Compression Test Results / Opinions

[Kcwiro]

Bike Info:

1980 GS750E ~40k miles with a busted odo... so not positive on mileage



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Ok, finally got out to the garage and compression tested the bike. Numbers I have got are the following:

With throttle open just FYI:-D... and warmed engine

Dry
CYL #1 - 120
CYL #2 - 110
CYL #3 - 120
CYL #4 - 120

Wet
CYL #1 - 150
CYL #2 - 150
CYL #3 - 165
CYL #4 - 155

Man. does that oil like to go poof when I crank the bike again after...

Anyway, Compression looks good. If not for the small leak at the base gasket, valve is leaking moderate+, head gasket is moderate as well. I am doing a take down of the motor to fix the leaks am thinking do I really want to fix the base gasket and move the pistons and all that or just go down to the head and call it good...torn between (I'm this far down so what not this too?) ... and ... do I really want to go through that much trouble and risk messing the pistons up or some bad thing like that.

Otherwise, carbs need some TLC cleaning/rebuild...and exhaust clamps need to be replaced since they are not closing the two pipes together enough to keep the exhaust from coming out the non-sealed gap on the pipes.


Fire away guys comments / suggestions

 

[49er]

Unless you are planning on doing a complete top end job, don't strip it down.
Once you slacken the nuts on the head, you risk causing an even greater oil leak on the base gasket. If you decide to pull it down, you should replace all the gaskets, give the barrels a light hone and re-ring the pistons, as a minimum.
I would try cracking the head nuts 1/4 of a turn each, one at a time and re-torqueing them to 29ft lbs. Work from the center outwards and diagonally. You may be able to stop your small leaks that way without dis-asembling the top end. Sometimes you get lucky.
Give the carbs a clean and tune. Replace those exhaust flanges and ride it.
Probably more fun to do the whole rebuild during the winter break though.
To hell with the expence, give the cat another goldfish.

 

[mark]

Good advice above. I'm in the middle of this right now, and its hard not to spend the money once you've got it all apart. Valves, cleaning, decarbon job etc. My compression was about the same as yours....but once it was torn apart found a huge nick in one of the cylinders, to fix that will require overbore, new pistons rings...$$ even if your putting it all back together yourself.

Its like going to the theme park. Once you're in there its hard to keep the money in your wallet. Fun though.

 

[Road_Clam]

Bike Info:

1980 GS750E ~40k miles with a busted odo... so not positive on mileage



------------------------------

Ok, finally got out to the garage and compression tested the bike. Numbers I have got are the following:

With throttle open just FYI:-D... and warmed engine

Dry
CYL #1 - 120
CYL #2 - 110
CYL #3 - 120
CYL #4 - 120

Wet
CYL #1 - 150
CYL #2 - 150
CYL #3 - 165
CYL #4 - 155

Man. does that oil like to go poof when I crank the bike again after...

Anyway, Compression looks good. If not for the small leak at the base gasket, valve is leaking moderate+, head gasket is moderate as well. I am doing a take down of the motor to fix the leaks am thinking do I really want to fix the base gasket and move the pistons and all that or just go down to the head and call it good...torn between (I'm this far down so what not this too?) ... and ... do I really want to go through that much trouble and risk messing the pistons up or some bad thing like that.

Otherwise, carbs need some TLC cleaning/rebuild...and exhaust clamps need to be replaced since they are not closing the two pipes together enough to keep the exhaust from coming out the non-sealed gap on the pipes.


Fire away guys comments / suggestions

You are ready for a top end IMO. 120 psi is kinda low. My 1100 with 17K would read 145ish dry. Adding oil than finding much higher readings on the compression further signals you need rings (or a valve job). Listen to the compression strokes, you should be reading max psi after 4-5 compression strokes. A bike that takes 7-10 strokes to get max readings also signifies a tired top end.

 

[SkeletonLake]

Imho, the results indicate an engine that is wearing at a uniform rate. Check the plugs to see evidence of serious oil burning and when you run the bike keep an eye on oil consumption. As my 850 ages, I switched to 20W50 and found if burns less oil. The heavier weight means I have to wait for the engine to warm up before the clutch operates smoothly though. With a manual, good tools and patience (and $) you can rebuild it to be leak free; as they say depends how much you want to spend.

 

[Kcwiro]

Read through all the comments posted thanks for all the input....

I should also note I had to let the engine turn over 10 times to get the final numbers I had. After 5 strokes of the motor I was near 90 PSI on the dry test.

Oil loss at this moment is if I ride the bike 10-15 miles my boots and pants get a pretty good coating of oil on them...if I stop I get that nice smoke from some of the burnoff. Don't think the re-torquing will save me on this one...thanks for the tip though.

I definately agree with the if it isn't broke don't fix it approach...but I had a general idea of what I might have to expect if I decide to go forward with this rebuild...winter up here in the northwest has not fully se tin yet but with all the rain and wind its kept me off the road since I'm reliatively new to riding.

In regard to honing the cylinders... is that something I can do with a attachment to a drill or is there something special I have to get?

Once again thanks for the advice I appreciate it

Have found a decent price gasket set top\bottom with help of the partsnmore thread to try out. I'm sure i can count on advice on anything I run into while I make this. This will also give me the chance to redo some of my grounds since i think I ahve some bad ones givent my R/R wiring is showing some melting / burning and bad ground is the likely culprit there.

Carbs... ugh... I will handle those after I get the rest of this stuff completed... best to do the sync for that on a well running engine anyway.

 

[bwringer]

I've been very happy with honing using a Flex-Hone a couple of times. You can get a really nice finish with a cordless drill.

A flex hone sort of looks like a big bottle brush with balls of abrasive on the bristles. You oil everything up, get it spinning on your cordless drill's low speed, then run it back and forth through each cylinder at a steady rate of travel for 15 - 30 seconds each. You have to keep it moving to get that nice cross-hatch pattern -- don't let it stop in the bore. You have to put the block up on something so the hone can protrude out the bottom at the end of each stroke.

The flex-hone is a good fast way to do finish honing easily, but you'll need to take the block to a machine shop to have measure it and decide whether you need to re-bore.

You'll also want to research the availabilty of standard size and oversize rings and pistons when deciding what to do.

 

[Kcwiro]

I've been very happy with honing using a Flex-Hone a couple of times. You can get a really nice finish with a cordless drill.

A flex hone sort of looks like a big bottle brush with balls of abrasive on the bristles. You oil everything up, get it spinning on your cordless drill's low speed, then run it back and forth through each cylinder at a steady rate of travel for 15 - 30 seconds each. You have to keep it moving to get that nice cross-hatch pattern -- don't let it stop in the bore. You have to put the block up on something so the hone can protrude out the bottom at the end of each stroke.

The flex-hone is a good fast way to do finish honing easily, but you'll need to take the block to a machine shop to have measure it and decide whether you need to re-bore.

You'll also want to research the availabilty of standard size and oversize rings and pistons when deciding what to do.

Already ahead of you on that one. I have managed to locate a local I can obtain pistons and rings from standard and oversize are obtainable :-D

What store can I get one of these flex hones from? Cost? Just so I know what the good deal is. Also when using the tool do I go up and down through the cylinder or do I just push through one way?

 

[jetta90]

Don't want to hijack the post but quick question that might help Kcwiro as well ................................. when doing a compression test on this bike with CV carbs.......................with the throttle wide open and the engine spinning do the carb slides open fully as well????
If they don't open fully when cranking then the cylinders would not be getting full air during the intake stroke. Wouldn't the compression readings be lower than if the slides were fully opened?
tx

 

[49er]

Jetta90

That's not hijacking, you make a very valid point.
When I did my compression check before I pulled the top off my 850, the difference between throttle wide open and closed was only 10psi. That was with a set of VM carbs.
It would be interesting to know at what vacumm level the pistons will activate on the CV carbs. I suspect that it would be well above cranking speed.
Those flex-hones are good. The more consistant you are with the speed of your strokes, the better the hatch patern will be.

 

[rustybronco]

One other thing over looked, check valve clearance.

 

[Kcwiro]

Was reading through the manual on the bike and measuring engine parts to determine ware was done via a micometer. I figure this is the same deal as a dial caliber but I wanted to be sure so I know what to snag since according to my manual I lack a few basic tools.

A tool list of what others use on their rebuilds would greatly help me out in determining if I am missing anything else to make my rebuild easier.