'96 FXD w/1340cc pops on decel....what will cause that?

 

Assuming you have a carburetor, it needs adjustment. What kind of air cleaner and exhausts do you have? What kind of modifications does it have?

 

It's either lean, usually likely caused by a leaky intake seal, or it has an exhaust leak. An exhaust leak will usually be more like a backfire, as air gets into the exhaust on decel and allows unburned fuel to ignite. On an Evo, it's an intake leak 90% of the time.

To check for an intake leak a lot of guys spray carb cleaner or starting fluid at the intake seals with the bike idling. If the idle changes, you have a leak. I use a propane torch (unlit). Same effect, no mess. Just use cardboard or something to block off the back of the air cleaner so that whatever you use doesn't suck in through there.

 

Decel pop can mean a lot of things... some good, some bad.

I will say this... a properly tuned motor should have some decel pop.

I'm going to agree with checking for an intake leak, but we need more information about the bike first. Any carburetor work? Stock CV carb? What size jets? Cam?

 

My 97 flh does the same, it's efi, do the same things apply?


Intake gaskets were just replaced, as well as backing plate gasket.
Did it before replacement and still does it, I don't really pay it much mind, but if it's fixable....

 

Nothing wrong with decel pop.

Some say it is caused by a lean condition when you have pop out of the pipes but I disagree, and so do several engine builders. Decel pop is caused when you have unburnt fuel get pushed out of the cylinder and into the exhaust, where you have a vaccum under decel allowing it to burn off in the exhaust. When you have a little bit of decel pop it usually means your carb/timing are just about spot on.

I get more decel pop in the cold air. When I left Houston 2 or 3 weeks ago it was still warm and I didn't get much popping, but when I had my first run in Tallahassee it was about 48* or so... all motors love cool, dry air.

 

I have found exhaust leak and idle set too low causing problems.

 

With a carbureted bike, some fuel is always going into the cylinder, even with the throttle fully closed. Decelerating at speed, that mixture is too lean to ignite consistently at the speed the engine is turning, so it accumulates in the exhaust, where, every so often it gets ignited by the "waste spark" and burns with a "pop". Remember, Evos are "waste spark" ignition meaning there is a spark on the compression stroke and another on the exhaust stroke.

I'm running an HSR-42 (Mikuni) carb on my bike. It will deceleration pop with a 17.5 pilot jet, but not with a 20, unless it's VERY cold out. Stock pilot jet is a #25, but my bike accelerates better with the smaller jet and an altered (earlier onset) accelerator pump setting.

 

I had no pop with any of the incarnations of EFI, nor do I have it with the carb. (Single fire ignition)

 

EFI actually shuts off the fuel when you close the throttle until engine RPMs drop below 1500 or so. No fuel=no "pop". Likewise, single fire ignition=no spark when the exhaust valve is open=no "pop".