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'79 gs 850, what heads fit?

  • Thread starter Thread starter corax
  • Start date Start date
C

corax

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hi, dumb ass me rebuilds the carbs & forgets to put vacuum caps back on the carbs. so i'm pretty sure the reason the valves burnt was cause it ran a little bit lean. now i'm looking for parts to get it running & found a head from an '82 850. anyone know what the parts interchange looks like, or where i could find a parts interchange?
 
Did you take it apart and find burnt valves? I don't think it would have run at all with all the vacuum ports open. These valves are extremely hard and usually just need cleaning (and valve seals).
 
Oh, yeah, low compression is most often due to tight valves that need adjusted, in case that's the symptom you have.
 
well, as far as it running, when i went to start it it started quicker than ever, & a motorcycle mechanic i know said it was probably due to it being lean from the uncapped lines. it ran good for about a mile or two, i stopped to get gas and it barely started. so i drove home and that was the last it ran. compression is around 20-40 psi in all 4. shooting a little oil in the spark plug holes does nothing to improve things. my best guess before tear down was valves, but i'll find out soon. i'm gonna pull the engine this weekend and figure out if it's worth saving
 
Well, that certainly sounds like a cause and effect situation, but I suggest that you investigate further. The 79 GS850 should have the standard 26mm carbs, and they don't have vacuum ports on them - there is 1 vacuum port on the top back of #3 carb, so only that carb would run lean. The vacuum ports for synchronization are on the bottoms of the carb-to-motor rubber boots - were these left off? Were you holding the throttle open when you were checking the compression? There is no way the bike will run with the compression you show, but I also think it's very unlikely you burnt these valves in one mile. It sounds possible that you ran out of gas in the bowls without the vacuum hooked up. Turn the petcock to PRI, count to 10 and try to start the bike. Let us know what happens.
 
let me start by saying THANK YOU. the sychro ports were still capped, the fuel tank was set onto reserve (because, for some reason the petcock wasn't flowing with vacuum hooked up - w/ it on reserve, i could see fuel
flowing through the clear in line filter). and actually, this bike does have
vacuum nipples on #2, #3, & #4 carbs, on the top/back side (i think they
are vacuum) and the carbs are the style w/o diaphragms (i did have them apart before to clean out sediment & it ran afterward w/ the mystery vacuum nipples capped, but i had these left over rubber vacuum lines shaped to perfectly fit the nipples and go over top the airbox, so i put the rubber lines on & that's when something went wrong--cause & effect).
 
The nipple that is ON the carb #2 is a vent carb #3 is the vacuum supply to the petcock carb #4 is a vent
 
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