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1978 GS750E Rich Carb

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The point is, kits are notoriously bad! Every Part I have ever seen from Suzuki or Mikuni is sold individually. If it was an aftermarket kit listed as Mikuni (for Mikuni) it could very well explain your issues. The factory parts rarely need replacement and clean up just fine. If you still have the old parts I would highly recommend cleaning and using them unless you are absolutely certain the new parts are genuine and specked exactly for your carburetor by Suzuki.
 
I just completed my test ride, I just did 12 miles under 4000 as I have a lot of new parts in the engine. The bike ran perfect, good acceleration no backfiring or misfiring, idles well at every stop. Have not checked the plugs to see how its running but don?t think I will get it to run any better. The carb kit I purchased was made up of Mikuni genuine parts, I purchased them from a Specialist Suzuki retailer in the UK. The parts all looked good quaility and the way it is running I cannot fault them.
 
can you give us the name of this "specialist Suzuki" retailer please? I am in the UK and it would be handy to know.
 
I use discountbikespares.co.uk, I have known them for years and always get good quality parts for almost any Suzuki. They stock mainly genuine parts at approx.20% less than a dealer like Fowlers. The carb kits are actually labelled as pattern parts from Japan, I don?t know the full story but there is some advertising restrictions in the UK who can use the name Mikuni. I understand only Allen Performance can use the Mikuni name as they are the official dealer. I ran into this issue when trying to buy parts for my Yamaha racing bikes, I ended up importing parts from the US where there a number of good sources at reasonable prices.
 
Had the same problem on my 1980 GS550ET but on number 2 cylinder which is the manifold used for fuel tap vacuum take off. At low throttle openings it ran on 3 and a bit cylinders, but when thrashed it ran lovely obviously the increase in air diluted the extra fuel coming down the vacuum pipe. Transpired it was fuel tap diaphragm, so against all advice I got a repair kit which cured the problem but only lasted 2 weeks, so I bit the bullet and got a new tap, now runs like a Rolls Royce.
 
Unfortunately my carb tuning is on hold until I fix a battery charging problem, got home from my test ride and battery was flat. I think it is only 40 yr old wiring that needs some new connectors! I did not get around to doing a proper coast down plug chop but looking at my plugs I am going in the right direction, No.1 looks a little weak so I have turned the pilot fuel screw out another 1/8 turn. The other cylinders still look a little rich. I am suspecting that the air filter is clogged. I will fit a new one tomorrow and see what happens. Based on all the feedback I get about vacuum pipes I am going to do a test by blanking off the vac pipe and running on prime. When I purchased my GS one of the previous owners had moved the vacuum pipe from the No.3 Carb take off to a take off on No.2 inlet manifold. I did not understand why but may be some history similar to your GS550. I don?t smell and see any fuel leaking out of the vacuum pipe but worth another look based on all the feedback you guys have given me, looks a common thread!
 
Yes I agree fully with you, however I don?t think the quality of the parts I purchased is my issue. I am very experienced at tuning carbs from my racing experience but I have never had to tune an idle that has both air and fuel circuits. Typically you only need to adjust the air screws and if it runs too rich change the pilot jet size up or down accordingly. In the case of the Mikuni carb used on the GS we have the pilot fuel screw as well as the pilot air screw. I know Suzuki recommend not to touch them as they are set by the factory but to clean the carbs you have to remove the screws.

I fitted a new air filter element today and made some small adjustments to the carb settings, the idle is now very good so looking forward to riding it once we get some dry weather.
 
Tuning the pilot fuel screws is typical and common. 1/2 from lightly seated is fine if that's where the bike runs best. Just be sure to verify fuel level using the clear tube method because a high fuel level causing rich running may be the real problem and setting the fuel screw lean is not the proper fix for that. Float height is just an approximation to get the fuel level close.
 
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At 1/2 turn out if the needle seats are in good shape you might experience some low speed surging in town traffic. It's hard to pick it up because you don't usually drive like that but if you do notice it just tweak the needles out about 1\8 turn.
 
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.
 
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!
 
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.
 
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.
 
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.
 
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 ??
 
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