Walbro Pop-off explained


Here are some quotes, just as he wrote them, from an article by E.C. Birt in National Kart News:

“And the pop-off pressure of the inlet fuel needle is the <<<most important phase>>> of any diaphragm-type carburetor. What, how much, and when this phase of the carburetor takes place, is what controls the way the carburetor will work all the way through the whole range from idle to maximum RPM. ————————” “How much pop-off pressure do you set in your carburetor?” “Well I have used anywhere from six to 15 lbs.” “The carburetor can be made fast with any settings in the range of six to 15 lbs. of pop-off pressure.”

If it’s so *important*, why is there such a wide range of settings that *work*. Maybe pop-off isn’t *important* at all? It could happen.

Just recently I’ve heard there are people running with no fulcrum arm spring at all in Northern Calif. Of course this is said to be illegal by high ranking Tech officials.

You know the IKF tech people hate it when you come to their races without some part that’s a none tech item. If it’s a none tech item, why does it have to be there?

Everything past here is just my way of looking at the Walbro WB3A carb. I think I know, with a fair degree of certainty, how it works, but if my ideas conflict with yours, test both at the track and see who’s right. If yours are working good, don’t mess with them, unless you want to see if you can improve your lap times. But do it on a *practice day*, please.

Some FAQ’s:

Why the fork on the inlet needle end of the fulcrum arm: If you let your chain saw or weed eater sit around for months at a time, the carb can get gummed up. Too much gumming up and the fuel pump can’t push the needle off the seat. When the atmospheric pressure pushes down on the diaphragm, (as will be explained later) the forks will pull the needle off the seat.

Why the fork on the diaphragm end of the fulcrum arm: If something, like a little fuzz or hair or damage to the rubber tip, should get between the needle and the seat, fuel will leak past the needle, this extra fuel will push up on the diaphragm and this will pull up on the fulcrum arm putting even more pressure on the needle. Hopefully this will stop the leak.

Why a needle, seat and fulcrum arm: Beside the first two reasons, we have to remember that this is an all-position carb. In chain saws the fuel tank is often above the carb.

1. Without a positive way to close off the fuel system, if you stop the engine and leave the saw sitting on the bench, you could drain the fuel tank into the engine.

2. Without a way of limiting the amount of fuel going to the engine, while idling or during part throttle operation, the pumper would just flood the engine.

The worst thing you can do, when trying to figure the Walbro carb, is to think that a vacuum sucks. There’s no such thing as suck, and every time you say that a vacuum sucks, or pulls, you’re not looking at the problem from the right end and most of your conclusion about how the Walbro works will be in error.

It is my contention that once the engine starts, the needle is off the seat and never goes back to the seat until the engine stops. The engine never sees the pop-off everybody is so fond of checking. When some gremlin is at work, making your day at the track less than enjoyable, and you ask advice of the local hot shoe, more times than not he’s going to ask, “What’s your pop-off”. Of course if you don’t know, that will be his “Out” for not being able to analyze your problem. He’s heard other hot shoes say that, (and they win a lot of races. They must know what their talking about.) and so that’s what he says. If you respond with something like, “I set it at 12 lbs yesterday” it makes his analysis of what’s wrong a lot harder. He’ll say something like “Try 9 lbs. and see what that does”. Problem solved.

This is, IMO, how it works. If I could include some drawings, this would be a lot easier.

As soon as you turn the engine over, the piston starts pumping a vacuum in the crankcase. With the throttle closed, the three paths open to atmospheric pressure, to fill this vacuum, (nothing gets sucked in or pulled in, it gets pushed in.) are the Idle jet, the transition jet and the air premix orifice. The idle jet by way of atmospheric pressure on the dry side of the metering diaphragm. (M.D.) The transition jet and premix orifice by way of atmospheric pressure in the throttle bore. As air is pushed into the vacuum through the air premix and transition jets, fuel is pushed through the idle jet, from the reservoir on the wet side of the M.D., by air pressure on the dry side of the M.D. As the M.D. is pushed down it lifts the fulcrum arm off the inlet needle. At this point, fuel pressure from the pumper side of the carb pushes the needle off the seat (the fulcrum arm has already been lifted off the needle and there’s nothing holding the needle in the seat.) and begins pushing fresh fuel into the reservoir. A balance is found between the fuel pressure in the reservoir, the air pressure on the dry side of the M.D. and the amount of fuel going into the engine. If fuel goes into the engine faster, the M.D. gets pushed down farther and the needle comes up higher off the seat letting in more fuel and the balance continues. (no fuel is being pushed through the high speed jet because the atmospheric pressure in the throttle bore is still higher than the pressure on the wet side of the M.D.) If we go to full throttle, we expose the transition jet to the engine vacuum and fuel is pushed through it, (by pressure on the M.D. and by fuel pump pressure.) trying to fill the vacuum. The combination of engine vacuum plus the fuel pump pressure, IMO, is still less then the air pressure on the dry side of the M.D., so the M.D. is pushed as far down as it can go. This opens the inlet needle as far as possible. This is how it stays until you let off the throttle and I can’t see how Pop-Off pressure can have any effect on this process. The 14.7 lbs. of air pressure on the backside of the M.D. can easily overcome the spring pressure of the fulcrum arm. I have no way of knowing what the pressure is on the wet side of the M.D., but even if it’s 13 PSI., (this would be a vacuum compared to atmospheric pressure) the 1.7 pound delta could still, very easily, overcome the fulcrum arm spring. And you have to consider that the increase in fulcrum arm spring pressure needed, to go from 6 psi to 14 psi of pop-off, is really minimal.

The actual pressure against the inlet needle tip with 10 PSI of pop-off, is only .032 lbs. (not in PSI, but actual lbs. of pressure on the small area at the inlet hole.)

The max. size of the inlet hole, from the IKF tech book, is .064 Dia. This is how the calculation goes. Pi x Radius Squared. = the area of the hole.

.064 / 2 = .032 *.032 = .001024 * 3.1416 = .0032169 sq.in. * 10 PSI = .032 lbs. 6 lbs = .0192 and 14 lbs. = .044

In any case, the wet side vacuum and the fuel pump pressure would determine how hard the M.D. is being pushed down on the fulcrum arm. Once the pressure, (and the mechanical advantage of the M.D. is more than 2 – 1 over the fulcrum arm spring) on the needle by the fulcrum arm is released, the fuel pump pressure can easily push the needle off the seat.

In the final analysis: Pop-off has been the HOLY GRAIL of a lot of tuners for a lot of years. Old ideas die hard and you’re going to hear a lot of people saying I don’t know what I’m talking about. The facts speak for them self’s. The carb works like I’ve explained. Try my ideas, if you go faster, you’ll know who was right.

Yes, a case could be made for a certain amount of spring pressure, we don’t want to flood the engine while idleing, (not much chance of doing this with forks on both ends of the fulcrum arm.) and we wouldn’t want to siphon the fuel out of the tank. We also need a positive pressure to re-seat the needle when we stop the engine. But is it *REALLY IMPORTANT*??? I would say, as far as improving your performance, *NO*. If you have enough, (about 8 – 10 pop-off) that’s all you need. A little more or less (2-4 psi) will make *no difference*. And according to recent statements I’ve heard, you need no spring pressure, (which means no pop-off pressure.) at all.

The best advise I can give you: Start using an Exhaust Gas Temperature Gauge (EGT). You’ll stop trying to tune the engine with pop-off pressure and start using the high and low speed needles.

Replace the pump diaphragm often. Every race if possible. Nothing has a better chance, than the pumper diaphragm, of going bad. Also the circuit plate check diaphragm. If it’s not laying flat against the circuit plate, put in a new one. These parts are cheap, always keep replacements in your tool box. At the first sign of fuel problems, this is the first place to look.

And the next time someone asks you “What’s your pop-off pressure.” just smile.

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