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Author Topic: High altitude timing / fuel mileage  (Read 3944 times)

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Offline SvenTomas

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High altitude timing / fuel mileage
« on: May 23, 2012, 08:45:13 AM »
Hello . . .

I need assistance with some timing questions.

After purchasing my little brown eagle, I was averaging about 24-25 mpg (21-22 local, and 26-29 hwy.).  Super stoked!

However, I then decided that since it was a 'high altitude' car (from Cheyenne, WYO), and I live at 2000 ft (and drive mostly between 2000' asl and 100' asl), perhaps I should investigate doing the high-low altitude conversion.

What I did instead was simply adjust the timing.  I adjusted it according to the label under the hood, and noticed that the timing was way off from what the label under the hood recommended.  This was part of the reason that I decided to not mess with the high-low alt. conversion.  Just wanting to get everything squared away.

SO . . . long story longer, the timing was set at about 0d BTDC with the dist advance hose off and plugged, and the vacuum switch assembly unplugged (per instruction), which was way out of spec.  Made the simple timing adjustment to spec and went away.

NOW . . . I'm getting 13mpg!!!  Horrified.  Obviously, I'm going to try to reset the timing to what it was. 

My question is this though . . . why???  Is there possibly something I'm not understanding . . . or is am I (or was the previous owner) on to some secret off-spec setting that boost fuel mileage way up??

I have to get this figured out because I need that fuel mileage back asap.

The odd thing is, that before I adjusted the timing, it was at 0d BTDC without the vac advance line to dist., and when I would plug the vacuum advance in (at idle speed) the timing would jump up to the specced advance (I can't remember the number, but like 13d BTDC??).  So, I have vacuum to the advance at idle.  So, I though maybe that was related to the the high altitude package, but it doesn't match what the factory settings say they should be???

Baffled.

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Offline ammachine390

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Re: High altitude timing / fuel mileage
« Reply #1 on: May 24, 2012, 01:24:29 AM »
I assume you are still running the original feedback carter bbd carburetor with the computer still hooked up. If so, there is more to adjusting the car from high altitude to low altitude. In addition to the timing being adjusted, the altitude jumper wire plug must be disconnected, which I believe is located near the computer which is behind the passenger side kick panel. The plug may be in front of the passengers foot well under the carpet. Also, some vacuum hoses need to be relocated, which I believe can be found in eaglepedia.
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Dan
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Offline carnuck

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Re: High altitude timing / fuel mileage
« Reply #2 on: May 24, 2012, 02:17:48 PM »
I have found that the closer to the point of ping you get with advanced timing (without going over) the better the fuel economy you get. The power goes up with the advance (to a point) because the engine produces NOx naturally from the higher combustion temps. When it gets too high, you get ping (and your emissions skyrocket)
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Offline BenM

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Re: High altitude timing / fuel mileage
« Reply #3 on: May 24, 2012, 03:40:42 PM »
If you have the "computer" hooked up, what's happening is that it can retard the timing on its own. Did you disconnect the plug from the sensors on the passenger side?

What I think is happening is that without the vacuum signal but with the sensors still plugged in the computer thinks you're starting the car and to help you it retards the timing.

Check out the Eaglepedia entry: http://www.amceaglenest.com/~iowaeagl/guide/index.php?title=Correctly_setting_your_idle_speed_1980_-_1988
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Offline ammachine390

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Re: High altitude timing / fuel mileage
« Reply #4 on: May 24, 2012, 10:26:50 PM »
In the link that BenM provided, the picture for the 10inch-4inch switches is wrong. The picture shows the 3 vacuum solenoids located on top of the valve cover. The switches are actually on the passenger side firewall across from the distributor. But other than that it seems correct

Here's the link on how to adjust from high altitude to low altitude settings (and vise versa)
http://www.amceaglenest.com/~iowaeagl/guide/index.php?title=September_1987_-_IS_11E_-_Altitude_Performance_Adjustments
« Last Edit: May 25, 2012, 12:52:20 AM by ammachine390 »
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Dan
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Offline SvenTomas

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Re: High altitude timing / fuel mileage
« Reply #5 on: May 30, 2012, 08:36:29 AM »
Okay.  I'm going to go at it again as soon as I can this week again then.  I did have the vacuum switches (passenger-side fender well) disconnected, but I wasn't too sure on that so I'll check the link.

I did attempt to set the timing back to where it was originally, and mileage picked back up just a hair to 15, but I really need it to be back up in the 20s . . . was a huge part of the justification behind the purchase.

I'll post back my results.
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