1978 GS750E Rich Carb

[UKAndy]

Thanks for the advice guys, I have been trying get a good slow speed running because I fitted a new clutch and I found it almost impossible to ride when it was running rich as it stalled almost every time I pulled in the clutch. I have done 12 miles of mainly town riding not noticed any slow speed surging but that could be masked by the new clutch. I will take note as the clutch beds in.

[UKAndy]

After a number of tuning cycles I was still having an over rich condition on No.3 so I blanked the vacuum line from carb No.3 and ran on prime. It was not totally conclusive but the No.3 plug was not sooty just richer than ideal. I decided to go back to basics so I did a full strip down again to confirm jet sizes, needle position etc. Whilst searching the internet I came across a Cycle World article published in 1979 following the introduction of the 750N into the US. The article details the carburation changes Suzuki introduced to meet tighter emission regs in the US, the changes which are common to the UK models involved changing the jet and needle but must interesting a change to the float height from 26 to 24mm. I believe somebody earlier had asked why I was using 26mm, that is the only specification I could find in the Suzuki manual and there is no mention of it in the 750E upgrade. The carb changes according to the article in Cycle World were made in 1978 for the 750E. So when I rebuilt my carbs I changed the float height to 24mm and set all 4 carb pilot fuel screws to 1/2 turn out, pilot air screws are all at 1 turn out but may a tweek later. I have also replaced the air cleaner element. As I have the carbs apart a few times recently I decided to confirm the balance using my vacuum gauges. I quick test ride and plug chop showed an even burn on all cyclinders. I am still running on prime with the no.3 vacuum hose blocked as suspect the fuel tap is leaking fuel through the vacuum outlet. I am going to purchase a new fuel tap. I have also managed to located a genuine fuel height guage from Holland so I will purchase one to confirm fuel heights as advised. Thanks for all your help and advice I think I can put this one to bed!

[OldVet66]

Thanks for the update. Looks like you got it!

[KEITH KRAUSE]

As others said, a failing petcock makes sense. That cylinder could run well while warming up, but then run too rich at operating temperature.
I run a Pingel on my '79 GS1000. I've owned a Pingel for 20 years now. Needed it's first rebuild 2 years ago (hard to rotate lever). You mail it in with $15 and that covers the rebuild and return shipping. Yes, I have to manually turn it off (to be safe while sitting several days) after riding but it's instinct after awhile. I'm tired of failing petcocks.
As for the side air screws: you mention simply setting them at a uniform adjustment. That isn't right. You should set them using the "highest rpm method".
Set the screws to 1 1/2 turns out as a base setting. Warm the bike up completely. Set the idle to 1,000 rpm's with the idle adjuster knob. Start at any carb. Adjust the side air screw slowly until the highest rpm is achieved. Now reset the idle to 1,000 rpm's with the idle adjuster knob. Go to the next carb and repeat until all 4 are done. Highest rpm's are usually achieved between 1 1/2 turns and 2 turns out.
As for the pilot fuel screws underneath, they are set at the factory using sensitive emissions equipment and once moved, they're difficult to set exactly. I've found that they are commonly set between 1/2 turn and 1 full turn out from LIGHTLY seated. I'd start at 3/4 turn out. I would set the pilot fuel screws FIRST and then adjust the side air screws.
If there are no issues with the tuning/condition of parts, then starting the pilot fuel screws at 3/4 turn out will not result in sooty plugs even if 3/4 is not the sweet spot.

[UKAndy]

Thanks for the advice on the petcock, they are commonly used by the racing guys in the UK. Nice and simple.

I had my engine running reasonably well with my tuning cycles and balancing but, when I was just warming it up to adjust the air screws when it developed a terrible misfire. I had go through all the usual checks even doing a compression test which was very good. I had rebuilt the engine with new rings so I expected good results. After checking timing and all the electrics I concluded it must still be in the carburation. I had recently changed the air filter and clearly I had left too much oil on the air filter element. Hence for the rough running. I suspect some of my rich running has been poor airflow so I may investigate a K&N element if they are available. I am not a big fan of pods with all their issues.

[ASK]

Hi Andy, Did you finally find the solution for No. 3 cylinder running rich?? I am facing exactly the same issue and would appreciate if you could share how you managed to fix it?
I own a 1979 Suzuki GS 750E UK Model.

[uk gs nut]

No body has thought of the choke plunger rubber seal leaking. Ok when cold and running on choke, warmed up and a leaking choke seal will give rich idle mix with high vacuum pulling harder at the choke seal. Just my thoughts, what thoughts from the collective ??