Rejet to solve backfire? - VTXOA
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post #1 of 24 (permalink) Old 06-01-2017, 03:27 PM Thread Starter
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Rejet to solve backfire?

My 05 1300C still backfires even though I've removed the PAIR valve and capped off the reed valves, replaced all 5 (slip ons, crossover and crush) gaskets in my stock exhaust, and replaced the acorn nuts with flange nuts torqued at least twice after riding. Also using stock air filter. I've also adjusted the idle and a/f screw according to the manual. So for some reason I'm still burning fuel in my pipes on the deceleration. Bike has about 65K miles on it and I'm getting pretty good MPG at 40-42mpg.

I'm thinking it may be time to look inside the carburetor to check whether the pilot diaphragm may be torn or any other gaskets may be cracked or rotting. If I do dig into the carburetor, would it be helpful to increase the jetting to run a little more rich while I have it apart??

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Gaithersburg, MD
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post #2 of 24 (permalink) Old 06-01-2017, 04:46 PM
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I did everything to stop mine.. Pair, Intake, torqued exhaust, Jets..
Nothing worked.
Then I drilled my pipes !!
Side effect was the popping stopped 99%....
Must be a back pressure thing or?????
And it sounds much better to.....
Best of luck.......


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post #3 of 24 (permalink) Old 06-01-2017, 05:16 PM Thread Starter
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Ha, now that's intresting!

Regards,
Pete
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post #4 of 24 (permalink) Old 06-01-2017, 05:29 PM
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Air cut off valve perhaps
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post #5 of 24 (permalink) Old 06-01-2017, 06:00 PM
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Quote:
Originally Posted by frnkcat View Post
I did everything to stop mine.. Pair, Intake, torqued exhaust, Jets..
Nothing worked.
Then I drilled my pipes !!
Side effect was the popping stopped 99%....
Must be a back pressure thing or?????
And it sounds much better to.....
Best of luck.......


.
Did you do the "Honest Abe" exhaust fix? (
) I had a friend do that to his boulevard C50 and it stopped popping and made a better rumbling sound and didn't have to drill the chrome. Mind you I am not sure what the ends of stock 1300 pipe looks like so it may not work..
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post #6 of 24 (permalink) Old 06-01-2017, 06:34 PM
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Originally Posted by PoggiPJ View Post
My 05 1300C still backfires even though I've removed the PAIR valve and capped off the reed valves, replaced all 5 (slip ons, crossover and crush) gaskets in my stock exhaust, and replaced the acorn nuts with flange nuts torqued at least twice after riding. Also using stock air filter. I've also adjusted the idle and a/f screw according to the manual. So for some reason I'm still burning fuel in my pipes on the deceleration. Bike has about 65K miles on it and I'm getting pretty good MPG at 40-42mpg.

I'm thinking it may be time to look inside the carburetor to check whether the pilot diaphragm may be torn or any other gaskets may be cracked or rotting. If I do dig into the carburetor, would it be helpful to increase the jetting to run a little more rich while I have it apart??
think about that statement. you are backfiring because you have excess fuel in the exhaust. Do you want to add more fuel to the fire????

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post #7 of 24 (permalink) Old 06-02-2017, 10:30 AM Thread Starter
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think about that statement. you are backfiring because you have excess fuel in the exhaust. Do you want to add more fuel to the fire????
I'm an old programmer by trade so I probably misunderstood - I was under the impression that since our bikes were set lean off the assembly line, that there wasn't sufficient fuel to enable full combustion in the cylinder causing excess unburnt fuel to escape into the exhaust. So maybe you can help to educate me with what is actually going on between our carburetor and the cylinders that allows excess fuel to escape.

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Pete
Gaithersburg, MD
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post #8 of 24 (permalink) Old 06-02-2017, 12:22 PM
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Actually combustion is almost always incomplete, strictly speaking. What happens, specifically when decelerating, is that you transition from a high volume to a low volume charge. In that transition you always end up with a fuel rich charge and that gets discharged into the very hot exhaust. If there is enough O2 in the pipe, it will ignite the fuel, hence a pop. This is far more pronounced and far more likely with the pair valve operational, or with an exhaust leak because some fresh air gets introduced.

There is only one way to stop this, but the methods vary, and that is to create a condition in the exhaust pipe that is unfavorable to ignition.
1) Create a situation where there is not enough O2 present to allow combustion of unburned fuel. That is done by rendering the pair system nonoperational and ensuring you have no exhaust leaks that may allow fresh air into the system to allow combustion
2) Create a situation where there is not enough fuel available to allow for combustion. FI bikes may try a strategy of completely cutting off fuel at anything much above idle when the throttle is closed. Can't really do this with a carbed bike.
3) Create an overly rich condition where there is not enough O2 present to allow combustion. Again, with FI bikes there is a strategy to actually increase fuel at anything much above idle when the throttle is closed. Ever wonder why you can have a fuel pump and wires inside of a fuel tank? The conditions inside of a fuel tank are so rich that even a spark cannot ignite the fuel, There simply is not enough O2 present. Same thing in your exhaust pipe. Raising your AF ratio in your idle circuit can push your high RPM / closed throttle exhaust fuel ratio past what will cause a pop.

I'm not sure if you know any of this, so if you do, you can just skip this. The perfect AF ratio is 14.7:1 (by mass). That is 14.7 parts air, 1 part fuel. This is considered the stoichiometric mixture where there is no O2 and no fuel left. Combustion is complete. An AFR (Air Fuel Ratio) of 8:1 and richer will not ignite (referenced to gasoline). And AFR of 18:1 and leaner will not ignite (referenced to gasoline). The addition of ethanol will shift these numbers richer BTW. Best power is at about 12.6:1 and best economy is at about 15.4:1. That's in the combustion chamber, of course.

So if you can create a condition in your exhaust when you would normally see popping where the AFR in the exhaust is richer than 8:1 then you will effectively eliminate the much hated popping. This is what you would be reaching for by changing jetting. Your bike would rich a bit richer down low, but would be really rich during decel. That's how I first dealt with the problem on my 1800, by using a Cobra FI2000, like an electronic jetting kit.
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Last edited by Harkon; 06-02-2017 at 12:28 PM.
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post #9 of 24 (permalink) Old 06-02-2017, 12:28 PM
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Quote:
Originally Posted by frnkcat View Post
I did everything to stop mine.. Pair, Intake, torqued exhaust, Jets..
Nothing worked.
Then I drilled my pipes !!
Side effect was the popping stopped 99%....
Must be a back pressure thing or?????
And it sounds much better to.....
Best of luck.......
Same issue with my previous 05' 1300R. I did the PAIR valve removal, carb cleaning, and replaced the exhaust head gaskets.

With stock pipes, I would leave the office, start the bike with choke on, drive a few blocks and "lower" the choke. After an additional 4 blocks I'd slow down coming into downtown traffic and BOOM! The pipes would sound like a shotgun going off. I'd often look down half expecting the pipes to be blown off the bike.

My "fix" was to replace the rusted stock pipes with clayton moded stock pipes AND all five gaskets, torqued the nuts to the heads correctly...AND still got the "One-Shot" backfire... But now it was a muffled PHUMF sound I could live with. It would only do it once, on decel, after warming up and using the choke OR after constant highway speeds for more than a 1/2 hour or so.

My final fix was to get an 1800.

I'm gonna hit the highway like a battering ram, on a silver-black PHANTOM bike!
Meatloaf~Bat Out Of Hell

2002 1800 "S"
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post #10 of 24 (permalink) Old 06-02-2017, 12:29 PM
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Originally Posted by cavie View Post
think about that statement. you are backfiring because you have excess fuel in the exhaust. Do you want to add more fuel to the fire????
That is actually a valid strategy, yes.


'12 Road Glide Ultra, Kuryakyn Alley Cat intake, Jackpot Super Duals header, Jackpot HiRoller mufflers, PV Tuner, Daymaker LEDs
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