Pump Shot Recommendations

[Quezzmo]

If I nail it from a stop or slow roll it will die. If I take off with normal throttle pressure it stumbles momentarily then launches. I have a Holley 470 with a 50 cc accelerator pump with the factory .031 squirter and right now it has a orange pump cam in it. It's on a mild 350 with a Holley Red electric pump with a regulator set to 5 psi. Dual plane intake and a .462/.469 cam. I should be around 300 hp. I'm ordering a pump cam assortment. Should I order some different squirt nozzles also? Was installing the 50 cc pump a mistake? Thanks, Tom

 

[Mike]

Pump cams and nozzle sizes are just part of the tuning process. And what works well for one fellow might not work well for you.

Have you tried moving the pump cam you already have, to see what kind of effect that has? It costs naught and will let you see if you are tangling with a pump issue, or something entirely different.

A .028 nozzle might work better for you, from the way you describe the problem. But note I did say 'might', and that is not an absolute, by any stretch.

Something to remember is that when you are working with off-idle acceleration issues on a Holley, you also need to bring the power valve into the equation.

Was the 50CC pump addition a mistake? Only in that it was money you likely could have kept in your pocket. But you fail to mention how everything worked with the 30CC pump, prior to the change. Did the pump change introduce this problem, or was it already happening, prior to the pump change?

 

[Quezzmo]

I didn't have the opportunity to run it with the 30cc pump. I bought the carb from a friend. He changed the pump and then decided against using the carb. Since he altered it he couldn't return it so he sold it to me unused and cheap. He was born rich.

I did move the orange cam from the #2 position to the #1 position with no noticeable change.

You mentioned the power valve. Holley puts in a 2.5 which seems really low. I'm pulling 15" of vacuum in park. I need to check the vacuum at idle in gear. I'll pick up a longer vacuum line for my tester this evening.

 

[railroad]

Holley carbs do fairly well right out of the box for stock to mild engines. If you have the original pump, I would put everything back to stock. See if a passenger or adjacent motorist can watch your exhaust, watch for black smoke right now. good luck,

 

[Screaming Metal]

What Mike and RR said....
Usually the only need for the 50cc shot is when you need to keep a lean-out from occuring during the big vacuum dump or having a long shot for tuning needs....as Mike said. The PV's can be overlaped by the pump (used as a tuning aid)....I have used them as a crutch in the round robin matches....to get extra fuel into the equation....
I would suggest since you have the 50 installed, try the smallest cam in #1 position. I'm pretty sure that it will be too much for your mild motor. If it is, go back to the 30, with the stock cam....in the 1st position, try it, if not better, then move it to #2, and try it there. Keep going up, and move from 1 to 2, until you don't have a bog.
When you go back to the 30, put the stock squirter back on....
With a mild motor, if you go thru the cams, and things aren't better, you need to look at your power valve.
*****Matter of fact, in your instance, it would be best for you to put things back stock and tune from there.....
Hook up your Vacuum gauge and get the appropriate PV in there also.

 

[Screaming Metal]

****Scratch my previous post....one on my guys was asking me a question and I took too long to elaberate on things....anyway....****

Just put the carb back to a stock baseline, doublecheck your timing, make sure you have some decent plugs in the mix.
Try to lower your fuel PSI to 2 1/2 when you set the carb back to stock....
Hook your vacuum gauge up and get the correct PV in there....
And then tune away....

 

[Quezzmo]

I tried a brown pump cam and standing beside the car I could see the black puff of smoke when I popped the throttle. "Bigger is not always better". I'm sure it's a rich bog so I'll work towards leaning it out. I ordered a .028 and a .025 nozzle and the assortment of cams. Also, my vacuum drops from 15 at idle to 10 in gear so I ordered a 8.5 power valve.

 

[Quezzmo]

My fuel regulator isn't rated to go as low 2.5 but I'll try. I'm curious to know the thought process behind dropping the fuel pressure.

 

[railroad]

As easy as it is to work on a Holley on a T, I would make only ONE change at a time. KISS

 

[Screaming Metal]

My fuel regulator isn't rated to go as low 2.5 but I'll try. I'm curious to know the thought process behind dropping the fuel pressure.

Ok, while Holley makes a great performance carb, in some folks eyes, they used to, the over the counter holley street performance carb is sensitive to fuel pressure.
The street carbs are not setup to handle the fuel pressure the racing carbs are....won't go into a elaborate explanation....street carbs are setup to run with stock street fuel pumps. While they can handle supposedly 3 to 5 PSI, most will leak at that pressure if not in absolute perfect shape. 2 1/2 psi is a good rule of thumb for street carbs....

 

[2old2fast]

Funny you would say that as most stock mech. pumps through the 60's-70's were rated at anywhere from 5-9 p.s.i. ?!
dave

As to the OP bog issue , I'd be looking at base timing , idle settings , ported or manifold vacuum for advance ...

 

[Mike]

As easy as it is to work on a Holley on a T, I would make only ONE change at a time. KISS

That is the way EVERY tuning procedure should be approached, not just carbs on a T-Bucket. When people start making wholesale changes, there is no way to keep track of it all, to know what may have worked, or to (more likely) know what stuck a fork in the mess.

Ok, while Holley makes a great performance carb, in some folks eyes, they used to, the over the counter holley street performance carb is sensitive to fuel pressure.
The street carbs are not setup to handle the fuel pressure the racing carbs are....won't go into a elaborate explanation....street carbs are setup to run with stock street fuel pumps. While they can handle supposedly 3 to 5 PSI, most will leak at that pressure if not in absolute perfect shape. 2 1/2 psi is a good rule of thumb for street carbs....

Has this been your experience? Personally, I never noticed that as an issue. If customers were using regulators on Holleys, I always recommended 6.5 PSI for everything. We would run the race cars at 7 PSI, but I never saw any leaking issues at 6.5. What have you seen leaking at those kinds of pressures?

 

[Screaming Metal]

Well, some of it is crap in the gas these days, some of it is cheap parts mfg'ing. I deal with alot of carbs, and as long as you don't put alot of pressure on the seats, your ok. They can make some expensive junk now days.... The race stuff we use is pretty good.
The older carbs yes, they could take the pressure, good components....
More and more of our stuff is being made across the big pond. Usually they manufacture junk awhile before they get all the bugs worked out. The new block cores that are coming from across the pond for the flatheads are good reliable. A few years back, there was core shift, porosity,
bad casting lines....even had trouble with some lifter bores.

 

[Screaming Metal]

We put a dry sump on a motor yesterday, the screw on filter housing that came with the system, we screwed the filter onto the filter housing, and the tapped nipple that the oil filter screws onto was not perpendicular to the filter sealing face.
Screwing it on by hand, there was a 1/8th gap on one side between the filter sealing face and the housing blocks sealing face.
We had to put the piece in the bridgeport and true things up....

 

[Quezzmo]

Parts will be here tomorrow. I plans to start by changing the .031 with a .028. Then start trying different cams. Last, the power valve. Unless you suggest a different order of operations.

 

[Mike]

Actually, I would change the power valve, first, since you know you need to make that change. From there, you can start tuning on the accelerator pump circuit. Reverse the procedure and you might end up with a big Band-Aid on the pump circuit, that will not work after you change power valves.

The thing to remember is that you will will still be dealing with a 5 CC pump shot, when you pin the throttle, but the smaller nozzle is going to give that 5 CCs a longer duration. There are a lot of variables coming into play with accelerator pump shot, so you may wear yourself down a bit, making the incremental changes required to get it all dialed in. Just remain patient, and it will all come together for you.

And yes, if you still have access to the 30 CC pump body and pump lever, you may find yourself going back to them.

 

[railroad]

There are some good Holley tuning sites on the net. Getting educated there is better than trial and error. I strongly encourage you to study a couple of these ASAP. Looking forward to you reporting how well you get it tuned.

 

[Quezzmo]

I got the off idle pump shot working much better. .31 nozzle and black cam. It takes off like a rocket but falls flat at 3k rpms. Holley says the 470 cfm carburetor, although mathematically suitable, was not meant for V8's because the 470 truck avenger has no high speed air bleeds and that on the high side it will always be pig rich. I'm thinking a street avenger 570 will be a better place to put my money and energy.

 

[Screaming Metal]

That or the good ole universal stand-in.... a vac. secondary single feed Holley 600....The Holley 600's can be picked up anywhere, theres alot of them out there, can be had for a song....parts are carried everywhere also....