C2 carb idle mixture adjustment

[Richard S.]

Appreciate feedback on relative balance (position one to the other) of the two idle mixture screws on a stock 3461S carb. In short how far off the seat have been noted on the typical 340 HP cars? This for a good running car not initial settings.
Thanks
Rich

[Edward J.]

Re: C2 carb idle mixture adjustment

All cars slightly differ Rich, but I would say 1 To 1 1/2 turns outs, I always use a good tach/dwell meter to attain the highest RPM while turning mixture screws with a warm engine, then reset warm idle speed my 340SHP car will idle @ 775 nicely but a lot of others say 850/900.

[Gene M.]

Re: C2 carb idle mixture adjustment

The most over looked part of adjustment is the throttle plate positions controlling the transition slot exposure. There are plenty of you tube on the topic to walk you thu it. The timing to get idle speed is also covered on the you tubes. The vacuum gauge to get the max with the needle screw adjustment is well covered too.

[Richard S.]

Re: C2 carb idle mixture adjustment

Thanks Ed for the feedback but is that what actually happened at final settings on your 340 HP car? Your idle speed is near the same as what I have on my car.

[Frank D.]

Re: C2 carb idle mixture adjustment

Typical starting settings are with the mixture screws LIGHTLY seated then backed out 1-1/2 turns...I then adjust for best vacuum.

Some say the screws should be exactly the same number of turns out when done. I don't subscribe to that...on 50+ year old carbs...
Yes, if you get too far "up" on the transition slot your idle mixture screws will become ineffective and you'll wonder why...

[Duke W.]

Re: C2 carb idle mixture adjustment

The applicable service manuals recommend an initial setting. For the 3461S it's 1.5 turns out from the seat, and that's where mine usually runs best, but it depends on ambient conditions including altitude.

As far as idle speed is concerned I recommend: as low as possible commensurate with acceptable idle quality. This is subjective and varies from person to person, but I find 900 is good for all mechanical lifter cam engines.

I also recommend that both screws be turned the same amount with each adjustment, say 1/4 turn, and both should be the same number of turns out from the seat at the final setting. You can also use 1/8 turn, but you may not be able to tell the difference.

On mechanical lifter cam engines it may be beneficial to run a bit on the rich side to minimize any start-off stumble with minimal clutch slippage, especially with tall axle ratios.

Duke

[Jim D.]

Re: C2 carb idle mixture adjustment

The applicable service manuals recommend an initial setting. For the 3461S it's 1.5 turns out from the seat, and that's where mine usually runs best, but it depends on ambient conditions including altitude.

As far as idle speed is concerned I recommend: as low as possible commensurate with acceptable idle quality. This is subjective and varies from person to person, but I find 900 is good for all mechanical lifter cam engines.

I also recommend that both screws be turned the same amount with each adjustment, say 1/4 turn, and both should be the same number of turns out from the seat at the final setting. You can also use 1/8 turn, but you may not be able to tell the difference.

On mechanical lifter cam engines it may be beneficial to run a bit on the rich side to minimize any start-off stumble with minimal clutch slippage, especially with tall axle ratios.

Duke

Well said and spot on as usual Duke.

[John F.]

Re: C2 carb idle mixture adjustment

Which port or vacuum line do you hook the vacuum gauge to?

[Jim D.]

Re: C2 carb idle mixture adjustment

Which port or vacuum line do you hook the vacuum gauge to?

Anywhere you have full manifold vacuum will work. Avoid ported vacuum.

[Duke W.]

Re: C2 carb idle mixture adjustment

"Tee" it into the VAC signal line. This will allow both the vacuum gage and VAC to see manifold vacuum. The engine must be in the normal configuration, which means the VAC must be connected.

It's amazing how many people simply disconnect the VAC signal line from the VAC and connect the hose to the vacuum gage thereby leaving the VAC non-functional.

My vacuum gage has a couple of feet of tubing connected to the base of the tee, and a short piece of hose on one end of the top bar. I disconnect the VAC signal line and attach it to the other end of the top bar, then connect the short piece of hose to the VAC. This is what is called a parallel connection, and the engine has no idea that a vacuum gage is attached and remains in the normal configuration.

For engines with ported vacuum advance (like '63 FI) and most emission controlled engines, the VAC signal like will not show full manifold vacuum at idle, so another source that does must be used. Teeing into the signal line for an external choke vacuum break, if equipped, is a good source.

Duke

[John F.]

Re: C2 carb idle mixture adjustment

Thank you guys. It is the base 327-300hp.

[Timothy B.]

Re: C2 carb idle mixture adjustment

On my 1963 300hp car with the 3461 Carter the emulsion screws are approx 3/4 turn out from seat.

[Richard S.]

Re: C2 carb idle mixture adjustment

Thank you Duke for the feedback on your 340HP car. Just what I was looking for. Sorry for the late response I was away.
Rich

[Richard S.]

Re: C2 carb idle mixture adjustment

Thank you for the information on your 340HP car
Rich