How fitting that such a bunch of bunkum should still be published by Hot Rod, the mother of "old wives' tales." I have the Hot Rod publication on my shelf that informs me that Y Blocks can't breathe, live, or rev.



All that the air flow rating of a carb indicates is how much air will the carb flow at a given pressure differential (1.5" for 4v carbs and 3.0" for 2v carbs). The flow rating does not say at what point the carb can efficiently meter fuel on a given engine, and this is the key requirement of a carb: It must be able to meter fuel over the intended operating range of the engine.



How is it that a Boss 302 works well with a 780 cfm Holley? How is it that Pro Stockers use a pair of 4500 Dominators? Why isn't a 302 overcarburated when it runs 4 dual throat Webers with about 2800 cfm flow capacity?



Carburetor size is greatly affected by whether one uses a dual plane or single plane intake -- why doesn't the formula reflect that fact? I believe that it is because those formulas simply don't address the issue of what carb to put on a car very well at all.

My Little Y Block Page

 

Thanks Ted. I have driven the car several more times now and it has run well. I am sure your diagnosis is correct.

 

 

I have also read articles that endorse a "leaner is meaner" approach...as long as you don't go too lean. The 390 cfm number never impresses anyone, but I looked around for other folks who have used this Holley on the 312 y-block my TBird has. A piece on Automedia.com reads

"First introduced for use on the hot "Thunderbird Special" 312-inch Ford Y-block engine that was optional in the 1957 T-Bird, the Holley 4150 was shorter in profile than previous units due to the low hood line on the early 'Birds. Dubbed the Model 4150, it went on to take the performance carburetor market by storm."

http://www.automedia.com/Carburetor/Classics/The/Hailed/Holley/4150//res20051201hc/1

Another Holley guy on the Internet wrote "My Pop took the 390 cfm Holley off my Pinto and put it on his 390 powered 62 Thunderbird. The mixture was right on." 

http://www.bob2000.com/carb.htm

Not exactly biblical sources, I understand, but there are people who've had success running this setup.

About the formulae used by Joey & commented on by Timmy. I don't know how old that formulae is, but it's just intended as a guidline, & if it's an old wives tail, it's still being published by Hot Rod magazine currently. (?! )  There seems to be some evidence on this site a in rodding magazines, that many think bigger carbs must be better. Apparently it ain't always so. A great example of this is the last Engine Masters contest in PHR. The winning entry out of 19 contestants, running displacements up to 509 cu.in., ran the smallest carb, a Holley 850 & 2nd lowest compression ratio! The engine developed the best average power & torque from 2500 to 6500 rpm... Just food for thought.

Paul

HI, I`m glad you solved your issue. However with all the hoo-hoo you went through I think that one thing may have been lost. The 4160 is a vacume secondary carb and a 600 CFM version will be equal or better than a 390CFM. These carburators are self limiting and only flow what the engine asks. If you need 200 CFM that is what you get. This is not VOO-DOO just what happens in the real world. Pete

Sounds like your 'chugging' was a result of being too rich.  The #65 power valve opens at a lower vacuum point (6½ inches of vacuum) and you were apparently at that point where the power valve was opening during light acceleration or cruise where the extra fuel wasn't necessary.  I have permanently mounted vacuum gauges both on my '55 and my tow truck to monitor the engine vacuum which also gives me a good indication of when I'm into the power valve or not.  I had an instance on one of my earlier tow trucks when the vacuum gauge needle started bouncing wildly and it didn't take long to find a broken valve spring on an intake as a result.  Gotta love them gauges.

Glad you isolated the issue.

Lorena, Texas (South of Waco)

Thinking that I had tried everything within reasonable constraints and gotten nowhere, I tried (again) to figure out what was going on here, sort of star over. With nothing to lose, I took the bowl and metering block off again, and removed the #75 power valve. If understand things correctly (at this point I was so frustrated I began to doubt everything), if there’s a lean stumble or bog with a 75 then the next logical thing would be to go to the #85, right? I had already done so, but I tried it again. Nope. Car ran lousy.

Having reached the “what the hell” stage, and since I am now able to change power valves in a Holly 4160 in 60 seconds flat with my eyes closed, I figured I would throw logic out the window and try the #65 which I had on my garage shelf. I realize that only when you’re accelerating without a stumble should you move in this direction, but I thought I had nothing to lose.

 

Well, gentlemen, not to take up too much more of your valuable time…the car started right up, purred at idle, took off like a bat, and accelerated smoothly, bottom to top. You know that feeling when you sort of can’t believe it? It ran so good I was driving down the road leaning forward with my head cocked to one side, waiting for the chug to start again. It didn’t.

 

So…what is it in the relationship between power valve, secondary diaphragm spring, and whatever else, that I am missing?

 

 

 

 

Hi Ted. Yesterday I spent the better part of the afternoon working on this again. I sprayed carb cleaner in all the air bleeds. This didn’t help, which wasn’t too surprising as the carb hasn’t got much use on it, and as I had already used Sea Foam.

 

Then I checked the bowl levels. For a moment I thought I was onto something there as the rear bowl was a little higher than the front, but it was slight; and after I leveled them off, the chugging still remained.

 

 

 

Thanks John. Well, the distributor is a mechanical advance Mallory, so there is no vacuum advance, and the feed for that line on the carb has been blocked off. Months ago I had bought an advance spring kit from Mallory and tested every combination of springs to arrive at the best possible advance scenario.

 

Still, I figured you can’t be too sure how everything interacts…so I took my timing light and lowered the initial timing at 2, 4, and 6 degree increments, and made a trial run at each interval. No dice. Car ran worse as I retarded the spark, so I concluded that wasn’t it and put things back.

 

 

 

Joey:

After all the responses with good carb suggestions, the last thing you probably want is another opinion.  But guess what, you're going to get one.  I feel the "chugging" you feel at lower rpm is not carb related, but related to your first post, the addition of a mechanical/vacuum distributor.  I think you have too much advance at that rpm.  Try unhooking and plugging the vacuum advance line and see if the plugging goes away.  If so, you will need to tune the advance curve.

John

John - "The Hoosier Hurricane"

Joey,

I'll add that the air bleeds are also worth considering.  You might try just shooting some carb cleaner into all eight air bleeds in the event they are experiencing some partial stoppage.  As an FYI, there is also a fixed idle circuit on the secondary side and if the air bleeds are not completely clear or the float level is not correct on the secondary side, then it becomes increasingly more difficult to tune the primary side.

I'm assuming you can screw in the idle mixture screws on the primary side and kill the engine at this point.  If not, then be looking for an excess of idle fuel mixture to be going into the engine by lieu of too much primary throttle blade opening or too high a float level on the secondary side.

Another point to consider are the metering block gaskets.  If they've been reused multiple times, they could be overly compressed to the point that they no longer seal the metering block to the carb body adequately.

Just some more thoughts to consider.

Lorena, Texas (South of Waco)

Joey: some time ago, it was fashionable to use a primary metering block on the secondary side of 750 cfm double pumpers, drill a couple of feed holes, and set up "4 corner" adjustable idle circuits on the secondary side. Guys running automatics in the drags got a better launch with this setup. Since then, Holley has offered the same combination.



But, even on their older carbs, many of them have a channel that runs down each side of the throttle body, feeding secondary idle slots from the primary side, so that the back holes are connected with their front counterparts. My list 8007 390 cfm came without a throttle body (or I don't have it anymore) so I can't say what it has. I'm using the stock 390 cfm 312 carb (which is a 4150 -- metering blocks on both ends) right now. I am wanting to put a 600 cfm throttle body on my 8007 for the next project. I will blend the bottoms of the body throats into the 1/8" larger throttle bores of the 600 cfm body.



I don't understand why your secondary modification would cause a stumble, as you say. Too bad that you had to give that A/F meter back! Good luck.




My Little Y Block Page