GT750 still idling erratically after much troubleshooting

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susej1981
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Joined: Tue May 04, 2021 11:15 pm

GT750 still idling erratically after much troubleshooting

Post by susej1981 »

Hi all, new to the boards and from the the US (Chicago specifically).

I need help getting my stock 1976 GT750 with 9,000 miles running. It runs, but not well.

My main problem is, it won't hold idle very well. I'll have to coast to stop signs holding the throttle otherwise it will die. Sometimes when parked running on the centerstand, the idle will be fine and out of nowhere climb to 3k RPM. It is also hard to accelerate from 1-3k RPM and anything after 3k rides normally.

Here's everything I've tried so far:

- Rebuild carbs multiple times (This involved doing it exactly like on Baz Wilkes youtube video, bench sync-ing them so they all open at the same time, new carb gasket kit, new oem O-rings on the needle jets as the original ones were hardened, made sure to blow out/use brake cleaner on the tiny pressed in pilot jets in the float bowl, check float height per the video, weigh the float bowls to make sure they're all pressing down the same amount, pilot screw 1 1/4 turn out, diapghrams have no pin holes/tears and pass the test of plugging the hole/slowly dropping, 110 main jet on outer carbs and 107 in the middle, middle needle position (also have tried one lower/leaner), tried to use all original brass pieces instead of the "Keyster" brand bits on my rebuild, has the correct slow jets with the two hole pair (not the three hole ones)
- New carburetor rubbers (from "Cruzin Image")
- Made a replica foam filter element as the original one was missing and lightly oiled it
- New NGK B6ES spark plugs and gapped per the owners manualImage
- Inspected the baffles to see if they're coked up (appear clean)
- Compression test (125psi on all three cylinders)
- Strong blue spark on all the plugs
- New 1k ohm NGK spark plug boots
- Verified it was getting clean fuel delivery (gas tank was rusty until I did a tank coating and cleaned the carburetors after)
- New battery
- Tried running it without baffles to see if there was any difference in performance which there wasn't
- No leak by the oil pump well
- Tried spraying carb cleaner by the boots to listen for a change in RPM's/air leak, made no difference
- Adjusted the timing gaps per the owners manual and made sure each cylinder was firing before top dead center
- Fuel strainer/filter on petcock is clean
- No outer crank seal leak visually, but only inspected it by pulling off the alternator to see if I could find anything (Condition of left/inner crank seals unknown)

The bike did sit for a long long time before I got ahold of it and started working on it.

If you guys can suggest anything else to try before I "throw in the towel" and send it to a mechanic to fix, I'd greatly appreciate it. Nearly no one in the Chicagoland area wants to work on my bike, and if they do- they're backed up with other service and can't look at my bike immediately.

Cheers!

-susej1981
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BAZ
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Re: GT750 still idling erratically after much troubleshooting

Post by BAZ »

Welcome to the forum, Please remove the carbs and spark plugs spin the engine and put your hand over the intakes you should get a suction around 1/4 power of a vacuum cleaner even on all three cylinders let me know how you get on. Put B8 Plugs in also Cheers Barry.
teazer
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Joined: Mon Mar 07, 2011 8:10 pm
Location: Chicago

Re: GT750 still idling erratically after much troubleshooting

Post by teazer »

Not sure where in the ChiTown area you are, but I'm up near 6 flags if you want to bring it over. We can do a leakdown test and replace any Keyster left with with OEM Mikuni jets, check fuel levels, and so on.

In the meantime, please do what Baz suggested to see how strong the intake suction is on each cylinder. If the carbs are off, leave them off and let's see how bad they are. Not sure if I have air jets (anti surge) in stock but they come in via the Mikuni distributor pretty quickly. That's not your issue, but might as well fit them while the carbs are off.

Since you have already done so much, you may want to check the carbs again. What I do is to blow WD40 (it's less nasty than carb cleaner) through one passageway on one carb and then repeat on the other two carbs. Start with one of the air jets and move on from there.

http://pinkpossum.com/GT750/carb/BS40carbs.htm for more details and part numbers for the jets.
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susej1981
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Joined: Tue May 04, 2021 11:15 pm

Re: GT750 still idling erratically after much troubleshooting

Post by susej1981 »

I'll have to test what you guys mentioned to try.

I hope maybe this video I caught of the carburetors in action might help someone point me in the right direction.

I've always noticed my right carb sounded "noisy" and know the sound is coming from the diaphragm piston and moves a bit more than the other ones in the video. The middle carb seemed to spray out of one of the jet holes and the inlet of the carb was wet with fuel. This video was caught before I did one of the few carb rebuilds. The last time I rebuilt the carb was "the best" as I was a bit more thorough with it.

https://www.youtube.com/watch?v=-6vSKfbsi8M
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slosher
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Re: GT750 still idling erratically after much troubleshooting

Post by slosher »

susej1981 wrote: Tue May 04, 2021 11:44 pm
I need help getting my stock 1976 GT750 with 9,000 miles running. It runs, but not well.

Hi there and welcome it looks like an "L" model which makes it 1974 ain't carb jets and settings and plugs slightly different than "A" :?
Someone will know for sure on here ;)
Only looking at the Tank to assume it's 1974 ;)
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canaletto5
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Re: GT750 still idling erratically after much troubleshooting

Post by canaletto5 »

Yup. which is why this is a handy document
Suzuki-GT-Series-Specifications-1972-77.pdf
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teazer
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Location: Chicago

Re: GT750 still idling erratically after much troubleshooting

Post by teazer »

That right carb slide is fluttering - will have a leak in the diaphragm for sure.

Bring it over and we can swap in a good one and see if that fixes it.
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