Holley Tuning Question

[mopowers]

Trying to tune the 750dp on my 340. The carb has 4-corner idle. Turning the secondary idle mixture screws all the way in will not kill the engine. The primary IM screws work as expected. I understand this to mean the butterflies are exposing too much of the transition slot to the point where the carb won't respond to changes in the idle circuit. The only thing is I've backed off the secondary throttle position screw to where it isn't even opening the sec throttle blades at all. Any ides on what else I can try?????? I started out with all transition slot "squared" and have fiddled with it since then.

I appreciate the helps guys. I'm still kinda learning as I go so take it easy on my dumbass.

 

[EL5DEMON340]

Usually the throttle plates should be open the same primary and secondary. I'm guessing your trying to adjust the carb due to it being rich??? If your 340 is on the mild side the 4 corner carb might be a bit much

 

[EL5DEMON340]

Make sure your timing is up around 15 to 20 initial if it has a healthy cam

 

[73Scamp318]

You might want to verify that your carb is set up for four corner idle. If you didn't buy it new with 4-corner idle, it may have a different secondary block on it that happens to have idle mixture screws.

I converted my 750DP to a four-corner idle last summer due to a broken base plate and I approached it like this...

Pop the carb off and square up all four transition slots. Set all four of the idle mix screws to 1 turn out. Any adjustments made on the primaries should be the same on the secondaries. I wound up setting my idle speed a touch higher front and rear and the idle mixture screws are a hair more than 3/4 of a turn all the way around.

This is what worked for me...of course, YMMV.

 

[mopowers]

Usually the throttle plates should be open the same primary and secondary. I'm guessing your trying to adjust the carb due to it being rich??? If your 340 is on the mild side the 4 corner carb might be a bit much

Make sure your timing is up around 15 to 20 initial if it has a healthy cam

Thanks for the input. The initial timing is set at 20*. I'm adjusting the carb because I just rebuilt it and put it on the engine. Just going through the steps to get it tuned in. It does seem a bit rich. It's a 4777-2 set up with 70's and 79's with a 65 pri PV and no sec PV. Air bleeds have not been messed with. I'll pop the carb off and even out the throttle blades and start over. So I take it any increase in idle speed needs to be made even between the primaries and the secondaries? I hadn't thought of that. Thank you.

 

[mopowers]

You might want to verify that your carb is set up for four corner idle. If you didn't buy it new with 4-corner idle, it may have a different secondary block on it that happens to have idle mixture screws.

I converted my 750DP to a four-corner idle last summer due to a broken base plate and I approached it like this...

Pop the carb off and square up all four transition slots. Set all four of the idle mix screws to 1 turn out. Any adjustments made on the primaries should be the same on the secondaries. I wound up setting my idle speed a touch higher front and rear and the idle mixture screws are a hair more than 3/4 of a turn all the way around.

This is what worked for me...of course, YMMV.

Thanks for the input! I inherited the carb (4777-2) from my uncle who passed away. When I recently rebuilt it, I replaced the old base plate with a new Holley one made for 4-corner idle. It has the plugs in the idle fuel channel isolating the front and back as seen here:

The carb also has a 4777-2 sec metering block with idle mixture circuit. I'm assuming that's all it needs?? Any additional info would be greatly appreciated!

Thanks again for your input! I think I'm going to pop the carb off and reset everything and start over.

Out of curiosity, if you screw your secondary IM screws in, will it kill the engine???

 

[70aarcuda]

depends...if the engine is getting enough air/fuel from the front idle circuit it will say running...if you screw all 4 in...it should die...

should set all 4 idle mixture screws 1 1/2 turns out...using a vacuum gauge..adjust them to the highest vacuum reading..with 4 corner idle..I usually end up around 3/4 to 1 turn on each corner..

 

[Snake]

Wat cam is in it,wat Iam geting at is what is your vacume at idle in gear and out,your power valve my be rich en things up.

 

[mopowers]

depends...if the engine is getting enough air/fuel from the front idle circuit it will say running...if you screw all 4 in...it should die...

should set all 4 idle mixture screws 1 1/2 turns out...using a vacuum gauge..adjust them to the highest vacuum reading..with 4 corner idle..I usually end up around 3/4 to 1 turn on each corner..

I totally get what you're saying. I'm just having trouble getting any sort of response out of any adjusting I do to the secondary idle mixture screws, even if I close down the primary mixture screws down to where it's starting to hiccup. I wonder if there's a clogged port somewhere in the body or metering block in the rear idle circuit. Would swapping metering blocks front to back be worthwhile???

Not sure what cam is in the engine since I didn't build it. That's something I need to check. I was told it's a hydraulic MP purple shaft. Just by listening to it, I'd guess it's the 280* MP purple shaft. But again, I need to check that to be sure. The most vacuum I can get at idle in park is around 10.5"

Thanks for the help. I'll get back when I have a chance to get to work on it.

 

[crackedback]

What happens if the IAB's aren't the same size front and rear?

The number of turns out on each end may be very different.

 

[Snake]

I have the 280 477 mp cam and i have 10 in park like you,in D it drops to 7 with the 6.5 it would flood it self out went to 4.5 runs like a champ,but you would not think it would not run rich at 10 at idle if the pirmerys or open say 1000 rpms it will hence why no response to mixture screws.my 2 cents.

 

[Oldmanmopar]

There is a method for this. I use it all the time if interested and no one else knows and chimes in. I can explain when I get time to type. You won't get them both to work if your only idling off of one set. Also a 4 corner carb is used for large cam motors that idle higher then the primary transfer ports allow before transition. This allows you to idle off the secondaries and still have the primary transfer ports for initial throttle movement. The start of this procedure is with the carb off of the car. When done correctly throttle response is much snappier.

 

[mopowers]

There is a method for this. I use it all the time if interested and no one else knows and chimes in. I can explain when I get time to type. You won't get them both to work if your only idling off of one set. Also a 4 corner carb is used for large cam motors that idle higher then the primary transfer ports allow before transition. This allows you to idle off the secondaries and still have the primary transfer ports for initial throttle movement. The start of this procedure is with the carb off of the car. When done correctly throttle response is much snappier.

Would you mind explaining it to me?? You can PM me or call me too if that's easier. I really appreciate it. I didn't get a chance to mess with it this weekend.

 

[pjc360]

lean out the idle circuit, put smaller idle feed restrictors In it and bigger idle air bleeds.

 

[mopowers]

lean out the idle circuit, put smaller idle feed restrictors In it and bigger idle air bleeds.

That seems like a pretty drastic change.

 

[pjc360]

I bought a quick fuel super street series 680cfm vaccum secondary carburetor for my 360 magnum and I had to go down 4 sizes on the idle feed restrictors to get my 4 corner idle mixture screws to work the way they are supposed to and to stop fouling plugs. But I also live 4000 feet above sea level.
My quick fuel came with .031 idle feed restrictors and .070 idle air bleeds, by the time I was done I was using .025 idle feed restrictors and .065 idle air bleeds.

 

[mopowers]

I bought a quick fuel super street series 680cfm vaccum secondary carburetor for my 360 magnum and I had to go down 4 sizes on the idle feed restrictors to get my 4 corner idle mixture screws to work the way they are supposed to and to stop fouling plugs. But I also live 4000 feet above sea level.
My quick fuel came with .031 idle feed restrictors and .070 idle air bleeds, by the time I was done I was using .025 idle feed restrictors and .065 idle air bleeds.

Before you made those changes, did you noticed any difference between the how the primary and secondary mixture screws reacted to changes??

 

[73Scamp318]

There is a method for this. I use it all the time if interested and no one else knows and chimes in. I can explain when I get time to type. You won't get them both to work if your only idling off of one set.

If I'm not mistaken, he's saying that if either the primaries or secondaries are closed down too far on a 4 corner carb, the idle mixture screws on that side will have no effect.

Before you start replacing restrictors, bleeds, and start drilling holes, I'd advise you to pull off the carb, tear it down, make 100% sure that all the passages are clear, then try setting the carb up.

I had a tiny piece of gasket start floating around in my carb and giving me idle problems while I was in the process of tuning it last summer. I fought with it for a weekend before throwing in the towel and tearing down the carb.

 

[pjc360]

With mine I could turn all 4 mixture screws all the way inn and it would still idle, And I was fouling plugs like crazy, once I stepped down a few sizes on the idle feed restrictors I stopped fouling plugs and my mixture screws were more responsive.

 

[pjc360]

If your holley has replaceable idle feed restrictors and replaceable air bleeds it only takes me about 45 minutes to tear into the carb and change the idle feed restrictors, you should have 2 per metering block.

 

[mopowers]

Thanks for the advice guys. My carb is just a run of the mill 4777-2 that's been converted to 4-corner idle, so the restrictors and air bleeds are not replaceable.

For now, I'm gonna do what 73Scamp318 suggests and square up the sec slots and start over making sure to make changes in blade position even between the secondary and primary sides. I may swap metering plates from front to back just to see if that does anything. I'll report back with results after the weekend. I've just been too damn busy chasing women lately to fire this thing up... priorities, right?

Thanks again guys.