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I have a 77 GS750 that I just put new rings in.Im not sure if it was running right before this because it had no comppression.I did get it running after the rering and it seems to run ok,maybe a little rich but it doesnt want to start cold.It cranks and pops out of the exhaust.After cranking it for a while it will finally start to pick up and then it will start and run pretty good.And once it starts you can shut it off and it will fire right back up.The popping makes me think its a timing issue??If anyone has any input it would be appreciated.Thanks,Andrew
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You put new rings in...As in full top end rebuild?? Did you adjust the valves?? -
I honed the cylinders,new rings and gaskets.I left the valves just like they were.Comment
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With low compression, checking the valves should have been done first. Also, carboned valves will lower compression, so it would have been nice to clean those up and replace valve seals when you were in the top end. Reringing is a more major step (and maybe unnecesary), but you did that first.
Check the compression. If it is good (130+), then the valves should be ok, but checking them is recommended. Hard starting when cold sounds like carb problems, like perhaps a choke feed hole in the bowls is plugged.Comment
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actually, hard starting when cold is a classic symptom of misadjusted valves...so are low compression numbers...I would have checked the valve clearance before i started into re-ringing the bike..If it wasnt blowing oil when it was running, it probably wasnt the rings/valve seals. BUT, since you already went about re-ringing it, I would DEFINATELY adjust the valves at this point. They need to be checked AT LEAST every 5k miles...this is SOP for these bikes, and considered routine maintainence...IF and WHEN your valve clearances are in spec, you still have cold hard starting issues, I would look to the carbs/ignition. ALso, listen for a metalic scraping racket, watch for blue smoke, its conceivable you pinched a ring and its dragging, but i dont know how likely..Comment
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I did clean up the valves when it was apart but did not do seals.The rings were definitly shot.They were worn down to nothing and sharp as a razor.I did not do a compression check yet since I reringed it,I was trying to run it a little to let the rings seat first.Would it be a choke problem with the backfiring out of the exhaust??Thanks for the info so far.Comment
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Back firing out of the exhaust is a lean condition...this can be caused by numerous things...including faulty valve adjustment (face it, there is no way around adjusting them..) intake leaks from around the intake boots and airbox seals (another common problem on mis-treated bikes) and carbs that are improperly adjusted/and or jetted for your intake and exhaust set up (for example, if you put that Kerker on the bike that is your namesake, and didnt rejet, you're lean...period. Pods will compound this problem 5 fold) Failure to properly jet the bike will result in hot combustion chambers, leading to burnt valves, and holed piston heads..and cooked rings... BUT, unfortuneately, in order to properly jet/adjust the carburetors, you MUST BE SURE THE VALVE CLEARANCES ARE IN SPEC
Hope this helps.
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Im am definitly going to check the valves first.The pipe was on the bike when I got it and no airbox so I added a set of pods.Im not sure how the bike is jetted but it does huff black when you crack the throttle if that means anything??Im an auto mechanic so the bike thing is a little different for me,but same basic concepts.Thanks for the help so far,I just have to start checking things and see what I can come up with.Comment
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Ok, well i have played with that particular bike, and the succesive years of 8v 750 many times, so if ya need anymore help feel free to ask. I can point you in the correct direction for jetting it, once you figure out where its jetted now... And, I can also assist in the valve adjustment process. Its a pretty simple, though probably not so common anymore shim over bucket design. Since you're a mechanic, it should make sense to you pretty quick, and not take you long at all. I can run thru the valves on an 8v motor in about an hour or so start to finish, depending on how many i have to change. You'll need to order a valve cover gasket, and take inventory of the current shims in the bike before you move on to deciding which are tight and which arent. You'll also need a set of feeler gauges that go as low as .03mm (roughly) or .0012" i think (i dont do SAE on a metric bike, requires too much thinking for me...lol) The clearance spec for the bike is between .03mm to .08mm tho its not uncommon for guys to run them as wide as .09 or .10..as they get tighter with wear.Comment
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OK thanks alot,you may be hearing from me again.My Clymer manual is in the mail so hopefully that will help me some.LOLComment
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It will be your bestest friend
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If it is popping back, check that the air screws are set right. Hard starting when cold, check choke tubes in carb bowls are clean.Comment
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