Backfire under load....rich or lean?


Information and questions on GL1200 Goldwings (1984-1987)
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zamboni920
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Backfire under load....rich or lean?

Postby zamboni920 » Sat Jul 21, 2012 3:33 pm



Just finished carbs on my 1984 GL1200I . See: viewtopic.php?f=5&t=12466
Everything very clean, polished and moving parts moves nicely. Replaced jets one size up. Slow jet from #35 to 38 and main jet from 108 to 110. All vacuum hoses good and in place. Synced carbs on bench by feeler gauge....seemed good and double checked....triple checked. All four butterfly valves starts moving in complete sync.
Now, on my test drive, it has definite backfire under load (accelerating) through exhaust. Not sure if this is lean or rich. tried applying choke while acc. and it seemed to increase backfireing. Makes me think it is rich. Idles fine...maybe a slight miss. Choke when cold increases RPM's , applying choke when warm, it decreases RPM and kills engine. Played with idle mixture screws. Currently set at 1 1/2 turn out, wich is where it seem to idle best.
I would say the backfire is most prominent at 1500 to 2500 RPM.



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virgilmobile
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Re: Backfire under load....rich or lean?

Postby virgilmobile » Sat Jul 21, 2012 4:49 pm

Sounds like you already diagnosed the problem.Its running a bit rich in that range.You may have to return to the original main jet.This is assuming that you left the slide needle in the original position.
I only re-jet if there is a problem with the mixture.
Lets hope that this didn't happen before the repairs and the new jet is the cause.(it didn't do that before I worked on it)

Obviously when a carb is jetted a larger size,the idea is to increase the airflow to match the new fuel ratio.Kinda tough on the CV carb without other mods.

That's why there called constant velocity carbs with precise main jet metering (the slide needle)

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zamboni920
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Re: Backfire under load....rich or lean?

Postby zamboni920 » Sat Jul 21, 2012 7:12 pm

I think you are absolutely correct Virgil.....it most likely are the larger jets. Oh well....fun experiment :) .
Wondering if, instead of removing carbs again, if I could lift the slide(O-ring....you know...) to increase airflow. This should be possible with carbs installed, yes?
If I have to replace the main jets with original....I think I should be fine with leaving the larger slow jets. The range of the slow jets ( 1/8 to 1/4 throttle ) seems fine.

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virgilmobile
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Re: Backfire under load....rich or lean?

Postby virgilmobile » Sat Jul 21, 2012 10:34 pm

Well,if your just experimenting,go ahead and raise the slides about 2-3mm.You should be able to find a "O" ring in your kit.
Personally I don't think it will help in that rpm range....
I'm gonna guess here,but when pulling the throttle and it hits 2500 rpm or so,the airflow is already lifting the slides a bit,so manually lifting them to that level would have no affect on that range.
It's already getting too much gas.Lifting the slides more just opens the main jet port further.

If it were possible,other than changing the main jet,the only other option might be to lower the slide needle.I'd guess 3mm or so to squeeze off some of the fuel that can get through the carb.
Remember,it's a constant velocity carb.the slide reacts to air flow and meters the fuel by lifting a tapered needle in the main jet port.More air flow,more fuel.
Lifting the slides should affect near the 1000-1500 rpm range,where the slides are the most inactive.

Now be aware.The low speed jet also runs all the time.From idle to red line.So increasing it's size also increased some of the fuel from idle through the range you are having problems with.
There is a overlap thing going on here.The fuel delivery starts mainly with the idle mixture screws,as the throttle is opened up,the slow jet provides fuel there and through tiny holes at the throttle plate.More throttle brings the mains into play till all of the are dumping fuel at wide open.

Just changing the main will help,but it may take both to cure it.Time to experiment.
Your gonna get good at R&R the carbs.

One last question.Did you check or readjust the floats?
If there set wrong,even a couple of mm. off can cause this problem...Too much available fuel.
I had to drop my floats in my 1500 carb by 2.5mm to clear up a rich condition at 1800 rpm.

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zamboni920
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Re: Backfire under load....rich or lean?

Postby zamboni920 » Sun Jul 22, 2012 12:11 pm

Update....
Took it out for a short ride today, mostly to verify the symptoms.....double check if you will. It turns out the backfire is not exhaust(wich it sounded like initially, quite audible) but rather popping through carbs. It is severe, with lack of a better description. Idle is fine.
Running at low RPM's and then throttle it up, gradually or quickly, it pops a lot and consistently. High, steady RPM's are fine. It is only during heavy load....accelerating from about 1500 RPM's.
Now I am not sure if it is fuel or ignition related.
I pulled plugwires, one at a time, while warm and running. All made RPM's stumble, except cyl # 1....the change was there, but miniscule. Checked spark on all wires and there were spark. It seemed weak across the board, but I think that is because it is daylight. #1 spark seemed exactly the same as the others. I did remove plug cap on #1 and cut about 1/4 inch off the wire. No change. Pulse generators were replaced about 3 years ago, during a major engine overhaul by certified Honda mechanic. At this time, also the ignition module was replaced. Makes me think this issue was present before the overhaul.
What do I check next?

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virgilmobile
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Re: Backfire under load....rich or lean?

Postby virgilmobile » Sun Jul 22, 2012 1:16 pm

We both know a backfire in the carbs can be a lean condition.
One cylinder not doing well?hmmm.have you done a compression test?No more than 5psi difference.Get that out of the way.one lean carb...stitck a angled mirror in the plenum and rap the throttle.all the slides should pop up.
Mine was lean but a little choke cleared it up.it was on all 4 cylinders.3 may be working ok and just one stuck down, so added choke enrichens the other 3.
I'm still thinking here.

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zamboni920
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Re: Backfire under load....rich or lean?

Postby zamboni920 » Sun Jul 22, 2012 6:44 pm

Compression test is sooooo much easier than another carb teardown :D .
Result..... Cyl # 2,3 and 4 read about 115 ....carb #1 (the one in question) read around 80 . Drytest. A squirt of oil in #1 and retest.....got it up around 95. I've got a problem. I was thinking ...remove crash bar, valve cover and head. Clean valves, lap or re grind . Hmmmmm, maybe there is an easier way to try first. Seafoam.....that is known for dissolving crud build up. Plug hole has direct access to the problem area.....and intake horn has direct access to valvestems and backside of valves.
Poured about half ounce into cylinder, tapped starter to distribute the Seafoam....another half ounce into intake, by way of vacuum sync screw opening, and tapped the starter again. I know seafoam is used in both oil and fuel, so I was not worried about it as a chemical. And with plugs out, I was not worried about hydrolock. Let it sit for about half hour or so. Cranked the engine about 30 seconds before plugs went back in, to expell leftovers. Plugs in....start up....lots of smoke (wich I expected).....but it runs sooooooo smoooooth .
Cleared up everything. No more miss, intermittent cut-out....and absolutely no popping or backfire. Pulls strong from low range to wide open. Beautiful :D :lol:

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virgilmobile
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Re: Backfire under load....rich or lean?

Postby virgilmobile » Sun Jul 22, 2012 7:36 pm

I'll bet if you open the throttle when you crank it you'd get a higher compression reading.
It's a mute point.you found the problem and solved it.
You get the gold star today.good job.

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zamboni920
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Re: Backfire under load....rich or lean?

Postby zamboni920 » Sun Jul 22, 2012 7:50 pm

I like Gold Stars :D .
Will check compression again, after a few miles hard driving to blow out any crud and carbon.
The larger jets stays....at least for now.




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