Excessive backfiring?

[HackSawPyro]

Ive got a 1100 midnight special. It starts up and runs.. single problem is that there is excessive (any is excessive IMHO) backfiring, out the exhaust, and back through the carbs.... What's the deal here? I took the carbs off 2 times and cleaned them thouroughly *sp?* It only does it on the left, and right/center cylinder (middle 2, right cylinder) Are these cylinder controlled by the same coil possibly.. Is there a possibility that my valve springs are junk? Any help would be appreciated.. I really wanna get out and ride this thing this weekend.. quick replies are much appreciated.

[newmaac]

What do the plugs look like? is it possible that it is simply a
"lean pop"?
Is the "backfiring" upon acceleration, deceleration, or continuous?

[mitch]

stutter

Hi, mine did the same thing and got proggressivly worse. so i did the coil rebuild that is shown here just do a search and it will come up . my bike works perfect noy hope u find the prob good luck Mitch

[Jerry]

When is it backfiring? If while backing off the throttle you may have bad exhaust manifold gaskets. I had this problem when I first bought my XJ, thought it was carbs. Found a couple loose bolts on #3 exhaust pipe, pulled it and found gasket burned away. Replaced all 4 gaskets and backfiring on decelleration was gone.

As noted in other posts, backfiring that occures at other times may have other causes. If you can narrow down the conditions it will be easier to troubleshoot. Good luck!

[GsBike]

loose breather hoze?

I know I'm new at the workings of the XS but, last week I took my carbs apart, cleaned them, put them back on and then began to syc them. Started the bike and within a minute of two I started to hear small popping noises in the exhaust and airbox...it got worse and started really backfiring. I had turned the tank backwards and put the fuel peacocks on 'prime' for the syc and it never occurred to me to block off the two breather hoses that run to the fuel vacuum assist. I plug them with my thumb and the backfiring stopped instantly.

You may want to check to see that all the breather hoses (and rubber caps) are snug (the ones that connect to the carb boots - between the carbs and the engine).

Just my limited 2 cents...

[HackSawPyro]

backfire

I know its not an exhaust leak.. (straight piped) It's backfiring when revving or coming down, out the carbs (See a blue flame shoot out the back of the carbs) and out the exhaust... I dont know what it is.. it does it on free rev (havent had the bike moving yet) Its kinda hard to do a chop test right now to check the jetting, because i cant really put a load on it yet (It needs brake work and clutch work).. Im lost.. It almost to me seems like it may be something to do with the timing? Any more replies are much appreciated.

[Jerry]

Probaly not ignition timing...the XJ uses an all-electronic system with the spark advance built into the 'black box'. Pick-up wires are not usually an issue as they are on the XS since there is no moving plate under the left-side cover. However, you could check the wires anyway to make sure they are not shorted or broken.

Have you checked the cam tension adjustment? If not, let us know and a full procedure will be e-mailed to you. Possibly a very worn chain is affecting cam timing.

[HackSawPyro]

Haven't checked it.. nor do I know how.. a detailed explanation would be of GREAT appreciation. Thanks again.

[Jerry]

I took a screen shot of the cam chain adjustment page from the manual. You can see it here:



Image is large, full-size at about 10 by 7 inches and 140K. I let it this way so you could see the details in the photo. Best if you print it out and have it with you when you do the adjustment.

Adjusting the cam chain is considered routine maintenance. I do mine about every 6K miles, every other oil change, just because it iis convenient. You shoud not need to replace the gasket under the timing cover, but it is still available from Yamaha or you can cut your own from a piece of gasket material if it is really messed up. Since you will be removing the cover often I don't recommend a gasket compound.

There are occasional problems with the adjuster, as it uses small threads that can be easily stripped if over-tightened. Look at the tightening specs...very low.

Once in a while the sliding shaft can get nicked by the stop bolt, which keeps the slider from moving correctly. The adjuster can be removed and the slider checked if need be. Usually not needed, have never done mine but a few others have. Good luck.

[HackSawPyro]

THe timing has the plate that is controlled by a vaccumm advance thing (I think thats what it is... it hooks to the plate and theres an arm that moves up and down, Im guessing by how much vaccumm the motor is pulling?) I adjusted the cam tensioner. It seemed to help a little bit, but it still backfires like mad. The motor also will only run with the choke on, as soon as you turn the choke off.. it dies like crazy. The plugs that I had in it were a cheap plug, and fouled out... I just got some NGK 8's to put in it, I will give that a try and see what happens. Also, the vaccumm lines that go from the intakes were all scattered here and there, I have all of them plugged now, except the one that goes to the vaccumm controled advance modulator (I think thats what it's called, at least i know thats what its called on a car?) I think i may have to pull the carbs off again and give them a good douchin' and check all of the diaphrams again, and then do it again most liklely. The bike sat for a long time, but, the price was right, a big fat zero bucks. (Okay, actually, i had to go drink a case of beer with the guy and his brother before he'd let me take it, but hey, im not bitchin')
Thanks again for all the replies guys, and keep em comin!

[Ken Talbot]

Yup, you're probably going to have the carbs off at least once more by the sounds of it. Before that, though, maybe go over the vacuum lines once more as what you've described still doesn't sound right. Three of the vacuum nipples on the intake manifolds should be plugged. One nipple should connect to the fuel octopus to operate the diaphrgm. Finally, the vacuum line from the advance mechanism connects to a brass vacuum nipple on the body of carb #2 - never to one of the nipples on the manifolds.

Back to you original post, cylinders #1 and #3 operate from one coil, #2 and #4 operate from the other col. A flakey pick-up coil wire will mess up 2 cylinders at a time.

Valve springs - don't get too excited about these, that would be a very long shot. There's lots of other things for you to check over first.

Are we having fun now?

[Jeff]

Since you mentioned that it won't run unless the choke is on...... that would indicate an excessively lean idle. My best guess would be that you have at least one or two pilot jets plugged. SInce the XS uses an idle enrichment circuit instead of a true "choke"..... the bike is attempting to use it as an alternative pilot.

[Shuriken]

I've discovered among all of my bikes, dirt or street, that if there are any holes or leaks anywhere in the exhaust system, especially close to the head, there will be some backfiring.

Also, when I was tinkering with my '80g carbs a long time back and trying to find a main jet size I liked, I had set the float levels too low while also having too small of main jets. The result was loud, powerful, and annoying backfiring.

When the carbs are dialed in right and there is a really good exhaust system mounted, there shouldn't be any problems.

Ben

[HackSawPyro]

Pulling my hair out!!!

Still backfiring! I eliminated the fuel octopus, got the timing advance hooked up, plugged the lines on the intakes for vaccumm (Where do theese go besides to the fuel octopus? there are 2 that are open, where does the other one go?

Its backfiring out of the carb, and theres a loud pop from the exhaust.. this thing is REALLY starting to get to me!!!

Please help!

[Shuriken]

If it is backfiring through the carb and your plugs are fouled, I have to suspect an improper fuel condition or poor ignition timing, or both at the same time. If the fuel mix is far to rich, the throttle response is mushy until high load and rpm. Rich conditions leave fouling evidence all over the spark plug. If it's lean we see superheated conditions, preignition, backfiring, etc.

If the ignition timing were too late, I would expect the spark to fire much closer to when the exhaust valve opens, blowing a lot of the useable compression and combustion cycle out the pipe before it can be used.

However, I think super rich conditions are less prone to preignition and detonation. I think backfiring can happen when the mixture is so lean that just the heat of the spark plug or exhaust pipe can set it off.

If the backfiring is occuring on only cyls 1 and 3, or on cyls 2 and 4, there is an intermittent spark issue from either of the two ignition coils. Many headaches dealing with coils can be solved by making sure all of the connections are clean and tight with no shorting in the wiring or spark plug leads. You can always use a colortune (glass spark plug) to see what the combustion cycle looks like as well as combustion color.

Inconsistent spark can produce a black/fouled plug as well as backfiring. On these old bikes, it is nearly certain you will have to go over the wiring and contacts. If the spark appears on one cycle and not on several others, then again on one cycle, the spark will have to burn the new mixture plus anything else that was leftover from the last cycle that didn't exit through the exhaust valve. This could cause your plug to go black.

However, didn't you say the backfiring occurs while revving in addition to backing off the throttle? If the bike pops while revving, I could suspect several other things:

One or more carbs have plugged fuel passageways and are not getting enough fuel to pull the bike. Are all the plugs the same color or just one or two?

Since it's a 4-carb setup, each carb has to work the same as the others. Did you say you tried to synch them? They have to be in synch.

Float levels? Are they at stock height and each carb the same as the other?

Main jets? Are they plugged? How about the needle jet with the little holes? If the bike only runs on choke, it sounds like a lean condition because of fuel delivery problems. The bike tries to idle using the pilot jets - if they are plugged it will not idle unless it's getting fuel from somewhere else or from the throttle being held open a little. If the pilot jets are plugged I'd be wondering if the other jets were plugged too.

How is compression? Do you have high compression on 2 cyls and super low compression on the others?

......

Really, it would be sweet if you could swap on another set of carbs just to see. But if I'm going to have to put money on it, I'm going to say it's in the carbs or one ignition coil. We call it Carb Hell. I've been through mine probably no less than 12 times. I haven't been inside of them for months and months now. Once you get them right they do fine. The only way to be sure is by the book; the carbs get removed, completely dissasembled, soaked in chemdip and the passages blown dry with compressed air. All the jets have to be removed and soaked. When reassembling, the tiny washers and o-rings have to be there. Everything should be set back at stock specs, this means float height and jet needle clip position.

How long has it been since the bike ran well? Was it sitting for awhile?

On a last note, my XS11 had at some point skipped a tooth on the cam chain sprocket. This happened as a result of a mangled chain tensioner. The bike was sluggish, rich, and terrible - until in 30 minutes I removed the valve cover and discovered the problem. After that it ran like a raped ape. You could always pop the cover just to look.

Thing is, it's necessary to be methodical, logical, and sequential so you don't chase your tail.

We've all been through it. That's the price we pay for an old muscle bike.

If you don't find plugged jets or a bad coil, be patient, the answer will come.

Ben

'80g rust rocket
'78 parts bike