1978 GS750E Rich Carb

[UKAndy]

I rebuilt the carbs with all new parts and I checked the needles were correct. My issue is on idle so the needle should not be in play until the throttle is open. Once I do my road test tomorrow and do a plug chop I should get a better idea of what’s going on. My vacuum hose is new. I have tried running with the petcock on prime setting with the vacuum take off blanked and it makes no difference. I also use this set up for balancing the carbs with an auxiliary fuel tank so I can adjust the carbs.

[Agemax]

exactly what parts did you replace? were they OEM Suzuki, Mikuni or aftermarket parts?

[OldVet66]

Are these new parts all factory parts? Sorry Agemax we must have had the same thought and you were faster.

[UKAndy]

I used a Mikuni Kit, the OEM parts were made by Mikuni so I guess you could call them genuine.

[Agemax]

i wasnt aware Mikuni made a "kit" for these carbs. where did you purchase these kits from please?

[OldVet66]

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.

[UKAndy]

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.

[Agemax]

can you give us the name of this "specialist Suzuki" retailer please? I am in the UK and it would be handy to know.

[UKAndy]

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.

[WOLFY]

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.

[UKAndy]

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!

[Agemax]

i have also used discountbikespares quite a few times now and they do sell quite a lot of OEM parts, however, for £12.99 for the complete kit, it is definately not OEM Suzuki, or Mikuni.

https://www.discountbikespares.co.uk/product/carburettor-repair-kit-gs750/

[UKAndy]

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.

[Nessism]

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.

[OldVet66]

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.