Backfiring in carb

Reply Subscribe

Thread Tools

Search this Thread

Oct 24, 2011 | 04:54 AM

#11

Spanners39

Seasoned HDF Member

Joined: Dec 2010

Posts: 7,736

Likes: 1,524

From: Coromandel Peninsula New Zealand

Quote:

Originally Posted by Jim Kraft

Yeah Spanners; That makes sense. That may also be why the rear cyl. is running rich. It might be getting some of the fuel charge that would normally go to the front cyl. Does that sound right?

More like its been set up rich to compensate and it didn't fix the issue....some bikes go years with this fault undiagnosed. I fixed a Sporty a coupla months back, it had been carb farting and back firing for 4 years (during which time it had 3 different owners and many changes of jets) until I changed the manifold rubbers

Oct 24, 2011 | 08:58 AM

#12

JohnnyC

Outstanding HDF Member

Joined: Oct 2010

Posts: 2,706

Likes: 14

From: Near Frankenmuth, MI

What happens to many (carbed bikes) is that as the air leak progresses you compensate with the choke. The cylinder that still has a good seal then blackens the plug (with choking compensation) and the bad cylinder fires better due to the richer fuel supply from using the choke. It's the only way to compensate for the leak and of course it gets worse.

There is an upgrade (aftermarket) of a different designed intake that is all solid and can improve air flow.

That's IF it fails the leak test.

You could have a carb someone has already tampered with (changed adjustments or jets). I hope not for your sake. With several metering jets, 2 or 3 ways an idle can be adjusted wrong, and in some cases parts may have been swapped out to improve performance, it can be tough to get it set right without knowing precisely what your doing.

Oct 24, 2011 | 03:52 PM

#13

Spanners39

Seasoned HDF Member

Joined: Dec 2010

Posts: 7,736

Likes: 1,524

From: Coromandel Peninsula New Zealand

I have never really found Keihin carbys, either butterfly or CV that hard to adjust.....its no biggie, just chuck in the standard jets as per the manual and go up from there until its right.

Oct 24, 2011 | 05:15 PM

#14

texashillcountry

Dirt don't hurt

Joined: Jan 2011

Posts: 21,062

Likes: 4,547

From: Haslet Texas

Quote:

Originally Posted by Spanners39

I have never really found Keihin carbys, either butterfly or CV that hard to adjust.....its no biggie, just chuck in the standard jets as per the manual and go up from there until its right.

I realize that you are a master at working on bikes but speaking for those of us who comaritavley know absolutley nothing about motorcycles the fuel/air mixture screw on a kehin is a PITA to get to which is why I switched to an S&S. If there was a trick to getting to the fuel/air mixture screw on a kehin I ain't smart enough to figgure it out.

No scarcasm here just an observation.

Oct 24, 2011 | 06:06 PM

#15

Spanners39

Seasoned HDF Member

Joined: Dec 2010

Posts: 7,736

Likes: 1,524

From: Coromandel Peninsula New Zealand

The trick is to make a tool to fit....I ground down a screw driver and sawed the handle in half :-)

Oct 24, 2011 | 06:47 PM

#16

texashillcountry

Dirt don't hurt

Joined: Jan 2011

Posts: 21,062

Likes: 4,547

From: Haslet Texas

Quote:

Originally Posted by Spanners39

The trick is to make a tool to fit....I ground down a screw driver and sawed the handle in half :-)

Figgures it would be something as simple as that.
Thanks for the trick.

Oct 24, 2011 | 06:52 PM

#17

miacycles

Elite HDF Member

Joined: Sep 2007

Posts: 4,468

Likes: 12

From: Bluffton, South Carolina

Johnny, The solid manifolds aren't that good, as the cylinders expand, they can't move and adjust so they tend to leak. The reason the manifold on the V-twins are designed the way they are is to allow the rubber seals to move slightly when the cylinders grow. That's what causes leaks in a seal that worked for years. They wear out!!! Hope this helps.

Last edited by miacycles; Oct 24, 2011 at 06:54 PM. Reason: typo

Oct 24, 2011 | 07:39 PM

#18

1skrewsloose

Stellar HDF Member

Joined: Feb 2009

Posts: 2,936

Likes: 20

From: Driftless Area

All this info is good stuff. I suspect I 'll be going thru this soon, 90 fxrs-sp. Love reading this stuff, hope I can find it when I need it. Great board!!!