I finally picked up a digital timing light/ rpm meter today, and went about adjusting the idle since I replaced all the plugs and just adjusted my advance.
I was shocked a bit to see how far off from each other they are. I had previously set my idle to 550 in gear according to the dash tach and thought it felt a little lower than what I’d want but just stuck with it. But with the numbers from the digital unit, I set the idle to somewhere between where it would end up if I just used one or the other. If I set it to 550 in gear per the dash tach, it would be super high according to the digital one. If I set it to 550 in drive according to the digital one, I have a feeling it would stall out. It already feels too low to me, as I have it set now. Here are the numbers I recorded, please share your thoughts on the specific numbers and on the dash tach vs digital tach question in general.
Distributor vacuum advance disconnected and plugged:
Digital tach in park : 780
Dash tach in park : 450
Digital tach in drive : 630
Dash tach in drive : 400
Distributor vacuum advance connected:
Digital tach in park : 780
Dash tach in park : 425
Digital tach in drive : 620
Dash tach in drive : 350
Before adjusting the idle and recording these numbers, I did the shop manual’s prescribed test for mechanical and vacuum advance in the dist, and both seemed ok (6 with no advance, 20 total with mechanical @2000 rpm, 30 total with vacuum @2000 rpm), and I also made sure I was getting max manifold vacuum by adjusting the idle mix screws (just under 18").
To me this idle feels low. Maybe that’s because I’m used to it having been set to 550 in gear according to the dash tach, but again, even that felt low to me. This feels really low.
As long as I’m not getting a big “thunk” when I shift into gear, is it cool if I bump the idle up another 100 rpm or so? Would you ignore the readings of the digital, ignore the dash, average them both a bit like I did, or just go ahead and set it where you like it regardless of the Ford spec saying 550 in drive?
One more data point… I just tweaked it to where it feels right to me, and here’s what I got
Vacuum 18.5
Digital tach in park : 1200
Dash tach in park : 800
Digital tach in drive : 850
Dash tach in drive : 500
That’s actually slightly lower than where it was prior to all this, which was 550 on the dash tach. But to me this feels more correct. No “thunk” when shifting into drive.
Go ahead and leave it there?
And if a digital meter is just counting how quickly it senses spark at the #1 wire, and then doing basic math to multiply that out to rpm, why wouldn’t it be exactly right? I would have thought it could get it perfect pretty easily, no?
Aha! Well I know my coil is outside the spec in the shop manual- primary is slightly higher than spec and secondary is slightly lower. So I guess that would be part of it. If I fully trust the digital rpm readings though, my idle would be so low I’m afraid of stalling.
Personally I would question the digital tach that you got. Is it new or used? If new, take it back and get another one. Im a bit old school, but analog (with needle) is better. This goes for sound equipment as well. You can’t really see proper fluctuations with digital as you can with an analog gauge. If at all possible take the thing back or get your money back if it is used. If you know anyone that has one you can compare it with first would be good as well. If you have a Parts Source automotive center around where you live, see if they have one you can borrow. Some automotive places have tools that you can borrow with a deposit and you get the money back in the end when you bring it back.
It should be about 700 RPM in park, 600 RPM in drive with the parking brake set. All headlights on, air conditioning on with high blower (if so equipped)
I’m going to borrow an old needle-type rpm gauge from my dad for another point of reference.
Royce, I don’t have AC, but did test the idle with the lights on and off. In park, there was a difference but in drive there was not. However, I was using the brakes to hold it still, not the parking brake. Should I use the parking brake and not the normal brakes? AT car.
I know that putting the brakes on is an extra load on the engine, I can see it in the rpm and all. I’m assuming that’s why I don’t see a difference with the headlights on or off in drive (only in park), because the load of the brakes swamps the load of the lights?
The idea is to load it as much as possible. If you only have a heater it still needs to be on high blower. There shouldn’t be any difference foot brake or parking brake, the six brake lights don’ take a lot of juice.
The dash tach is much more accurate at higher RPM’s than at idle. Some digital tachs have different settings for 4, 6, and 8 cylinder engines. Be sure that you have it set right.
So long as the carb is still running on the idle jets, (you can control the idle mixture using the mixture screws on the front bottom of the carb) I like to run a little higher than stock idle, particularly where the idle vacuum control valve is no longer in use. I try to optimize the vacuum reading and then go just slightly rich at idle. Tuning like this is an art and it will become more natural with practice. You will start to hear and feel what works best.