Bigger Carb...What's Wrong??

[Yamahauler]

I got a 38mm Mikuni Flatslide carb for my Tri-Z. Its off a 92 YZ250.
Anyways, I got it on, and stretched the boots around it all the way. They are all on completly...FOR SURE.
Anyways, when I first started it, (I had to prime it twice before it would go because of weak fuel pump) it just revs like crazy, like its got an air leak.
So tonight, I took the carb and everything off, checked everything over, and I replaced the intake manifold gasket, thinking that was the problem. So I got it started again and it has the same problem.
Is there any way the carb could not be feeding it gas?
I know there is gas in the bowl for sure.
Is it possible my rubber joins that dont have gaskets are leaking air?
Is there something I can put on them to seal them.
I dont think they're the problem, they're on the carb tight as hell and fastened down.
Any opinions? What should I check?:wondering

[BigGreenMachine]

Throttle cable is loose and free? Not binding anywhere I mean.

[MTS]

Im wondring if i didnt accidently put the floats in backwards....Or somethnig stupid like that, that carb is the most confusing carb i have ever taken apart....Or there could be a hole in the float?

[dizasterzrfun69]

sounds like my 89 quadzilla. i cant get it to start but when it does it revs like crazy and i have no control over it. I still havent found the problem so if you figure it out let me know. i was told it sounded like an air leak but i dont know.

Kevin

[Billy Golightly]

My 84 Tecate does this exact same thing and I am 99.999% sure it is not an airleak. I had checked the boot and and all gaskets multiple times. I've had the carb apart and back together several times too. It finally stumped me and I parked it. haven't messed with it for a couple years.

[ChrisD]

My guesses are this:

This assumes that you bike ran fine before you tried the carb.
(Base and intake gaskets are OK...no air leak)

1) make sure that the slide is closing fully. When you let go of the throttle, you should hear a "click", "thump" whatever you call it from the slide closing on the carb.

2) Do you know what pilot jet you are running? Is it too lean?

3) do you have a clogged jet that's not letting enough fuel into the motor.

4) You didn't stress crack the boot when you put the carb on. all it takes is a little too much air and you will get the out of control rev.

[hrc85250r]

it can also be an old used carb. when they get old the slide and carb body where the slide slides can get worn, causing the slide to move freely enough in there to vibrate when the motor starts vibrating. this will womstimes cause the slide to vibrate itself in place or sometimes cause it to "crawl" up the carb body.....my friends 87' banshee is a perfect example of this, sometimes it works awesome, but sometimes because of its age one or both carbs will vibrate the slide in place...hope this helps...

[short4stuff]

Slide in 180 off?
Seen it a fews times
Done it ONCE. lol

[90nut]

First thin you think of is an air leak like everyone has stated. I had a simialar problem with my 84 tecate also Billy, I will just throw this out here as some possible problems. I bought mty tecate and the kid said the carb was all messed up so he got a 85 carb for it. I bought it and replaced the intake boot, started it and it still revved to the moon. Cleaned the carb prob. 100 times, checked and double checked, all new gaskets in the top end and it would still do it. So I decided to put the 84's carb on it and see what happens. Put it on without cleaning it at all and fired up first kick! Ran like a top, idled great, etc. So I took the carb back off and started comparing it to the 85 carb. Turns out the needle jet retainer was slightly larger(taller) on the 85's carb compared to the 84's. I couldn't tell by visually tell by looking down the throat when that carb was installed though. So Yamahauler, i would dheck that out just to eliminate one more possability.

[MTS]

You did put that new retainer in right cory? and properly so the needle dosnt push UP in the slide....?

[TeamGeek6]

Air leak under the slide? Any slide carb can have a gap under the slide, a larger carb will expect a larger engine, and the leakage might be enough to drive a smaller engine too lean.

After my 34 PJ was bored to 36, I had to repair it to eliminate the air leakage. JB Weld to the rescue!

Air leak or too lean. Aint much else it can be. Thats not electrical.

[Unclediezel]

Pilot jet clogged or improper size ---And someone else said it already --make sure the retainer clip is on the needle correctly-----If not it will retract into the slide , instead of sealing off the jet.---Is the spring over the slide collapsed or broken?

[Yamahauler]

Throttle cable is loose and free? Not binding anywhere I mean.
Throttle cable is fine, slide is all the way down in the carb.


1) make sure that the slide is closing fully. When you let go of the throttle, you should hear a "click", "thump" whatever you call it from the slide closing on the carb.
I hear the thump


2) Do you know what pilot jet you are running? Is it too lean?
No sure of the size, but I'd imagine it is stock size of the YZ250.


3) do you have a clogged jet that's not letting enough fuel into the motor.
Ask Josh, I havent had it apart to clean it...he did it before he sent it:lol: I imagine all the jets are not clogged.



4) You didn't stress crack the boot when you put the carb on. all it takes is a little too much air and you will get the out of control rev.
No, checked all of it for sure, No cracks, boots are in great shape.


it can also be an old used carb. when they get old the slide and carb body where the slide slides can get worn, causing the slide to move freely enough in there to vibrate when the motor starts vibrating. this will womstimes cause the slide to vibrate itself in place or sometimes cause it to "crawl" up the carb body
I guess thats possible, but would it start at a lower rev, and then slowly build up? How would I check this?


Pilot jet clogged or improper size ---And someone else said it already --make sure the retainer clip is on the needle correctly-----If not it will retract into the slide , instead of sealing off the jet.---Is the spring over the slide collapsed or broken?
The retainer is in there perfect, I had the carb hooked up to the thottle cable and checked it lots of times to make sure the needle worked well. Also, the spring is not broken.



I guess the next thing to check is what MTS told me, put the choke on, block the intake off a little, and trying to start it, if it starts, it eliminates the possibility of air leaks.
I'll check all the stuff you guys said and get back to you hopefully tomorrow.

Thanks for the replies.

[Darius1502]

Check your pilot jet and the air screw. Screw in the air screw all the way in until lightly seated. Also check the idle screw.

[Kintore]

This happened to me on my Z, when I idle screw had poped out. But im sure you checked that right?

[Yamahauler]

Yep, the idle screw is still in.
I think I'm going to the shop today, I will try to get this done.

[Yamahauler]

At the shop right now,
I took the airbox off and covered the intake with a piece of carbboard. turned the choke on all the way and kicked it twice, it fired and idled. when I pulled the cardboard off more, it idle higher, but when I hit the gas with the intake covered, it sputtered and stalled.

Does this eliminate the possibility of an airleak?

should I start going up in pilot jets?

I'm going to tinker around with it a bit more.

[Yamahauler]

This is screwed up.
I put the airbox back on, had the choke on, and it fired up and idled.

Then I left it for about 10 minutes, when I was sealing the bead on my tire, I went back to try it and kicked it over and it revved to the sky. I checked the fuel line, and there is no gas in it. I'm starting to think it a fuel pump problem over anything.

I guess the next thing to do is rig up a temperary gravity feed fuel tank.