holley tuning issue at cruise

[mrmustangman357]

So my setup:

351W in a 66 mustang, T5, 3.73 gears, 3000 lb with driver
.530 list 230 dur cam
trick flow TW heads
edelbrock performer rpm air gap
38 degrees total timing (20 deg initial) set up with light distributor springs and vac advance.
Holley 750 vac. secondary carb - 4160 (the one with the secondary plate)
6 psi fuel pressure
has the factory setup: number 72 jets and 6.5 power valve
floats are set correctly
16" of vacuum at idle

Problem is that in any gear above about 2200 rpm, my a/f goes dead lean and the car surges and shakes. I set a vac gauge and it has 16" at cruise when this happens. Pretty much it has forced me to keep the car lugging in high gear so (I'm guessing) it stays in the idle circuit, which is tuned OK. Once I got on the highway doing 65 mph, I had to make the first exit because the shaking was crazy.

In neutral and just revving the motor, the motor goes lean as well.

Is this a simple case of needing to swap to larger jets? or should I look at other items that can cause this condition?

Thanks so much for the help,
Brian

 

[10secgoal]

Take a copper wire and take apart the strands out of the plastic. Stick them in the low speed air bleeds. That should make a change in the right direction if you are still in the idle circuit.

 

[mrmustangman357]

dustin: idle performance is good, nice stable idle. should I put the wire in the high speed air bleeds?

 

[2+2GT]

Controlling the mixture at cruise speed is precisely what the main jets are for. If you are lean at cruise, get bigger jets.

 

[jlangholzj]

Controlling the mixture at cruise speed is precisely what the main jets are for. If you are lean at cruise, get bigger jets.

+1 this is assuming of course that you've got the engine tuned for optimum vacuum with your current jets?

even so, if its as bad as you describe, i'd still look at getting larger jets.


however, I'm thinking that adjusting your secondaries could also help the problem? But since that its happening at such a "low" (relatively) rpm, I'd still suggest getting larger jets.

 

[D.Hearne]

I'm more inclined to thinking your vacuum advance isn't working. Have you checked it? 72 jets are perfect. Ditto on the powervalve. Could also be trash in the carb doing it. Or not enough fuel delivery, letting the bowls run dry.

 

[brianj5600]

Changing to smaller LSABs richened my cruise up. Testing with wire in the LSABs is easy to do. Much easier than changing jets and LSABs won't change WOT a measurable amount like a jet change.

 

[10secgoal]

Controlling the mixture at cruise speed is precisely what the main jets are for. If you are lean at cruise, get bigger jets.

Nope, if jetting effects your cruise, you've adjusted your idle speed too much with the primaries.

You still want the wire in the low speed. Work your idle screws around the air bleed when it's done. The air bleeds effect your curve. You need to bring yours down, and that's what the smaller hole will do.

 

[mrmustangman357]

I'm more inclined to thinking your vacuum advance isn't working. Have you checked it? 72 jets are perfect. Ditto on the powervalve. Could also be trash in the carb doing it. Or not enough fuel delivery, letting the bowls run dry.

I took it apart last night in anticipation of larger jets, everything looks clean and fresh. I checked all the ports and can't find any problems there. vacuum advance works good. I don't think its fuel supply problem because just free-revving the motor creates this condition.

Nope, if jetting effects your cruise, you've adjusted your idle speed too much with the primaries.

You still want the wire in the low speed. Work your idle screws around the air bleed when it's done. The air bleeds effect your curve. You need to bring yours down, and that's what the smaller hole will do.

My primary idle speed screw is backed all the way out. The secondary idle is opened enough to take care of the idling (at 850 rpm) since I found I originally had to crank the primary idle speed to get it to run right and I didn't like that. Could that cause an issue?

So if I'm reading right, I should put a wire in the low speed air bleeds (are these the idle air bleeds?), readjust my idle mixture, and drive it? Should I mess with the way the idle speed is set?

 

[D.Hearne]

I took it apart last night in anticipation of larger jets, everything looks clean and fresh. I checked all the ports and can't find any problems there. vacuum advance works good. I don't think its fuel supply problem because just free-revving the motor creates this condition.



My primary idle speed screw is backed all the way out. The secondary idle is opened enough to take care of the idling (at 850 rpm) since I found I originally had to crank the primary idle speed to get it to run right and I didn't like that. Could that cause an issue?

So if I'm reading right, I should put a wire in the low speed air bleeds (are these the idle air bleeds?), readjust my idle mixture, and drive it? Should I mess with the way the idle speed is set?

If your primary idle speed screw is that far out, you've got it tuned wrong to start with. You've now uncovered the transition slot with the opened throttle blades, something you don't want. You need to go back and retune the idle and timing to get it to idle without having the throttle open that far. Set the initial timing at about 12*BTDC, then tune the carb's idle, first with the primary throttle just where it slightly moves the throttle blades off the fully closed position. When it's idling, then adjust the idle with the idle mixture screws. If you can't get a suitable idle speed, try more timing. With the build you have, it should be fine idling at 500-600 rpms. My 331 does in gear with it's C-4.

 

[brianj5600]

I am cofused about where the primary and secondary blades are set at idle. Do you know how much transfer slot is showing the way your idle is adjusted? Air coming through the cracked secondaries is not pulling fuel. If you are not showing enough T slot it could be lean at light throttle until the mains start flowing.

 

[10secgoal]

Exactly. You have your idle set correctly. You want it open just a smidgen. You want that... .020 uncovered on the slot I think was the measurement.If you had it open too far, you would be that much closer to pulling fuel from your jets. But it would enrich earlier and use more fuel for no reason.

You just need to close the gap between the circuits. If you saw it on a dyno, you would see it slowly rise until it probably hit 15-1, stumble some, then the AFR would come down as the jets started moving fuel. You need to flatten the curve should it doesn't lean out. By making the air bleeds smaller, you are sending more air through the carb rather than the , metering block. Take a 12-14 ga wire apart and take one copper wire and put it in the four holes on the most outside of the carb. Make them long enough to hang over the main body and use your air cleaner to hold them in.

 

[mrmustangman357]

If your primary idle speed screw is that far out, you've got it tuned wrong to start with. You've now uncovered the transition slot with the opened throttle blades, something you don't want. You need to go back and retune the idle and timing to get it to idle without having the throttle open that far. Set the initial timing at about 12*BTDC, then tune the carb's idle, first with the primary throttle just where it slightly moves the throttle blades off the fully closed position. When it's idling, then adjust the idle with the idle mixture screws. If you can't get a suitable idle speed, try more timing. With the build you have, it should be fine idling at 500-600 rpms. My 331 does in gear with it's C-4.

to clarify, right now, the way it is set up, the primary idle speed adjustment is backed out, such that it does not raise the idle speed. I turned the secondary idle speed screw in to raise the idle speed to my liking (850 rpm in neutral). This allowed the primary idle mixture screws to work, which was something I could not do when I had set speed only on the primary side. I'm sorry if this is confusing as it is hard to explain when typing.

BrianJ, you wrote:"Air coming through the cracked secondaries is not pulling fuel." The holley 4160 I have does meter fuel in the idle side of the secondaries. I don't see how this is contributing to the problem when my idle mixture screws work fine and my idle a/f is good (12:1)

Soon I will put the whole thing back together and try the wire in the idle air bleed. Is that number 1 in this photo?
http://image.chevyhiperformance.com/f/techarticles/9591285+pinline_medium/p158299_image_large.jpg

It seems like I would like to try that before messing with the main jets and messing up the circuit elsewhere

 

[jlangholzj]

12:1 seems a bit off to me....shouldn't it be ~ 14.5 :1 if i remember right?

 

[D.Hearne]

to clarify, right now, the way it is set up, the primary idle speed adjustment is backed out, such that it does not raise the idle speed. I turned the secondary idle speed screw in to raise the idle speed to my liking (850 rpm in neutral). This allowed the primary idle mixture screws to work, which was something I could not do when I had set speed only on the primary side.

This right here tells me you've done something wrong, or there's trash in the metering block. Nothing to do with the air bleeds.

 

[Dark Knight]

Change the power valve to an 8.0 or 8.5. 12-1 AF is rich, not lean. Are you watching fuel pressure at cruise too? That cam is pretty small for a 351, you shouldnt have to mess with the air bleeds. What altitude are you at? If you run it at that rpm in neutral does it do the same thing?

 

[jlangholzj]

Change the power valve to an 8.0 or 8.5. 12-1 AF is rich, not lean. Are you watching fuel pressure at cruise too? That cam is pretty small for a 351, you shouldnt have to mess with the air bleeds. What altitude are you at? If you run it at that rpm in neutral does it do the same thing?

powervalve shouldn't matter....all that does is adjust it for when the engine is under load or revving. if he's having this issue when he's just cruising down the road...the PV should be irrelevant.



start AAALLL over...lol... unplug everything (vaccum) set your timing right, then start from square one again with the carb like some have said, 20* does sound like a bit much, usually they're in the 8-12 range. This could be the reason for teh extremly rough run that you're getting, too much advance.


sheesh...holleys...gotta love to hate em

 

[10secgoal]

This right here tells me you've done something wrong, or there's trash in the metering block. Nothing to do with the air bleeds.

How do you figure ? If the t-slot is showing like it should be, and it doesn't idle high enough, you have to open the secondaries to get the correct idle speed. You don't just open the primaries farther. If it was something in the metering block clogged up, he would have idle problems, and stuttering long before 2200.

The PV will do nothing, and the size of the cam really doesn't have anything to do with it. I've heard the car, and I'm sure it has a good amount of overlap with ton of exhaust dilution and poor vac. The fact that it's a vac secondary isn't helping. A DP will come with bigger IFR's for this reason, smaller PVCR's so it can run bigger jets.

If you think something is blocking it, pull the idle screws completely out and put carb spray in there, then blow the hell out of it. You'll see it come out the air bleeds. Put it all back together and it will still do the same. Clogged idle circuits make the car idle like ****, erratic as hell and drive even worse, if it will idle at all. You can do all that just to make sure it's not that that as you would any standard trouble shooting, but don't think it's the problem. The air bleed is the #1 in the pic.
Just make sure you have enough T-slot showing or all the adjustments are for nothing.

 

[Dark Knight]

The PV will help overall. If it's running out of fuel, click it off and check the bowls. Have you had the bowls off? Might pull the primary one off and make sure something isnt floating around in it. I've seen a small piece of gasket block a jet and have the same thing happen.

 

[10secgoal]

The PV will help overall. If it's running out of fuel, click it off and check the bowls. Have you had the bowls off? Might pull the primary one off and make sure something isnt floating around in it. I've seen a small piece of gasket block a jet and have the same thing happen.

A PV change miiiight cover it up, but that's not the correct way to tune. You don't want your PV clicking on and over just cruising around. You want it when it sees a load. It will KILL the gas mileage if you do it that way. PV should open when the vac sweeps towards 0, when the jets are already flowing, not in the idle circuit. Same for the jetting. You could take out the jets completely right now and should have close to zero effect on anything until about 2200 or so when it leans stumble right before the jet pull over.