The Stovebolt Site Forums The Shop Area The HiPo Shop Holley 2BBl 500CFM on GMC 305CI Big Block V6

Need help from some of you old hotrodders or Circle Track guys (hint: Hotrod Lincoln).

I put a Holley 2 barrel (65-4412S) on the son's 66 GMC 305E V6 a while back, rather than rebuilding the old WW Stromberg. This carb worked really well out of the box for a long time....then...he changed the fuel pump.
I can’t see how that had anything to do with it, but since then, when it is cold, it won't idle, and has a ton of bog and hesitation on throttle tip-in, and will die unless you "make it run" with multiple quick jabs of throttle. Sometimes it even wants to backfire. If you can get it past the accelerator pump and into higher RPMs, it runs fine.
If you give it any choke when it is cold, it dies. It will start up and run, even when completely cold, but then dies. Seems too rich, but turning in idle screws ½ turn and it will die.
When it gets warmed up, it will rev fine, and will idle much better, for 5 minutes or more, but still eventually dies on idle. Seems to run and pull fine, and the bog, hesitation, backfire, disappears.
When warm, Idle is OK, but continues to be unsteady and sputters a bit--specs call for idle to be 400-450 RPM and it will quit if you try to get it to run that low, so idle is set about 550 RPM
To date, we have:
1. Checked fuel pressure and confirmed 5 PSI at idle.
2. Replaced all fuel lines between pump and carb intake
3. Set float level to just below the sight hole on the carb
4. Set idle adjustment screws with vacuum gauge to max vacuum. (The thing will pull over 20 inches of vacuum—no big lopey overlapping cam in this one)
5. Pulled plugs and noted way rich, dropped main jets 2 sizes
6. Drained fuel tank and replaced with fresh gas
7. Replaced the plugs
8. Checked the timing with a light and it is real close (again, can’t get the idle down to spec)
So I don’t know where to go from here. What I don’t get is how poorly it runs when cold, then when it warms up, it runs pretty good. I would expect some choke to make it tolerable until it warms up. I also do not understand why it will idle just fine for several minutes then eventually dies.
I am thinking it is too lean at throttle tip-in and my only thought is to explore the accelerator pump nozzle…
(I told my son when he gets a new exhaust put on the truck to have Oxygen sensor bungs welded in both banks. I am ready to buy a wide-band sensor…)
Any thoughts from you folks who have messed with these things (“magic” carbs) a lot more than I have?

I took a quick look at the 4412 specifications, and it looks like you're in the ballpark for CFM rating (500 CFM). The nest step down is a #7448, a 350 CFM that we used to use extensively on the round track engines. NASCAR specified that carb for engines up to 375 cubic inches to act sort of like a restrictor plate. A 350 Chevy will run 6200 RPM absolute maximum on that carb no matter what the gear ratio is! The 7448 will supply plenty of air to the 305 engine for any reasonable RPM range if you simply can't get the 4412 to work.

One thing to look at is the tension on the power valve spring. If it's got a pretty heavy spring on it, the power valve will open too quickly under load and make the carb run too rich regardless of the main jet size. A leaky diaphragm on the power valve will also make the engine run rich. If you need a few different power valves to experiment with I've got a race tuning box full of them, and probably 100 jets or more of different sizes. Drop me a snail mail address in a PM and I'll send you some. Good luck!
Jerry

Go with an 8.5 or 10.5 pv if you have that much vacuum just to get it driveable. Then once you can get it to cruise in drive you would devide the vacuum at cruise to figure the proper pv size. No real need to get the wide band system. Plugs are a fine indicator.

Jerry and Hollow,

Thanks for the input and Jerry, thanks for the offer to try some parts--that is very kind of you.

I just went ahead and ordered a Holley 10.5 power valve from Summit. The motor pulls close to 22 inches of vacuum under idle, so using the formula, that is what it needs. Plus they are not expensive and easy to install, so I will just try that.

Jerry, what do you think about the 500CFM carb? I have been second guessing my choice and believe the 300CFM may be the better choice. These old V6's are built for torque only--the redline on them is 3800 RPMs and net HP is only 140. So if you plug that into the carb calculators for 305 cubic inches, that comes out for a street engine at 285CFM (using Summit's CFM calculator). Maybe I screwed up on the carb sizing to begine with...any thoughts on that?

Thanks again. Will post back when we try the 10.5 PV.

Either Holley is capable of flowing more CFM than that engine needs. My 401 V6 in my fire truck is also running a tiny 3-bolt 2 barrel, not sure if it's a Stromberg or a Zenith, and it runs OK with that carb. The 7448 is rated at 350 CFM, which will make your engine a lot more responsive at part throttle. The 500 will tend to over-carburete unless you learn to feather-foot it as the RPM comes up. Just a little too much throttle opening will kill the manifold vacuum and result in a bog, or even a backfire. The downside is that the 7448 is billed as a "race" carb in most catalogs and the prices are outrageous.

If you choose to continue tinkering with the 500, I'd suggest a power valve that opens at a very low vacuum setting, maybe 5 or 6 inches of manifold vacuum. That will keep the main jets in play longer without getting the massive rich dump when the PV opens up. Some of the power valves I have on hand are adjustable.
Jerry

The formula for pv is based off cruise vacuum, not idle. I suggested a 10.5 just so can actually get it to cruise and then figure it from there. Happy motoring. I'll be re-gasketing my carbs this weekend. Accel pumps or metering block gaskets are seeping again. I let it sit too long I suppose.

There are pv deletes which requir more rear jetting if the pv is a pain.

Any luck with your carb?

Got the powervalve, have not installed it yet. Well, my son hasn't installed it yet, to be exact.

I get my parts tomorrow hopefully. I get to rebuild (2) 780cfm double pumpers. I'm so excited! Not.....

I think 500 cfm ought to be perfect, I ran 450 cfm on a stock 302 Ford, and it was great. That should be good cfm for a 305 V-6.

I'm very familiar with those carbs, they are 1/2of a double pumper and great carbs.
If it ran good for quite some time and only began acting up after a fuel pump replacement then something affected something right then. A typical problem would be teflon tape used on the fittings or new rubber hose installed without washing it out so that trash has gotten to the needle & seat.
Except you say the float level was checked, hmmm
If it were powervalve being the wrong size that would've showed up when you first installed the carb, it has no affect on the idle unless it has gone bad and raw fuel is seeping past it and into the engine below the throttle blades.
Thats a rare possibility, a blown powervalve.
Did you have the carb apart prior to the problem. Did I restate the conditions right? it was fine untill the fuel pump change? Did the engine backfire thru the carb when the fuelpump failed?

Ojh,

Yes you have the picture as I understand it--and yes it doesn't make sense.

Carb was not messed with and ran fine (just a bit rich at cruise) until fuel pump was changed.

I'd make a swag that the carb problem is in the ignition.
If you go back thru the symptoms and delete the fuel pump replacement then you'd suspect the ignition as well as carb, but when you couple the fuel pump replacement to the problem then you suspect only the carb.
As a wise man said 'most carb problems are in the ignition' and I'd bet he's right in this case.

We did pull the plugs, noted they were sooty and too rich, and replaced with new-gapped plugs. Wires, coil, condensor, points, distributor cap are all pretty new, maybe 18-24 months and less than 8000 miles. Timing was checked...

What are you thinking? Water under distributor cap? Bad coil? Dirty points?

I have worked on a sneaky, but not well informed, suspicion about the ignition, so much that I bought new Pertronix solid state points...they have not been installed. My suspicions are more about "just replace everthing and you will get it" method, not very scientific.

I'd think twice about letting a bunch of hard earned cash burn a hole in my pocket for the sake of replacing parts. It might be better to do more diagnostics and get a look at the whole picture.

Don,
I hear you, brother, and that’s why I am asking about the problem here is to get some advice/learn something to save me time and money and get the old truck back on the road. Which of the following ignition parts would you suggest diagnosing?
I am asking because these things are kind of cheap in the big scheme of things, and as a working man, my time means something as well. (So by the time I diagnose something, I could just replace it.) About the only thing on there that makes me think twice would be the coil. (Pricing is on OEM replacement grade parts from Rock Auto.com. I didn’t include the price of points as I have already purchased the Pertronix, and wanted to get those for upgrade before. But for the record, the stock points are less than $10 for a set.)

Given the truck runs pretty good when it is warm, if ignition related, I would assume a weak spark, so some parts would pass a diagnosis unless you could simulate the load conditions that create the weak spark. Am I missing something here with this line of thought?

Any help appreciated!
Thanks.

1966 GMC PICKUP 5.0L 305cid V6
Ballast Resistor STANDARD MOTOR PRODUCTS Part # RU10T T-Series $3.80
Condenser STANDARD MOTOR PRODUCTS Part # DR90T T-Series; Heavy Duty A $1.75
Distributor Cap STANDARD MOTOR PRODUCTS Part # DR438T T-Series $4.54
Distributor Primary Lead Wire STANDARD MOTOR PRODUCTS Part # DDL36 $3.70
Distributor Rotor STANDARD MOTOR PRODUCTS Part # DR314 $4.38
Ignition Coil STANDARD MOTOR PRODUCTS Part # UC12 $21.79
STANDARD MOTOR PRODUCTS Part # 9624 Spark Plug Wire Set OE Performance Plus $12.38
Total $52.34

If you are using a Pertronix ignition system I would advise to but the correct coil from them. You will also want to make sure that you have a full 12 volts going to the positive side of the coil. Also with the Pertronix ignition you must use suppressor core spark plug wires and not solid wire core wires. If it were me I would do a compression test to find out how things are mechanically with the motor. If you have more than a 10% variance between any of the cylinders that will indicate if you need to be doing some more diagnostics. I would also recommend checking the output pressure of your fuel pump.

Don,

Great advice, thank you. That is especially a good thing to know about solid core wires (that generate a lot of EMR) interfering with the Pertronix, but it makes sense now that you have mentioned it.

I did a compression test on the motor around 8,000 miles ago which was fine. While I wouldn't expect that to be different, I agree that you make a good point in case the timing chain has jumped. Those old motors use a double timing chain, but you never know, it wouldn't hurt to do again.

Fuel pump pressure at idle was 5.5 psi, did not drive it around with the fuel pressure gauge hooked up.

Thanks again for the help.

I am definately NOT an expert on Holleys; but if you were running a brand of carb with which I was familiar, I would suggest that some crud from the pump change has partially blocked the idle system.

Over time, sediment from the fuel forms on the inside of fuel lines. When the fuel lines are disturbed (ie removing fittings to change a fuel pump), the sediment is dislodged and flows to an opening smaller than the sediment (often the idle jet in the carburetor).

I would suggest cleaning the idle circuit in the carb.

Yes, the carb is too big for the application; but I vote with ojh - if this were the problem it would have arisen upon the initial installation of the carburetor.

And just for the record, I would install bushings in the throttle body of the Stromberg WW and reinstall. Other than the throttle body wear due to the aluminum throttle body, the WW is one of the finest two-barrel carbs ever built, at any price, anywhere in the world.

Jon.

Jon,

Thank you for the comments. The info about crud plugging up the idle jets makes sense and correlates to the fuel pump change.
I assume to clean idle circuit one would pull the fuel bowel and metering block from the front of the carb, pull the power valve and idle jets out of the metering block, and soak the metering block in some carb cleaner overnight and blow it out with compressed air. Does the main body of the carb need to be cleaned also?

Are there rebuild kits available for the WW? The old one leaked and was pretty dirty. If kits are available, do they include bushings (I assume brass) for the butterfly shaft, or does one have to make their own? I don’t recall if the shaft was loose on the old one, but I have seen carbs where that happened and they would suck air past the shaft and you had a bad “vacuum leak”.

I am not a carb guy—but am becoming more informed with this Holley. (I did soak and clean the Mikuni on my Kawasaki 15 horse V-twin mower motor and went from not running at all to where it runs fine only at full throttle (but still lopes part throttle and at idle).)

I will continue to tinker—this is my son’s truck and is sitting at his house so I have to drive to South Carolina to tinker.

Edited: I see there are rebuild kits available for the WW.

If you plan to leave the Holley on the truck, I would suggest a repair kit and thoroughly clean the carburetor.

Kits for the WW are readily available, but do not include bushings; however bushings are also available.

The identification number for the WW (required for a rebuilding kit) is STAMPED (not raised) on the air horn (top casting) of the carburetor. Since this is a GMC, the number should start with 23-

Jon.

OJH is the man--the ignition points were badly pitted. It would run fine when warm, but wouldn't keep an idle more than 5 minutes and would sputter and gag and quit on open throttle when cold.
Must have been a weak enough spark that under load it would flame out and not ignite when the motor was not warmed up.

Turns out it has been 4 years since I went through and did a complete tune-up. Seemed to me to only be a couple of years. So new points, condensor, cap, rotor, wires and plugs, and she runs like a Singer sewing machine again.

I was going to put in the Pertronix, but could not find where the ballast resister was (to pick up a full 12VDC for the red wire on the Pertronix solid state pickup). So I punted and put in normal points. There must be a resistor wire somewhere, as there was only 4.5VDC at the positive lug on the coil--but it must be a long one and bundled into the normal wiring harness?? I will Google...

One reason I had not pulled the distributor cap (other than thinking everything was pretty new in there) is that on the big block V6's it is in the back under the firewall and you basically have to lay on top of the motor (avoiding the eye-gouging air cleaner stud on the Holley) to get to it. Prepare yourself with a good flashlight and a collection of stubby flat screwdrivers to get to everything. You may even want to practice your cussing before attemping. The coil is back there too and is not the easiest to change out either. Not to mention it is a pain getting old and not being able to see up close anymore.

Re-jetting the mains on the carb down two notches certainly didn't hurt anything, from looking at the plugs it was pig rich at cruise. Will get the son to check them after he drives it a while.

Thanks everyone, for your thoughts and comments on this one.

My son still has the Bendix Stromberg WW in storage...I may get it from him and order a rebuild kit, just for a fun winter project. Sounds like another topic, as I will surely have some questions.

A little extra information:

Several years ago, right after we bought the remains of Stromberg, we were doing our best to service the various Strombergs.

The larger GMC engines (401's and 478's) used a Stromberg type WWC carb. Wonderful carb but VERY expensive. We had some adapters cast (the WWC uses a trapazoidal flange pattern) to adapt the cheaper Holley 4412 which is about the right size for the 401/478.

We sold 6 of these conversions, and customers were happy (at least with the price, the Holley was never a Stromberg) until the choke pull on the Holleys started BREAKING!!! (seems Holley thought a plastic choke pull would reduce the cost of the carb). They also were not happy with the gas mileage, as it was about half what the Stromberg obtained. They were happy with the power, which was about the same.

Holley wouldn't warrant the carbs due to breakage; but we had three older junkers with the metal choke bracket, which we sent out. When the others came back, we had to refund their purchase price, which we did.

Subsequently, I understand Holley went back to the metal choke pull.

So check the choke pull bracket on your Holley. If it is plastic, treat it VERY gently when pulling on the choke, especially in colder weather.

Jon.

Cool story!!

Holley has metal choke components...was a factory refurb from Summit bought in the last 2-3 years...

Ahh, a happy ending. That is good news.
Thats interesting that a WWC would get much better economy than the 350cfm holley - I guess the holley must be too large and inefficient. Maybe Jon will expand on that.

OJH - don't know why, at the time I was just trying to keep the customers happy. Stromberg did make some of the best carbs built. But Stromberg could never be accused of being inexpensive. On the other hand, Stromberg never (at least to my knowledge) tried to use cheap plastic choke pulls either.

We now offer the Stromberg WWC carbs as replacements for some of the expensive two-barrels used on some of the Cadillacs, Packards, Lincolns, Duesenbergs, etc. that are being used at drivers rather than numbers matching. Everyone that has tried them seems to like them.

Jon.