81 GS750E - Missing at Idle & Low RMP

[OriginalOwner]

At idle, it feels/sounds like it's running on 3 cylinders - idle is low and will sometimes stall at a light without some help/throttle.

Suspected a stuck float, because it smooths out around 4000 RPMs but it's probably not that as the carbs were cleaned and rebuilt and the problem is still there. The issue comes and goes but exists 85 percent of the time now. This has been going on for many years.

Not sure where to start? Any suggestions are welcome!


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[rphillips]

If idle is low... turn it up to appx 1000 rpm and see what happens. Oh, great looking bike, I had 2 "80" 1100's that looked just like yours.

[oldGSfan]

I use an infrared temp gun on the exhaust, they are pretty inexpensive and I have found them to be useful and accurate enough for my needs. I had 3 of 4 cylinders cool enough to touch and the 4th was climbing over 200F, doing all the heavy lifting. It would rev out, sorta, but idle was very lumpy. I took out the pilot screws and sprayed carb cleaner down them, then briefly blasted with compressed air. The bike had been idling at 1K and it raced to 4K RPM idle, yikes. I had to back off the throttle butterfly adjusters and ended up pulling the set and going through the carbs with carb dip can resting in my ultrasonic - magic. long story short, maybe idle circuit is clogged.

[tom203]

Good time to check vacuum petcock for correct operation. I fit fails and leaks gas into #2 cylinder, you got a 3 cylinder bike at low rpms . Likely clears at higher rpms.
Shame on you for putting up with this for years !

[OriginalOwner]

Thanks rphillips > yep, the 1980 1100's were the same paint code/color.

oldGSfan > I have one of those! And there I was yesterday, doing it the old-school way, getting my hands WAY too close to the hot exhaust, trying to discern a difference.

=======================

tom203 > Great lead. And I'm pretty sure it IS number 2 that quits as I used to tap the bowl with a wooden dowel thinking the float was stuck.

Does turning the petcock to the PRIME setting, bypass the VP?

To "check vacuum petcock for correct operation," where should I start? By replacing that hose or do I need to dig into the vacuum petcock itself? Is there a diagnostic bypass of the VP itself?? How will I know if the VP is good or bad? (Sorry for the stupid questions, after 43 years of owning this bike, this is new to me)

I remember seeing here, on this site, instructions for permanently bypassing the vacuum petcock altogether. Do you recommend doing that? THANKS!!


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[Rich82GS750TZ]

You can buy a OEM style model-specific replacement petcock from https://www.georgefixs.com/new-suzuk...s1150-petcock/

Just be sure of correct model and spacing of tank mount bolt holes. I have not bought one yet. But will when I need one. Have never read a negative comment about these petcocks from this seller.

[tom203]

Get some hoses that fit on petcock…one for the fuel outlet and one for the vacuum line to #2 carb. In “ON” spot no fuel should flow thru outlet hose …unless vacuum is applied thru the vacuum connection. If you carefully suck on vacuum line, fuel should flow, but it fuel is spotted in vacuum line , the petcock is no good. Don’t waste your time rebuilding it…just get new one! Georgefix guy sells two suzukigs versions…..one has more reserve capacity..I.e.you got slighty more fuel remaining when you switch over. Get new dowty washers under petcock mounting bolts…..common plumbing item.

[OriginalOwner]

Get some hoses that fit on petcock…one for the fuel outlet and one for the vacuum line to #2 carb. In “ON” spot no fuel should flow thru outlet hose …unless vacuum is applied thru the vacuum connection. If you carefully suck on vacuum line, fuel should flow, but it fuel is spotted in vacuum line , the petcock is no good. Don’t waste your time rebuilding it…just get new one! Georgefix guy sells two suzukigs versions…..one has more reserve capacity..I.e.you got slighty more fuel remaining when you switch over. Get new dowty washers under petcock mounting bolts…..common plumbing item.

Roger that. Report to follow.

THANK YOU!!

[Big Block]

Another, DANG THATS A NICE BIKE! You'll get it sorted I'm sure.

[zed1015]

At idle, it feels/sounds like it's running on 3 cylinders - idle is low and will sometimes stall at a light without some help/throttle.

Suspected a stuck float, because it smooths out around 4000 RPMs but it's probably not that as the carbs were cleaned and rebuilt and the problem is still there. The issue comes and goes but exists 85 percent of the time now. This has been going on for many years.

Not sure where to start? Any suggestions are welcome!

Classic blocked/partially blocked pilot jet symptoms unless you have a poor connection on one of the coils, HT leads or plug caps.

[Rob S.]

Another, DANG THATS A NICE BIKE!

And another..........

[OriginalOwner]

THANKS ALL!!

I put it on the PRI position and took a quick ride before I got started on the vacuum petcock test as tom203 instructed. It RAN GREAT! Like new!! So, suffice to say, that was a pretty good indication that the issue is the VP.

Got the hoses set up for the test: Fuel flowed freely in the PRI position (as expected). In both the ON and RES position fuel did flow upon applying vacuum but the rate seemed slow. As I applied more vacuum the flow DID increase but before too long, I had fuel in the vacuum line. And as Tom explained that means, "the petcock is no good."

I'm ordering the new one from Georgefix right now. I really appreciate TOM and everyone here, you guys are the BEST!

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[rphillips]

Congrats to original owner.. Pat on the back to tom203.

[OriginalOwner]

UPDATE: Runs like a raped ape!!

New petcock arrived from Georgefixs yesterday - installed today. Functions perfect and the bike seems like it's brand new.

Install was easy, BUT the only issue I had was that the "rubber" washers for petcock bolts kept leaking. I've been using flat, 1/2-inch plumbing washers for a long time now - sometimes they work great and other times it takes many tries to get them to stop leaking. Well, this was one of those times. Plus, they never seem to last very long before they dry-rot and the bolt heads are wet. Never to the point of dripping, but wet with gas.

What do you guys use? There MUST be something better?

tom203 > you mentioned "dowty washers" but I was embarrassed to admit, I don't know what those are? If you could, can you tell me exactly what I should use here? I'm pretty sure the OEM part is no longer available. I've read some guys here talking about a fix but they're never very specific. They make it sound like the fix is something I could buy at Home Depot? Is it?

Thanks again to all!!!

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[Nessism]

The stock petcock washers are dowty type; there is a rubber seal on the ID, and steel overall. Buy the OEM washers and be done with it.