Carb problems with non-adjustable air bleeds

[chevy370lsx]

Motor is a 13:1 flat tappet sbc with an old 850 c&s alky carb non adjustable air bleeds that I just put on, I just went thru and cleaned the carb. Cant get the motor to idle down with out dying almost like its a vaccum leak and throttle response is horrible, had the same problem with the last alky carb. Any how when I cover the 2 outside air bleeds the motor starts, runs, idles, and revs great. What I'm wanting to know what does this tell me I need to do in the carb? In the past I've just put some silicone in the holes to plug it off, I'd rather not do that just to know how to fix it the right way.

[dynoman14]

What you have done is close of the idle air, which makes me think the carb is running way lean. How big are the air bleed holes about .065-.125 is a range depending on metering block set-up. Also how big are the inner holes? I use hole gauges but the barrell end of drill bits work fine to get you in the ballpark.

Did you block off all 4 outside holes?

How big are the holes in the butterflies?

If you are running MeOH your jets will be in the 100-110 range right??? Power valves in the front and rear 4.5-6.5 range.

What happens when you adjust the idle screws on the side with the carb running.

What is your fuel pressure and what is the float bowl level?

I guess when we get these answers we can go to the next step.

Something is wrong because the idles on an alky carb are usually way big...

[chevy370lsx]

I just blocked off the 2 outside holes.
The holes in the butterflies and the actual air bleeds I'll have to find out when I get back out to the shop, its supposed to have a new throttle plate on it.

FP is 7-8 psi, float level is correct as well.
The last c&s methanol carb(750) acted the same way but was not as bad, it was the same type carb, non aerosol with non adjustable air bleeds. I had 98-99 jets with the afr 12.5-12.8 on the dyno with that old carb.

When I got this carb a few weeks ago it had 130 jets and power valve block offs on the front and back. I called c&s and asked them about the carb and they said start out with 90 jets, they also said it was built it 1984.

I put 98-99 just to get going with a 5.5 power valve that I had laying around. With almost 4 rounds out it was still very lean. Covered the 2 outside holes and the motor idled down like I wanted to. I ended up plugging them with some silicone, starts without giving it any gas, idles great and revs really good, no hesitation. I know this is not the correct way to fix this, thats why I want to know how to fix this issue as its not the first time I've done this.

[yelloblue2]

i thought alcohol cars ran around 13-15 psi.

[Earl Parker II]

I think dynoman14 is exactly right- either the idle air restrictions are too large or the idle fuel restrictions are too small.

To correct things, first remove the carb, invert it, and make sure that the butterflies are set in the proper position. You'll want to adjust them, front and rear, until you can just see the bottom edge of the transfer idle slot below the bottom edge of the butterfly. The idea is to have the very bottom of the slot, and no more, exposed to manifold vacuum at idle. With the butterflies in this position, the entire transfer slot is 'available' to do the job of bridging the gap between curb idle and the onset of main system operation. Believe it or not, this adjustment is absolutely make-or-break for maximum acceleration.

As for the idle air restrictions, here's a good quick-and-dirty for finding the correct air restriction size. Remove the silicone and find the size of all four idle air restrictions using drill bits (OK) or pin gauges (better). Let's say for the sake of discussion they're all the same size. Now, find some wire that is about 50% the diameter of the bleeds, i.e., if the bleeds are .100" the wire should be .050". Cut four pieces 6" to 8" long- exact length isn't critical. Insert one end of the wire well into each bleed, then bend/drape the other end over the outside of the carburetor in such a manner that it isn't going to move. In the above example you have now temporarily reduced the area of the bleeds by ~25%. The exact wire diameter isn't critical- close will do.

I don't know what C&S is using for mixture needles but turn them in until they're lightly seated, then turn them out 1 1/2 turns- that should be a good starting point. Start the engine and warm it up- you'll need some decent water temperature to do the following. Slowly, one needle at a time in 1/4 turn increments, adjust them outward and see if idle quality is affected, i.e., can you richen the mixture enough to cause idle speed to fall off. Note that 3 turns is probably about as far out as you need to go.

Let's assume you can't effect a change yet. Insert a larger wire in each bleed and go through the procedure again. Keep trying larger wires until you get some adjustability with the idle mixture needles. Ideally, you'll want to find a wire size that allows you to achieve the best idle quality, i.e., the highest idle RPM, with the mixture needle set somewhere between 1 and 2 turns out. You should be able to find a point that turning the mixture needle 1/4 turn either way from the optimum position will lower the idle RPM and hurt idle quality.

Once you've arrived at a wire size that works, calculate the cross sectional area of the restriction and do the same for the wire. Subtract the cross sectional area of the wire from that of the restriction, divide that figure by .7854, then get the square root of that figure. That will be the diameter of idle air restriction that you need.

A few things to note:

1. By far the best way to do this is with a manifold vacuum gauge. The goal is the highest manifold vacuum reading you can get adjusting each mixture needle.
2. Do the basic adjusting in 1/4 turn increments- when you get close to the optimum setting, adjust in 1/8 turn increments.
3. There's no rule that requires all four idle air restrictions to be the same size.
4. Once you know your manifold vacuum at idle, install a power valve that opens about 1.5" below that figure, i.e., if you wind up with 8.0" of manifold vacuum, use a 6.5" power valve, etc.

As for your fuel pressure, you probably have a pretty large needle & seat. The fuel pressure required or, more correctly, the fuel pressure that can be tolerated, is proportional to the area of the needle & seat (all else being equal). I can't speak for alcohol- my alcohol experience is limited to my wife's kart back in the late 80's. However, with gas, a .110" needle & seat will tolerate no more than 6 psi of fuel pressure- any more and the float cannot close the needle against the seat. With a .120" needle & seat that figure drops to 5 psi. My guess is that your needle & seat is at least .120", likely larger. Someone with alcohol experience can doubtless supply the correct figures but I'm thinking that, at 7 - 9 psi, the pressure is overriding the float's ability to close the needle against seat. This creates a too-high fuel level in the bowl, resulting in poor acceleration and the inability to properly tune the idle and main systems. Like idle system setup, proper fuel pressure is make-or-break for maximum acceleration. Note: if you do alter fuel pressure, but sure to re-check for proper float level.

Understand my experience is with gas, so please be sure to consult someone knowledgeable concerning the correct pressure with alcohol. I just wanted to illustrate the issue with too much fuel pressure.