Spark advance at elevation

Ahoya to all!

Tuned up my stock '73 Cj5 w/258, replaced: cap and rotor, points and condenser, plugs and wires, ignition coil. Plug gap .035, dwell is 32, idle speed at 700.

The question I have concerns ignition timing. Tune up specifies 3B, I live at 6000'. One book referenced, states most engines will tolerate from 1/2 to 1 degree of extra initial advance per 1000 feet of elevation change. For safety sake limit extra spark advance to a maximum of 3-5 degrees. The other book states to compensate for elevation by a little extra advance of your spark. Crank up about 7 degrees more advanced at 1500 rpm (not at idle.)

Do I advance spark at idle (700) or at 1500 rpm?

 

The information I have from older Ford FSMs states you can increase initial spark advance 1 degree for each 1000 ft over 5000 ft up to 10,000 ft.

The 81 Jeep FSM I have does not mention elevation adjustments but shows 8 Degrees initial advance +/- 1 degree for the 49 state models with the 258 and manual transmissions with the total degrees advance with dist vacuum hooked up between 35 - 43 degrees at 2000 rpm.

It also shows the dist vacuum unit should pull on between 20 - 26 degrees at 16 in hg and the mechanical advance should throw in between 6 - 9 degrees at 2000 rpm.

8 initial + 9 mechanical + 26 vacuum = 43 total degrees at 2000 rpm.

I personally would adjust advance at idle with dist vacuum plugged.

Hope this helps.

 

Thank you Brian.

 

in the late 80s on BBD computer 258s, the factory high altitude compensation preferred method was to attach a supplied ground wire to the block (which pulled the metering pins back) and advance the timing 5*.
You should feel confortable with no worries just advancing the timing 5* on your year of Jeep, at idle.
I personally advance my V6 the 5* when I vacation there for two weeks.

FYI in 1982/1983, there was a factory timing sequence on a 258 that required disconnecting more components, and actually checking the timing at 1500 RPM; only Jeep I can recall that the timing was checked at that high of RPM. It may have been just a CA thing as well, not 49 state.

 

Thanks Jim!

 

I live at 5000 feet. At the shop, we always advanced everything 4 degrees over the factory spec during a tuneup unless there was some compelling reason not to. About the only vehicles I recall that regularly couldn't handle it were mid-70's to 80's Ford small blocks.