I have an 82 Z28 with an engine from a 74 Chevelle. It has a mild street/strip cam and 3.42 gears. When I bought it I was told that the carburetor was a Holley 750 double pumper. When I hit the gas hard, there is hesitation like I am flooding out the engine, and when I launch off the line 1st gear seems really sluggish, like I'm pulling a heavy trailer. I asked a mechanic and he said that if it really is a 750, that it might be feeding the engine too much fuel when I really romp on the gas, hence the "flooding" feeling and the sluggish take offs. He suggested a 650 CFM carb.

So, a few minutes ago I went on Holley's website looking at carbs. They had a really nice page about finding your carb's model number. So I went out, took off my air filter, and took a look at my carb. Turns out, it is NOT a 750 Double Pumper, and that it is fact a 600 CFM carb, Model 4150.

So here is my question: I've gone from thinking that my Carb was too big and was flooding the engine, to thinking that my Carb is to small, and isn't feeding the engine enough gas. Is a 600 CFM carb big enough for a 350? If it is, what would cause my car to hesitate/stutter when I floor the gas pedel and be super sluggish to take off?

Thanks in advance for helping, I've been trying to figure this one out for a while.

 

Either carb would work on your engine. They will produce slightly different torque curves and different top end horsepower, but either can be made to function.

If someone says "that 750 is too big and dumping in too much gas"...run away from their advice because they likely don't know what they are talking about. The amount of gas entering the engine has to do with the various passages in the carb and the vacuum signal the carb sees. It could very well be that bolting on a 750 could cause the engine to run lean (too big CFM= low booster signal = low fuel flow).

That being said, we don't know nearly enough about what you have currently. Start at the basics. Here is the order I would go in...

1) Timing curve - Base timing? Max mechanical advance (and by what RPM)? Vacuum advance hooked up?
2) Set idle and idle mixture screws (need a vacuum gage to do this correctly).
3) What jets, PV, secondary spring and pump cam/squirters are in there now?
4) Drive it...how does it react? Vacuum readings while driving around?
5) Tune WOT (This is the easiest to get right)
6) Get the cruise and transition areas working (this is tougher to do and may take multiple changes)

There is a holley tuning thread at the top of the Carb section...read through that too. There is a lot of good info in there.

 

It sounds to me like your accelerator pump may need replaced. It gives the initial shot of fuel to the carb when you stomp on it. When they go bad you will get a hesitation just like you spoke of.
I would at least check that out first. You can google it for the 4150 and it should show you where it is and how to take it off.

 

What is the LIST number?

How fresh is the carb?

Can you see a good stream of fuel coming from the squirters?

Make sure the ignition is up to snuff.

 

X2 on the accelerator pump.. Had the same issue with my 350 in my 74 chevelle, new pump, hesitation gone.

 

The LIST number of the Carb is 4776-5, and the Date code below that is D690. I guess that means it was made December 6th of 1990? If thats true then it's a 20 year old carb.

When someone is in the car and hits the gas for me, I can see a really nice stream of gas coming out of the squirters, so I don't think that's the problem. The ignition is a Mallory ignition system, and the spark plugs seem to be fine. I took them out the other day to check them. Accelerator pump, hmmm I'll have to check into that it sounds promising.

As far as what jets are in it, the idle mixture, vacuum, etc I have no idea. I'll have to get back to you on that.

OH! Before I forget, there is one other problem that I'm pretty sure is related to the Carb. When I brake hard, my car stalls out. If I stomp on the brakes to avoid a deer or someone pulls out in front of me, I stall out nearly instantly. When I am coming down a long hill and I brake softly everything is fine, but when I really start to brake harder and feel the G Force from it, the car stumbles and stutters and the RPMs go way down, then it stalls. I've heard of people having this same problem on all sorts of carbed cars, if they brake hard gas pours into the carb and floods the engine or something along those lines. Is this happening to me, and if so how do I fix it?

Thank you so much guys I love ThirdGen.org, so many nice people willing to teach clueless people like me.

 

You just verified the accelerator pump is working properly. The pump is what makes fuel come out of the squirters. Of course, you have to do that with both the primaries and secondaries. Check it with the engine shut off so you can verify both are working.

Check the float levels.

If the ignition timing isn't set correctly, nothing you do the the carb will fix that.

 

Thank you Five7kid and everyone else that has helped, I will check all the things you have suggested. I really appreciate the help, thank you!

 

check all your hoses make sure there isn't a vacuum leak. you might be having problems with your float levels.

 

Get the carb sized right for the engine is critical if you want the best performance and ease of tuning. This based on two things: engine displacment and target rpm of the cam and intake.

For example a 350 with a target rpm of 5900 should be running a 600 cfm carb, not saying a 750 carb won't work just not to the best ability and tuning is much more difficult and time consuming.

I have formulated a chart based on this principle (volumetric efficiency) and has worked very well for me over the years. I can email if anybody wants a copy.

For our heavy cars I would almost always use vacuum secondaries.

 

How are you determining the VE at max RPM?

There are people out there that advocate carbs MUCH bigger than what many would prescribe (David Vizard is one). The key is to have a booster with enough signal to operate correctly. I am talking about running a dominator on 7000RPM, 350 chevy! This would never work with standard dog-leg or straight boosters. I have never purchased one of these specialty carbs (Bo Law, Troy Patterson, etc.), but it does intrigue me.

 

Note: I corrected my original post to the exact numbers, I was going off memory.

I would be glad to explain how I come up with my calculations: this has been pulled from a few race engine builders and my own experiences so I can't take all the credit

One 350 cylinder holds 43.75 cubic inches of air (350/8 = 43.75). We are dealing with a four-cycle engine, only four cylinders of an eight cylinder engine draw air for every revolution of the crankshaft. So we have 43.75 x 4 cylinders = 175 cubic inches of air the engine will draw through the carb for each revolution of the crankshaft. We must convert cubic inches of air to cubic feet of air we divide by 1728 (there are 1728 cubic inches of air in one cubic foot of air).

175 divided by 1728 = .1012 This means that with one revolution of the 350 crankshaft the engine will draw from the carb .1012 cubic feet of air.

Now take .1012 and multiply by the maximum number of revolutions the engine will run.

From my example two post above

5900 rpm x .1012 = 597.08 cfm

This particular combo would be best suited for a 600 cfm carb.

An important note that if the maximum amount of air this combo can draw into it's cylinders is 597 cfm at 5900 rpm then no matter how large a carb is installed on the the engine will only draw in 597 cfm at 5900 rpm.

Of course this is at 100% VE which most engines are between 80% and 90% VE,so even 600 cfm is a little big. This shows how important component selection is and number one how the engine is assembled to make the engine efficient which is a whole other conversation.

Concluding for me most people waste money on a bigger carb when what they have will do just fine, spend the money from carb upgrade on a better exhaust, bigger cam or better flowing heads all increase efficiency if done right.

 

If the plenum is large enough that flow through the carb is relatively steady, and you don't mind the carb being a 1 1/2" inHg restriction at that speed. It also assumes that there's no fuel suspended in the flow displacing air too, but that's a pretty trivial thing.

 

I wasn't trying to shoot you down TransAM Joe, I was trying to spur a discussion. What I have seen recently are a lot of guys pushing carbs that seem way too big (using your logic above) and yet these people are making more power and torque across the board...not just at high RPM. We see this with factory setups too. Did you ever see the size of the throttle bodies on newer LS engines? Of course, they only pass air. When we use VE in your equations, we are assuming that air speed (in the intake tract) is meaningless. It is not. The manifold needs to recover after every opening and closing of the valve. In other words, the engine is a dynamic model, and the model above is static.

As I said, I do not fully understand all of this, but it is interesting. Here is what I know from personla experience...I put a 750 Mighty Demon with annular boosters on my 350 (6800RPM max). It flows similar to a Holley 800. 6800x.1012=688CFM. I picked up torque and power across the board with this carb. I thought it would be too big, but it is not. It did take a lot of tuning to get it to run properly. A 650 double pumper probably would have been easier.

 

David Vizard would be correct. There is no set size that can be determined based on such calculations because there are way too many variables to imply a 600-cfm would be the ideal choice for such an engine in the example provided. 100% VE is irrelevant because we are referring to 0" of vacuum, and 600-cfm can handle the needs of upwards of 30-psi in boosted applications for a 350, so at 0" of vacuum 600-cfm can be considered more than just a little big. Understand that so long as the booster signal is correct, the amount of carburetion cfm merely effects where the power is in the RPM band, meaning cfm flow effects vacuum and/or the resistance and it only really moves the average and peak power around the band when it is altered, but the entire valvetrain also has a say in that, not just the carburetor, and stall and gearing also have a say as to where you want that power. Consider those who are running cold air intakes leading to a 600-cfm carburetor's inlet, and not an open element, as suddenly now the carburetor can be viewed as an "extension" of the intake manifold because the diameter of the cold air intake tubing becomes the orifice restriction, and not the carburetor itself. Too many variables, but there is no way to just say that a 600-cfm would be better than a 750 without knowing every single thing about the car....

 

with the stalling, the float levels are too high, especially the rear, gas is sloshing forward and flooding the engine. Floats are to be adjusted while running and you want the gas to be just barely trickling out of the sight hole.

Whats details on the motor? might have wrong size squirter, mis-adjusted accelerator pump, wrong power valve, etc.