Spark plug gap, different values in TM's

[Sledge303]

I did some searches regarding this issue, and found no mention of the fact that depending the year of the TM-20 level, the plug gaps are different.
In the 1971 -20 TM its different depending on what SECTION of the TM you are reading.
1971 Tech specs says 0.029-0.032
1971 actual servicing instruction says in the text 0.027-0.030
The 1982 (with revisions) 20 series TM calls for a gap of 0.032-0.036
If you run the Champion XMJ17 plugs the manufacturer gap is 0.026

With gap ranges documented at 0.026-0.036 what factors should we be considering to make the appropriate choice?
Altitude
Octane
Age of coil
Distributor with breaker points
Distributor with electronic ignition
Compression test values
Flat terrain
Hilly roads

With ethanol free (non Gov't subsidized) gas hitting $4.50 a gallon I would like to run efficiently as possible.

[rickf]

.032 is a pretty much standard gap for a standard coil type ignition. There have been a lot of errors in those manuals over the years. In one year they said to set the carb float at 3/32" when the proper setting is 9/32". And it was never corrected in that manual. I forget what year that was but is was in the 60's I believe.

[Sledge303]

I appreciate the info / answers.
I was hoping for a bit more of a discussion about the topic.
Usually there are some good tips about squeezing a bit more from the motor than just the factory settings.
I read lots of chat about coils and the ignition system, not a lot of shared experience about spark plugs.
My MUTT lives at 5K ft here in Colorado Springs.
There was a time when civilian vehicles sold here received a "high altitude emissions" package.
My M151A2 is early, so doesn't have the emissions add-ons and uses a mechanical fuel pump.
I think the carburetor is the emission type though (see picture)
The previous owner bought a used distributor with the electronic control as he didn't like setting points.
I think the advance weight springs are a little tired, judging by how slow I need the motor idling to get all the advance out of the distributor.
The motor also hangs for a minute at a low RPM when returning to idle.
The replacement springs don't seem to be available in the USA NSN 5340-00-350-2288 / Autolite PN IGB284
I think engine timing should be set a degree in advance for every thousand feet?
Its been a long time since I had to think about timing and plug gaps!
The previous owner also purchased new Champion XMJ-17 plugs, and I really don't like/trust Champion plugs.
I wish a pallet box of Autolite 2245's would be discovered somewhere!
I do run ethanol free fuel as it is available nearby.
OH, the previous owner had a fuel pressure regulator on the output of the fuel pump (I have removed it now) but it is in the picture.

IMG_20210808_1759577.jpg

IMG_20210728_1945040.jpg

[svramselaar]

hi


this is what i found
Holley also suggests decreasing one jet size for every 2,000 foot increase in altitude. So if we were going to re-jet a carburetor for 7,000 feet, this would mean reducing both the primary and secondary jets by three numbers – or from the 73 down to 70.

below a big story about it

https://www.holley.com/blog/post/how_to ... _altitude/

at Holland it is easy it is flat

george

[rickf]

With the Zenith you have two jet choices, 30 and 28. The 28 was used on some of the 13660A carburetors but I have seen them scattered all over the 13660 range so I guess people changed them around as they saw fit at the time. I have tested them at wide open throttle which is the only place the main jet will come into play and I have never seen any difference because the exhaust manifold limits the engine to the point that the carb can never fully flow it full capacity. Basically you just need to run it for a while and pull the plugs and read them to see what you need for jetting. Most of the jetting needs can be done with the idle mixture screw.