Ty 250 Misbehaving Under Acceleration

[sirsherco1]

Gentlemen and others.

My TY250 i think it's an A, has a KT brand carb on it (never heard of them in little old NZ.)

Problem is: on the first and second rounds of a comp occaisonally when I give it some Welly, it goes really flat and won't rev out, and of course going up the steepest section it just runs out of puff and sometimes dies. I am sick of losing 5's, jets are clean, no crud in the filter, but, now this is strange, generally on the 3rd and 4th round it runs like a dream!

Crank seals ok not pressurizing gearbox or blowing lots of smoke. I note that the filter (clear see through) doesn't seem to be more than 1/3rd full but when i tap the float bowl more fuel runs in. Yes i do run with the full filler cap slightly off tight incase there is a vacumm inside the tank.

Main jet is so big that there is not even a number on it.

Also occaisonally it's really flat after a small climb.

Any thoughts or recommendations gratefully received.

Sirsherco1 Christchurch NZ.

[feetupfun]

You can measure the fuel flow rate available to see if that is the problem.

Dont clean anything or fiddle with anything until you have done the test or you might never know what the problem was.

Take the (TK) carby off the bike so you can have it held over an icecream tub at the normal height and attitide it would be on the bike still with the fuel hose connected.

Remove the float bowl drain plug

Hold the carby over the tub and turn the fuel tap to on.

See if the fuel flow rate out the bowl drain is weak (restriction in fuel flow), or starts off good but decays over time (insufficient venting of tank).

Another thing that semetimes catches people out is if there is an uphill section in the fuel line, it can hold the fuel back if an air bubble forms at the highest part of the line.

[ducman]

Gentlemen and others.

My TY250 i think it's an A, has a KT brand carb on it (never heard of them in little old NZ.)

Problem is: on the first and second rounds of a comp occaisonally when I give it some Welly, it goes really flat and won't rev out, and of course going up the steepest section it just runs out of puff and sometimes dies. I am sick of losing 5's, jets are clean, no crud in the filter, but, now this is strange, generally on the 3rd and 4th round it runs like a dream!

Crank seals ok not pressurizing gearbox or blowing lots of smoke. I note that the filter (clear see through) doesn't seem to be more than 1/3rd full but when i tap the float bowl more fuel runs in. Yes i do run with the full filler cap slightly off tight incase there is a vacumm inside the tank.

Main jet is so big that there is not even a number on it.

Also occaisonally it's really flat after a small climb.

Any thoughts or recommendations gratefully received.

Sirsherco1 Christchurch NZ.

Well i hab exactly the same problem as you on one of my TY 250.It was a nightmare.Anyway the result was a aftermarket petroltap(chinese or whatever)an the trouble was after a while it get not enough petrol,especilly on climbs.So,what i would do is,put a brand new original Yamaha petroltap on and your bike will run okay.

Chrs:Mike

[sirsherco1]

Thanks gents, i had an inkling it might be an air bubble thats why i started opening tank filler cap slightly.

The fuel tap is original but doesn't have the brass tube on top of the tap inside, for the main feed or, the filter for reserve.

1 thing that points to fuel flow is the way the bike runs fine

when the tank is getting down in level , say 3rd or 4th round.

Thank you both for your replies, i will check and report back, the fuel flow is a good one to try i hadn't thought of that.

By the way it starts brilliantly when warm, needs a tiny bit of choke on first start up but idles like a champion otherwise.

Who said bikes don't have a personality?

Cheers, mike.

[tony27]

Are you coming up to Kaikoura for the 3 day?

I'll have my/John Lawton's 250c there & if you haven't got it sorted by then we'll see what can be sorted

Tony