i have a holley 390 on my six. It cranks and idles but it idles at 2000 rpms or more. Any suggestions on what might be causing this?

[quote]It cranks and idles but it idles at 2000 rpms or more[/quote]



Most likely throttle cable is too tight not allowing throttle to return to fully closed position, or one or more vacuum leaks from several possible locations.

First inspect throttle cable for slack when throttle is in closed position. Adjust as needed, or remove restrictions hanging it up. I had to shim the accelerator peddle away from the floor to get enough cable travel to fully open throttle when floored to alow secondariy's to open at WOT. Also check that high idle cam is free to fully retract to its hot position once engine is up to temperature, and set properly. Clean with carburetor cleaner as needed, and look for binding choke linkage.

Rough adjusting high idle with carburetor off is easiest way. Once carburetor is installed on manifold, fine tune high idle by fully opening throttle plates by hand and turning its adjustment bolt with, I believe, a ¼â€￾ wrench to around 1400 rpm. Also make sure electric choke’s lean/rich adjustment is set properly. It is possible that the bimetal spring is not engaged with the outer housing properly making adjustment impossible. This will require removing three screws securing round choke housing.

Second, spray carburetor cleaner around base of carburetor, and throttle shafts to detect for vacuum leaks. You may not find one just by spraying, as my 390’s base was so warped, that the leak was so large, no amount of carburetor cleaner would show a leak… Removal of carburetor was the only way to find the problem.

Check the flatness of the bottom of the carburetor’s casting mating with intake manifold using a known straight edge and a strong light. This casting can become deformed from over tightening when not using a torque wrench. If deformed, file with a #12 mill bastard file in north-south, northeast- southwest, east-west, and southeast-northwest directions until flat. This can corrective action can take a while, and you will need a wire brush to clean the file as the project progresses. Once flat, be sure to blow out shavings from idle circuit, and transition slot as well as any other places.

Check the setting of secondary throttle plates. These need to be cracked open so that they don’t full close to allow the secondary idle circuit to function, but not too loose to cause an excessive lean condition at idle. There is a small set screw accessed from the bottom of the carburetor on the choke side for this adjustment.

Inspect throttle shaft condition; high mileage units can become egged out.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

hi, is this a new or used 390? do you have the list number by airhorn? it should not have drill holes in primary valves if it was new for your car. holes are sometimes drilled on used carbs. check secondary set screw setting under carb to see if its open to far. post what you find. ronnie

What sort of engine build do you have? Close to stock, or aggressive cam, high compression, oversized valves, and headers?

As for drilled primary throttle plates, if cam causes a loping idle with low (0 to 8 inched Hg swinging needle) vacuum, idle circuit mixture screws won’t do their job. My engine is a loping mess under 1000 rpm, mixture screws had little effect until I progressively drilled the throttle plates starting with 1/32â€￾ drill to let in more air to lean out the mixture. Drilling produces a controlled vacuum leak if you will when secondary throttle plate gap can’t let in enough extra air at idle.

Don’t worry if your plates are drilled, one can easily solder up the holes to return original function to carburetor.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

we had someone build it for us and it is bored 30 over and the largest hydrolic cam that would fit. it also has headers on it. there is a slight exhaust leak on the header flange closest to the fire wall. could that be a problem also? the holley is used and does not have the vacuum that goes to the distributer

Is this Hydraulic cam in your '74?

If so, did you change out the pushrods, rocker shaft/rockers and pushrods for parts from an hydraulic system?

Hydraulic cams and lifters can not be used with mechanical rockers and pushrods, at all. Ever.

Just getting clarification. If you already knew this, then disregard.

If you have hydraulic lifters in a mechanical valve train, do not run it until you either convert all the parts to hydraulic, or switch back to mechanical lifters. The biggest hydraulic cam is a whimp compared to a mild mechanical anyway. Wouldn't waste my time with a slushy engine personally, but that's my paradigm.

CJ

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Part of Tyrde-Browne Racing

honestly i dont know much about the internals but the guys that built it build race engines and really know what they are doing..im 18 and this is my first car so im just trying to get a better knowlage of things.and the carb is the last thing we have to do and work the bugs out

By the time I was 18 I'd built lots of engines. Time to find out.

Pull your valve cover and look at the pushrod end of the rockers. If they have an adjuster on there, don't run it until you know.

Talk to your engine builder and find out what they did and who they got direction from.

I have an idea they listened to the sales people at Clifford.
Clifford has a history of selling hydraulic parts to people with mechanical valve train engines. I've personally talked to them repeatedly about this. You will damage your engine. Don't run it until you know what's in there.

If the current owner of Clifford did this again, I think some effected people need to start a class action. It will damage lots of parts.

Slant Six hydraulic systems are fed oil from the rocker shaft down through the push rods. Mechanical pushrods and rockers are not set up to do this.

Get your socket set out and look at the rockers.

CJ

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Part of Tyrde-Browne Racing

okay ill go talk to them soon and also take the valve cover off and take a look. thanks

Clifford did the very same thing to me,,sold me a hydraulic cam for my solid cam and valve train setup, since been changed to the right cam, they cost me time and money in the long.

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Brian Long

Anytime. Find one of the gurus close to you and get some help. Not many engine builders know the specifics of slant six engines.

I was surrounded by gurus and mentors from the age of 9, (Earlier if you count my A&P father,) so building engines is something that isn't too tough. It makes my perspective somewhat different.

If your anywhere close, I'd be happy to help. I'm sure you can find some folks on this forum in your vicinity that know what they are doing.

CJ

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Part of Tyrde-Browne Racing

the guy i bought my headers from doesnt live too far away and he races slants so i could ask him if need be

That sounds like a plan!

Give him a ring, and tell him about what is going on.

Fix your leaks. That's important. The carb your running has lots of numbers associated with it.
An off the shelf 390 Holley will run terrible on a slant six, and requires a bunch of mods to make it work right. Depends on what your starting with.

First thing to find out is: Does it have a jetted or plate Secondary. Does it have out of the box jets, or have they been changed to something else? What power valve is in there, what pump cam, shooter, and screw position? What secondary spring? If it's a used carb, does it have the anti-blow out kit installed for the PV? Any other mods to it? Aluminum or iron throttle body?

If your starting from a plate secondary, the body may or may not have the blow out check installed. If it doesn't, get one. Invest the money in the following:
1. Jetted secondary metering body.
2. Quick change secondary spring housing
3. A mopar throttle lever kit.

How much compression did you have built into this engine?

To run any of the four barrel carbs that are out there, you want a bare minimum of 8:1. A stocker with a composite gasket will net you 7.x:1.
To realize any gains, you want 9:1 compression or better. I like 9.5 to 10:1 personally.

The following are my list of bare minimum instruments to tune with:

1. Tachometer
2. Vacuum guage
3. A/F mixture guage. (Narrow Band will do, but wide band is the best.)
I run a narrow band on my build. It tells me enough for base tuning. E.T. tells me the rest, but one has to go to the track to learn that.

If you want to save yourself a bunch of time tuning, go buy an Edelbrock 500 AFB. Run the base jetting and metering rods. It will work fine.

The Holley has the potential to be faster. The Edelbrock doesn't need messed with.

I run a Carter 500 AFB. Base jetting and rods. The pump shot has been modified a bit, but only based on the lever hole. Wonderful!

CJ

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Part of Tyrde-Browne Racing