Noticed something unusual with the Holley

[Paul_G]

I recently noticed that the car will pull a little harder when I let up on the throttle. When I noticed this I was out cruising, in high gear, just floor the peddle but not enough to downshift, the car pulls hard like it always has. But if I let off the gas just a little, the car seems to pull a little harder.

What might be causing this? Thanks.

BTW, it's a 360, A518, Holley 670, Eddy RPM air gap, long tubes, 3:91 gears, MSD 6. 72 Charger, 4000# car

 

[Photon440]

Perhaps the jets are a little on the lean side. Flat out, they can't keep up, but when you let up a little, they're able to flow enough and you have a better A/F ratio.

Either that or the throttle plates are going over center on WOT.

 

[nm9stheham]

In the carb, look into the power valve. It almost sounds like it is not opening properly.

Another possibility is that the vacuum advance is not coming in properly. Make sure the vacumm line is on the ported vacuum, not manifold vacuum. Then use a vacuum pump (or your mouth along with a guage) to pull a vacuum on the distributors vacuum advance while idling to make sure it comes in smoothly.

Finally, you may need to look at the spring in the vacuum pod for the secondary action; it may be too light and allowing the secondaries to come in too quickly and thus causing a bog. I would actually suspect this first, now that I think about it more. (I am assuming this is a 670 Avenger....)

 

[Paul_G]

I put a kit in this carb not long ago. New 6.5 power valve. It runs very well at low and midrange. No skips bumps or hesitation. I did have to jet up a lot from stock. Stock jets were 65/68. With that set up it had a bad stumble off idle and surged at cruise. Jetting up helped that out. Right now it has 68/74 jets in it. It is pretty smooth now, other than the peculiar "let up and go faster" thing it is doing. I wonder if the secondary is jetted up too much?

 

[1wild&crazyguy]

First I would go smaller in primary jet size till it surges and then go up 2 sizes larger jet and look at the plug color.
Thats a good way to get real close if not right where it wants to be.

For the secondary jetting 'unless removing an existing power valve from the mix' shouldn't have to increase jet size more than about 4 sizes from factory, otherwise the carb is too small for what you're doing.

About the stumble off idle, that could have been also related to the pump shot 'nozzle size' ,pump cam, power valve.


Personally i would run 750cfm on 360.

 

[451Mopar]

If you are using vacuum advance dist, when you let up a bit it may be adding some advance?

 

[lewtot184]

they'll do that if there is a check ball in the secondary vacuum pod.

 

[nm9stheham]

^^^ As I recall, there is no check valve in the Holley for the secondary pod.

 

[lewtot184]

^^^ As I recall, there is no check valve in the Holley for the secondary pod.

some do, most don't. some people recommend them, i don't. they delay the opening, which may be ok in some circumstances, but they also delay the closing. ['ve played with them in 6-paks; very dangerous!

 

[nm9stheham]

some do, most don't. some people recommend them, i don't. they delay the opening, which may be ok in some circumstances, but they also delay the closing. ['ve played with them in 6-paks; very dangerous!

<<< Exactly! I am thinking the OP has an Avenger model...not really sure.

 

[Paul_G]

First I would go smaller in primary jet size till it surges and then go up 2 sizes larger jet and look at the plug color.
Thats a good way to get real close if not right where it wants to be.

Personally i would run 750cfm on 360.

Jetting up the primary is how I corrected the lean surge and stumble off idle. Holley tech told me they set up the street avengers on the lean side. This engine had a 750 on it when I got it. It had multiple issues. Worn base plate shaft bushings finally made the decision to retire it. I picked up this SA 670 used on CL, put a kit in it. It may indeed just be undersized for the 360?

If you are using vacuum advance dist, when you let up a bit it may be adding some advance?

Not using vacuum advance on this engine. It doesn't like it. Base timing is around 18°, total at 36°, all in by 2000 RPM.

<<< Exactly! I am thinking the OP has an Avenger model...not really sure.

Street Avenger 670.

I spent some time on it this morning. First I checked the float levels. Secondary side fuel level was not dribbling out of the sight hole. Took about 3/4 turn of the nut CCW to get fuel to dribble out of the sight hole. Also, I remembered I had 72's in the secondary at one point, I put those back in and took the 74's out.

The primary side fuel level was low also. Took about 1/2 turn of the nut CCW to get fuel to dribble out of that sight hole.

Went for a drive to a couple local shows that were going on today. Smoothness and acceleration seems better than it has been for some reason. Fuel level adjustment? Couldn't do much full throttle today. Saturday shoppers and snow birds (eternally lost) are coming back in town. The few times I flat peddled it and slightly let up, the surge I was experiencing before could not be felt. What I can feel is the secondary's opening, or what I think are the secondary's opening. A second or two after full throttle I can feel a real kick in the pants. It has the plain vacuum diaphragm spring in it right now.

The 750 I have is junk. Might start looking around for another 750.

 

[383man]

some do, most don't. some people recommend them, i don't. they delay the opening, which may be ok in some circumstances, but they also delay the closing. ['ve played with them in 6-paks; very dangerous!

There is a reason for the check ball on the Holley vacum Secondaries. It lets the back barells open at the right speed and lets them close fast. If they drilled the orfice hole (vacum feed) at the right size for the proper opening rate then when you let off the gas the hole would be to small to bleed off the vacum fast enough and the secondaries would stay open to long when you let off the gas. So Holley makes the hole to big as it would open to fast and they put the check ball in to slow down the opening to be just right. Then when you let off the gas the check ball unseats and opens a large opening so they will close real fast.

See if they just put the right size hole they would still open fine but when you let off the gas the hole would be to small to bleed the vacum fast enough and the secondaries will close to slow and make it feel like your still on the gas. Thats why the hole is the proper size to open right with the check ball and they let the hole get larger when the check ball unseats (raises off the seat) so the back barells close fast enough.

Now many racers took them out and then the secondaries will open real fast and still close fast but for street driving they (Holley) feel it opens to fast and is not safe like that without the check ball.

Did you try to unhook the secondaries and drive it like that to see what it does ?? Ron

 

[nm9stheham]

^^^ Thanks for the explanation on this; I did not know the Avengers had the ball.

 

[747mopar]

I found my SA Holley to be really lean as well, wonder if the EPA has anything to do with it haha. Glad to here it's running better.

 

[lewtot184]

There is a reason for the check ball on the Holley vacum Secondaries. It lets the back barells open at the right speed and lets them close fast. If they drilled the orfice hole (vacum feed) at the right size for the proper opening rate then when you let off the gas the hole would be to small to bleed off the vacum fast enough and the secondaries would stay open to long when you let off the gas. So Holley makes the hole to big as it would open to fast and they put the check ball in to slow down the opening to be just right. Then when you let off the gas the check ball unseats and opens a large opening so they will close real fast.

See if they just put the right size hole they would still open fine but when you let off the gas the hole would be to small to bleed the vacum fast enough and the secondaries will close to slow and make it feel like your still on the gas. Thats why the hole is the proper size to open right with the check ball and they let the hole get larger when the check ball unseats (raises off the seat) so the back barells close fast enough.

Now many racers took them out and then the secondaries will open real fast and still close fast but for street driving they (Holley) feel it opens to fast and is not safe like that without the check ball.

Did you try to unhook the secondaries and drive it like that to see what it does ?? Ron

can't say i agree with all this, but believe what you want.