Backfiring At Lower Temps


Information and questions on GL1200 Goldwings (1984-1987)
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BikerNewsman
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Motorcycle: 1985 GL1200 Limited - Anniversary Edition

Previously owned:
1969 HD Sporty
1974 Yamaha 650

Backfiring At Lower Temps

Postby BikerNewsman » Sat Sep 08, 2012 8:47 pm



When riding my bike at lower temps, upper 50's - lower 60's (don't know about colder than that since it hasn't been that cool yet), it will backfire only when decelerating *most* of the time as I either slow pass 30mph or cross the 2,000 rpm mark. Forbit they both happen to line up because it goes from a moderate rumble to a loud pop. If I happen to notice I'm approaching either one of these situations I can pull in the clutch and let it slow past either of these points, then continue decelerating and that usually will negate the backfire. It never does it when the temps are at least in the mid 70's, nor does it backfire when cruising or accelerating. I'm told there's an anti-backfire valve on carbed models but this is a CFI and my repair manual doesn't mention such a valve so I don't know if it was just left out of the manual or doesn't exist on this bike. If anyone has had this problem, and has a resolution for it, I'd LOVE to hear it. With fall and winter approaching I really want to solve this before it becomes an embarrassment to ride. For now I simply try to avoid riding until the day warms up. Not good since I love early morning rides.

1985 GL1200 w/ CFI
Under 57,000 miles


"It ain't about the miles, it's all about the smiles".
My (non-commercial) motorcycle blog = http://dixieriders.wordpress.com/purpose/

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Re: Backfiring At Lower Temps

Postby WingAdmin » Mon Sep 10, 2012 9:15 am

Backfiring on deceleration is normally caused by an over-lean condition, and the fact that it happens more when it is cool out (cool air is more dense, which makes the mixture even more lean) supports that.

It could be caused by a number of things - anything that can cause a lean condition, basically. If your idle jet is clogged, it can limit the fuel getting into the carb(s).

It can also be caused by a vacuum leak - but I'm going to guess that it's an idle jet. If you pull the clutch in to get past your backfire RPM, the engine RPM falls quickly, to the RPM that will support balanced combustion based on the limited fuel being introduced. If you let the bike decelerate in gear, the velocity of the bike keeps the RPM's up as you switch to the idle circuit, which means you hit a sudden over lean condition, and you get a backfire.

That's my guess, anyway. Try a high-concentration of carb cleaner in the fuel and see if it will clean out your jets enough to fix the problem.

You can also try getting the engine warm, and idling, and spray some starting fluid around the bottom of the carbs, carb runners, etc. to check for a vacuum leak. If the engine suddenly speeds up when you spray near something, you've found a vacuum leak.

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BikerNewsman
Posts: 77
Joined: Sat Jan 21, 2012 11:22 pm
Location: Hondo, TX
Motorcycle: 1985 GL1200 Limited - Anniversary Edition

Previously owned:
1969 HD Sporty
1974 Yamaha 650

Re: Backfiring At Lower Temps

Postby BikerNewsman » Mon Sep 10, 2012 8:30 pm

Thanks. :) I'll try the higher concentration of cleaner in the fuel when I refuel tomorrow, and I'll spray some starting fluid around the lines and listen to the idle speed, but... You mentioned carbs... mine has CFI, so is there anything specific to the fuel injection that could also be a contributing factor??? Also, I read that too wide a gap in the plugs could cause backfire so I narrowed the gaps from mid-range to just above the minimum, but this didn't change the characteristics of the backfiring.
"It ain't about the miles, it's all about the smiles".
My (non-commercial) motorcycle blog = http://dixieriders.wordpress.com/purpose/

User avatar
WingAdmin
Site Admin
Posts: 17050
Joined: Fri Oct 03, 2008 4:16 pm
Location: Strongsville, OH
Motorcycle: 2000 GL1500 SE
1982 GL1100A Aspencade (sold)
1989 PC800 (wife's!)
1998 XV250 Virago (sold)
2007 Aspen Sentry Trailer

Re: Backfiring At Lower Temps

Postby WingAdmin » Tue Sep 11, 2012 10:00 am

Ah right, didn't look to see your bike was an EFI. Same thing applies - if the injectors are clogged up, they can cause a lean condition, and vacuum leaks can also cause a lean condition, so the same diagnostics apply.

User avatar
BikerNewsman
Posts: 77
Joined: Sat Jan 21, 2012 11:22 pm
Location: Hondo, TX
Motorcycle: 1985 GL1200 Limited - Anniversary Edition

Previously owned:
1969 HD Sporty
1974 Yamaha 650

Re: Backfiring At Lower Temps

Postby BikerNewsman » Tue Sep 11, 2012 9:15 pm

Thanks again. :) Will definitely check those out. Added the "souped up" cleaner today.
"It ain't about the miles, it's all about the smiles".
My (non-commercial) motorcycle blog = http://dixieriders.wordpress.com/purpose/

User avatar
BikerNewsman
Posts: 77
Joined: Sat Jan 21, 2012 11:22 pm
Location: Hondo, TX
Motorcycle: 1985 GL1200 Limited - Anniversary Edition

Previously owned:
1969 HD Sporty
1974 Yamaha 650

Re: Backfiring At Lower Temps

Postby BikerNewsman » Wed Sep 26, 2012 10:30 am

Well, here's an update. Doubled the amount of injector cleaner in the fuel for 2 tank-fulls, then added Sea Foam for the 3rd. Sprayed the starter fluid around the injectors and vacuum lines. Since I replaced virtually all the vacuum lines while "restoring" the bike after the PO let it set for over 2 years I didn't expect to find a problem there, and didn't. The backfiring problem is better, but still there. I noticed what sounds like an exhaust leak near where the pipes bolt to the engine when riding at higher speeds (55+). Checked the bolts and found a couple just slightly loose, so those were tightned. Still getting the leak sound, and backfiring, so I bought a set of gaskets for that connection. Will put those on in the next couple of days and hope that resolves the problem. While that's apart I'm going to go ahead and pull both pipes to check, visually, for any other minor exhaust leaks also. I heard there's an anti-afterburn valve that could be causing the problem, but neither of the 2 local Honda dealers can seem to find it on their fische, although my Clymer shows a pic of it. Anybody know a source, or a part number, in case the gaskets don't solve the problem???


"It ain't about the miles, it's all about the smiles".
My (non-commercial) motorcycle blog = http://dixieriders.wordpress.com/purpose/


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