Idle mixture screws question

[72Dart360]

How many turns out is the norm for most? Currently on my 4150HP 750 mech sec, I have the screws 3/4 out. Is that making my idle circuit too lean? I dont have a vacume gauge but if I turn them out any more the engine doesnt like it.

 

[rumblefish360]

How many turns out is the norm for most? Currently on my 4150HP 750 mech sec, I have the screws 3/4 out. Is that making my idle circuit too lean? I dont have a vacume gauge but if I turn them out any more the engine doesnt like it.

2 - 2-1/2 turns should do it.

3/4's of the way out is way to far and the father out it is, the richer the mix.

 

[72Dart360]

3/4's of a turn out. When I moved them out to 1.5 turn out the idle went from 780rpm to less than 500rpm. The initial timing is 15 and total is 35. The cam is a solid 252 duration at .050 with 550 lift.

 

[crackedback]

Put more initial in it. like 20-22 for starters...

 

[72Dart360]

Put more initial in it. like 20-22 for starters...

Only problem is that I'm using MSD ignition(box and distributor). 20 initial would put me at 40 total. Too high right? Or No.

 

[doktor_x]

I'd be curious about the history of the carb. If the idle mixture screws have been cranked down too tight in the past, resultant deformation could cause some weirdness in how many turns out it takes to make it work right.

Bear in mind that this is a somewhat way-out-there theory.

 

[72Dart360]

Not sure if the following has anything to do with my issue, but I tuned the carb utilizing Holley's instruction manual which has the primary and secondary adjustments the same. Before this tune I had the carb setup where the secondary was at half of the primary.

 

[wish4hemi]

Spend the money, buy a vacuum gauge. It's not like it's a $1,000 dollar investment. That way you don't have to guess around, the vacuum gauge will tell you exactly what you need.

 

[C130 Chief]

There's no "correct" setting other than the setting which gives the best idle. Since your combo seems to like very little, I would suspect you are running borderline rich at idle. But, since you can adjust for a good idle I wouldn't sweat it too much.

If you have a vacuum gauge, make sure the power valve is 2 numbers less than the idle vacuum reading. Also, Holley double pumpers are jetted for response over economy. You may be able to go a couple numbers smaller on your primary jets without slowing the car down.

 

[crackedback]

Only problem is that I'm using MSD ignition(box and distributor). 20 initial would put me at 40 total. Too high right? Or No.

Then you need to limit mechanical advance in the distributor. Get the initial dialed in first before wasting any more time tuning the carb.

Put a bigger advance limiter bushing on the distributor.

 

[Rob Eppler]

if you get a lean drop, it does not matter where they are. the engine likes it by the rpm or vacuum. when going out, if the engine sounds loapy/rich then it is! turn them back in [get the highest "clean" idle rpm/vacuum] and forget about it. Only one other question, is the power valve good or blown??? Blown will cause a very rich mixture and hard/long cranking problems.

 

[rumblefish360]

3/4's of a turn out. When I moved them out to 1.5 turn out the idle went from 780rpm to less than 500rpm. The initial timing is 15 and total is 35. The cam is a solid 252 duration at .050 with 550 lift.

Ohh, OK, your running rich to start with. 3/4 of one turn is very little to move it at all with those results.

Only problem is that I'm using MSD ignition(box and distributor). 20 initial would put me at 40 total. Too high right? Or No.

32*'s vacuum + intail (approx 5 - 7 degrees) and the rest is mechanical + vacuum and intail for a max of 52*.

If your intail is low, you'll need to change out advance springs in the distributor. This way you can move up the intail and get the quick advance going quick to 32*'s at or by 2600 rpm... or so. Your engine may want more or less, but this should be about where you need it.

The MOpar engines book has this layed out well. Get the vacuum gauge like everyone above was saying. It's a good tool to have around for this stuff.

 

[72Dart360]

Spend the money, buy a vacuum gauge. It's not like it's a $1,000 dollar investment. That way you don't have to guess around, the vacuum gauge will tell you exactly what you need.

I picked up a vacuum gauage and I have 6 in.

 

[72Dart360]

Then you need to limit mechanical advance in the distributor. Get the initial dialed in first before wasting any more time tuning the carb.

Put a bigger advance limiter bushing on the distributor.

I have the black bushing installed, which adds 10 degrees to the total.

 

[72Dart360]

if you get a lean drop, it does not matter where they are. the engine likes it by the rpm or vacuum. when going out, if the engine sounds loapy/rich then it is! turn them back in [get the highest "clean" idle rpm/vacuum] and forget about it. Only one other question, is the power valve good or blown??? Blown will cause a very rich mixture and hard/long cranking problems.

I have blockoffs installed on the primary and secondary.

 

[72Dart360]

Ohh, OK, your running rich to start with. 3/4 of one turn is very little to move it at all with those results.



32*'s vacuum + intail (approx 5 - 7 degrees) and the rest is mechanical + vacuum and intail for a max of 52*.

If your intail is low, you'll need to change out advance springs in the distributor. This way you can move up the intail and get the quick advance going quick to 32*'s at or by 2600 rpm... or so. Your engine may want more or less, but this should be about where you need it.

The MOpar engines book has this layed out well. Get the vacuum gauge like everyone above was saying. It's a good tool to have around for this stuff.

On the distributor I have light springs installed, so the total comes in quick.

 

[green1]

Put the power valves back in, so you can get the carb to work correctly.

 

[72Dart360]

How do you guys tune your Holley? Tell me how you would make adjustments as if you just received a 4150HP 750 mech secondarys out of the box.

 

[crackedback]

What happens if you screw the idle screws all the way in? Does it die?

Get the initial up first, it will allow you to close down the throttle plates. Put as much as the starter can handle or the highest vacuum reading you can get and dial it back 1" keeping idle speed constant. You'll have to limit the mechanical for total. You may need a larger bushing than the biggest one MSD sends, which I believe is 18* IIRC. You can make one by rounding off a nut.

You aren't going to hurt the engine with no load on it even if the total is 40-45, even 50*-. Just don't go out hammering on it until you get the total back in a safe range.

As a tuning tool ONLY, I put power valve plugs in both ends to get a clean idle on cars with big camshafts... you have a large cam! This totally removes a circuit that could throw a fly in the ointment. Get the vacuum readings and select the appropriate PV's to install. I don't ever run PV's in the rear. Just jet it up.