Yamaha engine


What should my pop-off pressure be on a Yamaha engine with a kb-do pipe?

Answer: Pop-off pressure huh? That is a real can of worms, First, every gauge and every person will get a different reading on the same carb. So, before we can go very far with suggesting where to set it, lets talk about how to check-it. Always check it with the metering diaphram as well as the fuel pump cover removed. First, put a few drops of marvel mystery oil in the fuel well underneath the fuel inlet needle. Check it with a good quality guage. Remember not to “creep up” on the pressure and always try to apply the pressure at the same rate.

We look at two numbers here. First is the number in psi where the needle pops-off. The second number is the hold pressure, also in psi. The higher the rate of the spring, the closer these two numbers will be to each other (Note: I said rate of the spring, Not the pre-load of the spring). The higher the pre-load of the spring, the higher the pop-off pressure will be. The rate of the spring is changed by the diameter and alloy of the wire used, as well as by the number and size of the windings. The pre-load of the spring is changed by stretching or compressing the spring. Also, the fulcrum arm height is the distance stated in thousands of an inch, measured from the metering gasket surface to the top of the fulcrum arm. The lower pop-off numbers allow the pump to overcome the spring pressure and supply more fuel. Fulcrum arm heights of a lower numerical number let more fuel sit in reserve for instant supply to the venturi when the demand calls for it. Generally, lower pop-off will result in a motor that runs a little “richer” on the bottom end of its power-band and a little “leaner” in the upper rpm range. After the pop-off has been changed to your liking, reinstall the metering diaphram and its cover, then before you tighten it, very gently, with a pointed object, reach into the hole in its center and make sure that the diaphram is centerd in the fulcrum arm’s fork (a ball point pen will work for this). Then, tighten the assembly and recheck your pop-off. It should be within one psi of where it was when it was apart. I also would like to add here that the racers that have the most trouble with pop-off, are usually the ones that are always messing with it.

The fulcrum assembly has to “wear in” to itself much like the rocker arms on a car motor. Each time that it comes apart, this “wear in” process has to start all over again. I currently have several shop carbs set-up at different pop-off readings. So, the only time that they will have to come apart is for tech at a national event. For the kb-do, with the method checking pop-off as described above, I suggest 8 1/2 pop-off, and holding 6 to 6 1/2 pounds, and fulcrum arm height of .040” to.045”.

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