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General Category >> Rubber Side Down! >> Backfire after jet-kit.
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Message started by mys40rox on 09/27/07 at 19:30:55

Title: Backfire after jet-kit.
Post by mys40rox on 09/27/07 at 19:30:55

Okay here is the Problem. I put on a Mac slip-on and Jetted the carb and now I have even worse backfire than before. Before the Jet kit was installed it only backfired when I turned off the bike. Since the install the my bike backfires on almost every shift and upon deceleration. I've taken it back to the shop that installed the jet kit and they said the backfire was from the muffler not being sealed correctly. I had them re-install muffler and still no luck they said there is nothing they can do? it makes no sense. I bought the muffler directly from Mac so I know it fits right. the pipe sounds awsome when its not backfiring.

please help, what can I do to fix this mess?

Title: Re: Backfire after jet-kit.
Post by Rockin_John on 09/27/07 at 22:15:47

Warning: a lot of people don't like my solution. I got tired of fooling around with a leaking stock header and Dyna muffler, and reading dozens of posts about how to fix it. Then I just had them TIG welded together by my neighbor, who has a major weld shop. Problem solved. No more leaks. Just be really sure the pipes are together in the position you want them; because it takes a cutting wheel to separate them and do it over. (I got it right the first time).

Title: Re: Backfire after jet-kit.
Post by Reelthing on 09/28/07 at 00:06:06

Well it could be sucking air at the header pipe muffler joint - it can also suck air at the head/head pipe flange - might need a new crush gasket there - thay can look good and not work very well -

by the way what jet kit did you have installed?

Title: Re: Backfire after jet-kit.
Post by mys40rox on 09/29/07 at 09:15:58

I had a dynojet kit installed.
thanks for help.

Title: Re: Backfire after jet-kit.
Post by slavy on 09/29/07 at 13:28:46

There are a lot of posts about the backfiring.
Usually the jet kits don't change anything about the pilot jet and usually the backfiring is caused by extreme lean condition in the idle cirquit. It can be caused by too much air/vacuum leak/, or by too little fuel /too small pilot jet/
Why don't You try one size up on the pilot jet ?
It is a very cheap and easy try.

Title: Re: Backfire after jet-kit.
Post by Savage_Rob on 10/01/07 at 06:35:15

The bike is lean to begin with and swapping the exhaust to a MAC makes it leaner. Rejetting will give you a richer mix at the high end but you still need to do the half-spacer mod and tune the carb. And, as stated above, any exhaust leaks can cause backfiring too. First though, you need to do the half-spacer mod and tune the carb. After that, if you still have the backfiring, replace the exhaust gaskets and tighten securely. Don't over tighten and don't tighten when the engine is hot.

Title: Re: Backfire after jet-kit.
Post by toddlamp8 on 10/02/07 at 08:58:39

But didn't earlier bikes (ie my 1986) come with bigger jets? Therefore is it still necessary to swap jets when putting on a different muffler?

Title: Re: Backfire after jet-kit.
Post by Savage_Rob on 10/03/07 at 08:18:26

toddlamp8 wrote:

But didn't earlier bikes (ie my 1986) come with bigger jets? Therefore is it still necessary to swap jets when putting on a different muffler?

They're not as lean to start off with but, when you change to a freer flowing muffler, they are leaner and should be rejetted. In order to alleviate lean conditions in the midrange, the white spacer mod should still be done if that model doesn't have the adjustable needle. If it has the adjustable needle, adjust that instead of shaving the spacer.

Title: Re: Backfire after jet-kit.
Post by bill67 on 10/03/07 at 08:26:42

Which models have the adjustable needles

Title: Re: Backfire after jet-kit.
Post by Savage_Rob on 10/03/07 at 10:02:28

I believe only the late 80's models had them and I don't know if all of them did or not.

Title: Re: Backfire after jet-kit.
Post by toddlamp8 on 10/04/07 at 17:14:56

I am getting backfiring after the muffler install and i'm getting crap gas mileage. Under what circumstances could a HD muffler richen the air/fuel mixture? Could backfiring/flames out the muffler be caused by a rich condition?

Title: Re: Backfire after jet-kit.
Post by Savage_Rob on 10/05/07 at 05:23:40

toddlamp8 wrote:

I am getting backfiring after the muffler install and i'm getting crap gas mileage.

Stop using crap gas.

Sorry, couldn't resist... ;)

Title: Re: Backfire after jet-kit.
Post by Savage_Rob on 10/05/07 at 05:31:27

toddlamp8 wrote:

Under what circumstances could a HD muffler richen the air/fuel mixture? Could backfiring/flames out the muffler be caused by a rich condition?

A rich condition is simply too much fuel or too little air in the fuel/air mixture. Achieving a rich mixture where none previously existed with only a change of muffler would mean the new muffler is more restrictive. This is usually not the case but it answers the question you asked. As for backfiring from a rich condition, yes it can happen - though it's really post-ignition. Heated unburned fuel can be ignited when it mixes with air in the exhaust, either via a leak in the exhaust allowing air to be sucked into the stream or nearer the opening of the exhaust. More common with the LS650 (and when going from a stock muffler to a freer flowing one) is backfiring from a lean condition if the bike was not rejetted/retuned when the muffler was changed. Exhaust leaks are also pesky.