Won't Idle, Backfires

[Cabot]

This one has us stumped.

2003 YZF 250.

Ran great last year.
After the last ride last fall
-Gas drained from the tank.
-Carb completely taken apart and cleaned. (TPS was not touched)
-Valves checked, in spec like they have been since we bought it.
-New throttle cables

This Spring,
-Fresh Premium gas put in
-Starts harder then it used to
-With choke on, it will run decent
-choke off - it will not idle
-when given gas, it revs up nicely but backfires like crazy after throttle is released. (both choke on and choke off)

-We went from a 40 to a 42 to a 48 Pilot Jet, with very little difference.
-Played with the Idle Mixture Screw, didn't help any.
-Disconected the TPS, same problem.

The timing hasn't been touched. It wouldn't just jump teeth while its sitting in the garage.
Air leak? Highly doubt it, but it will be checked.
Last time the spark plug was pulled, it was a nice brown papery bag color.

Sounds like the pilot jet is clogged, right? its not. Everytime it was taken out, it was clear.
Sounds like the gas is bad. bad gas right out of the pump? The gas is a couple weeks old, no more then 2 weeks though. but the bowl was drained, and the pilot jet is clear.

We want to get this thing running so we can go riding on Saturday!! :bang:

Any help would be greatly appreciated. :cool:

 

[biglou]

I don't know if it's true on the 250, but on the 426 and maybe the 450 it was possible to put the slide in upside down causing these same symptoms. It might be worth a look.

 

[whenfoxforks-ruled]

Accelerator pump circuit, or valves?

 

[spark250]

I had an 02 that was REALLY touchy when it came to air temp. I always had to set the air screw and jet around the conditions.

Also, if you are thinking it's the pilot, sometimes you can still see through it and it looks clear but that might not be the case. a tiny amount of gunk can mess things up. Sometimes on something I think is clogged I will run some piano wire of the same size through the jet.

I'm in michigan and we've had some pretty cold track days so everything is runnin a little lean right now.

As you mentioned certainly check for any air leaks.
Even though you didn't move it, it could be worth the exercise to reset the TPS according to the manual.

Sorry just kinda rambling any suggestion I could come up with, hope you find the problem

 

[Ol'89r]

-Starts harder then it used to-With choke on, it will run decent
-choke off - it will not idle

Sounds like the pilot jet is clogged, right? its not. Everytime it was taken out, it was clear.
:cool:

Check the pilot air passagways. The pilot jet may be clear but the passagways could still be clogged.

Also, your valves may need some attention. Not just adjustment but visual inspection. If the valves are not seating properly, it will be hard to start and won't idle correctly. Do a compression check.

 

[Cabot]

Valves are in spec, but compression test was not done. The air passages were very clear. Took the idle mixture screw out, made sure the O-ring was in there correctly. air through that passage, along with the pilot jet passage AGAIN. Float bowl removed, and everything cleaned inside AGAIN.
AP working like it should.
Its not sucking in air anywhere.

Next step, compression test/leakdown.

 

[oldguy]

When we had both the 250F and 450F apart tis winter for rebuilds when we put them together we couldn't get either to fire. I ended up (at Eric Gorr's suggestion) running the fuel screw all the way in and then backing it out to 2 turns. Next he had us turn the idle all the way in and then back it out all the way then finding the middle point set it there. Next we used the tip over method to make sure fuel was getting to the carb. Next step was choke on, 3 quick twists on the throttle, and 3rd kick they both fired. Not sure if it was coincidence but Eric G said there was a setting of some sort in the carb that needed to be "woken up". As I said I don't knowe how much BS he was feeding us but it worked

 

[Cabot]

I don't know if it's true on the 250, but on the 426 and maybe the 450 it was possible to put the slide in upside down causing these same symptoms. It might be worth a look.

Theres a plate on the engine side of that slide, its upside down :bang:

When looking at the carb, it seems right the way it was, but the book doesn't lie!

Thanks alot!! we probably would have overlooked that forever! :cool:

 

[sharky243]

LOL I've got an 03' doing the same damn thing. Carb is spotless. The only way I can get it to run half decent is by turning the mixture screw out like 5 turns. It ran well with the jetting thats in it last summer. The hot start plunger was seized in the carb, and the port where the hot start plunger goes was corroded. I'm thinking that its pitted bad enough to suck air through the hot start even when it's closed. New (to me) carb on order hope it cures the problem.

 

[Cabot]

Hope you solve your problem.

But this bike runs great now!!
I'm thinkin the 48 is a bit too fat now, we'll see how she is.

 

[whenfoxforks-ruled]

Stock leak jet?

 

[axgrinder6]

valves no dout

 

[oldguy]

Hope you solve your problem.

But this bike runs great now!!
I'm thinkin the 48 is a bit too fat now, we'll see how she is.

Cabot you may be fine with the 48 for now (cold weather) but we just solved a summer long problem with our 2007 YZ250F where it would not restart if it was run hard and stalled hot unless it was bump started. It would always start first kick cold and this problem came up this season after it was big bored to 262. Replaced the coil and rechecked the valves many times. Finally I got to the point of thinking I would have to sink a couple hundred into a stator when I pulled the pilot and it was a 48 (42 was stock). We had never touched the jetting so figured it was stock. I put a 42 in it and it seemed to start just fine last weekend even when a 12 year old girl stalled it in the whoops.

When I told Rich our situation with jetting he immediately diagnosed the problem as to fat on the pilot because the big bore will richen the intake at all levels making it probably a 52 so the hot start leaned it back to a 48 which was to rich to fire hot