1977 GS750 Build. Not Running Well

[Guest]

Hi All,

I am getting ready to do some somewhat extensive work to my GS750 over the winter. I bought it in April and it left it through the summer but I want to get it running right before I dig in and start doing some cosmetic/maintenance work. When I got it it seemed as if it was running on two cylinders and was backfiring horribly/wouldn't idle, and would bog down completely when trying to rev the engine. Turned out to be bad spark plug boots and bad rubber carb boots. The bike has since had the valves adjusted, carbs cleaned and sync'd, and new rubber boots to connect the carbs to the manifold/engine. It now idles smoothly, and on first start up runs close to perfect. However, after you shut it off and start it back up within an hour or so, it runs like crap. It runs okay when just running not under load, but under load more than 1/3 throttle bogs the engine and it sounds like its running on two cylinders again, then it backfires on acceleration and has no power. It does have Pods, and I am currently trying to source an intake box to go back to stock. But in the meantime does anyone have any advice? Am I on the right track with the airbox/is there a way to test if it that is my problem?

Let me know your thoughts.

Thank You

[tom203]

What size main jets are you running? So you replaced the carb boots at cylinder head, those o-rings at head interface too?
With decent pods, you should be able to get this thing right. Have you replaced the points/condenser ?

[Andrewb.95]

What size main jets are you running? So you replaced the carb boots at cylinder head, those o-rings at head interface too?
With decent pods, you should be able to get this thing right. Have you replaced the points/condenser ?

I had a shop do the carburetor work and check on some other things while they had it, Phoenix Cycles in Westbrook. They never said anything about the carb jets being larger (granted they may not know) so I am under the impression that the jets are Stock, But I am not entirely sure since I have not cracked them open myself. The rubber boots that connect the carb to the cylinder head is the only thing that has been replaced in that area, not the O-Rings. I have not replaced the points or condenser. I was thinking that maybe it is not strong enough, but I had the same shop test them and was told that the points and condensers are in good shape. The bike does have new ignition coils.

[tom203]

Just replacing the boots without throwing in new o-rings seems unlikely , but does it show up in any supplied parts in your billing? I'll assume the bike arrived to you with the pods....previous owners can be very creative with main jets..most want bigger often just drilling them out. Best to know what you got. I doubt that you needed new ignition coils, as they are very reliable while old points/condensers can harass you.

[Big T]

New points and condensors would be a good start
Even better, a Dyna S

[Andrewb.95]

my friend took it out a couple hours ago and it ran very well. I got home an hour ago and Had him take it out and it was running terribly again. Yes, Pods were from the previous owner, and I don’t have the bill from the shop because my friend picked it up and did not get a receipt, I can probably call and get that info though. I just sprayed carb cleaner around the intake/carb boots and didn’t get the rpm to change, so it doesn’t seem like a MAJOR Leak is the issue. And another thing, the bike does not seem to charge the battery, we ride it around for 10-20 min and when we turn the bike off and try to start it immediately after it barely cranks for 1/4 turn or less. Not sure if a problem with the wiring could be causing this poor running on 2nd start up issue, but the battery is new. But I’m just baffled that the problem only starts with the 2nd start up. I don’t have much experience with carbs, and was trying to avoid taking them apart since the shop replaced the o rings and I’m assuming looked for broken jets and other issues but if I have to tear them down I will. and I see a lot of areas on the carbs that look like they should have vacuum lines connected but nothing there and when I sprayed the carb cleaner in there the engine would stall, so I’m not sure if these should be capped off?

[allojohn]

I'm afraid you have no choice but get right involved with this bike if you want it running right. It's 42 years old...... so it's had 42 years of owners d!cking with it. There's a lot of pain, head-aches and swearing ahead of you - but you can do it with the help from the fine people on this forum. Start with the carbs then move on to the next issue. First up you need to find out what jets are currently in there, so take them off and open them up.
'77 was a funny year for the 750, Suzuki messed around with the jetting so you could have a main of 97.5, 100, 102.5 or 105 - none of which will run right with pods. Your pilot could be 15, 22.5 or even 27.5, who knows?
Do yourself a favor and download the workshop manual here, have a good read through and familiarize yourself with the machine.... or sell it on and forget about it.

[allojohn]

BTW, #3 carb has the vacuum line to the petcock, #1 carb doesn't have one and #2 & 4 vent into open air.

[Andrewb.95]

That’s what I was thinking but not hoping for. I have some time tomorrow so I guess that will be my project for the day. I found a factory air box which even with buying new rubber for will be less expensive than getting new k&n pods so I think that will be the route I go. I don’t want more power so pods are kind of useless to me.

[allojohn]

Pods add little/no power they’re more of an aesthetic change. I hope you find stock jets in there!