Idle & RPM

[GregH]

So, when one leans out the idle mixture, would that affect the rpm everything else being the same?

[Dave Neptune]

It can and should have a "slight" influence in RPM's until it begins to shudder to a stop. Maybe +/- 50 RPM.

Dave Neptune

[GregH]

OK, not as much as I was thinking.

[Dave Neptune]

Many things perhaps

Greg, you may have there issues as the +/- I gave you are for a good tune as far as all else is concerned. Timing, sticky advance weights, float level, any kind of a vacuum leak and the carb internal seals all can cause adverse idle conditions which will render proper adjustment of the idle mix most difficult and almost impossible to get set.

What is the state of tune from above? Any recent work on the carb? Did you check for any vacuum leaks?

Can you describe what has been done and what is currently being tried?

Dave Neptune

[JOHN COOKSON]

After you lean the idle you might also notice:

Cleaner running spark plugs.

Hard starts, especially when the engine cold. If this happens check to be sure the choke is closing all the way.

TRUE GRIT

[GregH]

Sorry folks, seemed to have missed seeing the replies here!

I'll get back to you shortly as I hope to get the engine spring commissioned this coming weekend. And then I can report on the current settings.

[Wisakedjack]

I am looking into this issue as well. I've noticed last season that my idle RPM was a bit high (1000+ rather than 800). This season the engine is running a bit rough at idle, which may still be somewhat due to water in oil issue that I had earlier. I am primarily looking into possible sources of air leaks in the carburetor. But I also came across some older posts by Dave Neptune where it was suggested that RJ12C gap should be close to 0.040 with EI. Default gap for RJ12C is 0.03. I think I saw Dave say that increasing gap would improve engine running at idle speed. Is this the case?

[roadnsky]

...I think I saw Dave say that increasing gap would improve engine running at idle speed. Is this the case?

I'll take the liberty of answering since I have saved a few quotes from Dave on this topic...

"I have been gapping my plugs at .038~040" (larger gap acts a bit like a hotter plug)
...gap the plugs to around .040, this will help the idle by raising the temp of the plugs tip a bit and creates a better burn for a cleaner plug."

"Note take advantage of the EI by opening up the plug gap a bit .040/.045 will work fine on the top end and idle much smoother too."

[Dave Neptune]

Extra gap clarity

The additional gap will raise only the temp of the tip's as there will be a bit better fire (burn) at low idle speeds. It takes a bit more energy to jump the gap and that additional energy is what helps.

With a std ignition .035 is very doable as the RPM's and combustion pressures are not high enough to bother the plugs firing at higher RPM's that an A-4 will never see.

With an EI I feel safe running the gap all the way to .040" and did so on my boat for over 30 years with stock plugs and poor compression!!!

You will also hear that a tighter gap will increase horse power. This is only true with older "std ignitions" and a hi-performance engine. These engines develop enough pressure that makes it harder for the spark to jump the gap and a bit tighter would allow the spark to jump and all was well except that idle quality would be affected. Now with the super hot ignitions on everything we see gaps of over .070" and plug life of a hundred thousand miles or a whole lot of hours.

In my view an EI is a must as it is far more reliable and simpler to diagnose than points and condenser. Also with an EI your dwell will be fixed unlike with points. And old distributors are no good for setting the points by gap as the cam they run on is worn. This is why you see so often on this site the question "what is the dwell?".

Dave Neptune

[Sam]

Dave, just thinking out loud here. We have a short season here in Chicago and I have a clean good running virgin 1966 A4 w/ Prestolite distributor w/points and some minor dark fouling on plugs by season end. If I assume my distributor cam might be worn a bit can I approximate a better "dwell" by tightening the point gap slightly from the .020 [compensating for cam wear]. I still time the engine using the " N Dutton" method by rotating the distributor to enhanced rpm at cruising speed then back off a "tick". Not very scientific but a little better than what I used to do back in the day with my first car, 1963 Chevy Corvair, by opening the door and adjusting the timing until the door stopped vibrating. [Apologize for diverting the thread. Generally on things if I am 90% or so on target it is usually good enough]

[Dave Neptune]

Sam, a dwell meter is cheap on line. It can be used to confirm dwell after setting the points. The meter will let you know if you need to open or close the gap. Usually a tighter gap is needed to compensate for wear.

For not much more than the dwell meter you could convert to the EI and use you saved maintenance time for liquid bread or dialing in the tune.

Dave Neptune