Main jets

I have been reading various threads regarding main jet sizing. Some suggestions will tell a person to change from a size 60 to a 120 this seems like a major difference. how does one decide what jet to use. i know the smaller the jet more fuel goes to the motor. My system came with a 79 in it but i was told by a another racer it should be 130. i am just looking for advise on why such differences in the sizing.

 

when you set up a fuel system it all starts with a base line...as in total hat area total port area and what your pump flows, how many gals. you put to the motor how many gals that is returned to the tank, and pump pressure with an enderle around 140 to 160 psi seems to be a safe zone with a waterman you can use alot more pressure...pressure will help atomize the fuel, after you have a good base line tune up you can then work on pump pressure I use a davenport program and it works great, if your pump pressure is to high you can go to the hat and ports and + or - % so if you are to high on pump pressur you type in for instance -20% and it will calculate it for you and never change your tune up or gpm to the motor it will tell you what nozzels to change and what pill to use...if you get lost call me I can talk better than type..Dave..620-249-5909
www.myspace.com/daveloweswr

 

Unless the person telling you what jet to use knows how much the gallonage the pump puts out and knows the nozzle area and knows how much boost your putting in plus several other factors calculated I would'nt listen to him. The thing you need to figure out is-how lean or rich is your motor. The easiest way to find out is buy reading your plugs. If they look brand new its to rich.You want to start to change the color of the plug on the metal around the porclean, then your starting to get close.Never make huge jumps in main jet size.A 120 jet does not flow twice as much fuel as a 60, but a lot more.

Dale

 

pump pressure

my pump pressure is low at only 85 through the lights and 55 at the shift. i do run a 150 main jet and a 55 high speed to get the plugs to read color. I have 46's in the hat and blower and 35's 38's 40's & 42's in the manifold. 540 BBC conv head, 8-71 Kobelco, 30% over, 23 lbs of boost in the lights at 7400. 6.70 @ 198. If the pump is weak and not putting out the pressure, do you think the nozzel area is too big? don't want to go much smaller on the port nozzel since the orfice area will be able to clog with the slightest piece of debris. (2) of the smallest holes (35, 38) are the coldest plugs. any ideas?

 

RPM ports on boxes

Does the port on a 44 box for the rev limiter, see the same signal as the port on the Retard box. Need to run a RPM box for shifting. BuT my boxes are non- rev limiting. Have two good boxes, and really dont want to buy another box if I don't need to. Looked @ all the paper work I have and can"t find anything on it. Thanks Don donimus333@yahoo.com

 

Boxes

I am sorry for that reply, I meant to start a new post, and i didn't x out. I will try tat again. Don

 

3-D, Sounds to me that your setup is working pretty good. I wouldn't worry too much about the pump pressure,in that the fuel going through the blower is atomized well, and the fuel in the ports going through the small jets at that pressure should atomize as well.If your pump was going bad you would be chasing the main jet smaller to keep it from being too lean.For that amount of boost, it sounds to me its running good.

Dale