Backfire

[WhY_ZeE]

Backfiring during decelleration.. Normal ?
01 400ex
Stock motor and exhaust.K&N filter + pre filter, lid removed
stock pilot, pilot screw 3 turn ½ out, main 165 , 3rd pos needle.
using dyno jets... any comments ?

[White_Knuckles]

Does it gurgle and pop a little on decelleration or does it let go gunshot bangs? The stock pipe should not ka-blam with the baffle insert installed. If you pulled the spark arrestor it'll backfire.

Here's what happens:

When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high airflow rate through the small gap formed by the throttle valve and carburetor throat.
Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.

[WhY_ZeE]

Originally posted by White_Knuckles
Does it gurgle and pop a little on decelleration or does it let go gunshot bangs? The stock pipe should not ka-blam with the baffle insert installed. If you pulled the spark arrestor it'll backfire.


Just doing one backfire,loud enough, when decelleration but not always though..
The spark arrestor is still there...

[White_Knuckles]

That's unusual, you need to solve for lean idle condition.

Re-check the fuel screw turns, many count 1/2 turns as one. Try replacing the lid to test if the lean condition is the deal. Last bump the pilot up to a 40-42.

What is the history? When did it change and start backfiring?

Your current reported settings should not create this problem. Keep us posted.

[WhY_ZeE]

My friend's 400ex has the same setting as mine and doesnt backfire..
I don't remember when it started backfiring...:(
i guess i'll try to put the lid back on and see if it still does..

[StreetLegalEX]

Mine does that sometimes when cold.....might be lean?

[hawiianpwr]

I'd go up on your pilot. Try a #40

mine gives off a bang probably like every 20-30min or so but i have no spark arrestor, is that regular or will that hurt my engine?

[White_Knuckles]

Originally posted by XxHonda_RacerxX
mine gives off a bang probably like every 20-30min or so but i have no spark arrestor, is that regular or will that hurt my engine?

It is normal for straight through type exhaust like yours. Read the theory in my above post. No, it won't hurt the exhaust valve or engine as some have claimed. The pop is contained within the pipe only.

If it is excessive you may be way lean and also hear ping or detonation on acceleration. This is the engine killer and must be corrected. The muffled metallic explosion this time occurs inside the combustion chamber and can break internal parts.

is that sound easy to hear cuz i definantly do not wanna mess up the engine