87 250sx jetting question.

[John 4]

Hey guys. I got a sweet 87 250sx. The problem I am having is that it is backfiring on deceleration. Now let me tell you what the machine has...
1.) a new K&N air filter.
2.) I beautifully installed a re-packed cobra exhaust from a 200x.
3.) it has a new carb from FlyingW which is I trust completely and ran spotless with the oem air filter and exhaust.

With that said...I believe FlyingW used the proper jets that the machine calls for, but the backfiring started when I installed the K&N and cobra exhaust. I know it's a lean condition, but I am hoping someone can give me some idea what jets I should install.
I hope to hear your opinions, thanks!

http://i816.photobucket.com/albums/zz81/Bigjohn010672/4D1C88A2-8134-4D53-8A27-20267D52BFC8-18512-000003692AC42994-1.jpg

[Gearheadtom]

An exhaust leak can cause that as well, either where the headpipe bolts to to motor, or where it connects to the muffler.

[John 4]

An exhaust leak can cause that as well, either where the headpipe bolts to to motor, or where it connects to the muffler.
Thank you for your reply, but I feel very confident there is not an exhaust leak.

[Larry T Moore]

where'd ya get the cool blue gas cap?..i want one for mine

[Flyingw]

Clarke has them.

http://clarkemfg.com/accessories-gas-caps-c-16_18.html

[Flyingw]

not for metal tanks though

[yamaha225dr]

Deceleration back fire is usually from a lean pilot jet. You may be able to riches your idle mixture screw enough to rid the problem, try turning it in about half a turn and see what happens. If that doesn't fix the problem then step up a size on your pilot jet.

[userj8670]

I have a supertrapp mine and had the same problem. The mix screw will not give it enough gas to fix the problwm...been there done that

[barnett468]

Hello




I got a sweet 87 250sx. it is backfiring on deceleration. has
1.) a new K&N air filter.
2.) I beautifully installed a re-packed cobra exhaust from a 200x.
3.) it has a new carb from FlyingW which is I trust completely and ran spotless with the oem air filter and exhaust.

the backfiring started when I installed the K&N and cobra exhaust. I know it's a lean condition, but I am hoping someone can give me some idea what jets I should install.

Since we don’t know exactly what size your jets are all I can suggest as a starting place is what I listed below, others opinions will vary.


1. Start with the needle in the middle position.

2. If it requires only 0 - 1/3 choke to start easily with bike cold and air temp around 60 degrees where it is starting, then go up 1 size on the pilot.

3. If it requires more choke to start easily under the above conditions then go up 2 on the pilot.

4. Go up 3 sizes up on the main if you are and have been using the air box lid.

5. If you removed the air box lid with the installation of the air filter than go 4 up on the main.


Run it for 45 minutes at all rpm’s and check the plug. If it has flat spots upon acceleration it is lean, if it burbles/ sputters upon acceleration it is rich.

It's better to start a little rich than a little lean.

[John 4]

I have a supertrapp mine and had the same problem. The mix screw will not give it enough gas to fix the problwm...been there done that
Ok, so what was your solution?

[barnett468]

Hello John 4

I posted while you were posting so you missed it, see post #9.

[John 4]

Hello





Since we don’t know exactly what size your jets are all I can suggest as a starting place is what I listed below, others opinions will vary.


1. Start with the needle in the middle position.

2. If it requires only 0 - 1/3 choke to start easily with bike cold and air temp around 60 degrees where it is starting, then go up 1 size on the pilot.

3. If it requires more choke to start easily under the above conditions then go up 2 on the pilot.

4. Go up 3 sizes up on the main if you are and have been using the air box lid.

5. If you removed the air box lid with the installation of the air filter than go 4 up on the main.


Run it for 45 minutes at all rpm’s and check the plug. If it has flat spots upon acceleration it is lean, if it burbles/ sputters upon acceleration it is rich.

It's better to start a little rich than a little lean.
I am using the airbox lid and it does take most of the choke to start. With that said should I go up 2 on the pilot and go up 3 on the main?

[barnett468]

Hello John 4



I am using the airbox lid and it does take most of the choke to start. With that said should I go up 2 on the pilot and go up 3 on the main?

That is what I would do based on ALL your information others might not.

I would then check plug color and carburetion as mentioned and do fine adjustments as necessary.

Start with a new plug so you get a true color reading.

1. near white = lean

2. lite tan = good

3 dark tan = slightly rich

4. dry black = way rich

5. moist black = burning oil and possibly rich.

[barnett468]

Hello John 4

Just curious, what number was your plug color before installing pipe and air filter?

[John 4]

Hello John 4

Just curious, what number was your plug color before installing pipe and air filter?
I'm sorry I don't know because it ran so good I just never felt compelled to do a reading.

[barnett468]

Hello



I'm sorry I don't know because it ran so good I just never felt compelled to do a reading.

Lol, if it ain't broke don't fix it!

How does it run now, flat spots or burbles under acceleration?

[John 4]

Hello




Lol, if it ain't broke don't fix it!

How does it run now, flat spots or burbles under acceleration?
It actually runs awesome! Starts right up beautifully, Very good throttle response, excellent power throughout the entire spectrum and revs out nicely. The only problem is the popping when I decelerate. I did notice that if I turn my idle WAY down so it won't idle...the popping pretty much goes away, but who wants a bike that doesn't idle?

[yamaha225dr]

It actually runs awesome! Starts right up beautifully, Very good throttle response, excellent power throughout the entire spectrum and revs out nicely. The only problem is the popping when I decelerate. I did notice that if I turn my idle WAY down so it won't idle...the popping pretty much goes away, but who wants a bike that doesn't idle?


When you loosen the idle adjuster the needle is dropping farther down into the jet and the throttle slide is blocking off more airflow through the carburetor, thus somewhat richening the fuel/air mixture.

[John 4]

Ok, so ultimately if my plug is reading good and i can cope with the popping what harm will this condition cause? I'm just curious!

[tri again]

I'm close to the same boat.

so is this what we need to go through if we can't find an oem
exhaust or airbox and filter?

I also understand, real or imagined, that there is no real
set formula.
In other words, 2 identically modified trikes may need different jets.

Thanks and good luck.

[yamaha225dr]

The deceleration backfire is more of a nuisance then anything but it's an easy fix. Jetting is all about the temperature, elevation, humidity and condition of the engine, so yes two identical machines sitting side by side may require slightly different jet sizes but there shouldn't be a big difference though.

[barnett468]

Hello John 4




The following comments and suggestions are based on my experience only, others experiences and opinions will vary.




Ok, so ultimately if my plug is reading good and i can cope with the popping what harm will this condition cause? I'm just curious!

Changing the parts because may now cause your engine to run hotter and have inaudible detonation caused by running lean. Detonation can seriously damage an engine. You can’t fix what you can’t hear.

Changing jets is easy, however if for some reason you decide not to jet it properly as recommended you should at least install a 1 step colder spark plug IMO.





DID ADDING THE PIPE AND AIR FILTER CHANGE MY JETTING REQUIREMENTS

Yes, in your case it “leaned” it out. The pipe and air filter you installed are less restrictive. The pipe allows exhaust gasses to exit more quickly which makes your engine run cooler. An engine that runs cooler than another requires more fuel than a hotter one. A lean engine will run hotter than a rich one. The air filter allows more air flow which mainly comes into effect at higher engine rpm's which can cause lean conditions at higher rpm. One of the reasons mfg's run new cars so hot is because hot engines require less fuel which means better mileage and lower emissions.





IF IT’S LEAN WHY DON’T I NOTICE A PROBLEM

To a typical recreational rider, a slightly lean bike can feel identical to a slightly rich bike, and it takes a person experienced in jetting to tell the difference. There is also not a huge difference in performance between 2 jet sizes.





WHY DID YOU RECOMMEND THE JETS YOU DID

If you re read my post, I asked very specific questions. My recommendations were based upon ALL the info you supplied and my years of experience. Others would make a different recommendation however no one would recommend jetting leaner.

[John 4]

Thanks for all the replies guys. I'm gonna look into going up a few jet sizes!

[barnett468]

Hello John 4



Thanks for all the replies guys. I'm gonna look into going up a few jet sizes!

Sounds like a prudent idea to me. Pay close to your throttle response before and after, you might notice more power with a more responsive throttle. Hot lean engine = sad engine, cool rich engine = happy engine!