VCCA Home Forums TECHNICAL - 1945 AND NEWER 1946-1948 Idle speed and vacuum

I notice the service manual calls out the idle speed to be 450 to 500 rpms for my 48 Fleetline with 216 engine. This seems so low, I notice that at 750 my engine is happy. If I get that low, I get erratic puffs from the exhaust but this may be something with the Fenton headers.

I do plan on removing those and installing the factory exhaust.

Odd thing is the vacuum gauge is steady 20 inches at all times though. The points, timing, valves, idle mixture are all set. Spark plugs are nice and clean, no carbon fouling. I dont get any misfires off idle, power is good on accel through all rpm ranges too.

I'm curious what you guys have set for your idle speeds and what your vacuum readings are at idle?

Thanks

I set mine at 700 RPM also for several reasons such as better oiling and cooling.

I'm with Gene on this... and for the reasons he gives.

I thought the idle on my ”˜37 was erratic/uneven when I set it at 500 rpm with a tachometer. So I disconnected the tachometer and set it up slightly to where it runs smoothly.

I get a steady vacuum reading at idle. If I remember correctly it is around 19”.

Great info guys! I will tinker with the old 48 again this weekend. I'll try to dial her to 700rpms and make adjustments as needed to mixture.

I'll keep everyone informed. Ohh...to adjust the valves while engine running, do you drop the rpms down towards 500rpms?

YES

Do you also use 700rpms when setting timing with a timing light?

I originally did my timing the old school method, set the octane selector to zero, then getting cyl 1 to TDC with pointer on the steel BB. Then made sure points were just open at TDC cylinder 1. Road tested and it was great.

Now I got this nice timing light and decided to try out my new toy. I had the RPMs set around 700 but noticed the timing mark was off on the BB, so I adjusted the distributor till BB lined up with pointer again. Now I realize that the the centrifugal advance starts at 600 rpms and this timing adjustment may be all wrong. I have not driven the car yet either so I don't know what the performance is like compared to the old school method.

I suggest that you want the idle below 600 rpm when you set the timing at the ball on the flywheel (5 degrees BTDC).

The centrifugal advance for your car starts to activate above 600 rpm. The vacuum advance might start to move if the throttle plate is open too far. That exposes the vacuum port in the carburetor base.

After you set the base timing I would loosen the mounting bolt for the distributor and advance the octane selector setting to 8 degrees for your car. Then tighten that bolt and reset your idle speed and mixture for the smooth idle you want.

Thank you so much! I will do this.