recently my 04 sportster has developed an intermitent backfire issue. After riding about 2o minuites when I decelerate around corners or start up at a stop light the bike misses a bit and backfires. The bike runs great for about 20 mins and then acts up. Set up with V&H long shots, Ness Big Sucker air cleaner, carbs were re jetted.
recently my 04 sportster has developed an intermitent backfire issue. After riding about 2o minuites when I decelerate around corners or start up at a stop light the bike misses a bit and backfires. The bike runs great for about 20 mins and then acts up. Set up with V&H long shots, Ness Big Sucker air cleaner, carbs were re jetted.
There is a differene between a backfire, and a caugh.
Is it a cough out the carb, or a pop out the exhaust?
If it is coughing out the carb, it is probably lean. Have you cleaned the air cleaner lately? Try turning out the idle screw half a turn.
If it is out the exhaust check for a exhaust leak.
Back fire is usually caused by an idle jet lean condition, however, a leak in the exhaust can cause it also..
When in de-cel you're going from, lets say, a correct mixture (for the RPM's you're at) to a sudden lean mix for the those RPM's..
Supposedly what happens is the very lean mix that is not burned is pushed out at the exhaust stroke and then will light-off in the very hot exhaust pipe..
Why that de-cel lean mix does not burn I do not know.. I've read that it is simply toooo lean to burn in the combustion chamber but does light off after getting in the hot pipes.. Believe me,, I have really researched this..
With a leaking exhaust system, the air leaking in at the leaking connection will mix with the suddenly lean mix in the pipe and light it off.. Apparently it simply intensifies the already lean idle mix prob..
Do this carefully--- light a Bic or something similar and without burning your fingers set it close to all exhaust connections.. If the flame blows around close to a connection but burns easy a few inches away you have a leak at that connection..
I have always conquered De-cel back-fire by fattening the idle jet even with a slightly leaking exhaust.. But I'm still fighting a de-cel B-F with this 127" I'm running.. Over time I have read that sometimes you cannot get rid of it even with perfect jetting and NO leaks simply because of cam design, overlap, ignition and timing, etc..
I have changed ignition systems 3 times and of course timings, jetted to a SUPER rich idle jet, changed the whole exhaust system and gone from a 45 Mik to a Super G, but only succeeded in cutting the B-F in half..
My next change will be to another 48 Mik carb..
My point---- I'm starting to believe that sometimes it cannot be conquered, BUT I bet you can cuz your motor is not as radical..
HC, posted while I was typing,, Good point HC,,,,,,, is it really a B-F out the pipes or a cough out the carb...???
__________________ "Fill your hands you son of a bitch"
Rooster Cogburn
Last edited by frisco-rigid; 05-30-2008 at 01:41 PM.
I'm getting both a caugh out the carb, and a backfire out the pipes. Air cleaner is new, and pipes seem tight. I'll double check the pipes, and check the idle screw.Thanks
Yea, looks like Frisco and I jumped on this at the same time, like Frisco said, if the gas is too lean it will not fire while compressed, and builds up in the exhaust. And then ignites.
The idle jet screw is at the bottom of the carb. It is covered from the factory, but if it has been rejected by someone who know what they are doing then it will be knocked out.
The idle jet screw is different than the idle adjustment.
Back this out half a turn, and see what it does.
Keep track how far you turn out this screw, incase you want to put it back where it was.
Before I adjust the idle jet I like to screw it in to count how many turns it is out, but don’t recommend doing this to new wrenches, because you can ruin a carb by turning in to hard and ruining the seat.
This can also give you a clue if your idle jet is too small, to large by how far out it runs best.
Jetting needs changes with temperature and humidity, so a perfectly jetted bike in the winter, can need tweaked in the summer.
Your drag pipes are going to cause a little crackle on deceleration. But the cough needs to be cured.
Forget what I said about the air cleaner, a dirty air cleaner makes the bike run richer, Daah.
Um,, I didn't realize the V&H long-shots are drag/straight pipes..
All bets are off if they are..
Drags will rob power and are notoriously hard to jet for..
Like HC said,, the screw is diff from actual idle jet adjustment..
To fatten the idle jet you must change it..
AND yup--- the cough out the carb means lean in that RPM range..
HC
Which rigid.?
There is a photo of the 93" shovel in the photo album back at about the Nov. 2005 area.. Pre-Mikuni..
I have a newer one but will not even attempt to post photos again in this crappy attachment manner here..
I built it in 1978 in a '52 wish-bone..
I built the 120" rigid in '02 in a very crappy Atlas frame,, never again for them..
__________________ "Fill your hands you son of a bitch"
Rooster Cogburn
[QUOTE=frisco-rigid;22366]Um
There is a photo of the 93" shovel in the photo album back at about the Nov. 2005 area.. Pre-Mikuni..
I have a newer one but will not even attempt to post photos again in this crappy attachment manner here..
I built it in 1978 in a '52 wish-bone..QUOTE]
HEY Frisco, I looked for your photo, but could not find it. Have you tried storing your photos in a web site like Photobucket.com then linking them here?
Sure would like to see them, love associating people with their bikes.
Hubble,, please forgive me and HC for Hi-jacking your thread temporarily..
HC,, I think the Album is sooo crappy I won't even go in to look for it myself..
It it labled "......200,000 miles"..
Somewhere between let's say April and Dec. '05.. I would start in Dec. and work back.. That might help..
It used to be easy to find stuff in that mess, for me at least, but no more..
It is my belief that every time they have "up-graded" this forum it has actually been negative..
And No,, I simply won't do all that posting there and linking here and back 'n forth crap to see something or to post something..
Sometimes the Simplycity of these computers and the associated forums gets more Complicated than it's worth.. The album for instance could be waaay easier.. Ah,, for me at least..!!
__________________ "Fill your hands you son of a bitch"
Rooster Cogburn
backfire issue 
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