I'm running a '74 225 from a Charger in my '65 Dart, and up until recently, I was running a decent Holley 1920 on it. Unfortunately, the casting on the metering block for the power enrichment arm wollered out, and is no good now.

So, I rebuilt the 1945 that came with the engine, and I've got a few questions. It had a 63 jet in it. Using my wideband, I've found that it cruises at around 15-15.3:1, and is in the 13.5-13.7 area under full throttle. Under light to moderate acceleration, it leans way out to 16-16.5:1 and stays there until I back off again. I can obviously feel this as a slight surge when accelerating.

The idle mixture screw also is backed out to 3.75 turns out from seated to get the lean-best idle.

The engine has 18.5 inches of steady vacuum at idle, and I have set the idle to around 725 RPM.

So, should I jet this thing up even more? It likely is the '74 version of the carb, which doesn't seem to have a good reputation.

Sorry for the long post...

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1965 Dodge Dart 170 Wagon
1965 Ford Mustang
1965 Buick Skylark Hardtop
1965 Chevy Corvair Monza Convert.
1953 Buick Special Riviera

Using your wideband data and assuming this is accurate, your main jetting and idle circuit is lean. Light cruise draws fuel from both the idle and main circuits. As you tip into the throttle further, fuel ceases to flow through the idle circuit and comes exclusively from the main circuit.

If you remove the carb top cover, you will expose the idle feed and main feed circuit tubes. It's been a while - I would have to confirm this with a diagram, but this how I remember it. Located in the tubes are the idle and main feed restrictions. To change how the carb manages A/F, you will need to modify these restrictions.

This is how you should proceed:

Run the car up to stabilized operating temp. Adjust the idle mixture screw for best idle. Remove the air filter top cover. Gradually increase the idle speed until you begin the see fuel coming out of the main booster. Lower the idle until the this stops. Alternatively, you can use 1500 RPM as long as no fuel is coming out of the booster. What happens now when you turn the idle mixture screw in or out?

If turning the mixture screw in raises the engine speed, then your idle circuit is rich. If turning the screw out increase engine speed, then the idle circuit is lean and you will need to increase the size of the idle feed restriction in the idle transfer tube. Being you have the mixture screw out almost 4 turns, you should find your idle circuit lean. In fact, it may be lean to the point that turning the screw out @ 1500 RPM will have no affect.

Most likely the idle circuit is way lean. I would suggest increasing fuel flow through the idle circuit by 15% to start. To do this, you need to use a small drill to determine what size the idle feed restriction is inside the idle transfer tube. Once you know the drill size and, therefore, the diameter, increase the restriction by 15%. Fuel flow is proportional the cross sectional area of the restriction. The cross sectional area is equal to pi * r squared. Figure out what drill size you need to increase the cross sectional area by 15% and use this to enlarge the idle feed restriction.

You don't need to remove the carb to drill the restriction. The restriction is brass. After drilling - with the idle mixture screw out, by the way, cover the idle transfer hole and slot with your fingers and use a can of Gumout or any car cleaner to blow out the residue.

Repeat the above test @ 1500 RPM. The ideal is to have the engine speed decrease when turning the mixture screw either in or out from your best idle setting. Depending on air bleed size which is unique to you year carb, you should find that best idle is achieved with the mixture screw turned out about 2 1/2 turns.

Put it all back together and see whet your part throttle A/F is. Now adjust you main jetting to give you what you want.

Yes, you could just jet up and this will give you more or less what you want, but your off idle and cruise will still be lean. And yes, richening the main circuit by jetting up will also richen your WOT. See what you get before we talk about this.

I richened the idle circuit in my 1945 and it made a big difference in throttle feel at low speed, off idle and light cruise. This is where fuel is coming from either the idle circuit or both idle and main circuits together. I also started with a number 63 jet and wound up with a 68 or 69.

Mitch

Thanks very much for replying Mitch.

I pulled the cover off the carb and found that the jet was actually a 621, and I swapped that for a 64. The difference was obvious...no more jerking on light acceleration. I don't know why I thought it was a 63...dumb day I guess.

I'll absolutely check out the idle feed restriction (the car's alternated into storage right now, but I'll have it back in a week or two).

Thanks!

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1965 Dodge Dart 170 Wagon
1965 Ford Mustang
1965 Buick Skylark Hardtop
1965 Chevy Corvair Monza Convert.
1953 Buick Special Riviera

Hey Mitch,

Do you remember approximately how small the idle feed restrictor is? I want to have the right drills on hand before I tear anything apart.

Aaron

_________________
1965 Dodge Dart 170 Wagon
1965 Ford Mustang
1965 Buick Skylark Hardtop
1965 Chevy Corvair Monza Convert.
1953 Buick Special Riviera