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High Elevation Carb Tuning

Discussion in 'Early CJ-5 and CJ-6 Tech' started by FinoCJ, Sep 13, 2017.

  1. Sep 13, 2017
    FinoCJ

    FinoCJ 1970 CJ5 Sponsor

    Denver, CO
    Joined:
    Jul 18, 2013
    Messages:
    1,004
    Hi all - jeep has been running well and I've been getting it out quite a bit recently, including some high elevation runs (say over 13,000 ft). As I live in Denver at 5000' (but the carb is jetted for 7500' as I spend a lot of driving time around those elevations) I do some carb tuning throughout the day as the elevations changes from 5000-13000'. Obviously in a perfect world, I'd change jets as needed, but since that is a bit impractical to me at the moment, I make the 7500' jets work pretty well everywhere and tweak things like timing and idle screws.

    So, how much additional initial timing advance do you think would be appropriate for elevation change? My rebuilt v6 with 252 comp cam seems to like a lot of initial advance - 9BTDC at 5000' (it actually idles better or stronger with more vacuum with 12BTDC, but I think I start to get too much advance with 'all in' during driving conditions). I usually readjust the carb idle mix screws and initial timing at the trail head when locking in and airing down - generally around 8-9000'. I often bump it up to about 12BTDC. Now that I have been pushing even higher, I am having some issues with idling at over 12,000' and finally pushed the initial advance to 16BTDC, and it ran much better. Reasonable? It likes as much timing as it can get, but I don't want damage. Of course, during slow speed, lower rpm conditions like crawling along in 4lo, I don't think it gets anywhere near 'all-in', and thus running a lot of initial should be safe?

    What kind of carb adjustments would you expect? I guess I have been surprised as I think of higher elevation needing less fuel (smaller jets) for proper mix. Certainly the smaller jets help, but its the idle portion that I am wondering about. As I climb up to about 10,000' it seems the idle mix screws can be turned in slightly to help lean out idle, but as we push the super high elevations over 12,000', it seems I turning them in more doesn't help, and if anything I need to turn them out (maybe because I am advancing the timing as well it likes the extra fuel).

    Sometimes a I cheat a little bit and increase the idle rpm screw which opens the throttle plate a smidge and helps pull fuel through the main jets (at least that is what I think is happening). It also provides a bit of vacuum to the the ported vac can which would then advance the timing a bit more. So, maybe what I am figuring out...advancing the timing is more important than too much fiddling with carb idle mix screws or even the idle rpm screw (especially when you aren't changing the main jets).

    i think the real solution is TBI...but that is another lifetime away.
     
  2. Sep 13, 2017
    jpflat2a

    jpflat2a what's that noise?

    Riverside CA
    Joined:
    Jul 30, 2003
    Messages:
    7,440
    I have the 49 jets in my Rochester 2 bbl right now for Colorado elevation.
    If I were to stay more than 1 week, I'd install the 47's and advance the timing 5* or so.
    But being we're just crawling in low range up shelf roads, I don't think it's necessary.
    I have noticed hard starts at higher elevation, but I haven't had any idle issues.
     
  3. Sep 13, 2017
    FinoCJ

    FinoCJ 1970 CJ5 Sponsor

    Denver, CO
    Joined:
    Jul 18, 2013
    Messages:
    1,004
    I already have the 47 jets in it - might be a bit too lean for 5000' denver, but seems okay since its mostly driven at higher elevations. I think the 'book' says 49 jets for 5000' and 48 for 7500 and 47 for 10,000', but I found the 49 is too rich for denver. 48s seem to be perfect for 5000' and I went down to 47s to help with the mountain trails where it spends most of its time. Considering getting a set of 46s just for summer high pass trails over 12000'.

    But, yea I struggle with keeping a strong idle. Advancing the timing definitely helps - just don't want to go too far. I am adding in a little more than 5* timing (more like 6-7*) but seems like we are in the same ballpark and I think the comp cam seems to like more advance than OEM.

    I really worry as I don't feel like I have a good ear for too lean (pinging) - especially when in it driving around with D18 whine AND Tera low gears!
     
  4. Sep 13, 2017
    colojeepguy

    colojeepguy Colorado Springs Sponsor

    At the foot of...
    Joined:
    Apr 4, 2010
    Messages:
    3,657
    I'm a little bit higher than you at 6000 ft. but I run as much timing as it will take, I think I'm around 18 degrees initial. My carb is a holley so my tuning won't help you but it sounds like you are on the right track. You're never going to get a carbed vehicle to run right all across the board with our elevation changes...if you're planning on a high elevation run, bump the idle up to about 1000 rpm (or install a hand throttle! :cool: )
     

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