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Holley 390CFM tuning 258

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71hornet6 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71hornet6 Quote  Post ReplyReply Direct Link To This Post Topic: Holley 390CFM tuning 258
    Posted: Mar/24/2016 at 5:28pm
Hi I have a hot rod 258 I built for my Hornet.
It is a '72 258 rebuilt with 7120 4.0L Head and Clifford intake with Holley 390CFM and a custom cam. The specs on the cam were built for street 258 with 390 carb. Gas is 91 octane
I bought the carb "rebuilt." I've been through the carb a few times and had to replace wrong parts used in the "rebuild". I also have tried to do jetting and pump cams etc as recommended by Clifford but have tried several recommended jets and pump cams and can't get rid of the stumble.
The car has a stumble off idle and then a blast of power like it is getting flooded briefly then "catching up" to the fuel level.
I'm thinking these problems are due to initial timing and idle being off and thus affecting the transition to higher RPM

I have a Holley book that I read through and the advice for setting the idle screw is to set it for maximum intake vacuum at idle and I've noticed the vacuum and idle speed keep going up as I turn the idle screw. It seems like the idle wants to be much higher than 600RPM.
I'm wondering what you have set upon for your hot rod 258 in terms of initial timing and idle speed.

Thanks for your help
'71 FrankenHornet:
'72 258 '95 4.0L Head Holley 390CFM
AX15 with R154 gears 5 speed Hurst shifter
AMC20 rear axle Detroit TrueTrac
Dual Exhaust
Concord Discs Spirit AMX sway bars
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Mar/24/2016 at 6:33pm
The Clifford recommendations are for a racing motor. At least that's what I think. I had to use a couple sizes smaller on the main jets, but then I was running a stock cam and exhaust manifold also. Clifford intake on a J-10 truck with Holley 390. Don't recall what size jets I ended up with.
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71hornet6 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2016 at 12:28pm
I reread this post. Seems like a pretty similar setup to mine
http://theamcforum.com/forum/holley-390-tuning-on-a-258-advice-wanted_topic40432_page9.html
I'll double check all of those issues.

Through my struggles I had read a book about Holley design and tuning and it started to seem like a Rube Goldberg machine.

My Hornet was near completion then work got busy for a couple years so I would really love to get that thing on the road

Update: I found a post I put up when I last worked on the car... Nov '14
Kinda sad my Hornet hasn't been getting any love in a year and a half.
http://theamcforum.com/forum/holley-390cfm-8007-jetting-pv-pump-cam-etc_topic60497.html

It would be great to have a consensus instruction for tuning the holley with a hot 258 and cheap HEI

Edited by 71hornet6 - Mar/25/2016 at 7:49pm
'71 FrankenHornet:
'72 258 '95 4.0L Head Holley 390CFM
AX15 with R154 gears 5 speed Hurst shifter
AMC20 rear axle Detroit TrueTrac
Dual Exhaust
Concord Discs Spirit AMX sway bars
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pdok Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2016 at 6:00am
Tuning a carb over the internet is pretty tough. A couple things I'd suggest. Make sure your throttle linkage allows it to close completely at zero throttle. Back the idle speed screw off all the way to check, then about 1.5 turns in after it contacts the tab. Enough to get idle rpm.
Either disconnect the distributor vacuum or make certain it's not pulling vac at idle. It should be on a timed port, so no advance at idle. Base timing should be set before the mixture. I ran my HEI at 12, with pretty good results. Total timing was 36, and fairly light springs to bring it in earlier. Depends on a lot of things what works for your cam, etc. Doesn't matter for idle mixture though, just base timing.

Choke must be completely open, carb and car at operating temp.

You said you were seeing the idle speed increase when turning the idle screw...I assume you meant the idle Mixture screw...

If idle increases as you add fuel by opening up the idle mix, might have a vacuum leak. PCV is a typical leak, as is the evap canister houses or check all of them. Including the one that controls the heater vac switch under the dash if you have that. A leak will lean out your mixture.

You can check with starting fluid around the intake gaskets and carb base for leaks. You will never get it tuned right if you have leaks there. That's common with an intake swap.

Open the mixture screws about 1 turn out from closed.

Using the idle set pretty low, 500-600, and a vacuum gauge, open the mixture screw 1/8 turn alternating sides until the vacuum stops increasing. I check to make sure I can't get rpm to increase either. Let it stabilize on each setting. If vacuum is fluctuating a lot, you probably have a leak or a wacky cam that wants more idle speed.

All this assumes no issue with the carb. The off idle tuning is actually pretty tricky, but sounds like you've got all the pieces to dial it in.

This is basically how I set all my carbs for idle mix. Idle jet needs to be close, but that's going to mess with how far out you need the mixture screw too. I don't know the 390, just by reputation as a pretty standard Holley.

If you bump timing later on, you'll want to tweak idle mixture accordingly. If your hei is throwing inconsistent spark, that will play heck with all of this...but they're pretty bullet proof.
76 Grem X 258/904,4.0 head/MPFI, Comp X250H cam, Hughes springs, Clifford header, serpentine swap.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2016 at 7:25am
ALL carburetors or sort of Rube Goldberg by necessity! Simple ones like most lawn mower carbs are pretty straight forward, but they don't have to operate under such a wide variety of loads and conditions. Auto carbs typically have three circuits -- idle, low speed, and high speed. It's all controlled by air coming in -- vacuum and air velocity. The more adjustable the more complicated the mechanisms that do all this. Webers have more adjustments than the Holley and require a good bit more time to tune. The good thing is it's not real hard (even for the Webers) once you understand some of the basics and how the adjustments affect the carb. There is a learning curve, and you really need a tuning kit with an assortment of jets and such that are in the range you want. That takes a little research too. I haven't messed with a carb in years, so I'm no more help than the advice above...
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote firefly Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2016 at 2:19pm
accelerator pump
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ramblin64sw Quote  Post ReplyReply Direct Link To This Post Posted: Mar/27/2016 at 2:06pm
Hey 71hornet6, I also have the 7120 head, but on my 232, with a 262/270 hydraulic cam. I decided to run a pair of small Webers instead of the Holley 390, so I can't offer a whole lot of help. What has helped me, however, is a book on tuning Weber carbs by John Passini. A lot of good info on how the transition circuit interacts with float level, emulsion passage, accelerator pump,etc. Good advice on how to diagnose and fix transition circuit problems. While written for the Weber, I think you will find it a useful read. Good luck!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greyhounds_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Mar/27/2016 at 3:45pm
Tuning Holley carbs on a 258 is something I've spent way too much of my life on. 

As indicated by another fellow above, it sounds like accelerator pump shot volume or timing would solve the problem. Typically you need to give these engines a healthy pump shot, especially with a Clifford or equivalent intake that isn't heated.

You also need to have enough initial ignition timing, and probably about 38 degrees total. "Enough" will depend on alot of things, but expect 12-14 degrees for starters.

So in order, I would:

1) Get the initial timing set up right
2) Set the idle mixture correctly
3) Tune the accelerator pump

Also verify that the secondary diaphragm actuator still has the check ball or orifice installed, so it doesn't come flopping full open. That really shouldn't be what you are seeing here as the secondaries don't generally come in for a few seconds or so, depending on the spring, displacement, etc.

What color pump cam do you have, what position is it on, what shooter do you have, and what is the part number on the air horn of the carb?

Once I know that stuff we can start making a plan.
 









1968 AMX 390 w/T5
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71hornet6 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/02/2016 at 6:32pm
Originally posted by Greyhounds_AMX Greyhounds_AMX wrote:

Tuning Holley carbs on a 258 is something I've spent way too much of my life on. 

As indicated by another fellow above, it sounds like accelerator pump shot volume or timing would solve the problem. Typically you need to give these engines a healthy pump shot, especially with a Clifford or equivalent intake that isn't heated.

You also need to have enough initial ignition timing, and probably about 38 degrees total. "Enough" will depend on alot of things, but expect 12-14 degrees for starters.

So in order, I would:

1) Get the initial timing set up right
2) Set the idle mixture correctly
3) Tune the accelerator pump

Also verify that the secondary diaphragm actuator still has the check ball or orifice installed, so it doesn't come flopping full open. That really shouldn't be what you are seeing here as the secondaries don't generally come in for a few seconds or so, depending on the spring, displacement, etc.

What color pump cam do you have, what position is it on, what shooter do you have, and what is the part number on the air horn of the carb?

Once I know that stuff we can start making a plan.
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Thanks for your offer to help. I just checked in with the hornet and took the car cover off
I have about a month to work on the hornet before I have a kitchen remodel job
Someone left a cool 1971 Hornet/Gremlin Chilton manual under my car cover, but didn't leave a name!
Pretty fun to find that. I think it's a good sign!
Unfortunately my car has become so modified I don't know how much the guide applies anymore
First thing is I need to put in my AX15 R154 hybrid transmission I made.
I have the gear assembly in a vice covered with a plastic bag.
If you follow the forum, you may have seen my posts about these heavy duty Japanese 5-speed transmissions and how these two models can be combined for optimal gearing and strength at a junkyard price.

Anyway I'm at that phase where I've built a weird monster of a car and now I'm trying to figure out where I left off...
'71 FrankenHornet:
'72 258 '95 4.0L Head Holley 390CFM
AX15 with R154 gears 5 speed Hurst shifter
AMC20 rear axle Detroit TrueTrac
Dual Exhaust
Concord Discs Spirit AMX sway bars
Back to Top
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71hornet6 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/02/2016 at 9:02pm
Where was I...
'71 FrankenHornet:
'72 258 '95 4.0L Head Holley 390CFM
AX15 with R154 gears 5 speed Hurst shifter
AMC20 rear axle Detroit TrueTrac
Dual Exhaust
Concord Discs Spirit AMX sway bars
Back to Top
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