As you can see, carburetion is a topic of some debate. Smaller is truely better for most 300s.

If you are ABSOLUTELY stuck with the 600, and can't get your hands on something smaller, I think you're first step should be to cut the secondaries out - either with the black v/s spring or just wire them shut ( at the linkage). That will give you a "smallish" 2bbl effect. I tried this with my 390 and still had appreciably more power than with the stock (1bbl) in/out.

 

Little carb theory here: A carb works on Bernoulli’s principle that states pressure is inversely related to velocity. As air speeds up, the pressure drops. This is what happens inside your carb’s venturi (the part that necks down). The air going through their speeds up and the pressure drop draws in fuel and atomizes it. There is an ideal pressure drop that the carb is designed to work around, and this assumes a certain quantity of air passing through the carb. If there isn’t enough air moving through the carb then it won’t pull the fuel right. This pressure drop is what is referred to as the “signal”. This is what Silver Streak was getting with the bigger carb not keeping its tune. It’s not that the carb isn’t keeping its tune, it’s not getting a strong signal and the fuel flow into the engine keeps changing. I’ve see people go from 600cfm carbs to 750’s and had to increase the jet size because of the weaker signal not pulling enough fuel.

IMHO one of the reasons that people fall into the trap of getting a carb that’s too big is the hot rod magazines. They run an engine on a dyno and it picks power at 6500rpm with a bigger carb. The problem is they don’t test the engine below 4000 and that’s where most people drive.

Bottom line, the 600 will work, just be prepared to play with it to get the most out of it. I would run the secondaries, just with the heaviest spring, and get a jet kit that has a good range of jets, and make sure you get some reusable bowl gaskets. Find a place to do acceleration runs to say 30 or 40 mph and time it. Make a change and retest. Or spend the money and tune on the dyno.

David -= 83 F100 - 300 4spd 4x2 =-

 

Thanks for all the great info. I bought this carb and offie intake from someone who was going to put it on their 300, learning later that it was this large and the 300 doesnt need this much carb. I dont have the moeny to get another or anything. AS of right now, i am planning on putting in the heaviest secondary springs, rejetting and using a 2 stage power valve. Does that stuff sound about right? Can anyone give me some numbers to start with as far as jetting and the vacuum openings and stuff on the power valves? I am completely clueless when it comes down to the differant numbers and ratings on the jets and power valves. Thanks for all the great info though!!
Eric

 

Thanks for all the great info. I bought this carb and offie intake from someone who was going to put it on their 300, learning later that it was this large and the 300 doesnt need this much carb. I dont have the moeny to get another or anything. AS of right now, i am planning on putting in the heaviest secondary springs, rejetting and using a 2 stage power valve. Does that stuff sound about right? Can anyone give me some numbers to start with as far as jetting and the vacuum openings and stuff on the power valves? I am completely clueless when it comes down to the differant numbers and ratings on the jets and power valves. Thanks for all the great info though!!
Eric

 

sorry for the double post, server was going slow, hit submit reply one too many times

 

The order of vac/sec springs is...

----------------------------------- (350 cu.in.) (402 cu.in.)
----------------------------------- open ----- open
Yellow (short) - lightest ------------1620 ----- 1410
Yellow------------------------------1635 -----1420
Purple------------------------------1915 ----- 1680
Plain (standard)---------------------2240 ----- 1960
Brown -----------------------------2710 ----- 2380
Black - Heaviest--------------------2720 ----- 2390

The chart I have includes rpms to open and rpms to full open but they are for 350 and 400 engines - so they don't actually apply to the 300. You would see a similar increasing range for the 300. As you can see - the rpms (to open) INCREASE with the stronger springs, so a heavier spring would reduce the overall influence of the secondaries (the "too big" effect).

Jetting - I'm not sure with the 600 - The Holley folks suggest 51's for the 390.

Power valves - determined by your manifold vacuum - according to my Holley manuals " you should choose a power valve that is (opens) 1 to 1-1/2 inches lower than the reading on you vacuum gauge." The 6.5 power valve is generally recommended as a good start point if you don't know your manifold vacuum.

Another thing you will need to become familiar with is the accelerator pump - cams and squirters. The Holley manual by Jeff Williams is full of pictures and decent info. ISBN 0-931472-27-X . ANY Holley manual would be of great benefit to you.

 

For the power valve, get a good vacuum gauge and hook it up so a passenger can look at it while driving. Drive the truck around for a little and record the lowest steady reading, then select a PV that’s 2 in below that. Power valves are rated in inches of mercury and if it’s stamped 75 then it opens at 7.5 inches of vacuum.

For the vacuum secondaries, get a Holley kit 20-13 (7 spring kit) which is about $8 and start with the black spring. Keep going to a lighter spring till it bogs from too much air then back off to the last good spring. If you get a rich bog, then go to a lighter spring till it goes away. Given that your over-carb’d I’d say the black will be fine.

With a rich bog the car will just lie down and die. You might also see black smoke come out of the tail pipe. A puff when you step on it’s OK, but if it’s too rich you’ll leave a black trail behind you. If it’s too lean then it will surge and may pop (backfire) through the carb. Good tip: If you ever get a carb fire, crank the engine over and suck it into the engine. Rich = too much fuel, Lean = too much air.

As for jets, they are numbered relative to their size. A #70 jet flows more, or is richer than a #60. As for what you’re going to need? There are a lot of variables. Altitude, booster type (the little round thing in the center of each barrel) cam, and the fact that it’s over carb’d really make it hard to say. Those types of carb typically come a little rich from the factory, but since it’s over carb’d and will have a weaker signal it might lean it out. If you can’t find anyone here that’s running one, then I’d just bolt it on and see how it runs. You can get a jet kit that has two of every jet from #64 to #99 (36 total) for like $40.

Hey GeneStoner, what jets did you use in your 600?

 

The secondary spring has nothing to do with the fuel mixture. If you get a bog from the wrong secondary spring it's because the engine isn't pulling enough air to draw fuel through the carb, not a rich condition.

 

The secondary spring does have something to do with the mixture. It controls the rate at which air enters the engine to mix with the fuel, but I do see your point. I guess that I had a brain fart...


Disreguard the rich bog on the secondary spring.

 

Man, everyone, thanks for the great info! I am home now for summer from college, got home yesterday and spent today dropping the engine back into the truck after the rebuild. Friend of mine, mehanic, put the throwout bearing on backwards, so, after a long day today of getting the engine ready to go....i have to go digging again to replace it. Oh well though, stuff happens. I think I am going to go to the library and grab a Holley manual or something for when i tear into it. All your advice has been great, I now have a great direction to start in, you all have helped more than you know, Thanks!!!
Eric

 

I just found another Holley Manual - this one specific to the 4150 & 4160 - by Mike Ulrich. It will be part of my next order. If you can shoot me a carb. # (stamped on the air horn - top front), I can tell you what series you have.

Bummer 'bout the throw out...
On my very first engine swap, the rear main seal was somehow left out...

 

I have a 500 cfm edelbrock on my 300.
It runs just fine thank you.
Just get a holley book, it is one of those things that you can't explain over the internet too well. All you need is patience. Don't be suprised if it runs very poorly with the carb at first. Just keep reading those spark plugs.

 

Oh and NEVER change more then one jet size at a time.

 

Would a restrictor plate between the carb and manifold work to get the CFM down to what you're looking for? If it would, it seems that would be a pretty cheap fix. Just a thought.

 

A restrictor between the carb and intake would make the problem worse. The carb relies on airflow to function, so preventing air from flowing through it would cause it problems.