IFR Size

[MD383]

Hello. I have a Q-750 on a 400 sbc with a 244@50 cam with 292 duration. Carb has .031 IFR’s, .067 TSR’s, 71/81 jets, 8.5 PV, 73 IAB, and 28 HSAB. .054 PVCR’s. IFR’s relocated to bottom and two hole .028 emulsions being used. 9.92:1 compression with a edelbrock dual plane rpm air gap manifold. 4 speed with 3.31 gears and if I don’t slip the clutch enough it will buck off idle and when at lower rpm like 1500-2000. Cruise and WOT is fine. Increasing IFR size to .033 but I didn’t see a difference so I put the .031’s back. I heard that with cams duration over 233@50 should utilize at least a .033 IFR. Is this true? I also have a 327 with a 250@50 cam and the carb has .028 IFR’s if that’s the case then I should increase those as well. My spark plugs look good with the .031 IFR’s and I have .020 transfer slot exposure on the primaries and none showing on the secondaries. Timing is 20 initial 36 total and 10 vacuum advance. Is there a range for IFR sizing depending on cam size/cubic inches? Thanks.

[GTO Geoff]

The IFR is sized for the T-slot/off idle/light throttle cruise. It is NOT sized for idle, as many think.

[MD383]

[Markv]

I always thought the IFR would supply fuel at idle. What does control the idle fuel? The mixture screws only? Why do you smell raw fuel out of the exhaust after the idle screws have been adjusted? I think my mind is resetting.

[GTO Geoff]

The IFR DOES control idle fuel, but it also controls fuel to the T slot. The T slot will need more fuel as rpm & load are increasing. Because the idle fuel requirement for idle is less than the T slot, idle fuel delivery is made adjustable via the mixture screws. A properly calibrated carb will have a small amount, ideally about 0.040", of T slot showing at idle; so the T slot also supplies fuel for idle, & the mixture screws fine tune/trim the mixture. The problem occurs with bigger cams that need a higher idle speed/more fuel. More T slot is exposed making for a very rich idle & the mixture screws become ineffective at controlling idle.

[Markv]

Thanks GTO.

[tbarb]

I would try a smaller idle air bleed, maybe .067 to see if that's the right direction and remove the TSR or enlarge it much bigger. If the vacuum advance is connected to full manifold try plugging it just to see if taking timing out helps.

[rgalajda]

Idle mixture, here is the problem.
For a short cammed engine vacuum is much higher which tends to vaporize the fuel better, and add to that a stock heated intake manifold increases vaporization for an easily ignited fuel . Distribution cylinder to cylinder will also be good.
Higher compression of yesteryears reduces residual exhaust pollution.
Lastly a true dual plane ( fully divided ) intake has less cylinder to cylinder pollution at idle because all the cylinders are not connected to a common plenum.

An example. 350 LT1 1970 used about 240@.050” duration camshaft on at least 114 wide LSA with 11:1 compression . Fully divided dual plane intake manifold with crossover heat. Good vacuum and Good idle.

396/375 hp used about 240@.050” duration camshaft on at least 114 LSA with 11:1 compression. Fully divided dual plane intake manifold with crossover heat.
And these cams had slow ramps on solid lifters. Good vacuum and Good idle.

Now we install a high duration cam on a tighter 110 LSA with an unheated intake manifold and compression ratio is modest . The RPM AIR GAP intake manifold runs cooler and has a cut out ( not fully divided ). This results in less fuel vaporization and some exhaust pollution within the intake manifold due to the cutout .
Dilution of the incoming charge by exhaust residue left in the combustion chamber at the end of the exhaust stroke. In addition you have exhaust dilution due to exhaust flow reversion at the end of the exhaust stroke. Overlap and tighter LSA .
Lower manifold vacuum and cooler intake manifold temperature means less chance of good fuel vaporization for a smooth low rpm idle.
The Fix
Idle Mixture ratio needs to be richer and more air added , even with best idle timing .

I run a camshaft with 107 LSA and a single plane intake witch took a .037” IFR to keep it happy with 32 degrees idle timing.