backfiring/revving up to 5k rpm while idling

[pbaker27]

Sorry if this is the wrong place to post this, but need some help. So my bike is an 82 xj1100j, the engine is from a 79 XS. Its a bit of a franken bike, it wouldn't hardly run at all even with the choke out till I got an airbox(since the original one was missing), got it installed today and now it runs nice without the choke out. Except for two things, and wondering if anyone has any tips/advice on it. It does have the ignition/electrical system from the 82.

1. it is backfiring, which my buddy believes is due to the timing being off.

2. when it is idling it will rev up to 5k rpm until we shut it off.

Any thoughts/ideas/suggestions? I do have the Climers repair/maintenance manual for 79-81 XS models.

These are the last two real things i need to get figured out before I feel safe enough to ride it/take it to get new tires on it.

Thanks!

[James England]

When does it backfire? When you rev and let go the throttle?

Are you sure the throttle cable is routed correctly? You new airbox may be pushing on the cable....

[crazy steve]

If the motor has been swapped, which ignition system does it have? Was the XJ ignition transferred to the XS motor? If the timing cover is only about 1.5" 'tall', it was. If not, you may have too much advance. The high idle could very likely be a carb sysnc problem if the timing is good, lots of things to check. Frankenbikes can be a bit of a chore to sort out sometimes.

And you really need an XJ manual, there's significant differences that won't be covered in the XS manuals. You can download a copy by finding any post by member CatatonicBug and going to his web page as linked in his signature...

[trbig]

If it wouldn't run before, even with the choke out, then does with the airbox on, along with popping and revving at idle, it indicates an extremely lean condition. My guess is that the carbs need to be either cleaned really well or the idle mixture screws adjusted. If the carbs are from an XJ, are the mixture screws at the top front of the carbs still capped off? If so, the carbs are probably still setup good enough to run, but the carbs will need a good cleaning and a synch. At least with a '79 motor, you won't have to worry about a YICS system to plug off.

When I ran my XJ with early model cams, it would run OK, but I didn't realize that I'd lost all the bottom end power until I swapped back with the later model cams. You'd probably be a lot better performance -wise with the electrical setup to put in some later cams. That'd be easier than swapping out the whole timing system/TCI. Of course, first things first.. Get it running decently.

[motoman]

The member needs to answer Steve's ignition question FIRST, otherwise the rest is irrelavant

[trbig]

The member needs to answer Steve's ignition question FIRST, otherwise the rest is irrelavant

It does have the ignition/electrical system from the 82.

Did either of you read his post?

[motoman]

Did either of you read his post?

Thanks for pointing that out Tod. Even at that, still wondering what is meant by having the ignition/electrical from 82.........just some or all, like the XJ TCI, vacuum sensor and whatever else is needed to make that system play happily together. A little more detail bout the swap sure would'nt hurt for diagnosis sake.

[James England]

The problem with any Frankenbike is that one needs a detailed description of exactly what has been swapped/retained etc before being able to diagnose anything accurately. At this stage it could be anything.

The 5k rpm 'idling' indicates something is amiss with the throttle slide/s. I'd check that throttle cable routing, throttle stop screw etc and eliminate that first if possible.

[trbig]

So you folks tell me... what happens when you're idling a motor and it starts running out of gas, causing a very lean condition? Sputtering and high revving. What causes the majority of popping on decel? A lean condition. His bike wouldn't start with open carb bells, which makes a leaner mix anyway, and by the sounds of it, too lean to even run. Adding the airbox got it to where it richened it up enough to run.

got it installed today and now it runs nice without the choke out

The only difference between running and not running was install of the air box, which the only thing that would have done would be to have richened the mixture slightly. If that's all it took to get it to run, it means it was way too lean to begin with, and still too lean to where it revs and pops.

Or, we can head off chasing electrical gremlins, slides, and cables?

[pbaker27]

Electrical/ignition

Sorry all, it does have the 10m TCI from the 82XJ1100. I also believe its the original wiring harness from the xj. Beyond that, I apologize, but do not know enough to determine that. A friend of mine has been helping me get it running as he has worked on motorcycles and cars for 20 years. Me, I know cars decently, but not as much as I would like about motorcycles, so learning as I go. It will be a week before we both have time to work on it again, but I will point him to this thread, and have him put together answers to the questions.

[crazy steve]

Sounds like the first thing you need to do is verify just which parts are on the bike. Has it still got the original XJ carbs, or were the 79 units transferred also? Did you do the swap, or buy the bike like this? Was it running when you got it? If you actually have the XJ ignition fitted, there's only one way to install that so the timing has to be right.

Tod's right, it's likely a carb issue but we can't pin it down until we have a bit more info...

[James England]

slides, and cables?

I'd still check them. You may just have done something to the throttle cable when installing the airbox. Checking will rule it out... or in...

[pbaker27]

The engine swap was done by the PO. Here is my friend's explanation:

The engine assembly as a whole is from the xs. Whoever did the swap took the entire xs motor pulled it from the xs frame and dropped it into the xj frame.

So what we are working with is the xs timing internals and the xj externals. Basically if it was inside the engine it's stock xs and carbs, and if it was outside the engine it's xj.

The throttle works good and good throttle response. It was back firing at idle and climbed to 5k rpm. We can try richening up the idle circuit.

[trbig]

So.... XS pickup coils and vacuum advance on an XJ with an electronic boost sensor?? How would THAT work?

Have the carbs been thoroughly cleaned recently? You might not need any adjustment... just the jets and orifices all cleaned. It would do no good to try to adjust gunked up carbs.

[crazy steve]

So.... XS pickup coils and vacuum advance on an XJ with an electronic boost sensor?? How would THAT work?

It might work.... only for how long before the too-high pickup coil resistance damages the TCI remains to be seen. You need to be positive as to which ignition you have on the motor; XS or XJ. Actually, the same thing probably goes for the alternator as that's different too. You can see the XJ ignition/alternator covers here: http://www.xs11.com/forum/showthread.php?t=37241. If yours don't look like the ones shown in that thread (your ignition cover is deeper and the alternator cover is flatter), you have the XS parts fitted. Also verify which TCI is fitted; do you have a 2H7, 4R0, or 10M as identified by the first three digits of the part number on the box. Each TCI is distinctly different and running a 'mixed' system will cause timing issues...

You might also want to check the carb jetting; some owners have made the mistake of thinking carb jetting follows the year of the bike, but it follows the carbs. So if the early carbs were jetted to the XJ specs, it will be way too lean, which has been noted as one of the possible problems.