What if the check engine light is on and engine will slowly idle donwn until it quits

The backfire could be a problem with the distributor cap or plug wires, or lack of fuel. The cap or wires could get damp and short out until the engine gets hot.
The fuel pressure could be low or not steady until the engine warms up.

Sounds like your carb needs the fuel/air mix adjusted..uses all the gas at high speed but has excess at low speeds...find your adjusting screw and at med throttle slowly screw it clockwise until the engine starts to sputter then back it out until it starts to sputter again then continue to do this until you reach ideal mix where it runs the best it can..then go slowly through the throttle up and down..when set..unjust your idle speed screw to your ideal idle speed..if this dies not fix ..carb will bed cleaned or rebuilt. Good luck.. Note always push or pull your throttle handle slowly to position avoids gas load up causing sputter and backfire.

For this types of problem,there are few help inks to troubleshoot the problem.Click the link below to go through the troubleshooting procedure to resolve the issues :----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html-----------van idles rough? http://howtobyme.blogspot.in/2012/01/2004-ford-freestar-van-idles-rough.html
--------------runs lean and idles rough? http://technoanswers.blogspot.in/2012/02/ford-runs-lean-and-idles-rough.html
-----------Car still runs Rough after tune up? http://howtobyme.blogspot.com/2011/12/car-still-runs-rough-after-tune-up.html
------------Idle flair is noticed on car? Rough idle? car will not idle? Erratic idle speed? http://technoanswers.blogspot.in/2012/02/idle-flair-is-noticed-on-car-rough-idle.html
----------High rpm in neutral? Inconsistant high RPM in neutral? http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html
------------Engine Backfires? http://technoanswers.blogspot.in/2012/02/jeep-backfires.html----------- Nissan stalls and bucks on Acceleration?

http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-stalls-and-bucks-on.html

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Nissan Pathfinder Backfires? http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-backfires.html
---------How to adjust rpm? http://schematicsdiagram.blogspot.com/2011/12/how-to-adjust-rpm-on-2009-dodge-charger.html
------------Engine cuts out at 3000 RPM? http://schematicsdiagram.blogspot.com/2011/12/engine-cuts-out-at-3000-rpm.html
----------This will help.Thanks.

The two carbs are not balanced to each other. Balance as follows; Start the bike then increase the idle on the left carb to a high idle. Now unplug the right cylinder and the engine will slow down some. Now slowly turn the idle down on the left carb until the engine just dies. Hook up the right plug wire then do the same proceedure for the right carb. Now both carbs are balanced. Hook up the left wire and start the engine again. The bike will be running faster than you want so now adjust the idle screws on both carbs equally to get the desired idle speed. Please rate my answer. Thanks.

It sounds like a fuel flow restriction somewhere. Sure, it'll start every time and idle for a while but take it down the road a bit. If it quits, I'd suspect that for some reason it's not getting fuel to the engine from the tank. Loosen the fuel cap, the cap vent may be bad. The vented cap the right side tank cap. If it's fuel injected, it could be the fuel pump or something. It sounds like a fuel problem to me.

Good Luck
Steve

more than likely has a bad distributor assembly..with high mileage they lose there magnetism..remove cap and rotor and check center windings for signs of cracking...cardone makes a good replacement distributor

Check the seals/gaskets at the intake manifolds. That is the main thing that would cause backfiring and erratic idle speeds. When air leaks in at that point the engine runs too lean and causes backfiring.

If the bike runs then Top Dead Center (TDC) is not an issue. But to check piston stroke position, Remove the valve cap cover on the intake of the front cylinder. Remove both the spark plugs. Rotate the crank shaft untill the rocker arm starts to move. The piston is now starting down on the intake stroke. Rotate the crank further and the rocker arm moves in the opposite direction. The piston is now going upward on the compression stroke. Look at the marks on the flywheel. Rotate the crank untill the TDC mark lines up with the timing index mark. The front piston is now at TDC.

If your model has carbs then the backfire fix requires you to syncronize and balance the carbs.
SYNCHRONIZE THE CARBS
Remove the air filter and rubber fittings to allow you to see into the throat of both carbs. You may need a mirror to see inside. You need to be able to see the slides go up and down when you twist the throttle. Now turn the fitting where the cable goes into each carb such that there is just a bit of slack in the cable, 1/16" is plenty. Lock ONE of the cable adjusters down tight. All further adjustments will be made on the other carb.

With the motor turned off twist the throttle very slowly while looking at the slides. Both slides need to begin lifting at the exact same moment. If the slides don't raise at the exact same time then slowly twist the throttle until the locked adjuster slide just barely starts to move. Hold the throttle still and turn the adjuster on the other carb so that the slide on that carb just barely starts to move also. Now recheck the slide movement timing. Do this process until the slides on both carbs begin to raise at the EXACT same moment. Lock down the loose adjuster and re-check the slides.

BALANCE THE CARBS
Turn the throttle screw on the LEFT carb one half turn inward. Now start the engine and get it warmed up. With the engine running, remove the RIGHT spark plug wire. Adjust the idle speed on the left carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Now turn the throttle screw on the RIGHT carb one half turn inward. Restart and rev the engine. Next, remove the LEFT spark plug wire. Adjust the idle speed on the right carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Turn the idle screw on each carb EXACTLY ONE QUARTER turn outward and restart the engine. The engine will be probably be idling very fast. Adjust both the idle screws equally from this point to get to the desired idle speed. Congratulations, you have just balanced and synchronized your carbs.

Please rate this solution. Thanks sgtswampdonk!

With the key off and the engine "running down", the idle circuit fuel is still going to into each cylinder then out the exhaust. A hot spot in the exhaust can ignite the unburned fuel creating an explosion, ( backfire ). Let the engine run down to an idle THEN turn the key off.

Please rate this answer. Thanks!

Check the fuel pump.