Gunshot afterburn during high-speed decel - Steve Saunders Goldwing Forums

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post #1 of 13 (permalink) Old 03-26-2019, 08:45 AM Thread Starter
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Gunshot afterburn during high-speed decel

This GL1100 that I bought the last day of Feb. has a stainless steel aftermarket exhaust system that is causing me grief. It is slightly louder than stock, but not too bad, but it makes a single very loud bang when decelerating with shut throttle from speeds over 60. The air cutoff valves are in good condition and there is very little afterburn at speeds under 60, but I think the air cutoff system cannot handle these pipes. I've tried very rich idle, very lean idle, fast idle, slow idle, retarded timing, putting 6" extensions on the pipe ends, squishing the ends of the extensions, nothing helps. Would it help to put baffles in the back 8" pipes of the mufflers? I noticed they are wide open, but I can see a baffle further forward. I tried killing ignition while in the decel, but when I turn it back on I still get the bang. I'm going to try this more, maybe with some throttle first there won't be a bang and maybe I could rig up a vacuum-actuated switch for this, but it seems so unusual that even I am reluctant. Any advice?

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post #2 of 13 (permalink) Old 03-26-2019, 10:43 AM
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Is this coming out of 1 pipe or both? Have you checked for intake & exhaust leaks up at the engine?

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post #3 of 13 (permalink) Old 03-26-2019, 12:41 PM
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I tried killing ignition while in the decel, but when I turn it back on I still get the bang. ?
I used to do that on purpose to make a big backfire. Maybe one or more of the carbs is dumping more fuel than it should, high float levels.
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post #4 of 13 (permalink) Old 03-26-2019, 07:42 PM
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Could be an air leak at the header/muffler joint, since the mufflers have been replaced.


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post #5 of 13 (permalink) Old 03-26-2019, 08:25 PM
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Are you sure the backfire is afterburn? And not in the intake?

If it were the exhaust one would think that with everything you've tried you would have hit on something that had an effect.

If it is afterburn I'd be suspicious of something in the ignition system cutting out with sharp fall of RPM. Like yer briefly losing spark until what ever is stumbling catches up again.

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post #6 of 13 (permalink) Old 03-26-2019, 10:41 PM
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Next time out when you going over 60, pull the choke out about half and see if it cures it or not. If it does, you’re too lean.
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post #7 of 13 (permalink) Old 03-27-2019, 10:24 AM Thread Starter
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Here's a picture to show the stainless steel pipes.

I'll have to try that choke thing, I had not thought of it because there is never a lack of throttle response. It accelerates from 60 to 85+ very quickly just rolling the throttle on. It runs very smoothly with no indication of a misfire. It's always the same, no cutting out of ignition in any conditions. The jets are all stock and I gauged the holes in them. The float heights are good. There are no gas leaks. I do a lot of carburetor work and I'm very good at it.

The bang sounds like it is coming from the rear, I can't tell which side. One time it sounded like it came out both pipes. It's definitely not coming from the carburetors.

The joints between the mufflers and head pipes do leak a little, I can hear it when I put my head down by the floor in the garage. There is no facility to tighten them, they have no clamps. I wish the previous owner had kept the stock pipes, I would have welded them. His mechanic thought they had to be replaced. Maybe I could wrap cotton rags around the joints temporarily just to see if that helps.
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post #8 of 13 (permalink) Old 03-27-2019, 01:26 PM Thread Starter
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Well, I proved cotton rags was a dumb idea-they caught fire!

But, second try with fiberglass mat wrapped around and wired into place proved those leaky joints where the mufflers join the head pipes ARE THE CAUSE OF THE BANG. Problem solved! Now I could decelerate across the state without a single bang. Except now I have to find a permanent way to seal them.

ob1quixote, you are a genius! Thankyou so very very much!

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post #9 of 13 (permalink) Old 03-27-2019, 05:54 PM
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More than welcome!


Get some silicate exhaust sealant, available at auto parts stores. Pull the mufflers and clean up the surfaces, then wet them. Have a water filled squirt bottle and lots of paper towels close by.


Apply the sealant liberally, put the mufflers back on, then quickly clean up any mess with the water and paper towels. Have a popsicle stick to do a little "sculpting" and add sealer if needed. Give the sealed joints a final spritz of water.


Wait 30 minutes, then run the engine for 10 minutes.


Here is mine I just did, at the initial 10 minute run.




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post #10 of 13 (permalink) Old 03-28-2019, 11:33 AM
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Dave, your pipes look like these:

https://www.saber-cycle.com/store/product988.html

We'll have to chat someday as I have many relatives buried in minn, with my folks moving from there to calif in 1951 :<)

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