 -- 100/102
#3 -- 100
#4 -- 100

These are dry numbers -- I did not do a wet test yet.

I've read a number of the posts about compression on this forum and because the bike starts and runs good, I am wondering just how good/bad these numbers are?

Should I think about doing a top end job? If so, I would like to do it myself and am wondering what it might cost and what I would be getting myself into.

And... if I spend the time and effort to do a top end job, should I just rebuild the engine totally?

I've also thought about buying a GS1000 or GS1100 engine and putting it the bike? Any thoughts on this?

TIA.

Charles...

 

[Dave8338]

Those numbers look quite good for a dry test, the main thing is that they're balanced. If I were doing it, I'd check the valves for clearance, get new schims where needed and put it back together. Have you noticed that there is any blue smoke out of the exhaust? How do the plugs look color wise? Have the carbs been balanced?

 

[ccs]

Those numbers look quite good for a dry test, the main thing is that they're balanced. If I were doing it, I'd check the valves for clearance, get new schims where needed and put it back together. Have you noticed that there is any blue smoke out of the exhaust? How do the plugs look color wise? Have the carbs been balanced?

Hi Dave;

Thanks for the reply!!

The number look good? Hmmm... I've read on other posts that some get 125-145 psi.

As for the valves, I checked/adjusted them on Mar 10 @ 25,931 miles. The only problem I encountered was that my feeler gauge only measures to 0.04mm (not 0.03mm) and several of the valves might be 0.03mm (in spec) or 0.02mm (too tight) -- but I can't measure either of these clearances -- can't even find a gauge that measures that narrow (best I could find was 0.038mm).

As the bike sat for 2 months (Jan 14 through Mar 10), I then ran three bottles of GUMOUT (didn't do anything), and 1 can of SEAFOAM (which seems to have cleaned the carbs quite well) through the engine.

There is no blue smoke at all (no smoke of any kind, actually) coming from the exhaust.

The #1 and #4 plugs are carbon fouled (and have been since I bought the bike), but #2 and #3 seem to be OK. I just purchased a colortune off eBay, and some vacumm guages from JC Whitney) to adjust the idle mixtures and balance the carbs.

I did get the colortune into #4 cyl last night and the flame is way YELLOW -- so, I've got a little work to do -- vacumm gauges should be here this week.

Charles...

 

[Dave8338]

The 125-145PSI numbers are most likely from a wet (oiled) compression test. If you have the time, use 1/2 ounce of 5-30 oil in each cylinder and re-run your compression test. You should see the numbers in the 140+ range, if not, you may be back to the valves project. Too tight on them will give you a drop in compression, as you are describing. If the bike has been cared for, the rings should be fine at 30,000 miles. However, who's to say that at one time, the petcock or needles and seats failed, dumped fuel into the crankcase and the bike was driven that way. You may never no.

 

[headsbikesmopars]

Charles, did you open the throttle? Be surprised how offten that is overlooked. You will have low numbers if throttle is not opened all the way while testing. Engine can't pull in much air with the throttle closed. Good luck, Terry.

 

[ccs]

Charles, did you open the throttle? Be surprised how offten that is overlooked. You will have low numbers if throttle is not opened all the way while testing. Engine can't pull in much air with the throttle closed. Good luck, Terry.

Well... I *did* on the first test of cyl #1 (105psi).

Then forgot to open it for 2 (80psi), 3 (75psi), and 4 (65psi).

Geez, I thought... my bike runs too good to have these compressions, especially a max difference of 40psi between #1 and #4.

So I went back to #1 and... got 75psi... it was then I realized I forgot to open the throttle (DOH!!).

Re-checked all cyls and got the 105, 100, 100, 100/102 readings, dry.

 

[GregM.]

Hi, I have been concerned about too much oil useage since last Fall... and have been reading here off and on about comp. tests and such. My bike also starts and runs good. Power seems good, but then no real comparison test I can make here. My mileage is 39 - 43 mpg.

I finally did a test a few weeks ago on my '80 850G with 24K. Sounds like I did the test the same way you did, but you didn't mention "wide open throttle". Did you do that? I believe you're supposed to. (edit. we were cross posting, you did)
I did (plus the rest you mentioned), and here's what I got:
#1 - 145 lbs.
#2 - 115 lbs.
#3 - 140 lbs.
#4 - 140 lbs.
Then I added 1 to 2 cc's (less than 1/2 tspn) of engine oil, found out later I should've added twice that amount. And repeated :
#1 - 155 lbs.
#2 - 125 lbs.
#3 - 150 lbs.
#4 - 150 lbs.
Obviously I've got a problem with #2... I've been busy and haven't done anything about it. But I am curious, the way I'm going through oil is not good.

 

[Nessism]

Charles, did you open the throttle? Be surprised how offten that is overlooked. You will have low numbers if throttle is not opened all the way while testing. Engine can't pull in much air with the throttle closed. Good luck, Terry.

Yup!

100psi is quite low if you were opening the throttle - near the low service limit. Proper compression should be in the 150psi range. If I were doing the job I'd check the valve clearances first and adjust as necessary - do not simply dismiss this critical check because a proper gauge is hard to find.

Good luck.

 

[Guest]

Do as Dave said with the oil in each cylinder. If the compression rises it's about time to do a ring job and while it's apart do valve stem oil seals and clean up the head and check the valves.
If the compression stays the same you either need to adjust your valve clearance or your valves aren't seating because of either carbon buildup or burned valves because of bad clearance.
If it turns out to be the rings I'd say you have one more season left and make it your winter project next year.

 

[nabrams]

I'm betting on insufficient valve clearance is the primary issue - I'm supposing you've never adjusted them, as you haven't mentioned it.
It made a HUGE difference for my CB750.
And yes, those numbers are at the minimum level.

 

[Steve]

Personally, I would not worry about not having a feeler gauge that measures the smallest clearance possible. I have always run closer to the largest amount allowed. If the specs call for .002-.006, I will adjust for the .005-.007 range. This assures clearance for a longer time, and actually moves your torque peak down a few hundred rpm. \\/


.

 

[Dave8338]

Personally, I would not worry about not having a feeler gauge that measures the smallest clearance possible. I have always run closer to the largest amount allowed. If the specs call for .002-.006, I will adjust for the .005-.007 range. This assures clearance for a longer time, and actually moves your torque peak down a few hundred rpm. \\/


.

That'd be how I'd play the game...but then what do I know! :-D

 

[Jim H]

I think that the consistency of the compression readings is more imporant than the actual numbers. Charles, your readings look good. I have a 1982 GS1100G with 16,800 miles. It runs excellent, burns no oil and regularly gets 41-43 mpg. The valve clearances have been adjusted to the proper clearances. I got the following compression readings with the throttles open: 120, 100, 115, 105. I was concerned about the 100 reading, mainly because it isn't consistent with the other cylinders. So I did a leakdown test. Surprisingly, the cylinder with 100 psi came out the best with the leakdown test. It tested very good, and the other cylinders tested good. Suzuki's limit for variation of compression between cylinders is 28 p.s.i. (seems like a lot to me). My point is that sometimes compression readings can be misleading. Variables such as cranking speed can affect the results. Poor engine performance and bad gas mileage is much more likely to be due to carburetion or ignition problems than due to compression problems.
Jim H.

 

[49er]

My point is that sometimes compression readings can be misleading. Variables such as cranking speed can affect the results. Poor engine performance and bad gas mileage is much more likely to be due to carburetion or ignition problems than due to compression problems.
Jim H.

I agree here.

You did say in your second post that you had plug fouling problems with 1 & 4.
These plugs are both fired off the same coil/igniter. Your poor mileage/performance could be caused in this area.

If you were able to set any two of your valves on the same cylinder in the 0.03-0.08mm range then your compression figures are still very marginal.