Why is there fuel coming out my air filter? by Doc : V-Twin Forum: Harley Davidson Forums

HDMD88 · 07-07-2007, 12:35 PM

its called fuel stand off....and it gets little specks of oil all over the bike plus fuel drips out of the air cleaner.

"FUEL STAND OFF''....this is a characteristic of a internal combustion engine with a carb on it. It has nothing to do with your jetting or acceleration pump intake leaks or timing......it's a pulse wave that happens when a column of air(intake) traveling at 650 feet per second into a cylinder THEN the valve closes on this column of air. The column of air with a charge of fuel in it just doesn't stop, (who was it that said for every action there is a reaction?) any way when the valve closes the charged column of air traveling at a speed of 650 fts bounces off that valve and makes it's way to the other head where it bounces off that closed valve and the column makes it's way out through the carb....now here is where this gets interesting...the carb isn't too smart, It doesn't realize which way the air is traveling so as it goes over the venture it picks up more fuel in the column of air that already had a charge of fuel in it....when the column of charged air hits the atmosphere it's like hitting a brick wall so the air is reflected back into the engine as a little weaker pulse wave but since the fuel is much heavier than air the fuel continues out into the atmosphere soaking your air filter and the whole right side of the bike depending on how long you have the bike under a load at high throttle positions.
To witness this you can take off your air filter and do a few runs on the dyno and watch this STAND OFF happen before your eyes....a plumb of fuel will hover out side the mouth of the carb up to 8'' away depending on the displacement of the engine, cam overlap, and exhaust system used.
Now when you take the time to set up ''Intake Track Tuning'' this waisted gas can actually be pulled back into the engine and used instead of washing your air filter. Intake Track Tuning is using the pulse waves of the intake to assist in cylinder fill by timing the wave to hit the intake valve as it is starting to open.....these are the things you do on motors that need the the EDGE to be a winner.

Hope this helped you to understand whats going on with your bike, you don't see or smell this gas on a bike with a stock air filter because it's all tightly sealed, nor will you see STAND OFF in a EFI bike because there is no fuel to pick up on the way out.
If you continue to have a problem with the fuel on your bike and/or clothes take one of your wife's (or girl friends), light day pads and place it in the bottom of the football A/C cover....this will absorb all the fuel and oil from the breather, the fuel will evaporate and the oil will be trapped in the pad. Now one more piece of advice...........................change out the panty shield every 28 days.

Go on and laugh but this really works....don't use the pant shields with the WINGS.... the wing will stick out the cover and drip on the engine.

I got to take a nap now....but first here is how you figuer out intake tuning for those of you who have inquiring minds.

Intake track tuning is done by lengthening the distance from the middle of the intake valve to the atomosphere....ie if you add a 5 or 6'' velocity stack to the end of your carb that would be extending your intake track. The intake track on My Pro Modified Drag Bike is 19 3/4'' long, a stock bikes intake track is about 11'' long. Are you getting my drift....this takes a lot of mathematics to figure out the length you need for your bike. The formula figures in the 2nd wave or the 3rd wave or the 4th wave each wave gets weaker. I do have the formula for this if your interested but like you say getting rid of the drag pipes will help you the most....it won't totally get rid of it but certainly slow it down.
The formula is this: (and I'm giving you all three waves to figure out)
2nd pulse: (1100 x 1/2 Intake Cam Duration x .96) / RPM = length of Intake Track
3rd pulse: (1100 x 1/2 Intake Cam Duration x .750) / RPM = Length of Intake Track
4th pulse: (1100 x 1/2 intake Cam Duration x .538) / RPM = Length of Intake Track

The 1st pulse would be the strongest but to achieve using this pulse wave the manifold would have to be 30'' long....this is not practicable. Whats this all about...in a nut shell...a pulse wave generated by the air hitting the atmosphere (as we talked about earlier) caries a movement of air with it. Your trying to time this pulse wave (movement of air) to hit the intake valve as soon as it opens so it adds in cylinder fill....more air in the cylinder gives us more power. The 2nd wave carries more air than the 4th wave does but the 4th wave will have a shorter intake track than the 2nd wave will have. This formula will tell you how much you have to space out your carb from the intake valve in inches to achieve using one of these waves. After the 4th wave passes it no longer will effect the cylinder fill. Now how and why are there different waves.....the pulse wave travels back and forth in the intake track at 1100 fps...each time it changes directions it gets weaker, so we concentrate on the 2nd 3rd or 4th wave. Once the wave is figured out to hit the intake valve as it opens it enters the cylinder and its done....when this is achieved for your build the gas that hovered out side your carb will be pulled back into the engine. (You can actually see the fuel being sucked back in on a dyno)

Again this isn't something you would do with a street bike....this is for Drag Bike Motor Builds. My Pro Modified was a 116'' Sporty that made 182 HP and 167 tq and it took things like intake track tuning to get these numbers (among many other things)

TXCHOP · 07-07-2007, 02:28 PM

Doc, many thanks for the write up. VERY informative...I have witnessed my bike do that on the dyno with out the air filter....This explains it perfectly. I dont have a problem of it dripping out though.....

Great write up and hope you do many more...Ever think of writing a book. I know i would buck up for it no matter the cost.

adlerx · 07-07-2007, 04:47 PM

Thanks for the tip and info. My wife is a little older and doesn't use those things any more but if I ever get a girlfriend who is young enough to have em around, I'll give it a try....

Oh yeah,

nevermind.....

HDMD88 · 07-08-2007, 12:59 AM

Quote:

Originally Posted by adlerx

Thanks for the tip and info. My wife is a little older and doesn't use those things any more but if I ever get a girlfriend who is young enough to have em around, I'll give it a try....

Oh yeah,

nevermind.....

Well the DEPENDS are too much so keep those on you both!....

.....and change those more than once every 28 days.....

Geezer-Glide · 07-08-2007, 09:09 AM

Quote:

Originally Posted by HDMD88

Well the DEPENDS are too much so keep those on you both!....

.....and change those more than once every 28 days.....

Now THATS funny, I don't care who you are!!!!!!!

Great write up, Doc. Last time I had my bike dyno'd, I saw the stand off. I asked the service manager just what was up with that. His stock "Harley Answer"............ They all do that!

adlerx · 07-08-2007, 09:10 AM

Quote:

Originally Posted by HDMD88

Well the DEPENDS are too much so keep those on you both!....

.....and change those more than once every 28 days.....

Good point there Doc!

HDMD88 · 07-08-2007, 11:09 AM

Quote:

Originally Posted by adlerx

Good point there Doc!

Just funnin with you my friend....this place needs the lighter side once in a while.

Totenkopf · 07-08-2007, 11:35 AM

hey Doc!

good info!! thanks for taking the time to post it up.

we certainly take this into account on the little Pro Mod and the bigger Pro Gas bikes we currently campaign. Otto does the calculations and then we tune the length with spacers and velocity stack.

On my current 114" build, we just discussed yesterday the design of the custom intake needed to tune the length of the run. Prolly going to use a highly modified S&S G to feed it and then run a open velocity stack. The bike's mission is lightweight, short distance, land missle, in a custom build. It should be fun! But the point is, we are using these principles you outlined for the design of the intake system.

Geezer-Glide · 07-08-2007, 12:03 PM

Quote:

Originally Posted by HDMD88

Just funnin with you my friend....this place needs the lighter side once in a while.

Careful. Sometimes the lighter side gets you sent off to band camp for a while.

HDMD88 · 07-11-2007, 10:30 PM

Thanks Guys.....do you think this would make a good Sticky......I do

TXCHOP · 07-11-2007, 10:40 PM

I think you should have your own sections of sticky's.....But hey thats me and i like it when you talk carbs...Keep on talking carbs, i have a new ink cartridge ready......haha...

sportking · 07-12-2007, 09:48 AM

That was a great article . I have witnessed it for many years !
With the cover off I can see a small column , at iidle on my old Sporty

LittleBear · 07-12-2007, 12:33 PM

I had heard of reversion in the intake track causing this but not "Fuel Stand Off". Thanks for the additional information on how motors with carbs work.
By reversion, I am referring to the the intake velocity of the air/fuel charge is not high enough, due to low RPM of the motor, to continue filling the cylinder as the piston rises prior to the intake valve closing. The force of the piston rising forces the intake charge back out the intake valve, out the intake track into the carb, where it pick up more fuel and dumps it onto you air cleaner. As RPM increases, the intake charge is faster and overcomes the force of the piston rising with the intake valve open and continues to fill the cylinder. The later the intake valve closes the higher the RPM required to reach the equilibrium point and overcome the force of the piston rising. The point where some say, the motor, "comes onto the cam".

I seem to have experienced this more than "Fuel Stand Off" I believe, cranking the throttle at low RPM trying to pull a hill and having it spit fuel back onto me. I have also done a run with no air cleaner and have see fuel spit back out my carb at low RPM when I crank the throttle open in a higher gear, 4th gear at 2k RPM with a cam closing at 39* ABDC.

Please educate me if I am "full of it" which has been known to be the case.

Thanks DOC.

LittleBear · 07-12-2007, 12:44 PM

One other thing.

Where did the intake velocities of 650 fps and 1100 fps come from? Aren't they RPM dependent?

Not trying to dispute you DOC, just trying to understand. Ignore my Avatar.

HDMD88 · 07-12-2007, 12:47 PM

Little Bear,
Fuel stand off is just a name given to the fuel coming out of the carb. Reversion has a lot to do with the intake track too but normally this is the case when you have a lot of overlap in a cam and/or bad exhaust system that pushes exhaust gasses back through the carb. This is evident by the carbon on the intake valve and the black residue in the carb itself.
It's all in the pulses of the motor.