850 low compression

[Steve C]

My new to me 79 GS850, I just checked the valves, 7 out of eight had no clearance ,reshimed, bike started and idle fine after warming up after running for a few minutes I started getting some smoke right side so I decided to check the compression and I got 30 30 30 30 more or less which as far as i know is to low for a motor to run. I put some oil in and the readings went up to around 60.
So I guess the head has to come off but first I'll have a friend do a leak down.
Bike looks great though.

[SqDancerLynn1]

Something is wrong with the way you did the test !! Did you open the throttle ??? A must or you will get very low readings

[Steve]

How long has it been since the engine was run regularly?

If it has been sitting for quite a while, the rings might be stuck in their grooves. There are a few ways to try to free them up, ranging from covering the piston with Marvel Mystery Oil or ATF, using some sort of solvent to cut through the grime, or simply running it for a few (hundred) miles.

.

[Nessism]

Lynn's right. The compression test numbers can't be right. The engine wouldn't even run.

Soak the rings for a day or so, then take the bike out for a good hard ride - 100 miles or more. After the engine cools off double check the valve clearances.

A proper compression test must be done on a hot engine, with the throttle held wide open. If the bike has sat, some good hard miles will help free up the rings and reseat them on the cylinder wall.

[Steve C]

I'm think my gage most be off since my numbers are to close. And yes wot on a warm engine.

[Steve C]

Also I rode the bike home about 45 minutes on the highway doing 75 to 80. The smoke is what got my attention plus the leaky head gasket between #2 and #3.

[Tom R]

On a side note - When testing compression on bike with CV carbs (not sure if the '79 had them), won't the slides have a similar effect to a closed throttle even if you hold the plates open?

[mike_of_bbg]

CV carbs were released with the 80 models, so his bike should have VMs.

The slides on CV carbs don't ever close completely. In their most downward position they're still far more open than throttle butterflies at idle. But your point is noted: half-open or so would probably be fine. What is often neglected by the uninitiated is to open them at all!

On a side note - When testing compression on bike with CV carbs (not sure if the '79 had them), won't the slides have a similar effect to a closed throttle even if you hold the plates open?

[Steve C]

I did a new check with my friends snap-on gage, and got 125 ,125, 128 ,125, so thats a lot better.
Though the head gasket is now really leaking between 3 and 4, the blub of rtv I guess isn't holding.
I need to get my other project (75 Moto Guzzi) a little more together to free up some bench space