GT250B main jet

[karl pa]

I have been having an issue with my 77 GT250 lately,when I am 3/4 - full throttle on an incline the engine loads up like it is running to rich,had carbs off to check float level ,level good,could not see what size the main jets are. Bike is stock,I am thinking the main jets might be 97 so I ordered 95 ,was afraid to go to 92.5 in first step. Is there something else I should be looking at?

[Vintageman]

95 is stock for 77 Gt250 if I recall. It should be OK and a little rich I think given suz went from 92.5 to 95... for people were burning up top ends. (92.5 won't hurt either (maybe be cleaner) unless you like holding WOT for long period of time. Use real mikuni and start stock for piece of mind. I find when I had the GT250A/B engine the only way it likes to run is WOT fast little 250 and it's tuned a bit hot.

Here may be other reasons ->

When you are in there again verify your needle jet is the correct size. Verify the inside of the needle jet where it meters (first 1/4 inc or so) is shiny and smooth and round (need good eyes, magnifying glass, and good light to see down the barrel).

Verify the jet needle is correct size, not worn and clip is at the right position.

Verify your ignition timing, for example if too retarded it will bog/drown out as I think you are saying.

[jabcb]

Suzuki made a lot of changes when they updated the 73-75 model to make your 76/77 model.
I’ve had a 72 T250, 73 GT250 & 76 GT250. The 76/77 likes the revs & doesn’t have the bottom end of the 73-75 model.

Carb specs for your 77 model are in Service Bulletin Specification-12:
http://www.ozebook.com/compendium/techb ... 010-14.pdf

The changes made from 75 to 76 are listed in Service Bulletin GT-32:
http://www.ozebook.com/compendium/techb ... 027-33.pdf

Vintageman mentioned the main jet update from 92.5 to 95.
GT-32 lists the main jet as 92.5
Specification-12 is marked as updated and lists the main jet as 95

I vaguely remember having heard of piston problems in the 76/77 model.
So make sure your timing is correct, use genuine Mikuni parts if you can , and use the 95 main jets that you bought.

[karl pa]

Thanks for the advice,I put 5000 miles on the bike since rebuild,always had a little trouble with this but got much worse lately.The other year I did have trouble with my points seizing up do to grease drying out,I will replace them and go from there.
I was using Alpha Sports parts for reference,they show main as 97 so I may have 95,also they show jet needle as 5cn3-2,but spec bulletin 12 shows 5cn3-3,I have it set at 2,wouldn't 3 make my problem worse?
The bike is running better now that it is cooler.
I use the lower part of the throttle in my riding style,but this bike does like higher revs. thanks karl

[Vintageman]

If 95 or 92.5 doesn't clean it up I would suspect timing is too retarded and secondly needle jet is eroded making it too rich. That 250 is sensitive when too much fuel. Being a 250 it is sensitive to everything and all details when combined add up to what you described.

My 250A would take above half throttle even when it was not ready for it. You should be able to fix this problem and then it will be more enjoyable to ride .

[karl pa]

I checked the timing,and left cylinder was retarded a bit,set timing and still had issue. Pulled carbs again and checked things,everything looks pretty good,found 95 on the one main jet so I put in my 92.5 and moved the needle clip to the middle groove. I can't believe the difference,I took it for 100 mile ride and was much more enjoyable. Definitely matches my riding style better, plus fuel mileage went from 28 to 35 mpg.

[tz375]

That's good to hear. It suggests that the mains were too large before and may have been "cleaned" by a PO with a drill bit or thick wire. That would also explain why the PO dropped the needle to try to clean up the richness by dropping the needles. It took me a long time to learn to use only genuine Mikuni jets and only new ones.

We have no idea what horrors have been perpetrated on old jets, so it's cheaper in the long run to replace them when a bike is rebuilt.

You can do a simple flow test on old jets. Spray carb cleaner or WD40 through a good new jet and compare the flow to the same test on an old jet. Often the old jet will spray much wider or erratically. It's a crude test that can reveal grossly modified jets.

[karl pa]

So now my petcock diaphragm must have a hole,it is leaking gas from vent hole on petcock when the engine is running.I was going to bypass diaphragm anyway but now I need to make time and do it.