[GreatWhiteEconoline]

Been a few months since I last posted, I have the 460 up and running, have had it out for several "shakedown" highway runs. I figure it might be time to fine tune the Holley carb I bought for it. First question, what type of vacuum gauge would I need, for performing the idle mixture adjustment? Second, What would be the best primary and secondary jet sizes to use for it, guy I bought it from had it on a Mustang,(I think that's what he said) now I have it on a 460 in a heavy 1 ton van with 4.10 gears. . Would I need to possibly change power valve size as well?

Sorry for all the questions, just want to make sure I do it right

 

[TommyK]

You don't need anything special as far as a vacuum gauge is concerned. Your local auto parts store should have one on the shelf for about 20 bucks.
If you think the previous owner of the carb tinkered with it, your best bet is to return it to the stock Holley calibration (including the power valve) and start tuning from there. Pick up some new bowl and metering block gaskets. You will need to disassemble the carb to check jet sizes and PV rating.There should be a List # on the choke horn. You can go to Holley's website and find the stock calibration for your carb or post here and someone can tell you.

 

[GreatWhiteEconoline]

Ok thanks I'll look for the list #. It was overhauled by a very well known local area carburetor shop, thay's been in business over 40 years. Not sure if the guy I bought it from did anything else to it or not, his 460 was a performance build

 

[GreatWhiteEconoline]

I bought a vacuum gauge to check and adjust the mixture screws. Now, to refresh my memory, I hook the gauge hose to manifold vacuum,then turn each mixture screw in till the engine stumbles, then back each one out till I get a good reading?

 

[TommyK]

For a two mixture screw carb set the mixture screws 1 1/2 turns out. On a fully warmed up engine set the curb idle screw so the engine idles at the desired rpm. Note the vacuum reading. Turn both crews in a 1/4 turn. If the vacuum reading goes up you are going in the right direction (leaner). If the vacuum reading doesn't change try going in another 1/4 turn. If the vacuum reading goes down and or the rpms fall off you are going in the wrong direction and you want to turn them out to a 1/4 past your baseline or a total of 1 3/4 turns out. Repeaat until highest idle is achieved. You may find that the idle speed changes with the change in mixture screw settings. If that is the case readjust the idle speed to your desired idle rpm and continue adjusting the screws until the highest vacuum reading is achieved at the desired idle rpm. Make sure when you are done the mixture screws are set equally.

If you have a four mixture screw carb the procedure is the same but start at 3/4 turn out on all screws.

You haven't mentioned anything about ignition timing but you generally want to have that sorted before you begin carb tuning.

 

[GreatWhiteEconoline]

Sorry about that, the ignition timing is set at 8 degrees BTDC as per my van's underhood sticker. Mine is a 2 mixture screw carb

 

[dacofa]

I use my ear to do idle screws. Turn it in till the engine rpms changes. Then back out about 1/2 turn. Then out until it change. Take note how much you turn to get the rpms changes and put it half way. Timing? Put it where it likes it. Try 2 degs at a time and drive it for a day or 2 and see how she likes it.

 

[GreatWhiteEconoline]

You don't need anything special as far as a vacuum gauge is concerned. Your local auto parts store should have one on the shelf for about 20 bucks.
If you think the previous owner of the carb tinkered with it, your best bet is to return it to the stock Holley calibration (including the power valve) and start tuning from there. Pick up some new bowl and metering block gaskets. You will need to disassemble the carb to check jet sizes and PV rating.There should be a List # on the choke horn. You can go to Holley's website and find the stock calibration for your carb or post here and someone can tell you.

Thanks, I found the info on the Holley site, now to see if it is set up in stock form

 

[GreatWhiteEconoline]

I pulled the front bowl, to check the jet sizes. I found, that the previous owner, had installed one #75 jet, and one #77 jet. I scratched my head over that, and proceeded to put a matched pair of #72's as the specs showed for my carb model. Everything else looked great.Had correct PV in it. Now why in the h3ll would one put mismatched jets in????

 

[bbfjet]

Possibly an accident or trying to correct lean condition on one bank/cylinder.

 

[GreatWhiteEconoline]

Possibly an accident or trying to correct lean condition on one bank/cylinder.

Yeah I was thinking an accident too, as the guy had a performance built 460, and he was apparently doing some "experimenting". I put it back to what jets it came with from the factory

 

[dacofa]

Due to manifold design with dual plane, not uncommon.

 

[causticuc]

You said you have this in a van?
Might be a thermodynamic issue where the air is hotter or slower on one side than the other. Vans typically have horrible airflow and convoluted routing.

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