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PostPosted: Fri Jul 05, 2013 10:28 am 
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On the street

Joined: Fri Jul 05, 2013 10:07 am
Posts: 28
Country: U.S
Bikes owned: SR500, GT750, R65,
Hello everyone, My name is Mario, live in socal, just got my first GT750 and I need a little help. Hopefully some of you guys would be kind enough to help me out.

I purchased the bike running but after a few runs it started running intermittently, pulled the carbs and took out tons of dirt. cleaned and cleaned some more (not the first carbs I've had to clean multiple times) but my problem now is not clogging it's too much gas!!

I set the float height according to this vid https://www.youtube.com/watch?v=fuHOrBYOeqo and I used aftermarket fuel stop needle valves which are a bit longer. First my center carb leaked, fixed, then left, then just when i thought I got it all three were found dripping the next morning. I set the float height to 28mm from the gasket and after connecting the fuel line and leaving them fill for about 30 minutes no drips. I installed and she fired up on the 2nd kick and would rev beautifully on all cylinders.

I went back out a couple hours later to give it a real test ride, she kicked over but immediately i felt she wasn't running on all cylinders. tried to force the bike to burn off the gas that collected but by the time i went around the block i think i was just on 1 cylinder. Pulled the plugs, wet!!!! pulled the airbox, full of fuel. The carbs keep dripping out of the pilot jet which goes directly to the fuel chamber which means too much gas.

I'm going to add another mm to the float height and try again. I haven't replaced the valve seats but it can't be a coincidence that it's all 3 carbs dripping or a different one every time. The only thing I can think is the floats aren't working properly or the petcock.

The way I understand the petcock to work is when the engine is running it creates vacuum which opens the diaphram and allows gas to flow so when the engine is off it's essentially an automatic fuel shut off, correct? when you set it on the prime position it bypasses the vaccuum and just lets gas flow. Could it be stuck on prime? I'll need to rebuild the petcock. I can barely move it with my hand. it must be gunked up or rusty but the gas flowing is clear.

Guys, I know I'm new here but please help. I love this bike and I want to get over this hump.


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PostPosted: Fri Jul 05, 2013 11:17 am 
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To the on ramp

Joined: Sun Aug 05, 2012 1:18 am
Posts: 415
Country: UK
Bikes owned: GT380B, GT550B, GT750A, GSX1400
The petcock notoriously can leak when the motor is stopped. Test by removing the fuel line when the bike is stopped and seeing if the few drips stop, or continue. The float valves are not designed to hold back relentless fuel flow if the motor is stopped, though they should hold 'OK' for some time.

It may be that you have the float height set incorrectly (not saying the value you quote is wrong) as the method to use involves holding the carbs at an angle until the float tab 'just touches' the float valve needle (and not fully inverted carb).

Good luck....it sounds like you are excruciatingly close!

Oh, and welcome,
Mike (an infrequent poster)
UK


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PostPosted: Fri Jul 05, 2013 11:28 am 
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To the on ramp
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Joined: Wed Jan 18, 2012 8:01 pm
Posts: 450
Location: North of Albany, NY
Country: USA
Bikes owned: T500R (now), T500M (30 yrs ago)
I'm not an expert on the GT750, but Suzukis from this era all have this as a weak point. I've experienced exactly the same thing on my T500.

There are 2 systems that are suppose to prevent what you describe: the Float Needle/Seat, and the vacuum petcock.

The float needle should stop gas from entering the carb when the float rises. A worn needle or seat, dirt/rust caught between the two, or an incorrect part, can cause gas to continue to flow. I tried an after market seat/needle from a kit, and it caused nothing but grief. After pulling it out and comparing it to the original, I too saw it was a different length. I always recommend installing genuine Mikuni parts.

The vacuum petcock should stop the flow of gas when the engine is not running. However, these petcocks (now 40 years old) tend to leak past the oring that is supposed to seal the gas flow. Suzuki sold the petcock as a unit, but some folks have had success rebuilding them. There is info here on this site (as well as across the Internet) on rebuilding them. Just note to be extremely careful when disassembling the diaphragm, so not to damage it.

After rebuilding mine, it still would intermittently fail to shut off the gas. I've since replaced it with a manual off/on/reserve petcock.


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PostPosted: Fri Jul 05, 2013 11:37 am 
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On the street

Joined: Fri Jul 05, 2013 10:07 am
Posts: 28
Country: U.S
Bikes owned: SR500, GT750, R65,
It's nice to know everyone is one the same page on this. Thanks for your replies. I'm definitely thinking it's got to be the petcock. especially how it's basically seized. (I need a wrench to move it) I tried pulling it back and fourth to "free" it up but maybe I caused internal damage or it's stuck open. I really appreciate it. Time to get the petcock rebuild kit and also instal an inline fuel shut off!!! = )


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PostPosted: Fri Jul 05, 2013 11:55 am 
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Joined: Tue Oct 21, 2008 2:41 pm
Posts: 2177
Location: Tulsa, Oklahoma
Country: USA
Bikes owned: GT550x2, GT750, GS1000
I strongly suggest you do what ConnerVT did and install a manual petcock. Here is a link to a nice one about 2/3 of the way down the page. You will need the adapter plate as well. I believe it's the 44mm one you need. If you do go that route, remember to cap off or plug the vacuum line. Otherwise you will have a massive air leak.

http://www.partsnmore.com/parts/suzuki/gt750/?filters[fitting]=universal

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I've got a secret, but I'd rather have a beer.

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1978 GS1000C
1976 GT550 project


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PostPosted: Fri Jul 05, 2013 12:16 pm 
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On the main road

Joined: Tue Jan 22, 2013 3:40 pm
Posts: 169
Country: Ontario, Canada
Bikes owned: B100P,T200, 68,70,74 T500,GS650G DL1000
mfrias2nd wrote:

I purchased the bike running but after a few runs it started running intermittently, pulled the carbs and took out tons of dirt. cleaned and cleaned some more


Make sure your gas tank doesn't have any more dirt/crud in it, you can clean carbs all you want but if your tank is continuously letting dirt in, you will be back to square one in no time.

Fred

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68 B100P x2
68 T500 Cobra
70 T500
74 T500
76 GT185 x2
81 GS650G x2
04 DL1000


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PostPosted: Fri Jul 05, 2013 12:26 pm 
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On the street

Joined: Fri Jul 05, 2013 10:07 am
Posts: 28
Country: U.S
Bikes owned: SR500, GT750, R65,
They were definitely dirty inside and out as if it was offroaded but they cleaned up nicely and I made sure all jets and canals were clean and free of debris. There is also an In-line fuel filter and with the problem i have now I've noticed all the gas going in and out of the carbs has been clean and clear.


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PostPosted: Fri Jul 05, 2013 9:23 pm 
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Moto GP
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Joined: Fri Oct 24, 2008 9:11 am
Posts: 4479
Location: Mostly Barbados, sometimes Florida and western Canada
I believe your tank has some fine particles in it you may be missing, you mention it runs fine once the carbs are clean then the carbs start overflowing after a short time period of riding. So from that comment alone its got to be the needle jets are becoming jammed by some form of residue.
On the stock needle jet bodies there are some little filter screens fitted to the tops, they are there to prevent items of a certain size from passing into the orifice and clogging the taper section.
If you clean the carbs and hook up a test tank container to the carb fuel line and run the bike on the stand do they still overflow after the same period has passed that you would normally be riding.

One thing on this site that has had loads of comments is the vacuum petcock and its leaking problems.
I've taken plenty of them that were considered junk and restored them back to their correct function.These were units that were considered bad, a PITA, never worked right, never stopped leaking etc.
With a detailed restoration to these old abused petcocks I've done the required work on them and now they all function perfectly.

_________________
Two strokes, its just that simple.

The list:
69 Suzuki U70
69 Suzuki T500
72 Suzuki GT750 cafe
74 Suzuki GTXVR project
75 Suzuki RE5
75 Suzuki GT750
76 Suzuki GT750
81 Suzuki GSX1100
86 Suzuki RG500
88 Honda CR500
93 Honda CBR900RR
98 Suzuki GSF1200 X 3


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PostPosted: Fri Jul 05, 2013 9:32 pm 
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On the street

Joined: Fri Jul 05, 2013 10:07 am
Posts: 28
Country: U.S
Bikes owned: SR500, GT750, R65,
I've pulled the carbs off at least 10 times now to check float heights, needle seats and valves (to check for blockage) no more dirt at all. Nothing that could cause the valves to remain stuck up at least.

Here is the timeline...

Purchased bike
Bike ran a few trips GREAT!
After about 4 rides bike starts riding with perhaps 1 cylinder popping, as if it's drowing then firing. Intermittent
Install new intake boots clean air filter, play with nearly frozen petcock.
Carbs start dripping
Remove carbs to find they're so dirty the little mesh screen is nearly covered 100%
Cleaned carbs, installed new gaskets, adjusted floats and replaced valve needles.
installed. now carbs constantly dripping even when I get bike to run it bogs and drowns in gas.
Installed inline fuel shut off so at least they don't empty my tank into cylinders when my bike is not running.
Ran great then hit 2nd gear and bogs down again. find plugs still drowning.

Already ordered carb rebuild kit, petcock rebuild kit. hopefully that does it.


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PostPosted: Fri Jul 05, 2013 10:38 pm 
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Moto GP
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Joined: Fri Oct 24, 2008 9:11 am
Posts: 4479
Location: Mostly Barbados, sometimes Florida and western Canada
Ok so its back to the float needles and seats, you need to find out why they wont seal even if they are clean. Since its pouring gas, performing a simple needle seat test for each carb should easily show you the reason as to why they are not sealing.
Set up each carb with a short length of hose to the fuel inlet and attach a small funnel to the other open end. Remove the float bowl and compress the float to full up position. While holding it there with the carb upright add some fuel into the funnel, if it leaks the valve is bad or has residue in it, if it does not leak lower the float until its on the spring pressure of the pin only.
If it does not leak, the valve is sealing, if it is leaking then you know its either bad or has residue in it and you need to see if its clean or faulty.
By the way, what sealing washers are you using on the needle seat to carb body joint section???
Can you post a photo of the float needles

_________________
Two strokes, its just that simple.

The list:
69 Suzuki U70
69 Suzuki T500
72 Suzuki GT750 cafe
74 Suzuki GTXVR project
75 Suzuki RE5
75 Suzuki GT750
76 Suzuki GT750
81 Suzuki GSX1100
86 Suzuki RG500
88 Honda CR500
93 Honda CBR900RR
98 Suzuki GSF1200 X 3


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PostPosted: Sat Jul 06, 2013 4:23 am 
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To the on ramp
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Joined: Wed Jan 18, 2012 8:01 pm
Posts: 450
Location: North of Albany, NY
Country: USA
Bikes owned: T500R (now), T500M (30 yrs ago)
I'd focus on this:

mfrias2nd wrote:
I used aftermarket fuel stop needle valves which are a bit longer. First my center carb leaked, fixed, then left, then just when i thought I got it all three were found dripping the next morning.


I did the same thing, with the same result. Switched to Mikuni branded part, have not had an issue again.


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PostPosted: Sun Jul 07, 2013 2:06 pm 
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Moto GP
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Joined: Fri Oct 24, 2008 5:55 pm
Posts: 3375
Location: Lancaster Pa.
Country: US
Bikes owned: GT750 x2 97 -1200 Bandit 86 GSXR1100
How are you setting the float height .. with the carbs flat or tilted ?

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PostPosted: Sun Jul 07, 2013 2:51 pm 
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Around the block

Joined: Tue Dec 25, 2012 4:43 pm
Posts: 81
Country: USA
Bikes owned: Honda-350, SUZUKI GT550 x2, GS850, 92 VMAX,
Are you sure the floats aren't leaking? I have had several floats develop tiny pin-holes as they age. I have been successful at repairing them with solder, but it's a delicate & tricky task. Next time you have the carbs off and apart, shake the floats & listen for anything inside. Shouldn't be anything but air in there. Another way to check is to submerge the float in a pan of HOT water. The heat will cause the air inside to expand a bit & bubbles will come out any holes. Don't worry about "blowing" them up due to the heat. I'v done this several time to check my repairs & have never had a problem.

Good luck, & welcome to the forum. Keep us informed. We all like to here the ultimate "FIX" for problems. That's how we all benefit!


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PostPosted: Sun Jul 07, 2013 3:01 pm 
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On the street

Joined: Fri Jul 05, 2013 10:07 am
Posts: 28
Country: U.S
Bikes owned: SR500, GT750, R65,
Excellent technique. I will pull the carbs and inspect the floats. I'm waiting for the an actual rebuild kit. I will try and post pics.


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PostPosted: Sun Jul 07, 2013 11:25 pm 
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On the main road

Joined: Mon Jun 10, 2013 6:04 am
Posts: 137
Country: us
Bikes owned: 73 gt 750 74 gt550
good luck, im rebuilding my carbs right now too. good to know that my pet cock might be a problem as well.


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