250SX Jetting! NEED SOME HELP!

[S250X]

I recently aquired a 250 sx from my dad. It had a cobra pipe installed on it several years ago and it had never been rejetted. It had a 128 main jet in it and a 38 pilot. I bumped it up to a 138 main and a 42 pilot, replaced the stock filter with a UNI and removed the airbox lid. The bike idled excellent, accelerated fine, and had gobs more top end power. But the only problem was it would bog if I jumped on it too quickly. As long as I would get moving under easy throttle and then jump on it, it was fine. I adjusted the needle up one notch from stock position thinking maybe it wasn't getting enough fuel at takeoff, but it didn't make alot of difference. Yesterday I put a 135 main and a 40 pilot in thinking maybe the jetting was too much, but the problem was still the same, and the power was less. The hesitation goes away with the choke on, but i just can't see it being too lean still. Especially since I was running the stock jets before with no trouble. It may have had the hesitation with the old jets too, I don't remember since I didn't ride it alot. The spark plug is a tan color, so I don't think it is too rich either. Anyone have any experiences here?

[200SXRIDER]

Can you tell me more about the cobra exhaust for the 250sx Model Number and where he bought it from. I would like to find one.
Thanks

[Thorpe]

Sounds like a pilot screw adjustment...

[tri again]

for lack of a reply, I'll try.
or at least prime the thread.

Welcome, first of all.
and it sounds like you're doing everything correctly.
My choice is to stay stock just because of these issues, but I digress.

I have an sx with a cobra exhaust that is too loud for me but it is also the only trike I've
never had the carb off.
If I did, I certainly would have etched the jet numbers on the machine somewhere.

Surely someone will chime in with better advice than this.
is it an 85?
is it a Keihin QA 01 carb by chance?
if so, there was a cold start recall but all that means is
a real pain to start cold
but that slow jet is also too small (a press in)
and seems like it may be fed thru the choke "circuit",
which as you know, is more of an internal passage sharing
than a 'choke off the air' type flapper.

MAY be why it quits stumbling with a little bit o' choke.

Please Keep us posted as this topic pops up periodically.

x2 with thorpe btw.
If that screw actually works, antiseize may be a good idea on those microscopic threads.

[Thorpe]

Your pilot adjusts the mixture from off idle to about 1/4 throttle, thats why it sounds to me like pilot screw adjustment. Your needle, is adjusting the fuel in the 1/4 to 3/4 throttle range, and your high speed jet does the high rpms. It sounds like to me, you still need to jet slightly richer than the 138/42. Keeping in mind, these carbs did not have an accelerator pump, so its not getting that extra dump of fuel when you jump on the throttle. Here is an excellent read, that may help direct you in your tuning woes'. The Keihin QA 01 carb does have a cold start issue, but unless you are currently tuning your trike in the snow, its a moot point for now...

http://www.3wheelerworld.com/showthr...Jetting-Theory

[S250X]

I believe that exhaust was a Cobra JET megaphone system. We got it from Dennis Kirk around 10 years ago. I think Cobra has stopped making ATV exhausts now, and I see Dennis Kirk has no exhausts for 250sx's now. I can't see any part #s on it anywhere. Thorpe, I do think maybe it is the pilot screw. I am going to stick the 138 main and 42 pilot back in her and play with that pilot screw for awhile. Even if I can't totally get it out, it really isn't that bad. Its just kind of annoying. drjoe, it is an 87 and no it does not have that cold start issue.

[Thorpe]

The 138/42 jetting was probably enough to compensate for the cobra... Probably need a little more yet. What happens if you put your airbox lid back on? Its amazing how much more fuel they require with just those 2 little mods...

[S250X]

Well, it may still need more, but I talked to another guy on here who had the same bike same mods as me +11:1 comp piston and a cam, and this was the combo he was running. I figured his required more fuel than mine, so I ordered the 138 and the 42 and a couple smaller of each. I will probably put the bigger jets back in, tune the pilot screw and run it awhile. At least I know that it is not dangerously lean or rich right now. Do you think I should move the needle any more? It was set at 1 notch down from the top, and I moved it down 1 more to raise it up more.

[Thorpe]

I am pretty sure the middle notch was stock, and I would raise it 1 from the middle...

[S250X]

Oh really! I am not even in the middle then! My dad rebuilt the carb a long time ago. He probably forgot which one he was supposed to put it in. I will change that too then! Oh, and the air cleaner lid seemed to make it run worse, but the hesitation thing seemed about the same. I just bumped the jets back up to 138/42 and turned the mixture screw 2.25 turns out like someone suggested, and it runs better. Still some hesitation though. Thanks for all your help! I don't really have anyone around here who knows anything about these.

[Thorpe]

Just in case you dont have a manual for your machine, you can download one here...

http://www.3wheelerworld.com/showthr...pdated-9-13-07

Also... What are you using for a carb rebuild kit, and jets? (reason I ask, I had bought a "moose" kit from DK for my ES, and the jets were numbered the same, but the diameter of the hole was largely different in size)

[S250X]

I haven't actually rebuilt the carb. My dad rebuilt it about 10 years ago, and I am not sure what he used. But the jets I am using are genuine Keihin jets, so I should be good.
Thanks for the manual, I was going to try to buy one, but this is better. I rode it for about 3 hours today, and it really does ok once you are moving. It only bogs out when I gun it from a stand still!

[tri again]

The 138/42 jetting was probably enough to compensate for the cobra... Probably need a little more yet. What happens if you put your airbox lid back on? Its amazing how much more fuel they require with just those 2 little mods...

x2 An air cleaner, either dirty or the lack thereof, can make a huge difference in the mixture.
You may want to make sure the air channel through the frame is also free of bees nests and mice
if it seems to hate the airbox lid on.

I thought I had a chart picture that shows which jet / circuit works at which rpm
and throttle position and their overlaps but it must be on my old dead computer.

Congrats on having an 87 btw

[tri again]

I hope this jet circuit chart helps.

Can't believe I even found it.

[S250X]

Well, I adjusted the pilot screw, and I didn't seem to help much. The stock setting for 86-87 is 1 1/4 turns out, and I think I have it further out than that by a couple turns. It seems to pick up a little better that way, but it may just be my imagination. I do know that it needs the valves adjusted, could this possibly cause a hesitation problem?