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Old 04-01-2021, 09:12 AM   #1
Lochlin
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Join Date: May 2011
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Default Backfiring Causes and Corrections

Harley / Par Car carts are notorious for backfiring. While such backfiring isn’t bad for an engine per ser, it affects performance, annoys the riders and can scare the daylights out of those within ear shot of one. The good news is that backfires aren’t all that hard to rectify, it’s just that their causes are many and finding the right correction can be frustrating.

To fix a backfire we must understand its cause. This is the easy part. Invariably, backfires are all caused by the same thing; ignition (spark) is not occurring at the right time or is inconsistent. When a 2 stroke is “on the down swing” it sucks fuel into the cylinder. When it goes back on the upswing and the piston is at the top of the cylinder, a spark is needed of course. When this spark is missing, the motor goes back on the down swing, still full of fuel, and sucks even more fuel into the cylinder. When it goes back on the upswing it now has double the amount of fuel it should. Should spark happen at the point BANG from all of the excess fuel. It’s simple really.

Why is spark not happening? This is where things get a little bit complicated.

The biggest reason for poor timing of spark is……timing. That’s why it’s called timing. A poorly timed cart is perhaps the biggest cause of backfiring. There is specific sticky for timing so give it a look.

Another big reason for backfiring is when the ignition starts and stops. On HD and Par Car carts, there is a round disc that is located under the cart, below where the drive sits. This disc moves when the accelerator is depressed and closes a microswitch. This starts the ignition process. It is important that ignition starts before fuel starts entering the cylinder and stops after fuel stops entering the cylinder. The disc is on a threaded rod and is adjustable. Adjusting the disc toward the back of the cart will start spark sooner and stop it later which could help prevent backfiring. Don’t over adjust, though, as you don’t want a cart to have too many dry rotations.

Another cause of backfiring can simply be a carburetor that is in need of servicing in that the carb is allowing too much fuel into the cylinder. Most carbs have a fuel adjustment screw on them which can be adjusted for more or less fuel. It’s trial and error unfortunately. A good cleaning never hurts.

Lastly, backfiring can be caused by a throttle linkage that needs adjusted. Carb butterflies have a tendency not to close all the way over time. This allows a little bit of fuel to flow into the cylinder when a cart is coasting and the ignition circuit is off. When the accelerator is hit again, all of the leaked fuel ignites at once and a backfire occurs.

Below is a link to a Par Car service supplement in which accelerator adjustment and backfiring is addressed. Even though it’s for a Par Car the same procedures apply the HDs as they are basically the same cart.

https://www.buggiesgonewild.com/colu...UTILITRUCK.pdf
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Old 04-01-2021, 09:16 AM   #2
Lochlin
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Default Re: Backfiring Causes and Corrections

Mods, can you please make this a sticky but keep it open so that others can add additional information?
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Old 05-04-2021, 08:59 AM   #3
Lochlin
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Default Re: Backfiring Causes and Corrections

Bump.

Mods, any chance of getting a sticky for this one?
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Old 05-04-2021, 10:22 AM   #4
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Default Re: Backfiring Causes and Corrections

Quote:
Originally Posted by Lochlin View Post
Mods, can you please make this a sticky but keep it open so that others can add additional information?
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