Hello gentlemen,

A few months ago I received some good advice here regarding what to do about an acceleration flat spot on my '56 TBird.

I ended up adjusting the accelerator pump arm and changing to a hotter accelerator pump cam; finessing the initial timing; changing the advance springs on the distributor for more advance; and finally, changing the power valve in the Holley from a #65 to a #75.

I had some success with that. So of course I figured, If a #75 is good, a #85 might be better. So I tried that, but the damn thing ran so crappy I quickly went back to the #75. That same day, I bought a Holley spring kit and took apart the secondary diaphram assembly and switched to a slightly stiffer spring. This was because the secondarys seemed to be kicking in too early and causing a slight flat spot. The heavier spring did smooth the transition out when under load.

But at the same time I've noticed that when I'm just cruising at low RPMs, the engine seems to "chug" a little. It did not do this before.

First thing I did was to clean and gap the plugs, make sure the timing was where it should be, and then the dwell. All those are dead on--they really didn't need much attention anyway.

Then, with engine hot I tried driving with the idle fuel mix screws everywhere from 1 turn out to 2-1/2 turns out. I could definitely feel the difference in A/F as I tweaked, but still this didn't eliminate the chug. Next I checked vacuum. The only SLIGHT variation my gauge showed was on the hose off the carb that powers the wiper motor. This is a convertible, I never drive in the rain, so I blocked that off completely. But it didn't help.

Then, I bought some SeaFoam and poured a 1/2 pint through the carb, because I thought that with all this carb work, maybe I dislodged a particle somewhere. But that didn't make any difference. I bought the carb new and it has only 7,000 miles on it 

I admit I am stumped. It's not a big problem, but geez, it was big enough so that my wife noticed it when we went out for a drive. I'm wondering, Why should it start chugging at lower RPMs when it never did before? Any suggestions will be much appreciated.

Joey

Joey,

What is the CFM rating on your Holley? I use a 390 CFM and it was smooth out of the box. The main jet size is .50 or .51 while a 600 CFM carb is .60 or .62. I have not changed the power valve. I was told that because I have low vacuum at an idle ( 14 ) I should go to a lower number power valve not a higher.

Does your care idle "fat" or can you smell fuel and see a little black smoke? I would assume it doesn't cause you changed the pump stroke on the acc. pump.



Go to holley's web site and look at the 390 cfm carb and compare it to your's it may lead to some answers.



I did have a 600 CFM on my mild 312 with a Clay Smith Cam 270 degree with 440 lift and ended up going down to the smaller carb.



I hope this helps a little. Mine runs crisp with no flat spots and that's with the Ford-O-Matic.



Good Luck.


  Jim Rowe Elkhorn, NE

My 4160 is a 390 cfm also, new about 7,000 miles ago. Back then I reseached it using the old formula: CFM = RPM x CI / 3456 x efficiency. My 312 is bored .030 over so that comes to 4600 x 317 / 3456 x (about) .85. So the 390 CFM seemed VERY close, couldn't get much closer, and I've always liked Holleys. I didn't think the mild cam ( 275*/.423" ) would make much of a difference with my carb selection. So I bought this carb, installed it new out of the box, and the jetting remains untouched. I did move the pump shooter up from a 25 to a 31, but that was months ago.

Car is a manual shift and it idles good--I can idle it down to 400 RPM easy, although now its showing a bit more of a wobble than it used to. The exhaust smells OK--varies normally with adjustments to the idle mix. Back months ago when I switched the power valve to a #75, the low end response was pretty good. 

So really, the only components that are different are having changed to a more responsive power valve--as I said, that went OK--and then just recently changed the secondary diaphram spring from plain stock to one notch stiffer. The transition to secondaries is much smoother now, but my low end response has become boggy.

Could the replacement of the diaphram spring throw anything else off? I'm trying to visualize how, but...could the stiffer spring somehow warrant a preference back to the #65 power valve? I'm trying to figure this out, I haven't changed anything else in there, but....

Joey: find a bridge and drop that formula off of it. Let it sleep with the fishes. That old wives' tail should have died with grooving mains and knurling piston skirts, but didn't.



I used a 390 cfm Holley from a 312 on my 239 until the engine wore out, and it worked fine (after I tuned it).



The big issue is, why would you think that an 85 power valve would work better than a 75? Those numbers only indicate the manifold vacuum point at which the power valve opens: e.g., a "75" opens at 7.5" of manifold vacuum and below, and an "85" opens more quickly, at 8.5". The way to work with this is by driving, preferably with a vacuum gauge. Once the carb is jetted correctly, you want to be able to do a high gear, relatively easy acceleration. You accelerate as hard as you can without opening the power valve (which is why you need the vacuum gauge -- to see when it opens) and when the engine accelerates without a stumble, you move to the next lower power valve (e.g., from a 75 to a 65) When the engine stumbles, then you need to go back up one step. The engine must be warm and if your tuning is on the edge, it will make a difference whether it is a cold or warm day.



The power valve number has nothing to do with the size, i.e., how much gas will flow. It only controls the point of engine load (as determined by the manifold vacuum) at which enrichment occurs. As a theoretical rough and dirty rule, you want your main jetting to be near 14:1 air/fuel, and your power air/fuel to be around 12.5:1.



But let the formula sleep with the fishes.

My Little Y Block Page

Thank you for your response. I used the formula (along with the fact that a lot of people have had good results with the Holley 390 cfm on their 312s) when I had to make a decision about which carb.

The experimentation with power valves was suggested on this forum a few months back. Right now, I'm using a #75 and have a slight low-end bog. I am of course willing to swap out again; in fact I have both a #65 and a #85 on hand. All I'll have to do is buy new gaskets.

See, here's where I'm at a loss. I'm already bogging with the #75, and the #85 was terrible. Does it make sense to try the #65 from where I'm at now? Or is there some bigger issue somewhere else that I am missing?

 

 

Joey,make sure the accelerater pump is connected and working.Take a reading of a spark plug to help determine if it's too rich or not.Make sure you have no vacuum leaks.Then make the changes you think will be needed even if you have to start over with the carb set up the way it was originally.>GB

-Gary Burnette-

(I'm assuming that, when you take off the primary side fuel bowl or change/reorient accelerator pump cams, you are adjusting for the .015" clearance between the pump lever and the plunger on the pump arm.)



If your bog is right off idle or at the point that you are just stepping on the accelerator, I don't think that changing the power valve is going to solve your problem (recognizing that it is hard to talk about this without being there or driving the car). If your bog is at the time you step on the accelerator, I think that you will need to focus on the accelerator pump.



Changing the pump cam or changing which hole you use in the pump cam will alter the total volume that the pump displaces, and the shape of the cam will, to some extent, determine how much volume is displaced over a certain amount of throttle movement.



However, the other part of the equation is the size of the shooter. If the pump is trying to squirt more volume, but the size of the squirter will not allow that much volume to pass, the spring on the pump arm plunger will compress until that fuel can be squirted into the carb.



Most squirters are .025" in size. Different size squirters (the number is stamped on them) are not as easy to find as jets, but they are available. I've had to resort to drilling spares to try out whether a bigger squirter was needed. You know that drilling jets is a no-no, but in a pinch, if you have some extra squirters laying about and some wire drills and a pin vise, you can successfully check this out yourself by carefully drilling the squirter orfices.



You don't need a big change. I've solved most bog problems by going (or drilling) to a .031" squirter.



I was always able to make my combinations work with the stock white cam, except for one. This was in a big block with a 780 on an open plane manifold. In that case, I had to go to a 50 cc pump and cam, and go to a .037" squirter.



The .037 squirter is the largest that can be used with the stock squirter hold down screw, because the passage size makes larger squirters pointless. In that case, you have to go to the special hollow hold down screw to pass more fuel.



I very much doubt that you would have to go to those extremes.



To recap: bigger holes in the squirter won't pass more total fuel, but they will allow the same size volume of pump displacement (determined by the pump cam) to be injected into the carb in a shorter period of time. If your bog is right at the point where you are touching the gas pedal, I would suspect some squirter tuning would be a better approach than trying to fix it by messing with the power valve, which will affect other aspects of driveability, too.

My Little Y Block Page

Timmy, it's not right off idle, but rather when I cruise, and then apply a load at low rpms, say 1000 or 1200, the engine chugs. That's the best way I can describe it.

As mentioned, I have already gone to a .031" pump shooter. Also the accelerator pump has been adjusted at WOT; and having bought the Holley cam kit, I have gone through all the cam combinations and settled at #1 hole on the orange cam (which incidentally is right where I started). All these adjustments were made previously, and for a long while after that point, the car ran fine at the low end, with a flat spot at the point where the secondaries kick in. But now that I have replaced the secondary diaphram spring, I am for the first time experiencing this chug. And in my simple mind I can't seem to reconcile why it's happening.

I just re-re-adjusted the idle mixture screws for about the 20th time, setting them at 1-5/6 turns out each. Took the car for a long ride. No difference--still chugs.

Joey: I admit to being surprised at your getting a flat spot from the secondary springs. I used the lightest gray spring in everything, and have never once felt a hesitation when the secondaries opened (from opening too soon). Are you able to hear when the secondaries kick in?



Since you are running a 4160, the secondary metering plate will be "jetted" or calibrated to give about a 12.5:1 air/fuel ratio without the power valve. If you've made the secondary spring stiffer, and now you are getting a bog, that would make me think that the primary side was too lean to begin with, which seems to be the way you are thinking, too.



Going to the 85 power valve and finding worse drivability, however, makes me think that you are already very rich on the primary side. Since you seem to be running an A/F meter, what readings are you seeing as you go thru rpm transitions?



My approach might be to disconnect the secondaries and wire them closed, and work on the primary side until you get the driveablity that you want, and then attack the secondaries as a separate problem. But I confess that I do not understand how your secondary calibration could affect the low end in the 1000 to 1200 rpm range. A thought here: the secondaries are usually adjusted by having the stop screw set to just crack open the secondaries when they are closed (about 1/2 turn, as I recall). If your throttle plate is grooved from primary to secondary (as some are, and some are not) you should then get an idle mixture at all 4 corners.



I've normally found that Holleys come out of the box a bit rich. Yet, reading your description, I get this:



1. You get a stumble: you are lean

2. You get poor performance going to an 85 power valve: you are rich

3. The bog started when you went to a stiffer secondary spring: maybe you are lean, and the secondaries are covering up that lean condition, but I can't understand how they are opening at such a low rpm anyway.



What carb were you running before you put the Holley on?

My Little Y Block Page

The engine didn't have anything before I put the Holley on. I suspect that back in the day it was equipped with the usual teapot arrangement.

But please note: Performance of the secondaries is now excellent. The flat spot is gone completely. I moved from the stock (unpainted) spring to one notch stiffer. I can feel the secondaries kick in, but it's a smooth transition. That's not where the problem lies.

BTW, the meter unfortunately was not mine, I had to return it to my friend, so right now I'm doing this by trial and error and "feel". You are right though, I read in the documentation that by ajdusting the two A/F screws on the front bowl you are supposed to be getting the correct 4-corner mix. But still, this problem here is prior to the secondaries kicking in.

I'm going to go out and tweak it again now while I've got it hot. Maybe I can isolate something.