Pilot Jet Size for 459 with open header aftermarket aircleaner

[Ryanflathead45]

Hello All,

I'm having some issues getting my 459 predator to idle slow enough that the 780 comet clutch does not start to engage.

I installed a new carb that has the adjustable idle mixture screw. That alone did not help much. So I drilling out the pilot jet slightly.

That helped a little bit. But the idle is still too fast, and rough.
The mixture screw does not make much difference to the roughness of the idle.

I'm running a briggs style pulse pump. At first it was over powering the needle valve and flooding the carb. I restricted the pulse line and its not over flowing now, but maybe this is still part of my issue.

Let me know your thoughts. This is the first time I have ever ran a fuel pump, so its driving me nuts.

Thanks in advance,

[raydhd]

IMHO A better way would be to run a restricted fuel return line.

[Ryanflathead45]

I had not though about a restrictor for the feed line. Would I need to put a Tee in my fuel line and run a return line back to the tank too?

Before I opened up the jet it was popping and backfiring out of the exhaust. I assumed running lean.

Its not acting as bad as it was but still not smooth. The mixture screw changes it slightly, but its still bucking and surging.

[raydhd]

That is what I have. A tee in the line and a restriction in the return line going back to the tank. I had to do this regardless which fuel pump I ran. Also both my 2 circuit and 3 circuit carbs have adjustable main and idle jets.

[raydhd]

Another thought is Vegascarts has a technical blog "carb tuning 101" on their site talking about pilot jet drilling.

[Jbrowne479]

did you verify you have the throttle blade closed down far enough so that you are only running on the idle circuit? i would think if it was over powering the needle and seat, it would be running rich.

[Ryanflathead45]

A little update.

I put a tee in the line going to the carb, with a return line back to the tank.
I have fuel coming out of the return line, so hopefully that helps with the overpowering the needle and seat problem.

I went back to a stock pilot jet I had and it made the idle problem worse. So I put the one I drilled out back in.
I think its just a little bit too big. I can adjust the mixture screw now and notice a difference in idle quality...but can't find a sweet spot where it smooths out real good. Its like it goes from starving for fuel (govener surging, trying to keep it running). Then it seem to run rich and be coughing and sluggish sounding.

I have the idle screw backed out fairly far. I keep tweaking the mixture screw a bit, when I get it to smooth out fairly well I'll try and slow the idle down some more then adjust the mixture screw again.

I bought an adjustable main kit that comes with a pilot jet so I'm going to start all over when they come in.

My micro drill bits must be cheap and out of tolerance because the stock jet and the "larger" jets that came with my air cleaner are all coming in about the same size.