2001 400ex Carb on 1992 trx 250x

[magos]

Ok, I've picked up a 250x from a friend. The stock carb was cracked so he opt'd for a 400ex carb. Engine is stock, exhaust is a full Cobra system. No other engine mods except a K&N AF.

I've cleaned it VERY good and rebuilt it (new screws, gaskets, etc.)

Jetting & Air:

It's got the stock pilot of 38. Idles fine and I was using a 42 pilot before (still idled fine).

Main jet is a 138 (stock) downsized from a 150.

Pilot screw out 2.5 turns and I am running an open K&N (airbox removed)

Symptoms:

Crazy decel popping and a little popping on acceleration when I mash the throttle.

Next steps:

I realize I need to check to plug to see if I'm running rich (I think I am) but has anyone done this yet?

Thoughts?

[wilkin250r]

The stock carb is going to work much better for you on a 250x, but I realize that's not the question you asked. If you want to know why, I can tell you the details.

Popping on decel is generally an indication of lean idle, either jet itself or just a pilot screw setting.

Popping on accel, is it a pop, or a gurgle? Does it have lots of power, or does it chug a little. Anything at full-speed WOT, or is the symptom only at acceleration WOT?

[magos]

Thanks for the info and please do tell why not to run it. I understand this carb is HUGE in comparison but a hotcam is in the near future which may allow for the extra CFM/Carb size.

So the Pilot jet is the popping on decel culprit huh? I'm used to tuning with my computer (on my harley) so it's a bit of a change for me.

It does have quite a bit of power but I notice on the higher rpm side at WOT it gurgles/pops/etc. a bit. Is my main too big? Again, I need to check the plug, but I'm guessing it's going to be black as night.

Side note: I just read the thread no reading plugs and it was VERY interesting. With that, I have a needle clip in tbe middle (factory setting) which if memory serves me correct - it's the 3rd rung down.

Thoughts/Suggestions?

[wilkin250r]

I don't know the best words to describe it, but the 250X is an older style, and old methodology. The valves and ports on those motors are really restrictive, almost the exact opposite of what you find on the new 450s today.

At 250cc, the motor itself isn't very big, and the intake path is STILL too small.

So with the intake and exhaust valves being the bottleneck, on an already small motor, you'll NEVER be able to pull in enough air through the 400EX carb to really make it worthwhile.

In theory, a larger carb will improve top end performance and give you more peak horsepower, but you get worse low-end and midrange performance due to the flow characteristics through the carb.

If your valves and porting are choking off your top end performance, the bigger carb isn't doing anything, ALL you are getting is worse low-end and midrange, without the benefit of the top end.

Now, add some serious head porting (big $$$) and oversized valves, along with a cam and maybe a bigger bore, THEN you might see an improvement with a 400EX carb over the stock. But until then, all you are doing is hurting your low end.

[wilkin250r]

Originally posted by magos
So the Pilot jet is the popping on decel culprit huh?

The carburetor only knows throttle position. So when you are decelerating, you are off the throttle, the slide is closed. That means, regardless of engine RPM, your carb is operating on the idle/pilot circuit.

Now that is cleared up, let's throw the RPM of the engine into the mix. You are on the idle circuit (carb is closed), since you are decelerating from a high speed, your engine RPM is significantly higher than idle. That means you'll be pulling significantly more air, more venturi effect, higher manifold vacuum, than your normal idle conditions. This doesn't change your carb, it's still operating on the idle circuit, it will just AMPLIFY any problems or issues.

You are pulling LOTS of air through the idle circuit, and it's popping. Popping is generally lean (gurgling is generally rich). Sounds like you're slightly lean on the idle setting, which isn't really affecting idle too much, but when you amplify that problem (because of the deceleration), it becomes readily apparent.

[magos]

Ok, that all makes sense. I think what I'll do is just look for a stock carb and work with that. This is a PITA anyhow.

[wilkin250r]

A stock carb is going to be easier to work with, and you'll find better information (because more people have them).

I realize nobody likes to spend money, but you can probably find a stock carb fairly cheap.

I would help you with jetting, but I got rid of my 250X years ago, and I can't remember what my jets were. But you can try this section, or there's another section lower down called "Sport Quads under 350cc Sub-Forum" that you can post questions in as well.

[magos]

Great! I appreciate your help!

[87xproject]

i have 2 stock 250x carbs if interested

[magos]

Found a 96 300ex stock carb. It'll be here today and I'll be putting in one and testing this weekend. Hopefully, all will be MUCH smoother. Thanks for all the help!

[JOHNDOE83]

I have a jetting thread if your interested, you may be able to get it working properly?

It may even add some performance?

[magos]

Well, I installed a 96 300ex carb and it made a HUGE improvement (shocking huh?)

Well, now I just need to tune a little more. I have a 38 pilot and it's popping on decel and a little hesitant on off the line starts, so I think a 42 pilot is in order. Well, it's the next size up that I have on hand, so I'm going to try that. Once I'm past the that issue - it runs strong! Glad I switched. Now I just need to unload the 400ex carb - no use to me now.

[magos]

Does anyone happen to know the stock Pilot jet size for a 92 TRX250x? I have a 38 in there now and it is still popping on decel and a little slugish on starts. Mid to top is great!

I'm at 2.0 turns out on the air/pilot screw.

Thanks!