The Stovebolt Site Forums The Shop Area Engine Shop 4-barrel carburetor on GMC 302

Last year I rebuilt my military 302 with cast iron headers and a Clifford intake manifold mounted with a Holley 390cfm 4-barrel carb. At higher revs, 2000-3000, it pours out power, but running through the gears there is an ultra-annoying dead spot just following each shift at around 1500. Is this overcarburetion? Something I can solve by rejetting? Or do I need more carb heat or intake preheat?

This manifold is heated by engine coolant passing through a jacket in it.

The engine is in a '54 GMC 350 2-ton, and does a lot of hauling.

Isn't that 390cfm a bit under powered for a 302? I have a carter 1406 that I think is closer to 500cfm on a Stud 289 as a comparison. I've also read that the fuel load from a 4bbl tends to pool a bit in the manifold on long intakes like a GMC has, which is one plus for a 2 carb set up as it has a wider distribution 'footprint'.

Interesting thought - that would fit the symptoms of what I'm experiencing. It's almost like a turbo lag without the turbo. There's another post a ways back about carb flow capabilities that made the point about CFM = displacement x max rpm, which in this case is 302/1728 x 3300 = 576, so I could in fact be a little short on carb here. On the other hand, the original carb for the 302 is a Stromberg WW 2 barrel, which has to be less than this and was not associated with this momentary starve after a shift.

just keep your foot in it.

The volumetric efficiency of that engine is no where close to 100 percent (unless you have upped the compression to maybe 11:1, and had a custom-ground cam installed). More like 75~80 percent.

Thus the equation for needed CFM at W.O.T. would be:

CFM = [(302 * 3500 * 0.8)] / 1728

CFM = 489 maximum

I ran a Carter 400 CFM on a modern 300 CID with 9:1 compression and headers (no cam change) and it was just about right.

It could be your carb needs more tuning, it also could be the timing curve.


Jon.

Take the cubic inch displacement of the engine and double it. 302 X 2 = 604. That is the very minimum CFM of fuel management you need on a normal operating day with no real load. Raise that number 15% and you'll talk about an adequate fuel supply for operations under a load.
Normbc9

604 CFM on a 300 cid inline engine is ridiculous over(kill)carburation.

I would listen to Jon...

CFM = [(302 * 3500 * 0.8)] / 1728

???

http://www.thecarburetorshop.com/Carburetorsizeselection.htm

CID = 302
RPM = 3500
VE ~= 0.8

I used 3500 as the max RPM he would ever see. If we plug 3000 in for RPM then the CFM comes out to be 396.

Jon

I am a new member to stovebolt. I have a 54 GMC half-ton with a 270. I have owned the truck for the last 45 years. I am referring back to Ranger's original post that started this thread. Believe it or not, 6 years after his post I have just installed the same setup in my truck, and I am experiencing the same exact problem. I have nicson headers, clifford intake, and Holley 390 carb. I am experiencing the same engine cutout that he mentions. I already know, however, the problem is that the Holley carb requires a minimum fuel pressure of 5 psi but 6 to 6.5 is recommended. Since I am using my stock fuel pump, my pressure is at 3.5 psi, so I know I am starving the engine during acceleration in 1st and 2nd. I wanted to find out if Ranger ever got his problem solved and what he did. Did he go to an electric fuel pump and regulator? Did he change carburetors? I tried to send a private message to Ranger but I got the message that private messaging is disabled. I am hoping that Ranger sees this post and he or anyone else can reply.

What is your fuel pump output volume? If it will produce at least a pint of fuel at idle speed in 30 seconds, don't worry about fuel pressure. You won't be able to send PM's until you have made at least 5 posts- - - -keep talking. Check out the power valve in your carburetor- - - -it looks like you need one with a stiffer spring that will open up and bypass the main jets at a higher manifold vacuum. The carb is going lean at part throttle and causing the bog. Opening the power valve sooner and MAYBE going to a slightly larger main jet will solve the problem, but I'd tinker with the power valve first. About 30 years of round track racing with a Holley carb taught me a few tuning tricks, and power valve swaps make a huge difference when transitioning from closed to open throttle.
Jerry

How about a little heavier shot from the accelerator pump? I'm not experienced with the Holly 390 so I'm just thinking out loud.

There are a couple of different-shaped pump cams available, and at least two different-volume pump assemblies. It's better to adjust the off-idle mixture than trying to hide the problem with a too-rich pump shot, however. Getting the pump shot too rich can also cause a stumble.

Thanks for your post Hotrod Lincoln. Unfortunately, I don't have near the experience or even the confidence to mess with the Holley carb. I was leaning toward installing an electric pump and regulator to bring the pressure up to the 6 or 6.5 psi required by the Holley. The other option, but my least favorite and much more expensive, is to consider switching to a Weber DGAS carb. I have read on a couple of posts that the Weber requires very little pressure, usually around 3 psi so I thought it would work well with my stock mechanical fuel pump. As far as your question of my fuel pump output volume, I am not sure what mine is. I can tell you that Summit Racing has a Carter replacement pump listed for my truck. They show the specs on the Carter to be 15 gph. Using the equation from Holley on how to find gph required, my engine requires a flow rate 11.7 gph, so I think I am okay on my flow volume. I wish I could tinker with the carb based on your suggestions but I know that is way beyond my ability level. If you have any thoughts on going to an electric pump or switching to a Weber, I would like to hear what you have to say. Thanks again for your time.

Electric pumps are like giving a brain tumor patient an aspirin for his headache- - - -you're treating the symptom, not the disease. Holley carbs are about as simple a piece of equipment as anybody can deal with- - - -they're "one size fits none" unless the carb is factory-designed for a specific engine, like the governor-equipped Holley 4 barrel for the medium-sized Ford trucks, for instance. Holley performance carbs like the 390 CFM 4 barrel MUST be custom tuned for the engine they're installed on. Fortunately, just about any mouth-breather with a set of screwdrivers and a crescent wrench or two can operate on a Holley with a fair amount of success. It's highly unlikely you'll have any more success with an out of the box Weber- - - -you'll just be spending an exponentially larger amount of money for something that also won't work.

Swapping jets and power valves in a Holley is ridiculously simple- - - - -four screws and a crescent wrench or a box end wrench gets you to the main jets and the power valve, and all it takes is a couple of cork gaskets to get back in business. You don't even have to remove the carb from the engine. Round track racers and drag racers have been tinkering with Holleys for the 60 years I can remember, and probably long before that. Of course, a quarter mile needs four turns and a dirt surface before it qualifies as a race track!
Jerry

Like Jerry stated, any universal carburetor is going to REQUIRE tuning.

Holley parts are reasonable in cost, and available in just about any FLAPS around the world (maybe because every Holley around the world needs parts ).

Weber parts, on the other hand, or neither reasonable in cost or readily available (at least here in central Missouri).

There are lots of books written on the tuning of Holley carbs. Determine the list number of your Holley, and then look up the type (probably 4150). Once you have determined the type, buy one of the books. Sit down one night, ignore the obnoxious soap operas on TV, and read the book cover to cover. The next night read it again for understanding. Now invite a friend over that is thoroughly versed in Holley, buy him dinner, and get him to help you tune your carb.

And before doing any of the above, check your ignition!!!!

Jon.

The most common Holley 390 is list #8007. Many circuits are either adjustable or replaceable, except one: the full throttle mixture.
But isn't that supplied by the main jet? That is the only circuit for high vacuum cruising, and if you increase the MJ size mileage goes down.
But it doesn't supply all the WOT fuel, the remainder is supplied by the PVCR (yours is probably .038" now), and the adjusting tool is a drill.

Thank you Hotrod Lincoln, Carbking, and Panic for taking time to help. I have decided to keep the Holly 4 barrel 390, part #8007. Will do on the book and reading the instruction booklet but I do have a question. The instruction book and tech support at Holley both say that a minimum of 5 psi is required and 6 to 6.5 is recommended and not more than 7. I will follow up on your suggestions but shouldn't the first step be that of making sure the pressure to the Holley is according to spec? I thought I should make sure I am getting the recommended pressure to the carb and from there fine tune the carb to my application. I apologize for my ignorance on this subject. I have had my one-barrel for the last 45 years so this is all completely new territory for me.

Holley's recommendation on fuel pressure is a little bit ridiculous. Remember, they're building hotrod carbs intended to be run at or near full throttle all the time. I think their suggestions are outrageous and should probably be ignored, especially since they also sell electric fuel pumps. Anybody who promises to solve all my problems by selling me something? No thanks- - - -I don't need any ocean front property in Arkansas, either. I've run bunches of Holley carbs in all sorts of applications including round track racing and I've never run that much fuel pressure. The NASCAR affiliated 5/8 mile track at Nashville specifically prohibited electric fuel pumps, and we ran those small block V8's from 3K RPM to 6K or so twice a lap with regular mechanical pumps. Street-driven engines will seldom, if ever need that much fuel. This isn't some pipe dream from reading an internet blog like a lot of the "experts" here will quote- - - - -I'm speaking from real world experience. Get the mixture right through the entire operating range of the engine, and ignore the fuel pressure.
Jerry

Agree with Jerry on the fuel pressure.

If you are racing the truck then you might need the additional pressure, but one must look at the application.

3 psi should keep you engine very happy on the street.

Electric pumps CAN add their own set of issues. Check this link on my website: http://www.thecarburetorshop.com/Electric_fuel_pumps_and_old_cars.htm

Jon.

Thanks Carbking for the link to the carburetorshop. I read it and do agree with the cautions on installing an electric pump. Shamefully admitting I have no experience in carburetors, I felt that if Holley says the pressure has to be at 6 to 6.5 psi then I should start with that. But okay then. I'll keep my low pressure mechanical pump, leave the electric pump out of the equation, and see if I can get some help on fine tuning the Holley. But, can I ask, if my engine is running fine once I shift into 3rd, 4th, and 5th with normal demand on the engine (city streets, no hills or freeway entrance) , but is always cutting out when accelerating on normal takeoff in 1st and 2nd, where would you start as far as adjustments on the carburetor? I haven't got as far as testing on any hills in the area or trying to get on the freeway. I really appreciate all the help provided by this forum. Thank you for your time.

A 4 barrel on an inline six is a solution looking for a problem. You're probably right on the ragged edge of "too much carburetor " for the engine size. I keep going back to the power valve as a possible solution, and you keep ignoring me, so I'll mention it once more and then sit back and watch/listen. Another possibility is a slightly low float level, which is ADJUSTABLE on your Holley. With the engine idling, the gas should be even with the bottom of the hole in both float chambers when the plug is removed. It's adjusted by loosening the screw above the fuel inlet fitting slightly, and turning the adjusting nut- - - -clockwise to lower the level, CCW to raise it. Be prepared for a little leakage- - - -have a handful of rags handy, (and maybe a fire extinguisher). Note the edit- - - -I had the raise/lower direction backwards!
Jerry

OP - thanks for the added information.

Something else to consider, and I don't know on this vehicle.

On some mid to late 1960's Pontiacs, it is a well-known fact that one can get a stumble at the end of first gear under hard acceleration IF the fuel tank is less than half full. The pickup is in the front of the tank, and centrifugal force throws the fuel to the rear uncovering the pickup. No pump will pump fuel if the pickup is in air.

But listen to Jerry on the Holley. Other makes of carbs are more my forte.

Jon.

Jon, the 1962 389 Pontiacs with a 4 barrel carb got a reputation for getting terrible gas mileage. People would leave Nashville on the way to Memphis (200-something miles) and run out of gas before they got there. They solved the problem in 1963- - - -by installing a 4 gallon larger fuel tank! No modifications to the engine at all!
Jerry

Sorry HotRod Lincoln if you thought I was ignoring your suggestions. I was getting anybody's input before I proceed. I haven't started yet because based on the feedback from this forum, I just recently decided to forget installing an electric fuel pump as a first step. I'll forget the pump and try my luck with tuning the Holley. Thanks for your input. I am just trying to benefit from the experience and know how of others.

I've got quite a bit of experience with the Hoilley list #7448 2 barrel carb that's rated at 350 CFM. It was the designated carburetor for the circle track class we ran at Nashville Fairgrounds speedway for several years. I'm not sure if the same methods are used to rate the 4 barrel 390, but the numbers will be fairly similar, IMHO. A 300 cubic inch engine at 4000 RPM will have a theoretical CFM of 347. That's virtually impossible, since no street engine approaches a volumetric efficiency of 100%. 90% is much more in the ballpark unless some serious cam, valve, port and exhaust upgrades have been made. That brings the actual CFM at 4K down to around 312. The 7448 carb will let a 350 Chevy go to approximately 6100 RPM, which is WAY above its rating, but the manifold vacuum starts coming up as it exceeds its rated CFM. It eventually acts like a restrictor plate to limit top RPM. Can you give us an idea at what RPM you're experiencing the bog? I'm going to guess that it's probably in the neighborhood of 2K-2500, right? It appears to me that the midrange mixture is going lean- - - -too much air, not enough fuel, possibly because the main jets aren't flowing enough gas before the power valve kicks in at low manifold vacuum and allows the main jets to be bypassed during acceleration. A low float level in the primary venturis' float chamber can also produce a bog, since the venturi vacuum needed to draw fuel into the airstream is directly affected by float level- - - -the lower the float level, the more likely there will be a lean bog on acceleration.

Start your tuning by making SURE the floats are set right. A float level less than 1/8" below normal can cause major problems. Set it too high, and it causes flooding at idle. Once that's right, you might want to install a power valve with a stiffer spring that allows it to open at higher manifold vacuum. The valve is held closed by vacuum, and it opens to bypass the main jets and provide extra gas as the vacuum drops when the throttle is opened. Ford calls the same part an "economizer"- - - -same part, different terminology. It's there to give a mixture boost on acceleration, then it closes at high vacuum cruise to improve fuel economy. Have fun!
Jerry

Jerry & Jon,

What is your opinion of the Holley AA1 ("94", 2100, etc) 2-barrel carburetor used by GMC on the 302 (and used on several other vehicles/engines)?
https://carbkitsource.com/carbs/tech/Holley/2100-index.html

It is two-barrel; and adjustable jets are readily available.

For us regular rubes, can the adjustable jets make it suitable for a GMC 270 or Chevrolet 261?

If you can find one of those carbs calibrated for the 55 Ford 272 V8, it will probably work pretty well on a 261/270 engine without much, if any rejetting. The 272 carb has a 1 1/16" venturi diaimeter and when I tried to use one on an overbored 239 engine (252 cubic inches) instead of the original 15/16" venturi carb it was slightly over-carbureted. It had a flat spot at part throttle. Other than that little problem it ran well, especially at full throttle. The gas flow through the manifold for a six is a little different than a V8, but the volume of flow should be similar. The "hobby" class race cars we ran with flatheads used those carbs very successfully.
Jerry

Tim - NOT my favorite carburetor because of the power (economizer) valve design. This is the carburetor that coined the phrase "leaking down".

And I know you do not fit into the "regular rube" category, but adjustable main jets, while great on a single barrel, add an extra degree of uncertainty on multi-barrel carburetors. Most would be better served using fixed jets, especially on Holleys, where jets are so readily available.

Finally, for racing applications, the AA-1, etc. is fine, especially if this is what the sanctioning body requires. But for street use, I would certainly prefer either a Carter (WCD, or WGD), the Rochester 2G, or a Stromberg WW (definitely a "sleeper" and very obtainable).

Far more important than the carburetor selected would be the choice of manifold. NONE of the carbs mentioned above, are going to work as well going through a 2->1 adapter to a single barrel manifold as the appropriate single barrel. I wouldn't even try it without using a manifold with a two-barrel plenum. Did LOTS of testing with the 2->1 adapters, NONE of it successfully!

Jon.

Thanks, Jerry
Thanks, Jon

I knew your replies would be polite, but not what I hoped to hear (and, what I expected to hear).

I have had a 302 GMC AA-1 on my 270ci 1960 261 for 3-4 years. It was drilled for the vacuum-advance port.

I am a rube regarding my techniques for carburetor adjustment. I "lean out" the adjustable jets to get clean plugs after normal driving. Mixture is adjusted to obtain maximum vacuum at idle.

If I want more power at high speeds. I (temporarily) go for a richer mixture for high-speed power. That mixture for that high-speed-oomph is set "empirically" : checking plugs after high-speed/acceleration runs (looking for more "color" on the plugs - never enough to foul the plugs). The power/acceleration impresses my friends (and, the gas mileage suffers).

My rather ignorant methods have resulted in reliable starting and enjoyable driving. But, I think/guess that I have been dumb-lucky.

Thank you both, for your thoughtful answers (and, for being gentle on me).

Since it's necessary to drain the float bowl by removing plugs in the bottom of the bowl before tinkering with the main metering jets, maybe you're referring to the idle mixture screws? Other than making the engine idle properly, they have no effect whatsoever on fuel mileage or performance. Adjustable main jets need some pretty sophisticated exhaust analysis equipment (and maybe a dyno) to set them properly, and when more than one barrel is involved, it really gets complicated. Jon's comment on having the proper manifold is spot on.
Jerry

"adjustable main jets for a 94 carburetor"
(more sources-of and references-to the adjustable main jets)

All I know is that my engine starts easily, runs smoothly, accelerates nicely, and gets good gas mileage - all are better compared to the original 1960 Rochester. If I am trying to impress someone, the gas mileage suffers.

If I can get my act together, I'll try to attend "Homecoming" next year,

Nonetheless, as always, thanks for the comments, explanations, and suggestions.

If you really want to see what your carb is doing or not doing, ie a flat spot off idle invest in a hand held wideband. They are dirt cheap these days and besides drilling the downpipe and welding on a bung, will tell you what exactly your air fuel is doing in real time. No reason why one of these isnt in everybody's tool box to take the guess work out of carb tuning.

Tim - if you really want my opinion of the Holley 94 (and the Rochester B) check out this page from my website:

http://www.thecarburetorshop.com/Free_carbs.htm

We try to GIVE them away!

Jon.

That hurts, Jon

I like Jon's idea! I think that the next time I sell parts to someone and I have extra space in a flat rate box, I am going to cram it full of the crap which I have no idea why I have saved over the years. A sort of Pay the misery Forward.