Backfires on starting after warm


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Mooseman
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Backfires on starting after warm

Postby Mooseman » Sat Sep 24, 2011 5:18 pm



My 86-A, calif. model starts with a BANG when warm. This doesn’t happen all the time just enough to embarrass you when you least expect it. Happens when the bike is warm from being ridden and parked for 15 to 30 minutes. Touch the starter and and it usually starts right up. Sometimes I have to hit the starter 2 or 3 times and if I give just a little twist of the throttle BAM! A backfire that makes everyone jump. Any ideas?
Thanks


Enjoy the ride. They are all good, just some better than others.
Mooseman

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virgilmobile
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84 GL 1200 I

Re: Backfires on starting after warm

Postby virgilmobile » Sat Sep 24, 2011 7:12 pm

The backfire in the exhaust is interesting.For that to happen,fuel and air would have to accumulate in the exhaust and then be ignited.

My first thought is what I did to simulate your problem..Here's what happened.

I turned on the fuel,turned off the ignition(kill switch)and cranked the engine with 1/4 throttle for 5-10 seconds and then switched on the kill switch while still cranking.It ignited and popped both pipes pretty loud.

So by what you describe,I suspect that your ignition(spark)sometimes doesn't spark on the first crank.

The crank angle sensor or ignition pulse coil may be "thermal". The coils can measure ok when cold and then go open when it reaches a certain temperature.

Obviously it will be hard to find what is failing because it is so intermittent.
Other than randomly replacing parts,The only thing I can suggest is to locate the test points and what the readings should be and then quickly test them at the moment it quits.I'm referencing the pulse coil plug and meter readings.

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Mooseman
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Motorcycle: 1996 GL1500SE 88k
1986 GL1200A (SOLD) 81K
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1986 BMW K75c 25K

Re: Backfires on starting after warm

Postby Mooseman » Sat Sep 24, 2011 10:56 pm

Have kind of had the same thoughts. But as soon as it starts it runs like a champ. If I hear the backfire while cranking I know it's started and runs just like it should.
When I get the time I will check the plug wires and coils for output.
Thanks for the imput.
Enjoy the ride. They are all good, just some better than others.
Mooseman

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virgilmobile
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78 GL1000
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82 GL1100 I
83 GL1100 I
83 GL1100 standard
84 GL 1200 I

Re: Backfires on starting after warm

Postby virgilmobile » Wed Sep 28, 2011 3:35 pm

I'm gonna see if I can attach a LED with a dropping resistor across the pulse coil to see if it will light and not affect the ignition.

If this works,then it is a cheep,easy part that could be added to your bike to help diagnose whats failed.
When it won't start,look at the LED's to see if they are sending a pulse.

Because your problem lasts just a short time,by the time you hook any test equipment to it,it probably will start right up.

I'll check it out tonight.I've got the time and parts.

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Mooseman
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1986 GL1200A (SOLD) 81K
1985 GL1200A (SOLD) 85K
1986 BMW K75c 25K

Re: Backfires on starting after warm

Postby Mooseman » Wed Sep 28, 2011 5:23 pm

Sounds intresting. This only happens after the bike has been parked for 20 minutes or so. Doesn't matter the outside temp. Just kind of wierd.
Enjoy the ride. They are all good, just some better than others.
Mooseman

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SilverDave
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Re: Backfires on starting after warm

Postby SilverDave » Thu Sep 29, 2011 10:42 am

I had that for several weeks... and it kept getting worse... louder, more often, embarrasing, and harder and harder to start ...

Its clearly fuel, and air in the pipes ...
So its related to the spark !!

Eventually, I pulled the relative new ( 1 1/2 years ) spark plugs out ... yucky ... and replaced them with brand new gapped for the Electrical Connection 1200 harness ( 40 thou) and presto ... running perfectly again..

My mechanic friend diagnosed it... " Even though the plugs are newish, once they start fouling up... they just get worse and worse, building up a heavier and heavier coating from the poor starting "

Plugs are cheap... I just hucked them away . Its worth a try .

SilverDave

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virgilmobile
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82 GL1100 I
83 GL1100 I
83 GL1100 standard
84 GL 1200 I

Re: Backfires on starting after warm

Postby virgilmobile » Thu Sep 29, 2011 11:20 am

Plugs yucky? I guess I'd be looking into why.
A good engine properly running will run plugs sandy brown for at least 20 k miles.I do mean at least.My 1000 ran 85 k miles with just re-gapping.

Mooseman's bike does not fall under your symptoms.His will intermittently not start right away.Most of the time it fires right up. Thats why were leaning to a "no spark when warm" scenario.

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Mooseman
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1986 BMW K75c 25K

Re: Backfires on starting after warm

Postby Mooseman » Thu Sep 29, 2011 1:48 pm

Thanks for the plugs information. My plugs have always looked great. Still the problem occurs even after plug changes. I am kind of leaning towards a week starting spark sometimes. Problem was really bad awhile back till I replaced a weak stator. The battery would get a little low and the starter would take most of the starting juice not leaving enough for the ignition. But the charging system is working like a champ so that's not it. The starter doesn't drag or appear to be anyway. The quest will continue as I have time. But right now it runs fine and starts fine except for the occasional BAM! Will keep you posted if I find the culprit. I also wonder if the Calif. emission has anything to do with it? But come to think of it, I pulled the plugs a month or two ago and regapped them to the small side. This seemed to cut down how often it, BAMMED! So maybe I have answered my own original question. Weak spark during starting.


Enjoy the ride. They are all good, just some better than others.
Mooseman


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