holley accelerator pump question

[76bonanza]

How can you tell if you have the high flow screws in the discharge nozzels? Mine use an allen wrench does that mean they are high flow? Also I have #31 discharge nozzles and want to expermint with some others I have heard the heaver the truck the bigger the nozzels? Looking to try to get quicker response.

Thanks for an insight

[Jonce]

I would also like to know the same thing. I have an off idle hesitation. I currently have a 600 cfm vacuum secondary Holley on my GM crate 350 HO engine. I get the stumble shortly after hitting the gas pedal. I verified the accelerator pump discharge nozzle is squirting fuel and its set to .015" of play at WOT. Any help would be greatly appreciated. Note: I have not adjusted the idle/mixture screw - everything is factory set.

[jhow66]

Jonce before messing with carb try upping the timing. What is it set at now?
76b' do you get a clean responce went you blip the throttle now? Most of the time you only need the bigger screws with the Reo pumps (50cc)

[sodak69c10]

also if you are having a lean backfire (through carb) under hard acceleration try bigger squirter , make sure as stated earlier 1.that your timing is set 2. that your butterflys are shut completly when adjusting idle air-fuel 3. also make sure your power valve (holleys, demons, etc) is right for your vacuum, if you are running a big cam it is a good idea to get a vacuum reading and use a powervalve that is 2-3 steps down from that reading is what a carb tuning shop told me on my demon

also a good idea to check to make sure your cap and rotor are in good shape i had a rotor on my 283 that looked ok but when i pulled it off and looked on the bottom of the cap you could see carbon tracking wich caused it to surge under cruising/ or hard acceleration but it idled fine

[Jonce]

Originally Posted by jhow66

Jonce before messing with carb try upping the timing. What is it set at now?

Currently I have the timing set to 10* - GM's spec's for the 350 HO 330hp crate. I haven't had any lean backfires as of yet.

Looks like someone else is having the same issue with the same engine.
http://67-72chevytrucks.com/vboard/s...d.php?t=227404

[68Stepbed]

The 350HO crate in my dad's Monte Carlo SS does the same thing. I've tried to get him to let me tune on it, but he won't let me. It drives me nuts.

Jonce, I don't think the problem with you're carb is with the squirter nozzle, but in the pump cam itself. That carb has a pretty large squirter on it now, I beleive. I think the power valve is the wrong size also in those carbs. I think they have a 6.5 PV. An 8.5 or 10.5 PV would be better because those engines make good vacuum.

76Bonanza, I would try a larger squirter(I think .034 is the next size up). Only go up in small steps. Also check the PV as mentioned above and see if it matches your vacuum reading. Your PV size should be about half of what your vacuum reading is. (20" of vacuum= 10.5 PV, 16" of vac.= 8.5 PV, etc.) Also play with the pump cams, sometime you can reclock the stock cams for better performance.

Also check spark plugs to make sure you have the right jet sizes. You want to make sure your jets are right before making any other changes. Holley carbs are harder to tune, but they make some serious horsepower when tuned right.

[Jonce]

Originally Posted by 68Stepbed

The 350HO crate in my dad's Monte Carlo SS does the same thing. I've tried to get him to let me tune on it, but he won't let me. It drives me nuts.

Jonce, I don't think the problem with you're carb is with the squirter nozzle, but in the pump cam itself. That carb has a pretty large squirter on it now, I beleive. I think the power valve is the wrong size also in those carbs. I think they have a 6.5 PV. An 8.5 or 10.5 PV would be better because those engines make good vacuum.

76Bonanza, I would try a larger squirter(I think .034 is the next size up). Only go up in small steps. Also check the PV as mentioned above and see if it matches your vacuum reading. Your PV size should be about half of what your vacuum reading is. (20" of vacuum= 10.5 PV, 16" of vac.= 8.5 PV, etc.) Also play with the pump cams, sometime you can reclock the stock cams for better performance.

Also check spark plugs to make sure you have the right jet sizes. You want to make sure your jets are right before making any other changes. Holley carbs are harder to tune, but they make some serious horsepower when tuned right.

68stepbed, thanks for the reply. I will look into the PV. What's the proper procedure to measure vacuum? Do you do it at idle or do you have to do it at a certain rpm? Do you measure it at the ported vacuum connection on the carb (where the vacuum advance hooks up to)?

The stock squirter nozzle is a .31 for these engines.

[jhow66]

Use manifold vacumn fitting. While you have the gauge hooked up, adjust the idle mixture screws to get the highest reading on gauge. (turn both sides the same amt. each time.) (CCW=richer, CC= leaner) This will probably cure your stumble as everything else seems ok. Forgot to ask-new carb?

[Psycho71]

To get a vaccum reading, Ricky (the holley tech) told me to hook up the guage, warm up the motor to operating temp, then put it in drive with the parking brake set. Record the vaccum reading. He said to then divide that number in half, and that's the P.V. you need. Thus, a 15" reading would want a 7.5" P.V.




Hope this helps,
Jay

[76bonanza]

I also have a double pumper should the squirters be the same size for the primarys and secondaries? I was under the impression you needed the high flow screws for .034 and above?

[cst70rocket]

Not all carbs have same size squirters front and rear, and if you're using over a 31 on a 4150, you're only "crutching" another tuning problem. Unless you're running alcohol or wrong sized carb. If you run a squirter larger than a 37 you should have a hollow screw. I divide the vacuum number in half and subtract one, hence a 15hg would be 6.5 pv.

[76bonanza]

How much vaccum change is there between park and drive? I think at idle in park I am at like 18 not sure in gear tho. The guys at quick fuel were the ones that sugested moving up in squirter size they said the heaver trucks sometimes respond better to larger squirters.

[dan42]

ttt

[Jonce]

Originally Posted by jhow66

Use manifold vacumn fitting. While you have the gauge hooked up, adjust the idle mixture screws to get the highest reading on gauge. (turn both sides the same amt. each time.) (CCW=richer, CC= leaner) This will probably cure your stumble as everything else seems ok. Forgot to ask-new carb?

Yes, the carb came with the turn key 350 HO crate engine. I will work on this tomorrow afternoon after I get my new exhaust on

[rage'nrat638]

the squirter screws will be hollow on the inside

[Jonce]

Originally Posted by 68Stepbed

The 350HO crate in my dad's Monte Carlo SS does the same thing. I've tried to get him to let me tune on it, but he won't let me. It drives me nuts.

Jonce, I don't think the problem with you're carb is with the squirter nozzle, but in the pump cam itself. That carb has a pretty large squirter on it now, I beleive. I think the power valve is the wrong size also in those carbs. I think they have a 6.5 PV. An 8.5 or 10.5 PV would be better because those engines make good vacuum.

76Bonanza, I would try a larger squirter(I think .034 is the next size up). Only go up in small steps. Also check the PV as mentioned above and see if it matches your vacuum reading. Your PV size should be about half of what your vacuum reading is. (20" of vacuum= 10.5 PV, 16" of vac.= 8.5 PV, etc.) Also play with the pump cams, sometime you can reclock the stock cams for better performance.

Also check spark plugs to make sure you have the right jet sizes. You want to make sure your jets are right before making any other changes. Holley carbs are harder to tune, but they make some serious horsepower when tuned right.

Hesitiation off idle

Just an update on this, I'm still it he middle of tuning this carb. I started with a stock Holley 4160 - 600cfm, 4 barrel, vacuum secondaries, 31 squirter, orange accelerator pump cam in #1 hole, .015" free play between pump arm and WOT, idle vacuum at 14-15" hg in drive, power valve is 6.5, timing set to 10*, and idle mixture screws set to 1.25 turns out.

I now have in my posession some more squirters, power valves, and pump cam arms. I have a 28, 32, 35, and 37 squirter. I have a blue, yellow, and brown pump cam arms. I bought another 6.5 power valve as well as a 8.5 power valve.

I haven't had time to try all of the squirters or pump cam arms, I tried to keep only one variable at a time. I tried the 35 squirter and left everything else stock, that didn't seem to help. It almost made it worse now that I look back on it. I then tried a 8.5 power valve which didn't seem to help. I also brought the timing up to 12* in both scenario's and still the same effect.

Tomorrow I will try the different pump cams (plastic) to see what happens. I still haven't ruled out the timing or a vacuum leak. I just need a couple of hours to work on it.

[dubs68SWB]

My 2 cents is that you don't need flow thru squirters. The best thing to do it go to the Holley website and find out, with your list number, how your carb was setup from Holley and go from there with adjustments. Your flow thru, .37 squirters and usually for 50cc accelerator pump, which are usually way too big for the street. There is a reason why Holley puts the 30cc pumps and .28 to .31 squriters and most all of their carbs. A heavy truck with auto is probably going to need less fuel than more. It's too heavy to be able to deal with the extra fuel. I'm running a well built 460 bbc with a 4 speed and am in the process of backing off my fuel shots to get it to run better. Remember......most people out there are running too big of carb for there application. A leaner motor will make more power than a fat one (just don't burn it up). Too lean is obviously not good either.

[Psycho71]

One more thing to look for. The squirters should squirt fuel instantly upon the slightest movement of the throttle linkage. We had this issue on a buddies carb. We had tried every pump cam, and a new PV. neither helped. We then talked to Holley, they said to adjust the pump linkage until it squirts instantly upon the slightest movement. We adjusted it, and it fixed the problem. An easy fix to a problem we were overthinking. It also corrected the problem of the carb not wanting to take fuel until the motor was completely up to operating temp. It will now fire up cold and take right off. No waiting for warm-up, or falling on it's face when given gas (when cold).

Food for thought.
Jay

[Jonce]

No more hesitation - change acc. pump cam to yellow

After speaking with a few of my well informed car friends, I decided to buy a new carb off eBay for cheap - same model except now its shiny. I bolted the carb to the engine completely stock form - still had hesitation. I changed out the orange accelerator pump cam (stock cam) for the yellow acc. pump cam (most aggressive pump cam) and it runs great now. This leads me to thinking there is a restriction in a passage way or something to that effect. I have another V8 I can put this carb on so it won't go to waste.

Note - the squirter is still a 31 shot, power valve is still the stock 6.5 hg