Question on Holley 390 power valve sizing for my setup.

[cuban11182]

I've read multiple posts by JeepHameron tuning the 390 to the AMC 258 i6 / 4.2l . My CJ pulls 21 inches at idle and the Holley is still setup with the stock 6.5 power valve. I read what he had said about driving it with a gauge connected at 55 with steady throttle, then subtract 2 or 2.5 based on whether or not you offroad a lot. When I'm running I get 13 inches. Should I really put in an 11 or 10.5? Besides a VERY minor hesitation when accelerating (still tuning the cams on the accelerator), she drives really well with the offy intake, and DUI. Also my plugs are NGK V-Power set to .050.

Thanks,

Dave

 

[Fastatv]

I've read multiple posts by JeepHameron tuning the 390 to the AMC 258 i6 / 4.2l . My CJ pulls 21 inches at idle and the Holley is still setup with the stock 6.5 power valve. I read what he had said about driving it with a gauge connected at 55 with steady throttle, then subtract 2 or 2.5 based on whether or not you offroad a lot. When I'm running I get 13 inches. Should I really put in an 11 or 10.5? Besides a VERY minor hesitation when accelerating (still tuning the cams on the accelerator), she drives really well with the offy intake, and DUI. Also my plugs are NGK V-Power set to .050.

Thanks,

Dave

I would recommend continue adjusting your accelerator pump before change the power valve. Sounds like your pump might be coming in just a tad late.

 

[gutthans]

My tuning advice would be:

1. Shooter size should be .025 or .028
2. pump cam should be stock SET ON #1 hole
3. PV ought to be 6.0 to 8.0
4. reset the plug gap to either .040 or .045

Now, don't just go put all of that in...hopefully you are already close to that. That works on my stroker, and your camming and ignition will undoubtedly be slightly different than mine. Go to www.holley.com and download their tech on tuning if you don't have it.

You will not be happy with a PV over 8.5. Why? Because you will constantly be pouring extra gas through the carb around town...when the motor does not need it. It would be better to know how low your manifold vacuum goes at fuu throttle acceleration...then get a PV +2 above that. Your WOT pressure will be close to 5 inches I bet...6 - 8 is right in the ballpark.

 

[cuban11182]

I'll throw the orange one back on. I'm going to have to also check the arm linkage to ensure its at the recommended 0.015. One thing to note was that my original cam was in the #2 spot, instead on #1. From what I've read it should be in the #1 since we idle below 1000.

 

[Fastatv]

I'll throw the orange one back on. I'm going to have to also check the arm linkage to ensure its at the recommended 0.015. One thing to note was that my original cam was in the #2 spot, instead on #1. From what I've read it should be in the #1 since we idle below 1000.

Unless things have changed on Holley carbs, the arm linkage adjustment for the 0.015 is at the END of pump travel. When the carb is at idle, choke is off, the linkage should be adjusted to "just touch" the pump arm. If it isn't, that's possibly the cause of the slight hesitation.

 

[cuban11182]

Yes it is at full throttle

 

[Fastatv]

Yes it is at full throttle

OK, good...LOL...you would be surprised how many think that the clearance is at idle. Sounds like you really know what your doing but, maybe adjust the linkage to pump arm, maybe one half turn longer, see if that helps. On a jeep, or most other vehicles that will rarely be ran wide open, I wouldn't be very concerned about the 0.015. That clearance is to protect the pump.

 

[cuban11182]

Well that makes sense. Guess we will see what I find.

 

[gutthans]

Yea, since the 390 is the platform for NASCAR and meant for track racing...the #2 spot is as installed...but with a street idle, #1 generally works better.

Fastatv has a point about eyeball setting the pump arm...I turn it in until I can't get a .001 feeler in there then go 1/4 turn extra (at rest)...I do a visual check with a reference point for instant throttle movement after that.

If I recall, the sequence for road tuning is:

1. warm motor
2. set timing (adjust idle speed screw)
3. vacuum adjust primary idle feed screws (adjust idle speed screw)
4. road test for CRISP throttle response off- idle)
5. if boggy change shooter size down; if good go up one size
6. road test (adjust shooter up or down accordingly until happy)
7. adjust cam setting or profile
8. road test for idle bog and mid-range bog
9. repeat 4 - 8 until you have a happy combo (adjust idle speed screw)
10. drive fast for a day, read plugs; change jets (adjust idle speed screw).

All of this would be starting from plain stock carb settings, and is done in order because of the way the systems interact. It's pretty rare that jet changes will alter any of the other settings, BUT if your jetting was way off to begin with (and was way too lean or rich) you might wind up doing this again after changing jets.

I typically run a #51 in summer and a #50 in winter and don't reset anything.

 

[cuban11182]

I just put the cam back to orange, although I dropped it to one, timed at 10* and it ran really well.

It's not a speed demon, but it still puts a smile on my face when I drive it.

 

[gutthans]

I just put the cam back to orange, although I dropped it to one, timed at 10* and it ran really well.

It's not a speed demon, but it still puts a smile on my face when I drive it.

Are you running on ported or full manifold vacuum? (side of the metering block or bottom of the carb) to the distributor?

Do you know the total mechanical advance provided by the DUI?
Do you have the spec for the vacuum can that is attached?

If you are willing to play a little bit (and have not done this already), I think you can get more performance out of your combo by changing to full manifold vac and bumping up the initial timing. Set your initial timing to the difference between your DUI's total mechanical advance and 36*. For example, if the DUI unit puts 20* advance out, you would set the initial at 16* for a total running timing of 36*. Your vacuum can will need to provide less than 16* total advance at your idle vacuum so you don't have more than 52* (*vacuum + mechanical) idle through cruise.

You will have to drop the idle screw down about one whole turn as this will really speed up the idle.

i Would assume that you have the stock DUI dizzy springs??? You may want to select the ones that start your curve to advance around 800 and be all in by 2400 (not 3000 as the stock springs bring it in).

DON't do this is you are computer controlled.

 

[cuban11182]

Are you running on ported or full manifold vacuum? (side of the metering block or bottom of the carb) to the distributor?

Do you know the total mechanical advance provided by the DUI?
Do you have the spec for the vacuum can that is attached?

If you are willing to play a little bit (and have not done this already), I think you can get more performance out of your combo by changing to full manifold vac and bumping up the initial timing. Set your initial timing to the difference between your DUI's total mechanical advance and 36*. For example, if the DUI unit puts 20* advance out, you would set the initial at 16* for a total running timing of 36*. Your vacuum can will need to provide less than 16* total advance at your idle vacuum so you don't have more than 52* (*vacuum + mechanical) idle through cruise.

You will have to drop the idle screw down about one whole turn as this will really speed up the idle.

i Would assume that you have the stock DUI dizzy springs??? You may want to select the ones that start your curve to advance around 800 and be all in by 2400 (not 3000 as the stock springs bring it in).

DON't do this is you are computer controlled.

I'm running on ported. I'm able to get 21 in. in hg at idle. The DUI is stock, including the springs. Mechanical is 23 and vacuum advance is 15. Vacuum canister is the stock one that comes with the DUI. If my math is correct then I should be running at 13*, and I"m currently at 10*.

I've tried manifold, but the idle quality wasn't as nice. This is with me adjusting the carb. to manifold from ported to achieve the highest vacuum.

 

[gutthans]

I've tried manifold, but the idle quality wasn't as nice. This is with me adjusting the carb. to manifold from ported to achieve the highest vacuum.

Switching from ported (and remember to plug it off) to full manifold should only cause your initial idle speed to go high by about 500 - 800 rpm. It ought not to become rough, but since I can't 'see' what else is up, if you like ported...stick with it. Give 12 - 13 degrees a try for an initial...just take the Jeep out and plug up the vacuum so that you're on total mechanical only and verify that you get no ping when accelerating. . If you do, drop it 1 - 2 degree and re-run. When the ping is gone that's your setting.

At total advance of 36* plus 15* (cruise vac advance) you have 51* running vac plus mech and you should be at the limit of what the AMC 258 i6 / 4.2l will tolerate. Note: you will be needing 89 octane at this point, if not 91 grade.

I run 54* all-in @ 2400 rpm, but I also use 93 octane gas.

 

[cuban11182]

My fear is that I don't know what pinging sounds like. I hear a light tick under load, but that is at 6*, 8*, 10* and 12* so I"m guessing it might just be an exhaust leak since it's the stock exhaust all the way back (hence the reason why I asked about the 4.0 head so I can decide whether to get AMC 258 i6 / 4.2l headers or wait if I'm doing the swap). From what I've read it sounds like rattling, but I've been unable to determine based on the videos I've seen on youtube. Plus it's VERY hot here in Texas now. I know a guy that drives a stroked YJ and he runs his a 10* with a Holley 2 barrel.

Thanks for all the advice gutthans.

EDIT

I'm guessing this is what it sounds like: https://www.youtube.com/watch?v=ttFy48SViTI

If thats it, mine sounds nothing like it. Mine is a tick which to me sounds like a cracked manifold or a exhaust leak.

 

[gutthans]

That video was titled 'Spark Knock'...that wasn't 'knock'....It was either 'ping', or valvetrain noise.. Knock is a deeper, hammer hitting solid iron type of sound.

If you have a metal coffee can, get some tiny pieces of gravel and drop them in...lightly shake the can...that's 'ping'. I've have never heard a motor that would start and run, ping on idle.

Ping results from an over-advanced condition for the motor/gas combination. The 'wave front' of the 'exploding gas in your cylinder needs a certain amount of time to fully combust. As you increase dizzy speed the advance starts the combustion earlier because the piston is completing the cycle quicker. If the 'burn' starts too soon, the wave front hits the top of the rising piston and 'smacks' it. In tiny amounts, that's ping. With no load at idle, the motor runs off of your initial setting plus whatever vacuum advance is in. (ported vacuum is close to zero at idle...manifold is full on at idle).

You treat the two types of advance separately to determine what amount of each can be tolerated by your combo.

You'll set mechanical first. 10 * is very safe for a AMC 258 i6 / 4.2l /4.0. Higher (12 - 16) is usually better for idle stability and mileage, BUT, it will depend upon your dizzy total (your initial plus dizzy total should not exceed 36* unless you can get Aviation grade octane). You run either uphill or a full throttle romp (vacuum closed off!)from about 10 mph...listen for a noise that wasn't there at idle. If you don't hear the 'gravel in a can' you can take the setting a degree or two up and go again. That will be your mechanical setting.

Reconnect the vacuum. With ported nothing will change at idle...with manifold your idle should increase immediately. Readjust your idle speed screw to your idle (650 - 750 rpm). Now drive again...same routine PLUS go down a stretch of highway at a stead 30 - 50 mph and LIGHTLY accelerate. If you DON"T get that rattle sound, your fine. IF YOU DO, then because you have a vacuum can that is NON-ADJUSTABLE you HAVE to either reduce the initial idle timing, or buy a different can that will have the vacuum numbers your setup wants to run with.