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Discussion Starter #1
You might have seen my posts under Engine re Holley secondary power valve plug, but now I just want to get the #%*& thing running right before this weekend for the Goodguys show.

350, seems like a moderate build (but internals unknown). Holley 4150, List 9379, 750 CFM. Sits on a Holley intake manifold. Yes, it's over-carbed, so I just want to make it run as good as it can.

Engine was running real rich at idle and under power. I rebuilt the carb and noticed that it had size 70 jets - stock is 68. I now have the following jets: 64, 66, 68 and 70 to play with.

When I got it back together, I first adjusted the float levels, then the idle mixture screws (vs the idle speed and vaccume). Then I tried several different primary jet sizes (currently at 66) and adjusted the timing to where it seemed to be best (20)

The engine now backfires out the carb just off-idle when I goose it. Screwing in the idle/mixture screws INCREASES the idle speed and vaccume. It seems like it likes 1/2 turn out from bottom. 68 jets don't make a difference.

So none of this makes sense to me. Unless I didn't adjust the float levels correctly. Just how much fuel should be coming out of the check hole? I set it so there was only a VERY faint drip, which in hindsight could have just been caused by vibration of the motor and thus be way too low.

So I'll adjust them up to higher levels tonight. IF that doesn't work, what else should I try to adjust?

[ 03-25-2004, 05:12 PM: Message edited by: Luke68ss ]
 

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That's a common problem with overcarbing and lump cams. Sounds like your not able to transition from the idle circuit to the mains causing a lean condition and backfire.

On the bottom side of the carb (you have to remove it) there is a small screw which cracks open the secondaries a hair. I presume you set this to Holley specs which is 1 or 2 turns depending on the carb. Try opening the secondaries another 1/2 turn.

What this will do is bring the idle speed up so you can back off on the primary idle speed screw. This will put your primary throttle blades back in the idle circuit and significantly improve the transition to the main jets.

If that doesn't work, take a close look at your accellerator pump setup. Ensure you have the 0.015 adjustment properly set on the acc pump. Make sure you have a moderately sized pump cam installed in the #1 hole. I use a white or orange cam. You can get some pretty radical pump cams (like brown) but they really don't help as much as they hurt. The accellerator pump nozzle is another knob you can tweak.

Take a look at the tech reference. DjD collected some useful holley troubleshooting info. I think series II is what you want to look at.
 

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Originally posted by dnult:
That's a common problem with overcarbing and lumpy cams. Sounds like your not able to transition from the idle circuit to the mains causing a lean condition and backfire. Before moving forward, make sure you don't have a vacuum leak. Do the accellerator pump levers clear the intake manifold? I've seen holley pump levers hang up on the manifold causing a vacuum leak.

On the bottom side of the carb (you have to remove it) there is a small screw which cracks open the secondaries a hair. I presume you set this to Holley specs which is 1 or 2 turns depending on the carb. Try opening the secondary stop screw another 1/2 turn.

What this will do is bring the idle speed up so you can back off on the primary idle speed screw. This will put your primary throttle blades back in the idle circuit and significantly improve the transition to the main jets.

If that doesn't work, take a close look at your accellerator pump setup. Ensure you have the 0.015 adjustment properly set on the acc pump. Make sure you have a moderately sized pump cam installed in the #1 hole. I use a white or orange cam. You can get some pretty radical pump cams (like brown) but they really don't help as much as they hurt. The accellerator pump nozzle is another knob you can tweak.

Take a look at the tech reference. DjD collected some useful holley troubleshooting info. I think series II is what you want to look at.
 

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also make sure your front nozzles are squirting when you press the gas ... it takes a very small piece of trash to clog a nozzle and and that can make your carb pop when you press the gas because you are getting big gulp of air ... check this in addition to the adjustment on the accelerator pump ... You probably have this type of pop instead of a backfire, but be careful just in case because a backfire can be a fire hazard coming thru the carb
 

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Discussion Starter #6
Yeah - I moved my question here because my original one was answered and this section seemed more appropriate to the current problem.

Dnult - I do as you suggest. However, the accelerator pumps appears to work instantaneously with throttle movement with a healthly squirt, so I'm not sure that's the problem.

Re adjustment to the float levels: just how much fuel should be coming out of the check hole? I set it so there was only a VERY faint drip, which in hindsight could have just been caused by vibration of the motor and thus be way too low.

I didn't get a chance to adjust them yesterday, so any advice is appreciated.
 

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One last thing that comes to mind is to make sure that you have the proper gaskets correctly oriented on the metering blocks ...
 

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Originally posted by Luke68ss:

Dnult - I do as you suggest. However, the accelerator pumps appears to work instantaneously with throttle movement with a healthly squirt, so I'm not sure that's the problem.
So you check the secondary throttle blade opening?
 

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throw away the holley&buy a demon. i put a650 mighty demon on my 302& it ran good right out of the box. you can also contact them& they will recomend the best carb for your application.
 

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Originally posted by racermike:
throw away the holley&buy a demon. i put a650 mighty demon on my 302& it ran good right out of the box. you can also contact them& they will recomend the best carb for your application.
not very practical... Nothing wrong with the demon but nothing wrong with the holley either...
 

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Its an acelerator pump issue . . . or at least a problem that you can probably tune out with accelerator pump calibration changes.

If it goes for a split second, then pops back, then goes again try a SMALLER pump squirter.

If it pops first then try a LARGER squirter. You probably need to go to a pump cam that squirts more fuel and squirts it quicker when you make this change.

Pump cams control how MUCH fuel is squirted each time you floor the gas.

Squirters control how LONG it takes for the fuel to squirt out. The smaller the squirter the longer the pump shot.

You need to find the right combination of pump cams and squirters to tune out your lean stumble. Typically the out-of-box squirter and pump cam is pretty close. Sometimes the factory cam and a size smaller squirter will do the trick.

Just my opinion.
 

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I just had a thought. Make sure your carb isnt dripping fuel from the boosters at idle. Carefully look down the carb to see at idle. Could be your rich problem. Dont give it any gas while your head is over top of it. That could get ugly if it burps in your face. Also, you said that the idle and vaccume increases when turning the idle mix screws in. Usually the other way around. It shuts my motor down if I get anywhere near 1/2 to 1/4 turn. Runs to lean. Most Models turning the mix screws in leans the mixture. Turning them out richens it. Try 1-1/2 to 2 turns out.
 

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Originally posted by Luke68ss:
I set it so there was only a VERY faint drip, which in hindsight could have just been caused by vibration of the motor and thus be way too low.
You are about right with that float setting. Many people set the level too high because they're afraid of running out of fuel down the track.

Another possible backfire cause with later Holleys is the red/purple accelerator pump actuator spring is too soft. You can adjust it shorter (stiffer) and bend the arm that rides on the pump cam to get a full shot without having a soft spring. If you bend the arm so the tip rides on the pump cam instead of the flat part of the arm, you will get a longer pump shot. I prefer the white pump cam with #218 on it as a good cam to start with.
 

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Discussion Starter #15
Excellent advice guys

turns out that the main problem was that I had the primary float level too low and was starving the carb. Now that I got it, the idle/mixture screws, and the timing adjusted, it's not perfect, but it's WAY better.

Flashback to 35 min ago: freeway onramp, moderate launch, clutch fully released in first gear, it broke loose when the secondaries kicked in. That's with an M21, 3.73 posi and 255/70/15 tires. You KNOW that feeling! :eek:

thanks again!
 

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Discussion Starter #16
So now having driven it for the weekend, it's close, but not perfect.

If I go from idle to 3/4 throttle very quickly, it pops out the carb just a second after the motor starts to spin up. I would not call the pop instantaneous with hitting the throttle.

I'm still running size 66 jets, stock is 68.

Do I need a slightly larger squirter in the primary, or . . . ?
 

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Again I say . . .

Originally posted by Eric68:
Its an acelerator pump issue . . . or at least a problem that you can probably tune out with accelerator pump calibration changes.

If it goes for a split second, then pops back, then goes again try a SMALLER pump squirter.

If it pops first then try a LARGER squirter. You probably need to go to a pump cam that squirts more fuel and squirts it quicker when you make this change.

Pump cams control how MUCH fuel is squirted each time you floor the gas.

Squirters control how LONG it takes for the fuel to squirt out. The smaller the squirter the longer the pump shot.

You need to find the right combination of pump cams and squirters to tune out your lean stumble. Typically the out-of-box squirter and pump cam is pretty close. Sometimes the factory cam and a size smaller squirter will do the trick.

Just my opinion.
Its hard to guess exactly what change to make without being in the car and hearing/seeing it happen. I suggest you just get some squirters and pump cams and play with it.
 
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