Backfiring

[BikerNewsman]

Took my bike out for it's first ride yesterday evening after replacing the regulator-rectifier and correcting wiring problems. Checked the stator while doing wiring repairs. Output good, no grounding of the 3 yellow wires, and proper continuity between them. She ran great with absolutely no problems. Voltage stayed between 13.5v and 14.5v depending upon speed. Idled down at stops w/ voltage +/- 12.5v.

This morning she started fine, revved up to put about a 14.7v charge on the battery, then idled down. Ran great until I got around 1.5 miles from home, then started backfiring between 30 and 40% of the time when I'd let off the throttle in gears 2 - 4. Never heard it backfire in 1st or OD... well, it may have once in OD, but it was so quiet I wasn't sure if that's what I heard. Acceleration was great every time. So... I'm perplexed.

Other than a few test rides this is my first time back on the road in 15 years so it's possible when entering the curves I'm in the wrong gear, or allowing it to slow too much when approaching the crest of a hill. Any thoughts on this or suggestions regarding what I should check????

Oh, if somehow it matters, the temp yesterday was in the upper 70's/lower 80's, this morning around 60 degrees.

The bike - 1985 GL1200 LTD with CFI (no carbs) Mileage under 57,000.

[Roleketu]

When mine starting backfiring the cause was a bad head gasket. Coolant leaking into a cylinder stopped it from firing then when it did fire all the unburnt fuel lit and KABANG backfire. Compression testing and removing plugs and examining them should confirm.

[BikerNewsman]

Thanks Roleketu. Will check that out the compression this weekend. My son-in-law (a car mechanic) has one of those compression gizmos. :mrgreen: Plugs are new, and the ones I removed were in all but perfect shape, especially for having been in there for several years (by the PO). I'll still check them to be sure a problem hasn't developed.

Here's an update - Left my bike on the battery tender all day today. Rode it just before sundown, same route as yesterday, to see exactly when it was backfiring (speed, gear, rpm, etc.). She ran beautifully. Even tried to lug it a couple of times. Came back up without any hesitation. Thinking back, I'm sure the tender wasn't attached the night before the "problem ride". Could battery charge or condition have anything to do with the backfiring or am I looking at one of those weird flukes?

[Roleketu]

Mine didn't backfire all the time, it seemed random. Not knowing it what the cause was, I made a trip from Massachusetts to Niagara Falls, Ont to Lake George (for Americade) rode for several days at LG, then back to Massachusetts. Before I left LG, when it backfired, stalled, then restarted, it smoked white for a minute (coolant) then ran fine. Got me home and within a couple days had new head gaskets installed.

[BikerNewsman]

Hmmm, mine seems to be somewhat random also in that it doesn't do it every ride. Seems to want to "act up" only during morning rides when the temps are lower. Late afternoon, early evening, not a problem. Very curious. I noticed this morning (Friday) it only backfired, actually more of a single pop, when I was in either 3rd or 4th gear between 28 and 34 mph/around 2500/2900 rpms and letting off of the throttle. It's not smoking so far as I can tell. Glanced in the mirror each time it would pop and never saw smoke.

Oh, I did have the battery checked. Had a dead cell. Replaced the battery. It's still doing the above.

[bs175dths]

I am not sure if CFI have the anti-backfire devices mounted under the intake assembly like the carburetor versions do, but if they do, the device may be working intermittently.
Another source of back-firing is an air leak at the intake o rings.
As odd as it may sound, a small exhaust leak caused more back firing on my bike than the larger hole which developed later in another spot.
These GL1200 are quirky little stinkers!!

Good Luck and let keep us posted.

-Ride On

[BikerNewsman]

Small exhaust leak huh? Hmm, mine has 2 small exhaust leaks, one on each pipe around the leading edge of the rear tire. I've checked everything else suggested so far, except for an anti-backfire device (don't see it in my repair manual), so I'll resolve the exhaust leak problem tomorrow or the following day and see if that makes any difference.

I asked the PO about it. Was told it idled down too slow in extreme cold but never backfired. When I first got the bike I found the idle adjustment was set too low in the first place (bike had been normally ridden by them at a much lower altitude) and reset it. Figure that will probably resolve that problem. I'm certainly no expert, but I don't see how the 2 items could be related. Right?

[bs175dths]

There should be two round devices on the underside of your carburetor setup. (three in California) One is attatched to the underside of the rrd valve chamber and it is called the anti-afterburn valve. The other is called the slow air cutoff valve. If you have three, then you have a California setup and the third round device is called the purge control valve.
The anti-afterburn valve and the slow air cutoff valve both use a diaphragm to function and sometimes they stop working properly.

I suspected my air cutoff valve when mine would backfire while getting off the throttle on freeway interchange ramps. Boy was she loud and embarrassing. When I was replacing the header to muffler gasket, I found a leak at the muffler cross-over area. Once welded up, the back firing ended.

Good luck.

[BikerNewsman]

Well... with CFI there's no carb, so I doubt this setup is on my bike, but I'll check around the injectors to see if there's anything similar there. The two exhaust leaks turned out to be threaded holes at the bottom of the pipes so I temporarily plugged the holes with very short screws and washers to see what difference, if any, was made. I also reduced the gap in the spark plugs. They had been set at the mid-point of the specs range, so I took them down to just above the minimum gap. Didn't have a chance to ride her again until an experienced rider safety class yesterday (the 25th) so I was hoping at least one of these two things solved the problem. She didn't backfire at all during the day-long class which put me through all gear, speed, and rpm combinations that had caused the problem before, but the downshifting was not prolonged like on a steep hill where the backfiring first started. I'm going to duplicate the "scenarios" where the problem occured the next chance I get to see if this is truly corrected.

[unionjack]

So long as the exhaust is not leaking at the headers, then backfiring upon deceleration is usually the anti-back-flow valve on the bottom of the carb. set up.

A cheap and simple part to obtain, and a real PITA to get to, you will need to pull the carbs. to do the job.

[ghostrider52005]

On my LTD which like yours has no carbs :d An exhaust leak on the crossover, or at the headers will cause a backfire. Not like your being shot at but a backfire.