Holley Accelerator Pump Questions?

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dgc333

dgc333

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I recently swapped out the Holley 4160 series 600 cfm carb on my 360 to a 4160 series #3310-2 750 cfm carb. The 750 works quite well and I definately have a bunch more top end but I get quite a bog when I step on the gas which I didn't have with the 600. I figured that I need to work on the accelerator pump some so I ordered the cam kit and a couple of larger squiters.

The 3310 comes stock with a 0.025" squiter and the least agressive cam (white). Staturday afternoon I spent quite a bit of time swapping parts around but only made the bog worse. Going up from a 0.025 to a 0.031 then to a 0.035 squirter made the bog worse but when the engine caught it was much more violent. That is with the 0.025 squirter I would get the bog then the car would go. With the 0.035 squiter I would get the bog but it would then launch violently such that in first or second gear the tires would go up in smoke. The same thing would happen going more agressive with the cams but to a lesser extent.

I also tried various amounts of preload on the pump arm but that didn't seem to make much difference.

I am back to the white cam in the #1 position with the 0.025 squirter. It's annoying but liveable.

The one thing I notice different between the 600 and the 750 is the 600 has side hung bowls and uses a rubber flapper to allow fuel into the pump then close off the port when the pump arm moves. The 750 has a little ball bearing retained by a strap.

My question is has anyone ever experienced a problem with the ball bearing not reacting quickly enough so that the intial part of the pump stroke fuel is being allowed back into the bowl instead being directed to the squirter? The only thing I noticed playing with this on Saturday was the intial movement of the accelerator pump resulted in just a dribble out of the squirter then it became a strong stream. Squirters and pump cams and preload on the arm did not make any differnce with this. The 600 has a strong squirt as soon as the pump arm moves.

Also, I do not know the vintage of the 750 (got it used and refershed it with a quick kit) but do newer ones use the rubber bladder in place of the check ball? Seems that the bladder is a better design.
 
I am going through a similar problem but with a 2 barrel. When you step on the gas, does the car move forward o.k. then bog? or does it bog right off the throttle?
 
that is what i had(have). its still there but have the proform 750 main body now. i have the same slight bog on mine if i just kinda give it a little throttle. medium to alot is just fine.
 
with the 3310 if there's a bog the first thing I'd play with is the springs for the vacuum secondaries. buy the spring kit and while you're at it, get the quick change kit for the secondary springs. They're both inexpensive. put in a heavier spring until your bog disappears. Then if you want, put in a lighter spring and start playing with the acc pump and cams. good luck.

don
 
The main jets are 67's and I still have the stock metering plate in the secondary which is equivalent to 75's. I purchased a Proform billet plate that uses standard jets but it's interfering with the float, I want to figure out the bog before I worry about the metering plate.

I have the plain spring which is down from the black that was in there. A secondary opening bog typically happens after the car has started to accelerate and the secondary opens to quick (I have the quick change cover and spring kit). This bog is right off idle, once it's past it the acceleration is hard and smooth right to 6500 rpm.

The main jets shouldn't have an impact on the bog since this mostly happens off idle when the mains aren't even operating yet.

On Moparts someone mentioned checking to see if the anti-drain back piston was still under the squirter which I really figured was going to be the answer since the PO of the carb had taken out the check ball on the secondaries. It's still there (I also clean out the port and needle too) and I also checked the gap on the check ball in the pump, gap is supposed to be 0.008 to 0.015 and it was close to 0.030. I reduced that to just under 0.015. None of these have made a significant diffference.

A friend of mine suggested that maybe the power valve is not opening quick enough. Tonight I will swap in one with an earlier opening than the stock 6.5 tonight.
 
Your primary jets at 67 sounds small, especially considering your altitude. I'd start with 74's and tune from there. If the throttle is open past 1/4 of the way, your'e really not pulling fuel from the transfer slot. The engine is trying to pull fuel from the boosters but velocity though the boosters is low when rpm is low. If you increase the size of the primary main jets, it will be eaisier for fuel to flow through to the boosters under low velocity conditions. Try it, it's cheap and it's always good to have some jets on hand anyway.

The accelerator pump isn't meant to be the sole provider of fuel during the off idle transition, but to ease the dip in fuel delivery between the idle transfer slot and boosters kicking in. Some applications with a tight convertor and low (numerically) rear gear and smaller displacement engine with a big carb may require the use of a Holley or Demon with annular boosters.

Your friend may well be right in regards to the power valve, but I'd swap the mains first and work from there. If you use your power valve to compensate for main jets that are too small, you will dump huge amounts of fuel in under part throttle conditions killing your mileage and washing down the cylinder walls as well as causing the "My Holley runs rich so it sucks" syndrome.

Hope this helps. :)

Edit: How much intial timing are you running?
 
Check the timing and make sure you have plenty of initial advance on the crank.

In reading your initial post, you need to get that delay out of the acc pump system. There shouldn't be the lag in squirt. It may require working with the check ball strap a bit. You could tighten up that strap a little bit more to get some delay out of the acc pump as well.

What's your idle vacuum in gear if an auto?
Take that number and subtract 1.5-2.0 off of it and you are in the area of proper power valve selection. If a 4 speed go about 1/2 of idle vacuum.
 
Well I put an 8.5 (up from 6.5) power valve no difference. I put the black secondary spring in to eliminate that, no difference.

The stock primary main jet is 70 and I have never run into a general use Holley that didn't require jetting down but maybe I do need to go back up.

In any case I am taking a break from this carb and put the Holley 600 back on. No bog, pulls clean from any speed and has been trouble free since I built the engine.

In case anyone has any other ideas the engine is a 360 with 10.6:1 compression, a Comp XE268H cam, dual plane manifold, hedders and a 4spd. Idle vacuum is 14"Hg, the initial timing is 15 degrees BTDC, total timing is 33 degrees (magnum heads require less total) and it's all in at 2500 rpm.

I do not know the history of this carb I picked it up used as part of a deal on an engine I was selling. What I noticed today is that the outside air bleeds look like they have been drilled out (raised burr around the edge) and do not know what size they should be but look much bigger than the ones on the 600.
 
dgc333 said:
In case anyone has any other ideas the engine is a 360 with 10.6:1 compression, a Comp XE268H cam, dual plane manifold, hedders and a 4spd. Idle vacuum is 14"Hg, the initial timing is 15 degrees BTDC, total timing is 33 degrees (magnum heads require less total) and it's all in at 2500 rpm.

I do not know the history of this carb I picked it up used as part of a deal on an engine I was selling. What I noticed today is that the outside air bleeds look like they have been drilled out (raised burr around the edge) and do not know what size they should be but look much bigger than the ones on the 600.
Click to expand...

Your timing sounds right on for your configuration. If the idle air bleeds have been drilled out, it most likely the cause of the off-idle bog you're experiencing. Especially if it's combined with a worn throttle shaft bores. I know throttle shaft bores can be rebushed but it may well be junk if the bleeds were drilled. :sad: i've never heard of Holley being returned to usable condition after such butchery. Air bleed modification has it's place, and that place (for the most part), is on a carb with replaceable bleeds.

That's the problem with used carbs. You just never know who's has been fiddling with them or what their level of expertise was.

Let me check on which bleeds may have been drilled and get back to you.
 
All!!

Thanks for all the good info and ideas. I always learn something when I post a question here.

I think I will likely purchase a new carb instead of playing with the old 750. Maybe a new 3310 that I can use the quick change spring cover and electric choke that is on the old one.
 
my buddy just told me they are some from holley on ebay for 159.99. not bad $......
 
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