I have heard this same thing from some of my Harley riding buddies' bikes out on the road and now it seems my new BF750 likes to do it as well. A brief run at hard throttle (1/2-full) followed by a hard let off (back to idle) for a few seconds as the KEBC kicks in and the motor gears down is sometimes followed by a very loud "POW!!!" out the exhaust. I only have about 12 hours on the machine right now so hopefully this will go away with a little more runtime I was just wondering if anyone else has experienced this. The bike has NO mods at all, showroom stock.

God bless,

-BunnMan

 

Jetting lean? I know it's stock, but it's a possibilty.

 

Could have somethin caught in a jet or maybe you got some bad plugs. Check the plugs. Cheapest and easiest thing to check. Actually, never replaced plugs in a brute force, i know prairie's and V-force are a pain. Good luck!

 

Mine did a similar thing once the temp. dropped in our area (Minnesota). My brute is currently in to get the jetting right since the needle change was not enough. They also believe my "pop" was a product of the jetting. I will let you know if any adjustments clears up the "pop" as well.

 

This little gem comes from the mikuni website. http://www.mikuni.com/fs-tuning_guide.html





4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) 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.
2) 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 air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) 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.
4) 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.
5) 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.


Usually, a 1/4 turn richer on the mixture screw, will stop it. As the weather gets colder, the mixtures get leaner.

 

My P700 does the same thing. But it runs so well I'm not concerned about it.....because it only happens occasionally.

But if it did it a lot, I would seek a cure.

 

Perciate the great response there Hondabuster! That sounds like the case no doubt. Maybe I should look in to getting the jetting checked. This is where I might run into the downfalls of buying form an out of state dealer. I drove 5 hours to a neighboring state to save $1200 on my new Brute. I wonder fi the local dealer will try to stick it to me for such an act?

Thanks and God bless,

-BunnMan

 

This topic has been touched on several times, and we all had our theories about its cause (too lean, too rich, loose exhaust nuts, etc), but this was the first time someone brought a good explanation. Hondabuster, thanks for posting that. I was wondering if I should do something about the cannon fire out the exhaust. Now I won't worry about it so much.

 

Glad to help,
Sometimes it can be loose exhaust , or an intake leak, or plugged pilot jet, or air bleed,...anything that makes the idle circuit lean. Usually on a new machine, its just a too lean setting on the mixture screw. If 1/4 turn doesnt do it, go another 1/4 turn. If that still doesnt fix it, check for other causes of lean in the idle circuit.
You should also notice that it will cold start, much easier after going richer, and come off choke sooner.

BunnMan, Its not a big deal to jet yourself, heres a good article on jetting, and has symptoms of lean and rich, to help troubleshoot.jetting

 

if you are popping under decell, try pulling the needles up (shim or move clip if you have a dynajet) you may also have to richen the idle mixture up as well,