Racing Fuel Systems

Looking for a good starting tune. The T slots seem rather long. What is a good starting point for T slot restricters once I measure. IFR is currently .033 lower. No holes in throttle plates [yet]. IFR is .031. MJ are 78 F X 86 R. PV is 6.5 tuneable jetting. This is for a 446, indy heads cam 271@ 50 13.3-1. Timing 27 @ idle 1300. 35 total. Currently running a Pro Systems 950 [830] version 1.37 x1.75. It doesn't have a clean crisp idle no matter what changes I make. So I thought I would try another carb for a comparison. The P S carb needed .106 holes in throttle plates just to idle. I think the long duration is effecting the signal at idle.

It is going to need a loooot more idle timing with that cam & it should be sorted first before touching the carb. Advance the timing with engine idling, by slowly turning the dist until highest idle rpm is reached. Then check what the timing is. Is it a BB Mopar?

Not sure if you understood, but I was talking about idle timing, not WOT timing.
Locked dist gives the engine the idle timing it wants, but could cause light throttle detonation under load because the timing does not drop off. Using an adj vac adv unit gives the engine the extra timing, but ideally drops off with load.

I will start with quotes from David Vizard ( because he can explain it better than me )


“Anything that reduces vacuum (such as bigger cam) means that the butterfly needs to be open wider to supply the idle airflow called for by the engine because the vacuum (suction) by the engine is less. In this position , less of the transition slot is available for doing its job. The first fix is to use a four corner idle system where the primary and secondary barrels supply the engines idle air demand.

At a certain point the cam may be so big that even four idle circuits are not enough.
Under these circumstances it may be necessary to drill the butterflies to allow further closing of the butterflies in an effort to gain more transition slot use.

Idle mixture is effected by the dilution of the incoming charge by exhaust residue left in the combustion chamber at the end of the exhaust stroke. In addition you can also have exhaust dilution due to exhaust
flow reversion at the end of the exhaust stroke. This is where the overlap generated by the cam comes into play. Lower idle vacuum and cooler running intake manifolds means the fuel is also less likely to vaporize as well.

This makes it more difficult to ignite , and the fix is more fuel for any given amount of air.

IN THE CASE OF A BIG CAM AND SINGLE PLANE INTAKE , THIS REQUIRES MORE IDLE AIR WITH A LOT MORE FUEL
If the engine has a race cam that generates minimal idle vacuum , you should have additional air drilled into at least the primary butterflies , if not both primary and secondary butterfies. ( unless your carb has idle bypass air adjustment )

Start with a 1/16 inch hole and work your way up to 1/8 inch. If the problem has improved but not completed cured , start drilling the secondary butterflies. Be aware that only a big cammed all out race engine requires as many as four 1/8 inch holes.
A typical race engine need 12.5 to 13:1 for its best chance at a steady idle in the 1,000 to 1,100 rpm range.

The optimum idle advance is typically about 35 to 40 degrees for a short cammed street engine and ( though not commonly realized ) as much as 50 degrees for a street/strip engine . “



You stated in your post.
271@ 50 13.3-1. Timing 27 @ idle 1300. 35 total.

This is a race engine. Is 13.3 your compression ratio?

So you start with timing , move on to adequate idle fuel / air.
.050” transition slot front and rear butterfly opening to start
Transition slot tuning comes after idle is sorted out correctly

This ^^^^^^.

A lot working against you with that cam. As Mr Vizard says low vacuum and intake dilution from reversion exhaust gasses being drawn back in the cylinder (not enough oxygen making it into the cylinder due to exhaust gasses displacing it) asks for more air to pass through the carby - and then enough fuel to make sure all the oxygen can be consumed. It’s all working against establishing what we want to bring that engine down to and call idling. Even with the air needed (especially without) that engine will need a substantial amount of ignition timing to give what would be poor combustion a big head start. I don’t think a clean crisp idle will be all that attainable, unless our definition of that is different.

Four corner idle - are you using primary and secondary throttle blades 50/50 to adjust idle speed?

Being a cam for 7000+rpm what is the car used for?

I missed the 13:1 CR. That will reduce idle timing requirement, but not by much.

Yes it's 13-1. No provision for vacuum advance[MSD pro billet dist]. IFR .031 lower. IAB is 70. Idle @ 1300. Both throttle blades are square..106 holes in all 4 blades for idle air. Mostly drag strip with occasional street driving.

"Currently running a Pro Systems 950 [830] version 1.37 x1.75"

I'm assuming this is 1.37 venturi and 1.75 throttle bore. If so this is your saviour, producing a better signal.

.031" IFR would be on the small side for this cam. I think I would start there enlarging to .035" front and back