Backfiring Problem

[Kilhoffer]

So out at the dunes one day the banshee started what seemed like getting warm and backfiring badly out of one cylinder and less severe out of the other. Only backfires from about 1/4 throttle to 3/4 throttle when holding throttle steady. Idles fine and when pinned will take off though not as it used too.

 

Tried new plugs

Put new needles and seats as they were seeping

Head orings are good and sealed

Jets are clean

120 lbs compression

110 VP with bean oil as I have always ran

Checked for leaks around intake with ether

Reeds are in good shape

 

The best solution I found was to set one needle jet second clip to the top and the other at the bottom. This makes the bike run a lot better but raises concern and still isnt right.

 

Any pointers would be greatly appreciated

 

Thanks!

[Kilhoffer]

Guess I should also mention

 

4mil Hotrods

Ported Stock cyl

Vforce 2

T5's

Cool head

Unknown domes

Stock carbs 310 main, I believe

[The-Association]

Carbs are synced properly?

[WINDYCITYJOHN400]

Many things can cause a backfire, but I alway go to ignition based sources first.

 

#1 - make sure the flywheel didn't spin on the crank. (AKA come loose, go for a ride, eat the key) If the flywheel spins at all on the crank it can advance the timing ALOT.

 

#2 - Check crank phasing. If your crank isn't welded, the webs can sometimes shift and no longer be in phase. (No longer 180 apart) This can cause one cylinder to fire too soon and backfire.

 

#3 - Check to make sure your pickup coil hasn't shifted.

 

I start with that.

[WINDYCITYJOHN400]

I wouldn't keep riding it till you fix the issue.

Edited September 17, 2012 by WINDYCITYJOHN400

[Kilhoffer]

Carbs are synced by visual sight, i dont have a flow meter. In fact i purposely put them out of sync favoring on r cylinder then the other hoping the backfire would switch sides. It made no difference.

 

I will take a look at the electrical stuff this weekend and check the crabk seal whilr im there. Ive heard they can draw air sometimes.

 

 

I was told the crank was welded but never been that far in the motor. Is there a way to check the phase without pulling the crank out?

 

Thanks for the help ill let yall know what i find out

[WINDYCITYJOHN400]

A probe in a spark plug hole. Find top dead center for one side and the other should be a bottom dead center at the same time. (I saw you have a hot rods crank....so that's probably not the issue.)

[Kilhoffer]

Sorry guys, it took a little longer for me to get into the bike then i thought.

 

Here is what i found out

 

Couldnt get the flywheel off so im ordering a puller. Turns out it has a timing plate set at +5.

pickup appears fine and flywheel key looks fine but ill learn more when i pull it.

 

 

One this i noticed is the plugs seem to be wet and reeds are almost dripping wet after running.

I also notice fuel is blowing back into the airbox. Backwards of the direction it should be going. My hunch is

that it is running really rich but when i lean the needles it gets worse. Can domeone point me in the right direction for main

jets? The one in it now have no markings. Im 2000ft about 80the degrees

 

Correct me if i am wrong, if i were having ignition problems wouldnt it affect operation at all rpms? Im only have problems when holding

throttle steady at half throttle

 

Thanks for your help

[Kilhoffer]

Oh and who here has a good supplier for flywheel pullers and sync tools

[Jereme6655]

Jeff at Farm and Sand Toys has pullers and sync tools. Just give him a ring and he will get em to you...

[87sheerips]

NEVER use a 3 jaw puller to pull the flywheel. You need a thereaded center puller and nothing else. I agree with John check pickup coil and make sure you didnt shear the flywheel key. You sound way out of time if backfiring.

[Kilhoffer]

NEVER use a 3 jaw puller to pull the flywheel. You need a thereaded center puller and nothing else. I agree with John check pickup coil and make sure you didnt shear the flywheel key. You sound way out of time if backfiring.

[Kilhoffer]

I got one oredered is it possible to have timong problems in just one cylinder? Im only having a backfire on left side. Well its more of a misfire until fumes build up in the exhaust and ignites making a whoosh sound.

[Kilhoffer]

Got the flywheel pulled. The key was sheared. Props on that call. Thanks

[ticktock]

Correct me if I'm wrong but the center of the crankshaft is splined right? So you can't actually get out of phase unless it was put together wrong.

 

Sometimes when the pilot is undersized you will have popping on Deceleration. This is common in Harley's and 4 strokes.

 

Adam