GT250A - No power at low revs

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sportston
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Re: GT250A - No power at low revs

Post by sportston »

Yes float height set correctly and no signs of leaks.
Needle jets were removed and ultrasonic cleaned. No sign of crap in them. But since then, the carb has been replaced with a genuine NOS one.
The jets are all new, correct size and of acceptable quality. Sorry to contradict, but there really is no need to waste money on genuine Mikuni jets and needles, when some others are just as good. I do appreciate that some kits have the wrong size jets or badly engineered ones, but these are good and the current owner is trying to save pennies. However, the problem cylinder will now have gen ones as it is a NOS carb and the problem still persists, so it is not logical to conclude that the fault is due to bad jets.
Please accept my apologies if this post sounds argumentative. I only mean to throw my alternate opinion into the mix.
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Re: GT250A - No power at low revs

Post by ConnerVT »

sportston wrote: The jets are all new, correct size and of acceptable quality. Sorry to contradict, but there really is no need to waste money on genuine Mikuni jets and needles, when some others are just as good.
Can you tell us the source of these? I have been through several aftermarket kits, all from different sources, and none were correctly sized or well manufactured.
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Re: GT250A - No power at low revs

Post by sportston »

Keyster. But I swapped the main jets for new ones of the correct size for the GT250A model from my own stock as the Keyster set is for the earlier models. Just used gaskets, needles, float valves and, if I remember correctly, the pilot jets were the correct size so I used them too. The needles were identical to the original in all dimensions, I checked carefully. The gaskets were of adequate thickness and quality, although I have come across poor quality gaskets in kits from other manufacturers before, so I do take the point that the buyer should beware of cheap crap. The pilots I compared with my stock and are the same size in number and actual dimensions.
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Re: GT250A - No power at low revs

Post by Vintageman »

Ketsyer is one of the better, but all you need to do is look at the machining quality compared to EOM (Mikuni).

Buying individual mikuni jets you need is lower (check prices online... don't by from Dealer) cost than those kits. Money you save you can buy OEM gaskets (always shop wisely for price if that matters to you).

But again how did you solve your needle jet if that needed attention. Did you look inside where it meters? You need a bright light source and youthful eyes or few times magnify glass to inspect If that surface is not perfectly is smooth you are compromising performance. If it looks like surface of the moon it will be poor mid-range. Egg shaped high miles or needle not floating. Hmm, if the jet needle was bad very likely jet needle bad, but not always the case the other way around. If Jet needle was not bad swap back from keyster eventually... or save part for next possible owner. Or post on eBay for if nice worth some coin for people know quality of Mikuni and exact fit
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tz375
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Re: GT250A - No power at low revs

Post by tz375 »

Vintageman is absolutely correct. Aftermarket jets and needles are often/usually/always poor quality and rarely match OEM for size or flow rates. I have not tested every jet and needle from from all sellers, but 100% of teh jets and needles I have checked are not worth using.

Your experience may vary, but jets are cheap and there is nothing to be saved by fitting parts that are not right. You can test needles in a vernier or digital aliper. No need to actually measure them. Put one oem and one aftermarket needle together side by side and close the calipers at any point along the length and see if they both rest at the same place. Usually one is higher (thicker) than the other.

To test jets, we don't need a full on pressure and flow test bench. Spray carb cleaner or WD40 and then spray through the other and compare how well they flow or how far the jet travels. That make no pretense at being a flow rate test but give a pretty accurate comparison.

Other jet kits supply the same needle to replace several different ones across different years where Mikuni changed the needles. May be close enough, but close enough is typically not close enough.

Have you tested the carb passages with a can of carb cleaner or WD40 to be 110% sure they flow the same through all drillings - especially tehe tiny idle jet passageways?
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Re: GT250A - No power at low revs

Post by dollydog »

"The carb recon kits were of reasonable quality and the correct sizes. The old jets were damaged and worn so are useless. If it were a problem with jet dimensions it would not effect only the right cylinder, it would effect both as they both had identical jets."

didn't the 250a have different size main jets? one side was one size bigger than the other? 99% sure it did. not that that made any difference to us, or dozens of other owners, we changed the main jets [both] to 112.5 - no problems with running at any speed.

your problem i would say is electrical. either duff plug/ coil/ condensor/ points/ wiring. i would certainly start there [coil first] :D

cheers, dd.
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Re: GT250A - No power at low revs

Post by sportston »

Opinions differ on if both jets are the same or different. Some say the same, others say they differ. I kept them the same.
The new needles are exactly the same as the old ones. The needle jets were cleaned by ultrasonic as best as I could however I do not have a microscope to tell if they are perfect. They looked ok with the naked eye.
I did try substituting coils and eliminated that as a possible problem. I agree that a good spark is essential if you wish to get a bike running right, which is why I recommended replacing the crappy Newtronic ignition which fitted poorly and worked unreliably (see Trevor's other post with regard to ignition on this bike).
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tz375
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Re: GT250A - No power at low revs

Post by tz375 »

Agreed, that the ignition has to be suspect, but pardon me for harping on about this but have you tested the carb passages with a can of carb cleaner or WD40 to be 110% sure they flow the same through all drillings - especially the tiny idle jet passageways?

I can't tell you how many times I have reconditioned carbs and found that they look perfect but one or more of those tiny holes are still blocked where I can't get to it. That is particularly true with the "choke" or starter jet in the bottom of the float bowl. I don't keep stats, but probably 1 in 4 have blocked starter jets. Likewise the pilot jet outlet and bypass which are also tiny holes.

What I do is to spray carb cleaner or WD40 through the holes in one carb and then repeat on the other and if the difference in observed flow is significant. it's time to clean again. That side by side comparison can be very revealing.
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Re: GT250A - No power at low revs

Post by sportston »

tz375 wrote:Agreed, that the ignition has to be suspect, but pardon me for harping on about this but have you tested the carb passages with a can of carb cleaner or WD40 to be 110% sure they flow the same through all drillings - especially the tiny idle jet passageways?
Yes I did, and could not see any signs of blockages or significant flow reduction. But it was only a visual check not an accurate bench flow rate test as I don't have the facilities for that.
The carbs were completely stripped and cleaned using carb cleaner, ultrasonic cleanered, flushed and blown out on a high pressure airline. I was very thorough but it still ran as bad. Just in case I missed anything the first time, I did it a second time. There was some tiny bits of crap in the carbs when I stripped them initially. By the time I had finished, they looked as clean as a whistle, and should remain so since I cleaned out the fuel tap and strainers and fitted inline fuel filters and new fuel and vacuum pipe. One of the rubber flange mountings had a crack in it so it was replaced with a new genuine item. The other one was perfect so was not replaced, but both were re-sealed with a thin smear of fuel resistant sealant on the flange end.
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