To remove the cyl head or not? That is the question

[sacruickshank]

I'm currently rescuing an '80 850G with 41K miles. As purchased, cyls 1,3, and 4 are all firing nicely and have respectable compression, i.e. >120psi. Cyl2 is not firing and has 30-60psi, my gauge is a little flaky.

When checking the valve clearances, all 8 fail. None let an 0.038 feeler through and only one or two will let an .02 feeler mm pass. So all eight likely need at least xx5 change.

As usual, the valve cover gasket did not come off cleanly. Hard to say if it was the original or not.

So ... should I try to replace all eight shims and clean the gasket surface with the cyl head in place or remove it and see what's underneath? The latter would likely require a new upper head gasket, and maybe surface machining(?). Hard to tell if it was leaking, but that might be the cause (or at least contributing factor along with valve clearance) of the low compression in cyl2. Removing the head would also give me a chance to clean the cyl head fins up nicely.

I suppose a middle ground would be to remove the cam shafts but leave the cyl head in place. That would allow more room to clean off the gasket surface and leave the upper head gasket intact.

I have plenty of time since I'm still finishing another project and have modern bikes ready to ride.

Pic for reference

PXL_20250131_124414596.jpg

[sacruickshank]

Pic from other side

PXL_20250131_124430219.jpg

[Burque73]

Tight valves will definitely cause low compression. The valves can't close if they always have contact with the cam lobe.

You can try different chemicals to soften the gasket remnants. It may help to apply some heat as well. The main thing is not to gouge the gasket surface. I've tried gasket remover from the auto parts store, paint stripper, acetone and berrymans carb cleaner. Each seemed to help some, maybe a little. It just takes patience and finesse. Sharp razor blades only if you're super careful otherwise plastic putty knife. Some folks have mentioned a dremel attachment but I've never used one for gasket removal.

As far as removing the head for the gasket cleanup alone, personally I think that's extreme. Stuffing paper shop towels help keep junk out of there and a small vacuum to help clean out afterwards works. If you suspect the valves have already burnt from neglected clearance checks, then removing the head AND cylinders may be necessary.

[Rich82GS750TZ]

As Roger mentioned, If you remove the head, you must also remove the barrels/cylinder to replace the base gasket, because unbolting the the head also loosens the cylinders from the base and that gasket is likely to leak afterward. If you're going to go that far, consider new rings, honing the cylinders, and replacing the valve stem seals in the head. Replacing valve stem seals can evidently be done with the head in situ, according to some posts here. But easier to do if the head is off the bike anyway.

[sacruickshank]

Thanks all. I'll leave the cyl head on, but maybe take the camshafts off to make shim replacement and gasket cleaning easier.

[rphillips]

My thoughts, How bout adjust valves on cyl' # 2 then check compression, if compression is up and good, head shouldn't need pulling. If compression is still 30-60 psi. head will got to come off...

[sacruickshank]

My thoughts, How bout adjust valves on cyl' # 2 then check compression, if compression is up and good, head shouldn't need pulling. If compression is still 30-60 psi. head will got to come off...

Excellent first step. Will let you know the result

[Nessism]

Don't pull the head or cams. Adjust the valves first, and target .10mm clearance. There is likely carbon on the valves, so when you run the bike for a short while, the carbon will wear off and the compression will improve. Also, the clearances will close down some.

[Grimly]

Plus there's the old bogeyman of stuck rings on one cylinder.
I'd drench that cylinder with some ATF and acetone mix for a couple of weeks at least.

[Big T]

A simple diagnosis would be to hit up your local chain auto parts store and "rent" the borescope
Checking the condition of the valve faces and possibly the seats will provide a lot of information

I wouldn't scrimp on valve adjustment. Do all of them now, even if it means redoing them in a few hundred miles

I'd dribble some Marvel Mystery Oil down the bored first, so it gets spread as you rotate the motor during valve adjustment

Lastly, you should consider that #2 might be extra low because the petcock was leaking extra gas into that cylinder

Scraping the gasket is no fun, but it has to be done. When reinstalling, a bit of grease of both sides of the gasket will prevent future issues.

From my observations, GS models get parked due to:
Lack of valve adjustment
Petcock leaking fuel into cylinder (s)
Bad O rings in carbs leaking fuel