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GT250B carb tuning

Posted: Wed May 12, 2021 10:20 am
by GT250Mike
Hoping for a bit of guidance please.

Busy working my way through some problems on a GT250 B 1977 I have just bought which I dont think the previous owner ever had on the road. The bike starts up first kick and idles fine but is a bit more sluggish than I would expect up to about 6000 rpm when it takes off. The manual I have has the adjustments for the 250A but the carbs on mine seem different so I am after a bit of help. The mixture screws are set at 4 turns out on the left carb and 4 and a half on the right carb. The air filter had just an offcut of dry foam in so I have new filters on order. Also the baffles were missing, which I have just replaced with the correct wading.

Any help appreciated.

Re: GT250B carb tuning

Posted: Sat May 22, 2021 4:52 am
by dollydog
mixture screw? presume you mean the pilot air screw? shouldn't be anything like 4 turns out, more like 1.5 or 1.75 turns out. what jets do you have in? i use #30 pilot jets and 112.5 main jets with the standard airbox in it. 4 turns out will make the engine run very lean, screwing them in will richen the mixture. hope this helps :D
cheers, dd.

Re: GT250B carb tuning

Posted: Sun May 23, 2021 5:18 am
by karl pa
The 76 GT250A used a 92.5 main jet, after some issues with top ends burning up, Suzuki went with 95 main jets in the rest of the A & B model. The 95 main jets are on the rich side, but they are on the safe side, for the way these bikes like to run, wide open.
Is 4 turns on the screws where the bike wants to run or is that just where they were set ?
Are the slides in the correct carb with cutouts toward filter ?
The GT250A & B don't really like anything below wide open, so you won't have much low and mid range.

Re: GT250B carb tuning

Posted: Tue Jun 01, 2021 5:16 am
by GT250Mike
Thank you both for the replies.

Just about to have a proper look so your advice is appreciated. I think its running a bit rich but not had a proper run since I bought it. Picks up fine when you wind it up but anything below 5000rpm is a bit boggy. Trouble is, I haven't ridden a 2 stroke since 1987 so nothing to compare it with. will check jet sizes and try tuning the carbs.

Thanks again.

Re: GT250B carb tuning

Posted: Sat Dec 18, 2021 11:41 am
by Vintageman
Late to party. Did this get solved? Due tell

If no, Consider new needle jets. Sounds like they are a bit eroded inside? They affect 1/8 throttle up to 7/8, more so in the middle range of throttle and thus mid RPM all typical of when running medium to light engine loading.

You can check once you remove them, but you need a good set of eyes and lighting tube to see well inside tube. The part that counts is just below the shroud for a few mm down the barrel. That is to be a precision machine ID bore. It should be shinny smooth, not looking like the surface of the moon through a low magnification set binoculars: no pitting or dullness at all.

I have revived many machines like this and being that old the majority I see are eroded to some level. I live in USA New England so moisture and temperature swings here that can cause condensation in this region as it runs down and stick to the jet needle living inside the bore. If arid warm temp region maybe not an issue so much.

It's up to you how much less than pristine you want to put up with. Many times they are real scaly. If all you do is idle about or run WOT then no worries, else check Needle Jet (also Jet needle condition for both erosion and wear marks)

I have been on few boards over the years and seem to be the only one that brings this up at least that I have seen. I don't think am I believed either. I suppose I am not good at evangelizing. Maybe I should change my handle from Vintgaeman to IsYourNeedleJetBadMan and stop writing too verbose, bad grammar, too long (can't) finish reading preachy post.

I am not sure this is it, again due tell if solved, but if you have not tried the above it can help.

The GT250 A/B is a great "exciting" bike:
I have owned GT250B they can run well. The only issue is they don't have little to no torque below 4000 RPM at least, but don't have any real jetting issues anywhere. The T305 and T350 does, but stock top end, the GT250B on the pipe is very exciting indeed, They have good top end (little rocket) but being air cooled can have hard time shedding off heat respective to the power they make. A little rich on Main jet can add some added cooling and also back off to give a chance to cool.

I hope your sort it out