backfire on deceleration

[rman]

Hello all,
I can use a little assistance. Recently my bike XS1100F(Pig) has started to backfire during deceleration. I knew that the carbs needed to be synced so thought nothing of it. After syncing the carbs I still have the same problem. I changed the plugs, no help. I checked the wires down by the timing advance but did not find any indication of a problem.

If I run the bike at a low constant speed it begins to backfire at low RPMs (under 3500) . When the clutch is pulled the backfiring stops. In neutral I can rev the engine up and down nicely with no backfire.

Otherwise the bike runs nice, lots of high end stuff. I think it could pull out a little better from a dead stop but it is much improved from what it was like before the sync. Wants to rip my arms from their sockets if I WOT after first gear.

So I am a little at a loss on how to proceed.

Please advice what might cause this issue,
Thanks
Ray

[bikerphil]

Might want to open up the idle mixture screws a bit (CCW) to richen up the low circuit, 1/4 turn each to start.

[MNXJ1100]

Popping in the exhaust is a lean deceleration issue. You can raise the idle speed alittle so the throttle blades don't close so tightly. Also richen up your mixture screws a bit. That will help but may not eliminate it completely.

MN

[DiverRay]

Also, check for an exhaust leak. Bad gaskets at the head, or on the inside to outside pipe connection(like my MNS had) can cause this.

[subike]

Driverray has the reason, leaking exhaust. I have had this problem on both of my rides, both times it has been cured by the fitting of new exhaust flange gaskets.
Although I must say that at night when my custon use to backfire, the flame was impressive!
The other place to check for leacks on a multi 4-1 system is at the collector. over time the slip joints can start to leak causing a back firing problem. Dissassembly, cleaning and reassembly with a touch of exhaust cement usually cures the problem.

[b.walker5]

Same here. I've just cured that exact problem on mine with new flange gaskets.

[rman]

Hi all
Thanks for the replies so far. I had tightened all of the exhaust flang nuts prior to a short ride last night and also changed plugs. The bike had always run very rich and plugs would look almost inky black, however this morning before leaving for work I pulled two plugs and the looked a nice cream color.

So even though I am not sold on the exhaust idea I will double check for leaks esp at the collecter. I think running the bike in a darkened room is a great idea to find leaks if I can get it to backfire in neutral.

As far as the lean condition the air box is drilled with three 1/2" holes. If I close up those holes will that richen the circuit?

Thanks
Ray

[natemoen]

Yes covering the holes would make it richer

[rman]

Covered the holes in the air box, and checked for exhaust leaks. Well not much difference.

Lets see, I have 137.5 mains, 45 pilot jets. floats at 24mm, think raising the floats to 25 would help?

[natemoen]

Raising the floats to 25 would lean it out more. What do your plugs look like?

[XS1100_OEM4ME]

Valve

Covered the holes in the air box, and checked for exhaust leaks. Well not much difference.

Lets see, I have 137.5 mains, 45 pilot jets. floats at 24mm, think raising the floats to 25 would help?

When was your last valve adj? if you have an exaust valve that is not closing all the way it will cause ing in the exaust that is a backfire, just a thought

[rman]

Hi,
I had checked and reshimmed the valves last year. They should still be in barring a major malfuntion.

The plugs were just replaced. When I checked them this AM before work the look clean and are slightly off white or what I would call a cream color. I guess that would still point to a lean condition.

Was wondering if during a ride when the bike starts its backfireing if I should pull the choke out? It should richen the mixture and might then let me know for sure that it is an overly lean problem.

The idle adjustment screws do they allow more air into the circuit or more fule into the circuit when turned out counter clockwise?

Thanks for the help.

Ray

[TADracer]

My two cents, try a dead cylinder test.

The idle adjustment screw (the large one between carbs 2 & 3) lets more or less air by moving the butterflies while the idle mixture screws on top-front of the carbs adjust the fuel load at idle.

Popping on decel is normally a lean symptom of the pilot circuit, especially on these carbs that do not have a compensator valve. You are already 1 step richer running 45 pilots with only a drilled air box. You are probably OK on the main jet at 137.5 if all you have done is drill the air box. Besides, if you have not changed anything else and the machine just started doing this on its own, then I tend to think your problem may be elsewhere.

I would rule out the idle mixture which has alot to do with popping on deceleration. Idle mixture is controlled by only one of the four holes in the pilot circuit adjusted by the idle mixture screw. Try a dead cylinder adjustment. Make sure your carbs are synced then get the bike warmed up and idling at 1,000 +/- RPM. Then one at a time, screw in the idle mixture screw until it is LIGHTLY seated. The cylinder should go dead but the other three will keep the motor running (but very roughly). If the cylinder does not go dead then that carb may be your culprit. You are getting fuel from somewhere else (probably one of the other pilot holes just behind the butterfly). Means you are out of adjustment or a passage is blocked and mixture is being compensated by one of the other pilot holes which indicates the idle speed screw holding the butterflies open too much.

If the cylinder goes dead as it should then just back the mixture screw out 2 exact turns. You should then have a reasonable idle mixture. After all are done, one at a time open the screws 1/4 turn at a time and allow about 10 seconds for the motor to level out between each adjustment. Do this until the motor stumbles due to the rich mixture on that carb and then turn the screws in 1/2 turn and the motor should smooth out. This should put you on the rich side of the adjustment window. If any cylinder does not react to the adjustment, that is where I would zero in otherwise, I tend to think the problem might be elsewhere.

Plug color is not a good indicator of pilot circuit adjustment. The only way to check plug color to read the pilot circuit by itself is to start with a clean set of plugs and ride 20 or so miles without going over 1/4 throttle. Hard to do.

[dpotter58]

Do you feel it lag when you twist the throttle hard? I fought that backfire problem for over a year and found it was pick up wires, Whne I'd twist it hard the wire would pull apart then on decell it would lite the whole exhaust pipe full of fuel vapors. Try the pull test on the wires and see if they are good.

[rman]

Thanks all for the advice.

So far I have only about a half hour to 45 min each day to work on this. today I again checked the exhaust, I did find cyl 2 pipe was lose at the collector. I thought that I had found my problem, after tightening it up I took a ride and still had the backfiring. I even have the problem during low cruising tach between 3500 and 4000 in 1st gear. I pulled the enricher and things briefly got better but soon returned to the backfiring. It may be possible that the pipe is loose again I will check that tomorrow.

Prior to starting this thread I did a sync and adjusted the idle screws but If there was an exhaust leak then I guess that I will need to re do the adjustments.

I will check the idle screw tips tomorrow JIK, I also noticed when I parked the bike as I turned off the fuel shut offs that the fuel in the tube seemed low? I will also be checking the fuel routing and verify fuel flow. I wish I could just take the day off from work and just concentrate on fixing this problem.

Again thanks to all here and at the other site for all of the helpful idea and advice, keep it coming and let me know if you think I missed something. I am double posting both sites to maximise the advise and expertise.
Thanks
Ray