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Midrange miss


38plymouth

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2 minutes ago, 38plymouth said:

I don't know, both of my timing lights stopped working.  I need to pick up a new one.

Warning, skip if you don't like long answers with examples and skip if you're not into tuning an engine:

This is the information you need to know for the engine to run properly.  As for using the manual for this type of tuning, it will work so-so but you are definitely leaving a lot on the table.  If you're willing to actually tune your engine, keep reading...  Here's why you don't want to use an 80+ year old manual in this case.

 

History:

Over the past years I have applied my career in the chemical analysis business (aka refineries) to tuning my cars to today's fuels.  When your manual was written, the fuel available was what is called "straight run gasoline."  Straight run gas is basically what boiled off the top of a distillation tower and condensed into the fuel your manual was written around (think moonshining).  Very volatile and very low octane compared to what is available now and that's where the refining ended due to the technology of the time.  During WW2, there was a need to fly higher and faster which required much higher cylinder pressures to and in turn required high octane fuel.  It was found that high octane fuels could be found in the heavy oils at the bottom of the distillation towers (where the fuel oil and asphalt lives) but the molecules will have to be cracked using a catalyst.  Cat-cracking was born out of this need and the cracked fuel is now blended with straight run fuel to get desired octane and volatility which are all much different that what was available pre WW2 when your manual was written.  There is much, much more to this but I am not in class and it will get off topic, but you get the basics.

 

Tuning:

First, close your manual and put it on the shelf...  Leave the vacuum advance out of this part for now.  Get a dial advance timing light.  Make sure your timing indicator is correct.  

With the small bore of our engines, I have found they don't like a lot or need of advance at speed and you also figured that out.  I think I ended up about 20-22 degrees total advance @2500 RPM.  This is the most important number you need to know, all other numbers need to be tuned in with springs and governors, but you have to be willing... It is possible to have too much advance without pinging especially with low compression.  When I did my slant 6 distributor mod, I searched high and low for a 9R governor.  This governor allowed 9 degrees of DISTRIBUTOR mechanical advance which equals 18 degrees of crankshaft advance.  This allowed me to run 4 degrees of idle advance and maintain a good idle.  Since distributor governors are not available for our originals, the advance slots can be welded and filed to achieve the desired amounts of mechanical advance and idle advance.  This will allow you to run a much leaner idle mixture, much cooler temps and better drivability.  I will probably weld my 9R governor and file to a 6 to allow more initial advance, but I haven't gotten to it in the last several years.

The vacuum advance is an economy only thing and probably be used as-is with no mods.  For those that use the Slant 6 distributor, make a shim to restrict vacuum advance to about 10-15 degrees total.

 

A lot of time - Yes!

Noticeable improvement - Yes!

 

Hope this helps someone...

 

This is a lot of work, if you are not interested the manual settings will work ok.

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