I've got a '68 Mustang, with a seemingly stock built low to mid compression 302. The engine was in there when I bought the car, so I don't know it's history, but it will pull about 17" vacuum at idle. Currently running a Holley 4160, 600 CFM, vac secondary, I think it's a list # 9834, but I would need to verify - nothing exotic. Electric choke - operating correctly.

The thing that befuddles me with this carb is the fact that in order to get it to idle smoothly, both idle enrichment screws are set about 1/4 turn out. Some days it will idle reasonably smoothly, other days it will idle rough and need another very fine adjustment. It seems to me that I must have some type of internal leak which is supplying a tiny bit of extra fuel at idle, leading to the relatively hypersensitive nature of the idle enrichment screws. Any thoughts on likely sources of this unmetered fuel? Maybe I am jumping to conclusions. This issue remained even after a carb rebuild. I've also tried various adjustments on the secondary throttle plate stop screw thinking it was being fed too much air / fuel from the secondaries at idle, but this didn't have a significant effect on the mixture screws, just forced me to adjust the throttle stop screw to get back to my desired idle speed.

Ignition is recently tuned, Carb is running #66 main jets, I assume stock metering block on secondary - carb is used also. Fuel pump and filter are recent as well. Recently R&R'd the carb thinking there might be a base gasket leak, and also re-torqed intake manifold with no change in behavior. 302 is hooked to a seemingly stock C4 and 3.00:1 open rear. Running 12 deg initial timing, but behavior was the same at 6 deg and 10 deg.

Any insights would be appreciated. I've live with this for years now, but it is very annoying to be having to tinker with the idle mixture on a weekly basis.

Thanks,

Andrew

Oh Yeah, I'm running a 8.5 power valve, and it is not blown, checked and replaced more than once with no effect.

Thanks,

Andrew

I am fairly new to Holleys but I am getting very familiar with them. I run a 4150 735 cfm Holley on my 428CJ.

Having the idle air mixture screw turned out only 1/4 turn to get it to idle properly makes me think the main jets may be too big. The normal starting point for the idle air mixture screw is 2 turns out, then adjust from there.

I started with 66 main jets with my 428CJ engine and had to turn the idle mixture screw out a lot to get it to idle correctly and not stumble when taking out from a stop. I changed to 68 main jets and now it works well at about 2.5 turns out.

There may be other issues but you might consider going to smaller main jets to see if it helps. Not a big cost to try.

You can also go to http://www.bob2000.com/carb.htm for help with Holley carbs. There is contact information there if you need it. He a great help to me when I was refurbishing my Holley.

And I am sure I will get lots of response to this, but the real issue is both of these carburetors are to big for their respective engines.
Especially the 600 CFM holley for the mild 302.

Consequently you have to compensate by reducing the amount of air flow at idle (reduced idle mixture screws) which then makes the carburetor extrememly sensitive to minute change, Most people then try to play with the jetting which does nothing for idle speed and idle airflows.

By changing the jetting especially drastically you then upset the balance of the carburetor causing problems elsewhere. Jetting should never be changed more than one or two setting for the street.

Try to borrow a smaller carb from someone and set it up and see how it works, I can almost guarantee you will be more satisified.

Bill White
White Automotive

Bill, I agree that a 600 CFM carb on a 302 may too large. However, with all due respect, the 735 CFM Holley is the original carburetor that Ford used for the 428 CJ. If it is too big, Ford made the mistake.

When I bought my Fairlane Cobra 428 CJ it had a 650 CFM Holley double pumper on it. It ran somewhat OK with low gears. When I put taller tires on it, to get reasonable cruising rpm at freeway speeds, I had a problem of bogging at low rpm and some misfire through mid range. I decided to go back to the proper carburetor (Holley 735 CFM) and it works much better now after I did some minor tuning on it.