GT250 Mysterious carb leak

[montgomeryhank]

Hello all,
The issue is as stated in the Subject Line. The bike is a 1973 GT250 K stock everything save a few new bolts and fuel lines.

The Problem:
The right-hand carburetor is spilling fuel out of the overflow at the bottom of the air intake and also into the engine itself. It is not a trickle. The left carb (the one with the choke) seems to be working perfectly fine.

What I have already tried:
The carbs were recently professionally rebuilt. It looks like they replaced all of the jets, needles, and gaskets but left most everything else stock.
I went ahead and got a brand new OEM float needle assy. Seat and pin.
I set the float height to spec. wich is 13.7 for the '73
Spec did not work so I lowered the float arm .5 mil at a time until I had it at 11mil and stopped there because that's well off-spec and wasn't making
a bit of difference.
I started swapping parts from the good carb to the bad one at a time trying to isolate the problem. To no avail.

Essentially at this point, it seems like a stupid oversight on my part, the right carb is missing a part (hard to say because the diagram only illustrated the left carb). Or there is some catastrophic crack somewhere that is causing a pretty sizeable overflow of fuel. I'm hoping for stupid.

Any help would be appreciated!
Best, Hank

[keith_vdw]

Hi Hank,

Have recently cured the same problem.
First fill a cup and trow in the float. Check if it is still floating(!). If it sinks there is a tiny hole in it and adjusting float height isn't doing much.

WhatsApp Image 2020-12-22 at 08.21.17.jpeg

Next with the floatbowl removed, trow the carb in the cup with a piece of hose connected to the fuel inlet and blow on it. Check if needle valve AND seat are closing off. If you see bubbles you know where to look.

Concerning the float level your screwing around with...
You can start with the float height in the manual and check the fuel height (connected to an improvised fuel tank). This way you can also easily check for leaks.
Connect a second piece of hose to the drain hole at the bottom of the bowl. Now you have communicating vessels and can meassure the fuel level.
Fuel height should be 3mm below surface, so ±2mm lower than in this pic.


Cheers,
Keith

[montgomeryhank]

Wow, what a wonderful response!
I have already tested the floats as best as I can. All seem to float fine, no bubbles when held submerged, floating at the same level. I don't think it is the floats considering switching them out with the other carb did not solve the problem and did not harm the working carb.

That submerged blow-test you mentioned sounds promising. Will have to give it a go in the morning.
The entire carb submerged fully assembled I assume?

I haven't checked the level yet. I brought the float level back to earth after the drastic changes made no difference so I'll test that next as well.
Thanks Keith

H

[keith_vdw]

That submerged blow-test you mentioned sounds promising. Will have to give it a go in the morning.
The entire carb submerged fully assembled I assume?

Yes, except for the float chamber/bowl of course:) Carb turned upside down, so you can inspect the needle valve closely.
Best to check twice. Once completely assembled with the float in place, moving the float up and down by hand.
Next with the float removed and operating the needle valve directly (with your finger or gently with a screwdriver or whatever).

Good luck!

[jabcb]

The 73 GT250 has two individual plastic floats.
Suzuki did not do a good job at sorting out the different plastic floats. If you have the wrong ones, the fuel level will be way off when you set the float level per spec. You can sort this issue out by checking the fuel level.

This pic shows the difference:
Parts_0580 by jabcb, on Flickr

I’ve been setting/checking the fuel level lower that the posted pic.
The correct GT250 carb fuel level per another forum member:
viewtopic.php?f=9&t=13872&p=172459&hili ... el#p172459

[keith_vdw]

I’ve been setting/checking the fuel level lower that the posted pic.

Indeed, ±2mm lower then the pic i posted, 3mm below surface.
Also try holding the carb under different angles to check under what kind of angle fuel comes running out of the overflow. For example the angle when your bike is on the side stand. Always handy to check this while the carb is in your hands instead of noticing later on when everything is mounted back on the bike

[jabcb]

Also forgot to mention…
I’ve had problems with the float bowl gaskets interfering with the motion of the individual plastic floats.
So make sure that the gaskets don’t protrude into the float bowl where it could cause a problem.

[montgomeryhank]

So, I built a testing stand today and that made troubleshooting a lot easier. And...

Potentially solved. But we will see once they are on the bike + time. I think we can consider the problem in the 'stupid error' category though if I was more proud i'd say "why couldn't Suzuki/Mikuni come up with a float needle that closed properly". Anyway, it seems like a few firm (firmer than I originally thought/was comfortable with) taps with a plastic hammer seem to set the needle properly. From what I have heard this is a classif problem with these bikes. So now at least 90% of the time I turn on the fuel the carbs start to overflow and I have to set the needle this way? I can imagine that will start to get old real soon.

Thanks again for the input.
Jabcb, interesting point on the floats. Mine seem to be the correct ones. Probably original.

H

[keith_vdw]

So now at least 90% of the time I turn on the fuel the carbs start to overflow and I have to set the needle this way?

I dont understand what you are saying here.
You adjusted the fuel height as we suggested and its still pouring out of the over flow?
Did you permanently fix the leak of the needle valve housing or not?

[montgomeryhank]

It seems like the needle just doesn't like to seat most of the time and it needs a tap. I put the carbs back on the bike and ran it today. Both would start leaking gas and stop when I tapped them with a small hammer. The right was still much worse than the left but it is apparently a problem with both now.

H

[keith_vdw]

I wouldnt tap the needle!
Are you sure the leak is between the needle and the seat? It could just as well be between the seat and the carb body.
You can give the seat a tap but i would be gentle with the needle.
Are the needle and seat genuine mikuni or after market?