Compression Test Analysis

[Anonymous]

I have an 82 GS1100GK with 30,000 miles. A host of inter-related issues have come up at once, so I'm working through it to get it back. I recently did a compression check.

The test was done with all plugs out, the carbs off, and with the engine very cold - cold here, by the way, is about 35F right now. I don't have a compressor so there's no leakdown data, but here's approximately what I got:

#1-160 #2-130 #3-140 #4-140

All within range, but my understanding is that a pressure difference of 30 psi is out of spec (it's really less than that, probably within the 28 psi permitted variance, but it's close). I have a theory on this.

When I first got the bike, it smoked at start. It would smoke BAD sometimes, too. My mechanic suggested I switch to AMSOIL, and that would make the smoking go away. It did. So, between that and the blueness of the smoke, I know I was burning oil. Also, the bike was always parked on the kickstand. Thus, it leaned toward #1, and my suspicion is that #1 burned the lion's share of the oil.

Fast foward about 9 months, and I've got this compression reading. My theory is that #1 has alot of carbon in it, and that's what's causing its high compression. So, GSers, I have two questions for you:

1) Could you think of any other ways this could happen?
2) If it is carbon, how could I best remove it without taking the head off?

[nert]

as you know, a compression check should be performed on a warm engine (was running and shut off). So the 30psi differance may not be critical, as testing conditions are not quite correct.

We use Onan carb and comp cleaner (PN# 326-5278) to chemically decarbon small air cooled engines. We have performed before and after tests (removed the heads prior to chemical decarb and again after) to check the effectivness of "mechanic in a can". I am happy to report the cleaner works very well. Not as good as a manual cleaning, but very effective. Enough to avoid mechanical cleaning.

[nert]

as you know, a compression check should be performed on a warm engine (was running and shut off). So the 30psi differance may not be critical, as testing conditions are not quite correct.

We use Onan carb and comp cleaner (PN# 326-5278) to chemically decarbon small air cooled engines. We have performed before and after tests (removed the heads prior to chemical decarb and again after) to check the effectivness of "mechanic in a can". I am happy to report the cleaner works very well. Not as good as a manual cleaning, but very effective. Enough to avoid mechanical cleaning.

[Evil Hobbit]

Where do I get Onan carb and comp cleaner (PN# 326-5278) ? I'd like to give it a try.

[Jethro]

Yeah, I can attest that the engine should be warm. I did the first comp. test on my newly accuired 1100 when it was cold (didn't know better). The results were between 120 and 160. A few weeks later I did the test while warm and I got 172, 180, 178, 180. I'm happy with the warm numbers.

[Dink]

There are two cheap and easy ways to de-coke a motor with out stripping it.
1) add a small amount of diesel to the fuel tank, about 1 cup.
2) remove the air filter and spray water into the plenum,,with a trigger pack.
Dink

[nert]

You can get it from an Onan generator dealer, or possibly and RV service and parts department. Or try scott_eriksen@hotmail.com with your request. No, thats not me.

There may be other "like" chemicals available at an auto parts store. The product is a carburetor and combustion chamber cleaner.

[Anonymous]

Dink - what's a trigger pack?

I don't know that Aussie lingo .

Nert - I think Seafoam is similar?

[Anonymous]

compression

#1 is high because the bike has been leaning that way on the side stand so oil has drained past the valve guides and seals in that cylinder, oil puddles big time on #1 in the head, this has added to the rings ability to seal so you have high compression same as you would expect for a wet check, motor has to be ran to clear this stuff out and if its been sitting for a long time you have to run it for a while to get an accurate reading (reseat things), as far as carbon goes some of that carbon is around the ringlands witch is helping the rings seal by filling wear gaps I would not try and Decarbon the engine, plus what if a big hard chunk breaks off and gets stuck under the valve head, its your bike so do as you wish Iam just giving you the other side of things

[Dink]

Dink - what's a trigger pack?

I don't know that Aussie lingo .

Nert - I think Seafoam is similar?

A spray bottle ??
Dink

[Anonymous]

There are two cheap and easy ways to de-coke a motor with out stripping it.
1) add a small amount of diesel to the fuel tank, about 1 cup.

:arrow: Wooah Dink, I would NOT recommend to use 1 cup.... I would use half a cup at most, and do it every tank FILL for 3-5 more.
when I was at tafe I was told to use a max of 1 cup in 50 litres, that said,

as far as carbon goes some of that carbon is around the ringlands witch is helping the rings seal by filling wear gaps I would not try and Decarbon the engine, plus what if a big hard chunk breaks off and gets stuck under the valve head, its your bike so do as you wish Iam just giving you the other side of things

:arrow: I also agree with gee-s-is that it would loosen carbon around the ring lands and a big chunk could break free...

as you know, a compression check should be performed on a warm engine (was running and shut off).

:arrow: re: compression testing, I was taught at tafe to do compression tests with the throttle at wide open. loads of people dont do this so it gives in-accurate readings.

[focus frenzy]

There are two cheap and easy ways to de-coke a motor with out stripping it.
1) add a small amount of diesel to the fuel tank, about 1 cup.
2) remove the air filter and spray water into the plenum,,with a trigger pack.
Dink

I would like to ad that to decarbon with water the engine needs to be warmed up good.

[nert]

GS750
I was gonna mention about the throttle being open, but trnpkroadwarrior1019 metioned he had the carburetors off. Thats about as wide open as you can get

[RacingJake]

You got to be careful cause loose carbon can scratch up the cylinders walls causing more problems if it gets down around the top ring. Best bet is to just leave it alone until you tear down the motor.

[Anonymous]

There are two cheap and easy ways to de-coke a motor with out stripping it.
1) add a small amount of diesel to the fuel tank, about 1 cup.
2) remove the air filter and spray water into the plenum,,with a trigger pack.
Dink

I would like to ad that to decarbon with water the engine needs to be warmed up good.

definitely-- if it's not really extremely hot, you can potentially hydrolock the motor..which is..well, not fun.