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105 psi in a 1979 GS 1000 cylinder

J

jknappsax

Forum Mentor
Past Site Supporter
I bought a GS 1000 which had been fitted with a 4-into-1 exhaust and K&N pods, and the PO had never gotten the carbs set right. He said that it was running rich, and would foul plug #3 after a while. I pulled the plugs, and #3 looked fuel-rich (fluffy black, not oil-black). I decided as a baseline to check compression in all cylinders. Now, I DIDN'T run the bike for 10 minutes to make sure it was thoroughly warmed up, as I'm working in the garage under my house, but the numbers I got were marginal; 110 on cylinder #1, 105 on cylinder #2, 120 on cylinder #3, and 110 in cylinder #4. the funny thing is that the cylinder that is fouling a plug is the one with the best compression. I haven't tried a wet test yet to determine whether it is ring or valve related, but was wondering what you all thought about whether not being warmed up might give readings that are low, though I doubt it. I checked the compression on my 1978 GS 750 cold, by using the kickstarter, and got 135 on cylinder #1, 130 on cylinder#2, 140 on cylinder#3, and 135 on cylinder #4. If I can get the carbs set up right, I'm hoping to get through the coming summer as it is, then do the cylinders next winter. I know the service limit on compression is 100 psi. Any opinions on what I explained, and plan on would be greatly appreciated. Thanks.
 
Did you check the valves? Could be the valves are cracked open causing the low readings.

Not sure how much the warm engine effects compression but I'm sure it does or Suzuki would not spec a warm engine test.
 
I always have to remind myself

I always have to remind myself

to make sure that the throttle is wide open. Even if I know it I forget.

Posplayr
 
My '78 750 has only about 60 psi on each cylinder, the exhaust valves are not sealing at all. Been riding it, it's very gutless but other than that it runs fine. I am planing to go with 894cc 10.5-1 JE pistons from the group buy we are trying to organize, with a redone head it will no longer be gutless.
Your high cylinder may be because of more carbon in that jug raising the compression ratio slightly. Plan on fixing it, but shouldn't hurt much that isn't ruined already to ride it in the meantime.

You do have the valves adjusted correctly, don't you?
They leak from being set too tight too.
Riding it in this condition can give you a head in the condition of mine.
 
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All good advice

All good advice

I appreciate all the responses. I haven't checked the valves; that's next on my list (probably should have been first!). And I did (finally) remember to check compression at full throttle. Something I forgot to ask in my previous post; I've got to get the carbs set right. Will doing so at reduced compression mean redoing the carb tuning again later?
 
Youll never get the carbs right if the valves are out. Check the valves for needed adjustment first. May kill two birds with one stone that way, as you may find your compression issue is related, and the carbs will be a boat load easier once they are done. Otherwise, you'll chase your tail till you're nuts.
 
TheCafeKid said:
Youll never get the carbs right if the valves are out. Check the valves for needed adjustment first. May kill two birds with one stone that way, as you may find your compression issue is related, and the carbs will be a boat load easier once they are done. Otherwise, you'll chase your tail till you're nuts.
Click to expand...

Agree. :)

Too many people neglect the valves on their engines. The 8 valve engines run at .03 - .08 mm (0.0012 - 0.0031 in) valve clearance, which is incredibly tight. As the engine accumulates mileage the valves recede into the head decreasing clearance. Neglect this critical adjustment and the valves will not seat properly and burn. My guess is that this kills many GS engines.

Adjusting the valves is easy, you just need a $17 tool and different shims to set the clearance. By far the worst aspect of this job is dealing with the valve cover gasket; big pain to get it properly cleaned off the cover and head. If you can manage this, the adjustment is child's play.

Good luck.
 
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valve adjustment it is!

valve adjustment it is!

That was next on my list, so I better get to it! Thanks guys!
 
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