Fueling issues....

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Skyguyz
Posts: 145
Joined: Sat Jun 01, 2019 6:53 pm
Location: Las Vegas, NV. USA

Fueling issues....

Post by Skyguyz »

Fuel issue

Data:

-1975 GT750 - Engine completely rebuilt down to the crank.
-Carbs rebuilt by Baz
-OEM Style fuel petcock, (*See pix below)
-Clean fuel tank, no debris clogs at inside petcock filter
-Small plastic triangle inline fuel filter.
-Clear fuel hoses to see fuel flow

Symptoms:

Engine cuts out after apps 15 minutes of riding in 5th gear. Some bubbles suddenly appear in clear fuel line. Downshifting to 4th, same issue. Bubbles, (slight fuel starvation.)

Stop bike completely. Examine clear fuel line, no Bubbles, only solid fuel status. Start bike up, no issues. Runs great for a while then issue reappears only when droning along in high gear,
(100-120 Km/hr.) The bike runs great around town when going through the gears and multiple stops & starts. It only manifests the issue when cruising for a while.



Attempts to solve:

1 -Checked petcock filters for clogs / debris; Found none - no change is issue.

2 -Use only High octane non-ethanol gas - no change is issue.

3- removed clear plastic triangle filter and only one continuous fuel line from tank to carb, (Yes, I know the petcock already has a filter,) -no change is issue.

4- took vacuum hose from the # 2 carb and ran it to the # 1 carb - Slight change in status - ran better for a period of time then same issue reappeared.

5- took vacuum hose from the # 1 carb and ran it to the # 3 carb - improved for a while then same issue reappeared.

6- Checked idle jets for clogs, cleaned them - -no change is issue.

7- Removed the Baz anti-surge valves, no clogs, cleaned and reinstalled - -no change is issue.

8- ran bike with baz anti-surge valves removed - -no change is issue.

9- verified that the vacuum hose from petcock to carb is fully seated with clamps and no leaks & changed vacuum line several times - -no change is issue.

10- Disassembled fuel petcock and inspected all areas for debris. none found. cleaned and reinstalled. -no change is issue.

11 - Replaced fuel petcock with a well used but functional OEM Suzuki petcock from my 74 GT750 -no change is issue.

Any ideas guys? Thought?

Thanks in advance as always.

Sky


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PS: I have started to rebuild the 74 GT750 now that I have some time off.

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PaulD738
Posts: 3962
Joined: Thu Sep 25, 2014 6:57 pm
Location: Warrington Cheshire

Re: Fueling issues....

Post by PaulD738 »

Fuel cap breather clogged, it’s a bit dodgy I know but try riding with the fuel cap undone.
They're rubbish them Jap bikes lad they won't last five minutes! you want to get yourself a nice Royal Enfield!
A quote from my old dad 8-)

I started out with nothing and I’ve got most of it left!
Skyguyz
Posts: 145
Joined: Sat Jun 01, 2019 6:53 pm
Location: Las Vegas, NV. USA

Re: Fueling issues....

Post by Skyguyz »

Tried that and forgot to list that. No help there. Will try some other options tomorrow.
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Alan H
Posts: 12104
Joined: Fri Aug 05, 2011 6:38 am
Location: Wombwell, Republic of South Yorkshire

Re: Fueling issues....

Post by Alan H »

Try running on 'prime' so that it doesn't matter if the diaphragm in the tap works. I have an in line filter between tank and carbs too, just for extra insurance. It only takes a VERY tiny peice of muck in a jet to cause issues.
If bubbles appear in the fuel lines, then air is getting in. There's a leak somewhere.
Proof that four strokes are over complicated
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Dutch
Posts: 756
Joined: Thu Jul 28, 2011 8:28 pm
Location: St Helens Merseyside

Re: Fueling issues....

Post by Dutch »

Hi Sky,

A similar thing happens to me but only at high speed, my engine would just die on me and when I pulled up to investigate saw the temp gage on hot. Its only happened about 3 times since my rebuild in 2013, the last time about 18 months ago. I found this bullitin on the Suzuki website. I can't say I have cured mine but hope it helps you.

Regards Jeff
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GT750A
Skyguyz
Posts: 145
Joined: Sat Jun 01, 2019 6:53 pm
Location: Las Vegas, NV. USA

Re: Fueling issues....

Post by Skyguyz »

Dutch wrote:Hi Sky,

A similar thing happens to me but only at high speed, my engine would just die on me and when I pulled up to investigate saw the temp gage on hot. Its only happened about 3 times since my rebuild in 2013, the last time about 18 months ago. I found this bullitin on the Suzuki website. I can't say I have cured mine but hope it helps you.

Regards Jeff
Dutch,

This sounds exactly the issue I am having. I don't know where you came up with this but I will defiantly give it a try. I'll report back my findings. Thanks agin, this could be the lifesaver I have been looking for,


Sky
Skyguyz
Posts: 145
Joined: Sat Jun 01, 2019 6:53 pm
Location: Las Vegas, NV. USA

Fixed issue

Post by Skyguyz »

Thanks to "Dutch" AKA: Jeff, I was able to remedy my fueling issue. As per the service bulletin, (GT-21, below) I used a 6 mm drill and very carefully chamfer out the vacuum port on the main body side of the petcock. Apparently the fuel petcock I purchased was an OEM one but did not have the service bulletin correction. I would urge anyone rebuilding there GT750 to follow the GT-21 service bulletin if your fuel petcock does not have either the blue or yellow witness mark on the vacuum portion of the petcock, (See bulletin diagram and procedures.)

*If you decide to do this I would be careful and do realize that you only need a small chamfer. To do this place the drill bit very lightly on the vacuum port. slowly increase drill speed; Do not apply any pressure. The very soft aluminum will cut quickly. Have a vacuum at the base of the drill bit to catch the aluminum bits coming off the drill. Remember you do not want to cut so deep that you cut material from the flat side of the petcock. You only need a chamfer about 1/2 the tip of the drill bit. Blow out any shards with compressed air. Re-assemble and ride safe.

My GT750 runs perfect now and thanks for all the support.


Sky


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Chris57
Posts: 147
Joined: Wed Mar 16, 2016 8:04 am

Re: Fueling issues....

Post by Chris57 »

Sweet, the joys of a good forum huh :)
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Dutch
Posts: 756
Joined: Thu Jul 28, 2011 8:28 pm
Location: St Helens Merseyside

Re: Fueling issues....

Post by Dutch »

Great news Sky, I've had plenty of help from this forum too in the past so I'm pleased to be able to return it.

Lovely bike you have there too
Regards Jeff
GT750A
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