250sx dies after being held wide open

[beastybaconman]

I have a 250sx and usually it backfires a lot when I let off the gas to slow down, because its got a cobra exhaust, but sometimes when I go down the road and have the throttle wide open for a little bit, when I let off it just bogs. When it starts bogging I try to give it more gas to keep it going but it'll just keep bogging and sometimes the rpms will come back up, but the same thing happens when I let off the gas again. Then once I stop and it dies I have to kick it over a lot before it starts back up. It only happens going down the road, around my house it runs tit and starts first kick every time. Does anyone know what could be causing this?

[Flyingw]

the cobra exhaust has leaned it out. This cab be bad for your motor.

[beastybaconman]

the cobra exhaust has leaned it out. This cab be bad for your motor.

That's kind of what I was thinking, do you think re-jetting the carb would fix it?

[Flyingw]

Yes but you have to rejet the slow speed and main jets or go back to the stock exhaust.

[beastybaconman]

Yes but you have to rejet the slow speed and main jets or go back to the stock exhaust.

Ok, I don't have the stock exhaust, any idea what the jets should be for the cobra?

[Keepah Rolln]

Check your compression.

[beastybaconman]

Check your compression.

I'll check, but it seems to have plenty of power when I'm just driving around my house

[shortline10]

My guess would be a fuel flow issue or an obstruction at the float valve . Make sure the gas cap is venting correctly .

[beastybaconman]

I don't have a compression checker, but I just went out and kicked it over with my finger on the spark plug hole and it seems like it has all sorts of compression. The spark plug does look like it has been running a little lean, so I think I'll richen the mixture with the air screw a little and go easy on it, until I can get the right jets. And I'm pretty sure there isn't anything blocking the fuel flow, I just had the carb apart the other day and cleaned it out pretty good. Which way do I turn the air screw to richen the mixture?

[Flyingw]

Ok, I don't have the stock exhaust, any idea what the jets should be for the cobra?There is no magic number for any given exhaust. Every motor breathes differently. Fatten it up 2 or 3 sizes and do a plug chop. The plug will tell you how its burning. Since its popping on deceleration, then fatten up the slow speed jet until the popping stops.

[beastybaconman]

There is no magic number for any given exhaust. Every motor breathes differently. Fatten it up 2 or 3 sizes and do a plug chop. The plug will tell you how its burning. Since its popping on deceleration, then fatten up the slow speed jet until the popping stops.

Ok thanks I'll try changing them. When I took the plug out to check compression it definatly looked like it was burning a little hot.

[Trike_crazy]

I don't have a compression checker, but I just went out and kicked it over with my finger on the spark plug hole and it seems like it has all sorts of compression.

You can get a really cheap one from harbor freight. Or you can spend a bit more and get one from the parts store. If you are going to do any type of work on an engine it is a convenient tool to have. Checking with your finger isn't an accurate way to check. Not that your engine needs a compression check but I was just letting you.

[beastybaconman]

There is no magic number for any given exhaust. Every motor breathes differently. Fatten it up 2 or 3 sizes and do a plug chop. The plug will tell you how its burning. Since its popping on deceleration, then fatten up the slow speed jet until the popping stops.

I just took a look and the main is a 130 and the slow jet is a 38. How many sizes do you think I should go up? It's probably pretty lean right now because there isn't much for baffles inside the muffler.

[Flyingw]

Try starting at 134 and 42 and see what that gives you. Its a bit of trial and error.

[beastybaconman]

Try starting at 134 and 42 and see what that gives you. Its a bit of trial and error.

Alright thanks

[beastybaconman]

Just got the jets, they only had a 135 main in stock and I got the 42 slow jet. I took it for a spin and it seemed to back fire a little less but it still pops on deceleration. I kind of like the popping, but if it's bad for the engine how should I fix it?

[Flyingw]

It needs to go fatter..

[beastybaconman]

Just found a pretty decent size hole in the head pipe. I think that's probably why it's running so lean. Also is there supposed to be any kind of packing inside the muffler?

[Flyingw]

OH MY!!!! Yes those holes will contribute to a lean condition and yes, the muffler is supposed to be packed.

[UPNATM]

http://pages.ebay.com/motors/link/?nav=item.view&id=131263878834&alt=web

[beastybaconman]

Thanks for the link, but my dad welded it for me so it should be fine now. What should I use to pack the muffler?

[Flyingw]

Most dirtbike shops have FMF packing. Don't wrap it tight and don't wrap it loose.

[wide'n'low]

although this does sound similar to a lean condition, theres no way the cobra exhaust is running the carb that far lean on a 4 stroke....on a 2stroke yes, god yes.
just changing the wadding/lid removal/forgetting to oil your air filter can cause a dangerously lean condition.

are you getting a smell of fuel, could it be rich? as if its bogging due to washout/flooding...... too much gas entering the cyl can cause a bog to happen after wot situations, because the carb itself can only flow so much air. while the carb will flow as much gas as the float plunger/carb needle will allow gravity to deliver(which is always wayyyy more than it could ever use) better rich than lean...that's the way every carb is set from the factory, so it always runs without going lean due to possible environment changes such as temp, humidity, elevation.

it honeslty sounds like an issue with the float height/float plunger needle or both(running the bowl dry).
on a 4 stroke just changing the exhaust out, would require a change of needle position and or adjustment of the afr screw depending on elevation.
try turning your afr all the way in and backing it out 1-1/4 to 1-1/2 turns as a base point. and check to make sure youre not starving the float.

[beastybaconman]

although this does sound similar to a lean condition, theres no way the cobra exhaust is running the carb that far lean on a 4 stroke....on a 2stroke yes, god yes.
just changing the wadding/lid removal/forgetting to oil your air filter can cause a dangerously lean condition.

are you getting a smell of fuel, could it be rich? as if its bogging due to washout/flooding...... too much gas entering the cyl can cause a bog to happen after wot situations, because the carb itself can only flow so much air. while the carb will flow as much gas as the float plunger/carb needle will allow gravity to deliver(which is always wayyyy more than it could ever use) better rich than lean...that's the way every carb is set from the factory, so it always runs without going lean due to possible environment changes such as temp, humidity, elevation.

it honeslty sounds like an issue with the float height/float plunger needle or both(running the bowl dry).
on a 4 stroke just changing the exhaust out, would require a change of needle position and or adjustment of the afr screw depending on elevation.
try turning your afr all the way in and backing it out 1-1/4 to 1-1/2 turns as a base point. and check to make sure youre not starving the float.

I just checked the air filter and turns out it's just a nylon stretched over the metal cage thing, so that might be part of the problem. Also there isn't a air screw on these carbs, so should I try moving the needle?

[Flyingw]

If that's a stock carb, yes there is an air screw aft of the bowl on the bottom. 85 models the air screw is 2 turns out and 86/87 is 1 1/4 turns out. You can identify the carb by the number on the side of the bowl rim under the fuel hose nipple.

QA01A and QA02A are 85 carbs and QA07A and QA08A are 86/87 carbs.

[beastybaconman]

Looks like I do have the original carb a QA01A, but I'm still not sure where the air screw is. The only screw I see other than the idle adjustment is on the left side of the bowl at the bottom, is that it?

[Flyingw]

It looks like the screw isn't even there. Get yourself a carb kit and rebuild it. Here's my link to rebuilding these carbs. No pilot screw will cause all sorts of weird stuff to happen.

http://www.3wheelerworld.com/showthread.php/161746-ES-SX-350X-Carb-rebuild-Tutoral?p=1257154#post1257154

Carb Kit
http://www.ebay.com/itm/Honda-ATC-250-ES-SX-Big-Red-1985-ATC250ES-Carburetor-Stock-Carb-Rebuild-Kit-/400752443123?pt=Motors_ATV_Parts_Accessories&hash=item5d4eb4fef3&vxp=mtr

The QA01A carb is actually for a 250ES but they are interchangeable with the SX. The 85 SX used a QA02A. The screw on the side of the bowl is the bowl drain screw.

[Flyingw]

If you don't want to mess with the carb, I do offer a rebuild service for 90.00. I do test the carbs before I return them. Turnaround is usually a couple of days.

[beastybaconman]

Thanks for the help, I don't have much money right now so I'll just get a rebuild kit. There is a brass thing there that I thought was a screw but it's flat. The previous owner must have snapped it off or something

[Flyingw]

It its busted off, take a dremmel or something and cut a slot in to it. Don't worry about cutting in to the aluminum tower it screws in to. Just cut it deep enough to get a good bite on it with a screwdriver. Hopefully it will turn out. If it does come out, be sure to get the spring, small flat washer, and small oring out of the hole if they are in there.

[Chopsaw]

Thanks for the help, I don't have much money right now so I'll just get a rebuild kit. There is a brass thing there that I thought was a screw but it's flat. The previous owner must have snapped it off or something

I had one that had no pilot screw . Was just a plug .

[Flyingw]

I've had them come through with no screw at all.

[beastybaconman]

Just got the rebuild kit and put in a new screw. When I was putting it in I screwed it down until it got hard to turn and one of the ears broke off, so I figured that must be all the way down. I just put a little notch in it with a band saw and screwed it out 2 turns. Is the end supposed to stick out this far?

[Flyingw]

on the 85 carbs yes. The 86/87 carbs the screw goes in further.

[beastybaconman]

Took it for a test drive and there wasn't any poping or backfireing when I let off the gas at first when I was taking it easy, but once I started to get on it a little more and run higher RPMs it started to backfire again. Right now the air screw is 2 turns out how should I adjust it to richen the mixtrue? or do I still just need a bigger slow jet?

[Flyingw]

Bigger slow jet. Fatten it up some. What does the plug look like?

[Flyingw]

Have to ask, when you replaced the carb parts, did you pop the needle jet out are put in the new one? That's the brass barrel the needle passes through?

[beastybaconman]

Have to ask, when you replaced the carb parts, did you pop the needle jet out are put in the new one? That's the brass barrel the needle passes through?

Haven't pulled the plug yet I will tomorrow. All I did to the carb was easy-out the old air screw and put in a new one with the spring o-ring and washer.

[beastybaconman]

Just pulled the plug, it looks alright.

[Flyingw]

Ok, not too bad. At least its not white.

[beastybaconman]

Ok, not too bad. At least its not white.

It seems to be running pretty good I think half the reason it pops when I let off is because there's no packing in the muffler, but I'll try changing the slow jet. And I just figured out the exhaust is leaking where the head pipe is clamped into the muffler, is that a big deal or should I try to seal it?

[Flyingw]

THAT IS A BIG DEAL!!! That alone will cause the motor to run lean. I would suggest not running it until you get that fixed. Be very very careful when you loosen the nuts on the exhaust flange. The nuts love to fuse to the studs and when you go to break them loose, you run the risk of twisting the studs. Happens all the time. If you have access to a torch, heat those nuts and studs till they're red.

[beastybaconman]

THAT IS A BIG DEAL!!! That alone will cause the motor to run lean. I would suggest not running it until you get that fixed. Be very very careful when you loosen the nuts on the exhaust flange. The nuts love to fuse to the studs and when you go to break them loose, you run the risk of twisting the studs. Happens all the time. If you have access to a torch, heat those nuts and studs till they're red.

I'm just talking about where the two pipes connect with a clamp, not where it comes off the head

[Flyingw]

My bad!!!!! That's not so much of a big deal.

[Outlaw #24]

Not to be a Thread Dawg , But Beasty you need to learn something before you go any further ! Listen to these people that are responding to your questions! 1st, You have been told to repack the silencer (Muffler) or it won't run right ! 2nd , You said you didn't have anything except a nylon sock for a air filter ! Did you ever put a proper filter on it cause that will cause you problems is what you were told ? Before you do any jetting fix the obvious , such as header welded, Muffler packing, Filter element ! JMO but I hate hearing somebody give advice on hear and the people getting the advice not heeding what they have been told. Packing for the muffler is fiberglass insulation and pack the stuff into it , no matter how tight or loose it is packed is better than NO packing at all! BTW my wife's 250SeXy has the cobra exhaust and has a little popping at the end like you like ! Keep working on it and you will get it!

[beastybaconman]

Got a filter for it, will be getting muffler packing this week. I took it for a spin and pulled the plug, it's brown on top and there's black soot looking stuff around the edge so I think it's running a little rich. I might lean it out a little, but I'd rather have it run too rich instead of too lean.

[beastybaconman]

took her for a spin down the road the other night and it still dies. It runs fine when I'm just putting around doing wheelies but after I hold the throttle constant for a while, like when going down the road, when I let off the rpms drop and it sounds like I'm just coasting in gear, and when I give it throttle nothing happens unless I hold if for a while then the rpms will come back up a little, but it doesn't have the power it should. Once it dies I have to kick it over a bunch before it even fires again so I'm thinking it is either not getting enough fuel and I run the bowl dry, or it is flooding out. I pulled the plug and it looks just like it did before, brown on top and dry black soot around the ring so it looks like it's running a little rich. I know the muffler is supposed to have packing in it, and I'm going to pack it this week, but would the lack of packing cause this problem?

[jakep53]

its definitely not running rich it looks a tad lean but not too bad so I would richen the mixture up a little!!

[beastybaconman]

its definitely not running rich it looks a tad lean but not too bad so I would richen the mixture up a little!!

You're right, guess I'm not too good at reading plugs. I figured out what was wrong, the sediment catch thing on the fuel shut off was all full of crud. It was letting enough fuel in for just banshing around but couldn't supply the carb when going down the road, so it would lean out until it died. Now it runs fine and loves 5th gear wheelies down the road. All I have to do is pack the muffler and it'll be mint, thanks everybody for all the help.

[jakep53]

You're right, guess I'm not too good at reading plugs. I figured out what was wrong, the sediment catch thing on the fuel shut off was all full of crud. It was letting enough fuel in for just banshing around but couldn't supply the carb when going down the road, so it would lean out until it died. Now it runs fine and loves 5th gear wheelies down the road. All I have to do is pack the muffler and it'll be mint, thanks everybody for all the help.

glad you got it all figured out!!

[shortline10]

Post # 8 , I tried to tell you what your issue was in the very beginning of your thread :lol:
Glad you got it fixed and can learn from it as well :)


You're right, guess I'm not too good at reading plugs. I figured out what was wrong, the sediment catch thing on the fuel shut off was all full of crud. It was letting enough fuel in for just banshing around but couldn't supply the carb when going down the road, so it would lean out until it died. Now it runs fine and loves 5th gear wheelies down the road. All I have to do is pack the muffler and it'll be mint, thanks everybody for all the help.