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'77 Suzuki GS750 Trouble

  • Thread starter Thread starter Sir_Isaac_New
  • Start date Start date
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Sir_Isaac_New

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Bought my first bike this summer, a '77 GS750 in great shape.
Very new to motorcycles, and looking for a few pointers as I've been having some trouble with it.


  • Two months ago, bought it, and it ran great. Maybe a bit of a high idle (2k rpms).
  • Replaced the battery right away, as a recharge didn't bring it back to full.
  • Removed an old fairing, and replaced it with a used headlight assembly.
  • Couple of days later, after a long ride, it wouldn't start. Would turn over, but wouldn't idle. Had ~1/2 tank of gas in it, and eventually got it home by leaving it on prime tank.
  • Got it home, cleaned the sparkplugs, filled gas, reset the fuel/air mixture to service manual suggestion.
  • Started it, but was bogging very bad out of first. By the time I got it home, it would stall when I tried to accelerate out of first.
  • Now it will not start, with a ~full tank.

Any help is appreciated, thanks.
 
If you got it home by using prime I'd be checking your petcock to make sure it's not leaking. I'd also take your oil cap off and have a good smell to make sure no gas has contaminated your oil. Next, since you replaced your battery, I'd check the charging.
 
Please look at the Newbie Mistakes thread linked in my signature. It will help you understand what to do when purchasing a 35 year old motorcycle.
 
azr said:
If you got it home by using prime I'd be checking your petcock to make sure it's not leaking.
Click to expand...

Thanks, I'll check it out. Would this lead to a high volume of gas leaking onto the ground? There is not gas leaking from the bike while the petcock is in any position.
 
Not necessarily, it'll just fill up your crank. 'Nessism' gave some very good advice/information, check his thread out.
 
If you notice the smell of gas in the oil or the level rising through the sight glass, that's a sign fuel is leaking into the motor.
 
hannibal said:
If you notice the smell of gas in the oil or the level rising through the sight glass, that's a sign fuel is leaking into the motor.
Click to expand...

Thanks. If I find this is the case, I assume I should do an oil change immediately, and try to resolve the leak.
 
Yes sir! It's a bit of a catch 22. As soon as I noticed the gas, I changed the oil. Assumed it had leaked a lot of gas into the oil. But then I had to run the bike to make sure I fixed the problem. Since I didn't fix it the first time, more gas got in the oil. I suggest using cheap oil on the initial change while you're locating the problem and do a final oil change with the good oil once you know you've fixed it.
 
chuck hahn said:
# 1 here is your new petcock. rebuild kits are a general waste of money and down time so just bite the bullet and get the OEM and be done with petcock issues for the next 15 or 20 years. I agree its probably a fuel delivery problem due to a failing petcock. If it runs on PRIme and not the ON position thats a dead give away
Click to expand...

Checked the oil cap and there is no sign of gas in the oil. No scent, nothing obvious in the sight glass.
Pulled and cleaned my sparkplugs, and spark tested all of them. They are all functional, and none are fouled.
I opened the drain on my carbs and drained them. After that, to test my petcock I moved it from ON to PRIME and gas flowed into the carbs when switched, and off when switched back. Does this mean my Petcock is functional?

Still will not start on any setting.

Thanks
 
If you have it in the 'on' position and no gas is dripping out at all that part of petcock is working fine, to test the vacuum side you have it in the 'on' position and apply a bit of suction by sucking on the vacuum line and fuel should only come out when you apply vacuum. Have you done any voltage testing yet?
 
azr said:
If you have it in the 'on' position and no gas is dripping out at all that part of petcock is working fine, to test the vacuum side you have it in the 'on' position and apply a bit of suction by sucking on the vacuum line and fuel should only come out when you apply vacuum. Have you done any voltage testing yet?
Click to expand...

I have only tested the voltage on the battery which seems to be working well (it's brand new).
I am thinking it is carb/fuel related due to the bogging when taking off the last time it was 'functional'.
 
Leave it on Prime for ten minutes or so before trying to start it after draining the carbs, mine took a while to fill.

Do you have spark? Remove one plug lead and put a spare "good" plug into the end, hold the threaded area against the cylinder head fins (bare metal) and spin it over on the starter for a few seconds. That will eliminate that.
 
If it takes more than 1 minute to fill the bowls youve got problems. Either a kinked fuel line or the needles are barely opening. I count to about 45 then full choke and hit the button myhelf if they are known to be dry bowls from storage or maintenance.
 
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