1980 GS 1000G - Low compression

[free99]

Hello everyone, I poked around the forum a bit trying to find some info on compression testing a 1980 GS1000G engine for this bike I just obtained for restoration.

The bike has been left alone for upwards of 8 years, I found one thread about a 1000 with low compression mentioning running the engine to try to unstick the piston rings, etc.

My numbers are
[TABLE="class: grid, width: 500"]
[TR]
[TD]Cylinder[/TD]
[TD]Compression (psi)[/TD]
[/TR]
[TR]
[TD]1[/TD]
[TD]95[/TD]
[/TR]
[TR]
[TD]2[/TD]
[TD]60[/TD]
[/TR]
[TR]
[TD]3[/TD]
[TD]90[/TD]
[/TR]
[TR]
[TD]4[/TD]
[TD]60[/TD]
[/TR]
[/TABLE]

Does this mean I need to replace the head gasket? I cannot run the engine as there is no carburetor, nor airbox for that matter. Before sinking the costs on those needed parts, is there a way to determine whether this engine is irreparably damaged? What steps would you recommend from here?

Thank you!

 

[Nessism]

Standard first question that has to be asked is if you adjusted the valves, and if you did, did you find any with no clearance. MANY GS's die because the owners never do valve clearance checks and since the valves sink in to the head with mileage, eventually the valves don't seat, the compression goes down, and the valves carbon up and eventually burn. Check the valves and then get the bike running. You can't judge compression until the bike has some miles on it. Worst case you crack open the engine and do some maintenance.

 

[Steve]

One other thing to consider: since you know that the bike has not been run for several years, did you also do a "wet" compression test?

Squirt a couple tablespoons of oil into each cylinder and repeat the test. If the compression comes up, it's because the rings are now sealing better. I would just run the engine to free them up before making the decision to tear it all apart.

Nessism also mentioned valve adjustment. How many miles on the engine? The valves were supposed to be checked at 600 miles, 3000 miles, then every 3000 miles after that. Stock clearance is pitifully small, and the clearance always decreases. Because a clearance check added a considerable amount to the service bill, it was often declined. It would not take long (usually less than about 20,000 miles) before the bike started running poorly due to valve clearance issues and was parked behind the barn. Usually, it would be found about 8-10 years later by someone wanting to rebuild it and wondered about compression numbers. :-\\\

Suggestions:
1. Rebuild the carbs. If you don't feel comfortable blowing them apart into about 300 pieces and putting them back together correctly, there are a few of us here that are willing to do it for you.
2. Adjust the valves. No need to go any deeper than removing the valve cover, spark plugs and ignition cover on the right side. Check clearances, see what shims you have, order accordingly. See the offer in my signature for help on this.
3. Clean electrical connections. ALL of them. That will at least give you a running start on being able to check things when the bike gets running again.

Of course, there are other things that will need to happen, like change the tires, rebuild the brakes and a few others, but once you get it going, just run the engine moderatly hard, the rings will break free and do their job.

.

 

[Redman]

First: No. Low compression Doesnt not mean failed head gasket. Could also be worn or cracked rings. Could also be valves not seating well because was ran a while with no valve clearance.

You got some good advise there from Ed Nessman and Steve.

As Steve mentioned, try the compression test with a little bit of oil added in cylinder. If that greatly improves the compression test, then you can have some good idea the problem is the rings, or one problem is the rings.

FOr a quick check to see if any valve clearance is ZERO: check each valve (with its cam in appropriate position) quickly by seeing if can spin around the bucket.
IF can spin the bucket then know the clearance is more than zero, may or may not need to be adjusted for better operation, but is probably not the cause of low compression.
If can not spin the bucket (because spring is pressing on it) then know the clearance is zero and then that valve hasnt been closing well and can possibly be burnt and that could be your problem, and possibly be more of a problem than just needing to be adjusted. If is zero, adjust the clearance and check compression again.

 

[duaneage]

Not as common but possible is a timing chain out of time. Probably not in your case but one tooth off will cause this

 

[bobgroger]

Did you have the throttle fully open when checking? Makes a huge difference. Also, many bikes that have been sitting will go up 20-40 points after a couple hundred miles...

 

[free99]

Thanks folks,
I'll give it a look tomorrow morning and let you know.

Next step if it looks OK, I'll have to Frankenstein a pseudo electrical harness together and try to get my hands on a carburetor, just for testing purposes. No regulator/rectifier available, just a battery, coils, the crank position sensor and the CDI(?) module.

 

[Johno]

Standard first question that has to be asked is if you adjusted the valves, and if you did, did you find any with no clearance. MANY GS's die because the owners never do valve clearance checks and since the valves sink in to the head with mileage, eventually the valves don't seat, the compression goes down, and the valves carbon up and eventually burn. Check the valves and then get the bike running. You can't judge compression until the bike has some miles on it. Worst case you crack open the engine and do some maintenance.

The issue of valve specification on a bike laid up relates to my personal experience. I had a 81 GS1000G which I purchased new in the shed for nearly 14 years. The bike had only 27000 kms on the clock and I knew the motor was fine. Cold compression was only 90 psi on some cyclinders.

Carbs cleaned, valves adjusted, fluids replaced and compression came back to spec after a good hard run.

A low mileage motor will come good if you do all the maintenance.

 

[Steve]

..., I'll have to Frankenstein a pseudo electrical harness together and try to get my hands on a carburetor, just for testing purposes. No regulator/rectifier available, just a battery, coils, the crank position sensor and the CDI(?) module.

Sounds like you have a LOT more going on than just a suspected low compression situation. :-k

- Without carbs, you won't be checking anything except valve clearances.
- Without R/R, you can fire up and run, but won't charge a battery.
- You don't have a crank position sensor.
- You don't have a CDI. What you DO have is what Suzuki calls an "ignitor". It's nothing fancy, it's just a box of transistors that replaces the mechanical points from earlier models.

.

 

[free99]

Steve, in order:

• That's pretty clear
• Again...
• What do you call that set of parts which pick up where the crank currently is in order to tell the 'ignitor' when to crackle? I always thought it was a position sensor. At least, the name seems (or seemed) applicable.
• Did they introduce CDI in 1982 for the GS450? On my bike it says "CDI" on the side of that little box going to the coils.

 

[free99]

OK, good news (mostly):
Turns out the compression gage I rented from autozone was broken, new figures after adjusting valve clearances are (on a cold engine):
[TABLE="class: grid, width: 500"]
[TR]
[TD]Cylinder Number[/TD]
[TD]Compression (PSI) w/o oil[/TD]
[TD]With 1Tbsp Oil (PSI)[/TD]
[/TR]
[TR]
[TD]1[/TD]
[TD]146[/TD]
[TD]150[/TD]
[/TR]
[TR]
[TD]2[/TD]
[TD]108[/TD]
[TD]120[/TD]
[/TR]
[TR]
[TD]3[/TD]
[TD]120[/TD]
[TD]130[/TD]
[/TR]
[TR]
[TD]4[/TD]
[TD]130[/TD]
[TD]140[/TD]
[/TR]
[/TABLE]


Cyl #2 looks kind of.. off. I believe I'll be ok to actually get the bike running and then narrow the problem down (if it's real) later, unless someone suggests otherwise.

 

[Big T]

#2 compression may be the result of a leaky Petcock, allowing extra fuel down the vacuum line into #2

Another item on your checklist

But, the numbers are more than acceptable