RESTO Restoring Barn-found 1996 XP 800

[WhiskeyD]

I'm no carb expert but I believe the problem is aftermarket flame arrestors are less restrictive so you won't have as much manifold pressure to pop the needle(s). Most people run a much lower pop off than the factory recommends for that reason. I believe my ski has 2.0 N/S and around 20 psi pop off with FAs, all stock motor otherwise.

 

[libertcat]

Yeah, I think you are right... to a degree. My aftermarket gizmo might be a bit less restrictive, but the pressure of the air does not pop off the needle. The only pressure behind the needle is the pressure generated by the gas pump, pressure of the gas. Needle has to be strong enough to prevent gas under that pressure from flooding the carb. 20 psi will probably do it.... As air runs through venturi it sucks gas vapor with it (similar to how water running past tubes in the jet pump sucks water from bilge). Anyway... as gas vapor sucked from the jets, pressure inside the carb body will drop a bit, will create sort of vacuum. Atmospheric pressure will push the membrane (that black round thing) into slight vacuum, which will open a needle via lever. Opened needle will allow a bit of the gas to get into the carb body, which will elevate the pressure inside the carb body push the black membrane out and close the needle again.
By a big shot, atmospheric pressure, pushing black membrane is what opening and closing that needle. pop off pressure test does not take into account rigidity of membrane and atmospheric pressure. It is a VERY ROUGH assurance carb will work ok-ish. Meaning - spring is strong enough to keep needle close against fuel pump pressure and weak enough that black diaphragm can open it when gas is sucked from venturi.

Here is my "artistic" diagram of the carburetor theory

 

[AdamXP800]

When you hear bog bog bog bog bog brrr bbbrrrrrr brappppp you might think differently lol. I'm jk man good luck

 

[libertcat]

Yeah... I will take an oar with me... That's my ingenious plan

 

[Mickirig1]

The stock carb are set to the volume of air the stock air box will deliver. The pop off is where the pistons have enough vacuum / negative pressure to move fuel thru carb. Delivering the right amount of fuel at the right RPM. The diaphragm adjusts pop off vers the air pressure available to the intake in theory. The engineers made trade offs to get them to function well and engine longevity. The carb or carbs may have problems too. So your not getting the right performance you think they should.

 

[libertcat]

Agree on most counts... Except volume of air ... Regardless of stock or modified airbox, carburetor channels ~20 cubic feet of air a minute at idle, and ~110 cubic feet of air a minute at WOT. So spring/membrane have to be perfectly functional (able to close and open) in a very wide range of airflow. So factory or engineers don't really set car for some specific airflow, but for a very wide range.
And yes, you are right, pistons create vacuum, that moves air. Moving air grabs gas mainly via venturi effect, but also in part due to negative pressure. And membrane and spring modulate flow of gas in combination with atmospheric pressure.
As far as performance goes, I don't have any yet. So far ski runs on the trailer, runs very well, except a bit rich. Once ice melts on the lake I will try it out and report. For now work on graphics and minor electrical stuff.

 

[libertcat]

starting to "nuke" it >

 

[mikidymac]

You sound like you are into the tech of these things. If you haven't already done it you should read the tech documents from Group K. It is some very interesting reading and gives a good argument for the stock air cleaner for most applications.

 

[libertcat]

oh, thanks, will check it out. I read a bit on their blog, they are fun.

 

[wreckless]

Cool winter project.

 

[libertcat]

Powerplant is coming together >

 

[mikidymac]

I like it. Doesn't look like every other 96' XP.

 

[Matt Braley]

Right. I was not a fan of those pricy graphic kits but your really making this work. I love the way you have followed the theme through the seat cover as well as under the seat. Your gonna turn some heads out there, nice job!

 

[Mickirig1]

I like it too. Don't feel attacked with the comments. Most guys here don't read up on theory of carb delivery / function. These are not 4 stroke engines. A 2 stroke is a totally different animal! They need a balance of fuel delivery / air flow vers RPM vers load.

 

[1of500]

I like it too. Don't feel attacked with the comments. Most guys here don't read up on theory of carb delivery / function. These are not 4 stroke engines. A 2 stroke is a totally different animal! They need a balance of fuel delivery / air flow vers RPM vers load.

Common knowledge that when going to aftermarket flame arrestor you have to rejet and adjust pop off. I don't think anyone was trying to attack him just trying to emphasize the importance when setting up carbs with after market flame arrestor. If it's a stock engine it is a useless mod. Factory carb setting for 96 xp are dead on if everything else is stock and working properly.

 

[libertcat]

It also helps if people have common sense in addition to common knowledge. I said that needle is closed and carb already runs rich. Why would I increase jet size if it is already too rich. If on the water I see lean conditions and it can not be remedied by opening needles less then 1-2 turns I will step-up the jets. How any more clear can I be. I can't care less what the piece of paper say. If conditions are rich, you do not up the jet.
As far as pop-off goes, I already explained, largely a misconception. Fuel is sucked by Venturi effect, low pressure is generated by moving air, not by vacuum.
But even if this is hard to digest... Softer spring is designed to get more gas into cylinders. If anything, I want LESS gas at the moment, not more.
It runs great, it runs slightly rich. Why the hell would I want to exaggerate slight imperfection that I have?

 

[libertcat]

Registration is on

in melty letters... I think looks ok