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81 GS750E - Missing at Idle & Low RMP

OriginalOwner

OriginalOwner

Forum Apprentice
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!


GS750side2.jpg​
 
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.
 
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.
 
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 !
 
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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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.
 
tom203 said:
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.
Click to expand...

Roger that. Report to follow.

THANK YOU!!

:tennis:
 
OriginalOwner said:
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!
Click to expand...



Classic blocked/partially blocked pilot jet symptoms unless you have a poor connection on one of the coils, HT leads or plug caps.
 
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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UPDATE: Runs like a raped ape!! :p:p:p

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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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.
 
If you can't get the OEM or Dowty washers, use a smear of Hylomar Blue on the bolt face and both faces of a plain washer. It won't leak fuel, ever.
 
Grimly said:
If you can't get the OEM or Dowty washers, use a smear of Hylomar Blue on the bolt face and both faces of a plain washer. It won't leak fuel, ever.
Click to expand...

I read about Hylomar Blue, sounds like some righteous stuff!! :encouragement: From your experience, what's the better route? Buying the original part, upgrading to a "dowty washer" that works better and will last longer than the OEM, or going the Hylomar route. Also, is Hylomar hard to remove/deal with when returning to the area? Thanks!!



Rich82GS750TZ said:
# 8……………
https://www.partshark.com/oemparts/a/suz/50d40086f8700230d8b4e2de/fuel-cock also on ebay. https...
Click to expand...

Thanks for that, Rich!. After looking harder I was able to find my old parts diagrams and I got the part number was able to search it. Do you suggest using the OEM washers or something else? Which source do you recommend for making the purchase as far as price and dependability?

Thank you!!!

:cocksure:

​
 
I've bought many orders of parts from partshark. Never had a problem. Shipping will cost more that the 2 washers. Check the ebay add, see if it includes shipping. I don't recall if I've ever bought anything from that seller. Ebay is definitely a buyer-be-ware type deal. But that seller has sold a bunch of items and has100% positive feedback. But we're not talking about big bucks here.

If you want to go cheap and quick, no guarantee that it will work, But I have used these for similar washers that go under the long bolts on the right and left side top of the Valve cover. I used the washers from metal roofing screws. They have a rubber washer attached to them. Any hardware store will have them. Just have to unthread them from the screws. They have a concave shape and will flatten out when torqued down. Will they work on the petcock bolts? I don't know. They may be too thick. Problem is, if they don't seal, it's a pain to empty the tank, remove and redo.
 
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