battling with idling GT750

[francoisloubser]

My GT 750 is now running fine with one minor problem. At idlingthe left and right cylinder is running fine. The middle one is doing the following: If I unplug the middle one the revs fall with about 200. If I unplug the left or right one seperately the bike wants to die. This is at about 1250rpm. If I up the idling screw slowly it will suddenly go to 2100rpm and if I unplug any plug the change in idling is the same.

I have cleaned the pilot jet and paths, pilot screw etc. It is as if the petrol flow is not 100%. Any idea because this makes no sense????

At speed there is no problem it seems, goes like a rocket. It idles fine but I want it 100%, except if this is normal behaviour. I think that if I up the idling then at about 2000rpm another circuit kicks in, that is why all cylinders are running fine then. Any ideas will be appreciated.

[Barry S.]

You can sync the carbs doing that by adjusting the carbs individually while running on one cylinder at a time.

[francoisloubser]

I have narrowed the symptons down to this. The bike idles between 1250 and 1500. At this stage if I disconnect the middle plug(cylinder) revs falls with 150 to 200rpm. If I disconnect any of the other 2 it wants to die at idling. If I then turn the idle screw in slowly, at about 1700rpm the idling will jump to 2100. then all cylinders are running fine.

Does anyone have more info on sync of the carb,s????

[two-stroke-brit]

all i do is this .(this was given to me by a fellow board maember).

It's most likely your idle screw adjustment, most guys miss the first part of actually "syncing" carbs.

Start from scratch, and run the idle screws out untill the don't touch the slides. Very slowly screw in, one at a time, the screws until they just "touch" the slides. Use your finger on the side to feel the movement. Now start the motor and very carefully screw in the idle screws EQUALLY until you get the idle rpm you want.

Now, you can sinc the slides to open at the same time.

The reasoning is that if the idle screw adjustments are different the slides will be at different heights. If you adjust the sync from there, they will raise at the same time, but the starting point heights are different.

hope this helps.
mark

[Suzsmokeyallan]

Mark his bikes a 74 model so its got the CVs on it and not separate carbs like the 72 and 73 models.
The BS carbs have a torturous path though the body for the pilot circuit so you need to make sure its 100% clean or it will give you HELL
You can sync the butterfly valves when off the bike to a close enough setting by using the "light" method or by a small drill bit placed in the bottom of each butterfly and adjusting them all to the height off the main one that has the idle knob directly attached to it.
This will get you very close to a decent sync that will allow the bike to idle, naturally a mercury or ball sync will finish the fine tuning 100%

[two-stroke-brit]

opps my bad .
mark

[TLRam1]

Make sure your pilot circuit is clean first.

[tz375]

The symptoms suggest that the center cylinder is barely running at idle but kicks in as revs rise. That suggests a partially blocked slow speed circuit. Terry and Allan already mentioned it.

BS40 carbs have a very long complicated idle circuit which includes 3 bypass holes and one additional outlet. The main outlet connects to the adjusting screw and is easy to see and test. The three tiny bypass holes must also be clean.

With the carbs off, remove the float bowl and spray cleaner and air through the small brass tube going from the bottom of the flange up towards the idle screw. Make sure that fuild blows out of al teh holes in the roof of the carb outlet.

When bench synchronizing the carbs, all three throttle plates (butterfly valves) should just uncover the first bypass hole.

[francoisloubser]

Thanks all for the advice. I have cleaned the carbs over and over. I have found a set of carbs from a 76 model. will they be compatible. If so I will clean them up properly. Then I will also have spares, as these are extremely scarce. And I can experiment with them.

[francoisloubser]

Is the first bypass hole one of the 3 small ones, or all 3 small ones. Really small set in a triangle or is all three the same. Does these 3 determine idling or is it the job of the pilot outlet???

[Coyote]

I have the Suzuki triples manual on CD for free if you want it. It really goes in to depth on getting those CV carbs set up correctly. Here's a little blurb form the manual

[francoisloubser]

Thanks Coyote, Is it to big to mail?? Would love to get a copy of it.

[Coyote]

Yes, It's 80MB. I have a program that will split it into 5MB chunks. That last file is an exe that will reassemble it. We can go that route if you want OR you can PM me your address and I can mail it. I am assuming you are in the USA?

[tz375]

The larger hole is the pilot outlet.

The three bypass holes act to supplement fueling at very small throttle openings and are crucial to the transition from idle to main circuit.

76 are slightly different. The throttle plate is #120 vs 11 which I take it relates to the angle at the edge face. Bodies are the same as are all the jets except for pilot jets but that could be because of changes in exhaust system in 76.

Carb sets are available on ebay quite cheaply - normally

[Suzukidave]

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