Don Terrill’s Speed-Talk

levisnteeshirt wrote:without the baseplate drilled for 4 corner ,, its not going to work right with that metering block ,,, ya need to get a Holley book , it shows you how to do it , or find a metering block that doesn't have the screws for the back ,, the secondary block kits i've bought in the past , doesn't have the screws ,, so ya have 2 choices , get a metering block , or have the plate drilled ,,

I screwed up. Forgive me, I've been sick the past two days. My metering block on the secondary side of my vacuum 750 have NO MIXTURE SCREWS. It has replaceable jets, I was getting myself confused. Sorry! Mixture screws in the primary block only.

Thanks,

The O2 sensor goes on the exhaust pipe. Can go after the header. Must be a heated type to read correctly at idle. 3 wire or 4 wire. A NTK #21006 replacement is like $30. the gauge you got is fine. You just want to be real close to the 14.7:1 crossover point. Center light on the three light gauge. You can also read the O2 sensor output using a volt meter. Use your fingers over the idle air bleeds to shift the idle rich-lean and see the gauge react.
The exhaust must not leak to be accurate. a 1 wire O2 sensor (without heater) will not read accuratly at idle.

Buy some small precision numbered drill bits to measure the size of the air bleeds on the stock carb body.
or take it to a machinist and have him measure for you. A numbered drill bit set is like $10 not that hard.
Measure the drill bit diameter (or what ever else use use to gauge the idle air bleeds on the stock donor body with a dial caliper.

Try #70 or #71 primary jets in both carbs. (same ET, better MPG) The AFR gauge will show you the way.

The final primary idle air bleed should be a bit bigger than 1/16" .062" (.064 to .067") when you get it all done.

When evaluating a air bleed size with the AFR gauge be sure to check at idle and just off idle
Be sure the engine is fully warmed up and there are no carb or manifold vacuum leaks.

Measure pri and sec air bleeds in the stock 3310 750vs carb body and post the result.

What you need to understand is that the sizes of the idle jets and bleeds should be set primarily for the proper function of the transition circuit. Fuel requirement at light throttle loads as well as the duration of operation of the transition circuit are set by the combination of the idle jet and idle air bleed. Fuel requirement for any engine is load dependent, and since the load is lower at idle mixture screws are placed in the circuit to taper the fuel needs to the particular engine.

O2's are not a good way to set the fuel delivery of the idle and transition circuit, it is possible to see AFR above stoich at light load conditions. Tune it for what the engine wants. An easy way to test is to start the engine cold, bring the RPM's up to around 2000 RPM in park or neutral or just before the main circuit starts dribbling fuel. Put your finger on one or two of the idle air bleeds, when cold the RPM's should increase a little. Once the engine is at operating temp set the mixture screws for best RPM with the least amount of turns out. Bring the RPM up again, when you put you finger over the bleeds it should not increase RPM or slightly decrease. This will have you in the ballpark for tune, you can try leaning it from there by going up on the idle air bleeds around .003 at a time until you find a drivability issue (stumble or surge at light throttle cruise conditions) then go back. Idle jet changes affect the overall amount of fuel available, the idle air bleeds affect how much is used and to a degree the duration of the transition circuit operation. A good starting point for a 750 to 1000 4150 style carb on a single carb app is around .037 for the idle jet and .070 for the idle air bleed. The position of the idle jet can have an impact on the operation of the idle/transition circuit and even on the main circuit, so tuning the whole carb can vary. The picture below shows the typical idle circuit jet locations, I prefer to tune with the jet in the lower position as it provides a more stable fuel delivery for a low vacuum performance engine.

Mark, have you seen any holley's with the idle jet placed in the main body?

That would be difficult since the idle circuit pulls fuel from the mainwell at the bottom location shown in the picture above, does not pass to the main body there, goes into the idle well before splitting between the idle mixture screws and the t-slot passage. Because the mixture screws can be completely open to the fuel delivered there or shut it off there is no need for a separate restriction. A restriction leading to the t-slot on the other hand is common, I have them in any Dominator I build as in the picture below. A 4150 has a shorter slot, smaller area, and usually needs no restriction.

Mark, thanks for the response. I've got a 750 holley main body that has restrictors in that passage leading to the transfer slot but they are installed vertically just above the baseplate, and the primary metering block on this carb doesn't have any idle feed restrictors top or bottom.

Is that a gas or methanol carb?

Bob, it's also possible the idle feed restriction is still there, a pressed insert into the idle well itself. You will have to pull one of the plugs out of the top of the metering block to see it.

Took the car out today. I swapped in the bleeds drilled out to .063, put in 80 secondary jets, reset idle and it works much better. There is still a slight something but with the wife and daughter along for the cruise I didn't mess around too much. I did notice I have a vacuum whistle. It seems to be in the area of the primary butterflies. I have idle set at 800 RPM but if I drop it back to 750 it whistles. It seems directly affected by the idle screw. Any idea?

Thanks,
Doug

primary throttles too far closed at idle. secondarys too far open. remove the carb and flip it over and reset the throttles at idle.

Make the pri and sec throttles about even so about .020" of the idle fuel transfer slot is exposed on each.
The sec side is set by a set screw in the throttle base. You want to preset them about even.
They both need to be in this sweet spot ( transfer slot exposure at idle)

This will also contribute to a off idle flat spot.

Once you have done that, you need to finish the air bleed fine tuning job, using the AFR gauge/O2 sensor.
It is likely just a tad too rich on the just off idle afr, now, with a .062" drilled primary idle air bleed . May need Slightly bigger pri idle air bleed. hone by hand with the .062 dril bit.
You want the off idle AFR between 15:1 and 13:1. Too lean (idle air bleed too big) create misfire and bad throttle response.
too rich (idle air bleed a bit too small) tends to foul plugs, may gurrgle at cruise, wastes fuel.
Very small changes in the idle air bleed size makes a big difference.
The afr gauge and a vac guage allows you to get it just right.

Again look at the stock carb body. What are the primary and sec idle air bleed sizes on that body?

If you want to do a lot of street cruising and want a sharp runner get a vacuum advance distributor or add vac adv to your distributor and tune it too.

What is the spark timing at idle (base) and at max mechanical advance?
The distributor probabily needs some work.
The accel pump cam (colour and mounting location) and linkage adj may need further attention. may need a .031" shooter.
Sometimes the accel pump arm that rides on the pump cam cam gets bent and has to be bent back to spec.

Old plugs should be replaced. .035" gap. Plugs may need heat range selection adjustment.
When setting the .035" gap make sure the ground strap does not extend past the center electrode on the plug.
Having the carb out of wack really messes with the plugs and fouled plugs rarely recover.

Are you using the PCV system? or...? Ya it matters.

If your local gas station is still selling winter gas, it will create drivabilty issues now that the weather is warmer.
Winter gas tends to vapourize way too easily in warm/hot weather.
This is a issue for a few weeks in the spring, every year until the gasoline is adjusted at the pump for summer weather.

After market aluminum intake manifolds with heat riser passages and plenum heat often need the amount of carb plenum heat/exhaust riser passage heat flow adjusted to get the carb body and plenum temp just right. Too hot is just as bad as too cold.
Holey carbs often need a heat shield under the carb that keeps the heat off the fuel bowls if the bowls sit close to the runners on the manifold. A wood carb spacer helps too.

Just a silly question..... Has anyone gotten better power from the Pro-Form body over the Holley?

how far are the idle mix screws open ? i like 'em a 1/4 more open than max vacume myself

jmarkaudio wrote:Bob, it's also possible the idle feed restriction is still there, a pressed insert into the idle well itself. You will have to pull one of the plugs out of the top of the metering block to see it.

I'll check that, I think this might have originated as a Marine 750 but to me the block looks the same as any other 4150/4160 primary block I've seen except there's no visable idle feed restrictor.