1979 GS850 Engine Swap

[Guest]

Hey Gang, new to the group and have a question for those that know:


I have a survivor but compression is low. I was looking at an '83 GS850 motor with transmission on ebay. Do you know what engine years are compatible with the 79 frame?

~Mark

[salty_monk]

I doubt there is any significant frame changes that would affect fitting it. You should also be able to fit an engine from any of the G or GL bikes incl the 1000 or 1100. As long as it’s 8v and shaft drive.

You’ll lose your kickstart obviously and you’ll have to make some wiring changes and/or change over ignition components.... that’s the biggest change I can think of. Mechanical vs electronic advance.

[Nessism]

This forum is about site questions, not technical questions. Regardless, before checking compression did you perform a valve adjustment? Valve clearances tighten with mileage and many people never do maintenance thus the question. If the clearance is low or non existent that means the valves are going to be coked up and being held off their seat and the compression will be low or non existent. Also realize that if the bike has sat for some time the rings may be sticking a little which can lead to low compression.

Before condemning the engine I'd verify the valves are okay and run the bike for some miles (assuming you can get it running that is) to be sure the rings are not sticking. Then test compression again with a hot engine and holding the throttle wide open. If after all that the compression is still under 100 psi you have a serious engine problem.

To answer your question, the 1979 850 has different carbs than the later bikes, so the boots on the head of the newer engine won't fit your older VM type carbs. The later engine uses an electronic ignition too so you either need the ignitor for that system, and the requisite harness changes and coils, or you need to retrofit the later engine with the earlier engines points system.

[Grimly]

You'd need the drive shaft from the later bike, too. The flange is different, so you can't re-use the '79 shaft.
Apart from all the minor details, it will slot straight in, the small stuff isn't a show-stopper.

[gsfan]

Thanks. I figured out I posted in the wrong forum but very much appreciate the feedback. It is running, but I haven't checked the valves yet. I'll do this first.