1979 400 Bronco
Emissions sticker says 6 degrees BTDC @ 500 rpm in P or N.
Vehicle also has High Elevation Stickers saying carb idle mixture screws have been adjusted to high elevations. Factory carb Motor craft 2150.

I am at 7800 ft of altitude and darn thing runs like crap at 6 degrees @ 500 rpm idle. But if I go much over 600 rpm the ported vacuum port starts pulling vacuum, which I assume is not good. I really do not care about the ported vacuum pulling vacuum at idle as my distributor is electronic computer controlled. I just do not want my carb to be out of tune via blades being open too far at idle.

To get good idle and performance with a 500 +/- 50 rpm I have to have idle timing set at 20-25 degrees BTDC. Best is at 25 degrees BTDC. 85 octane fuel and steady 14 IN HG vacuum at idle. Remember 7800 Feet elevation. The mile high city of Denver is a half mile below me.

Could someone suggest a reasonable Idle timing and a total timing figure I should be aiming for?
All stock motor.

 

AT least 14 degrees at your altitude
Rule of thumb is add 2 degrees for every 1000 feet you go up in elevation
So 6 + 14 is 20 and that might ping so you would back it off in 2 degree increments
I have a 69 Boss 302 at 5000 feet and run 22 initial

 

Take the carburetor off and drill a .125 hole through each throttle blade. This will allow you to get the throttle blades closed farther for the same amount of idle speed.

 

You are overthinking this rock2610d,
Why are you so worried about going above 600 rpm just to get the timing set?
Even at 7 or 8 hundred rpm you are not going to have enough vacuum on the spark port line to give all in vacuum advance
You are supposed to be doing this timing set with the vac line disconnected and plugged anyway
Same as the mechanical advance. It's not all in until around 1500 to 1800 rpm and have springs to overcome at that

 

Thank you all for the info.

I can tackle the carb stuff, I will probably have to re-adjust the idle mixture screws to allow more air so the idle speed gets in range. That is a different question though as I need to figure out at what RPM the idle mixture screws should be adjusted at. I work off max vacuum and then adjust rpm to 500 per sticker.

As I stated in OP I do not have a distributor that has a vacuum advance or mechanical advance. It is an electronic distributor with computer control. I am working up a timing table to optimize the timing curve for my elevation. The distributor is a progression ignition unit. Progression Ignition - Ignition Distributor, Ignition Timing

What I need is a base idle timing number which you all have already noted.....approximately 20 degrees BTDC, I also need the amount of vacuum advance the OEM distributors apply (without any mechanical advance). I tried getting this with my OEM distributor, but it appeared damaged and did not advance timing but a couple degrees when I put full idle vacuum to it.

I also need the max timing advance number. I assume low elevation engines would be around 34 degrees, so should I add 14 degrees or so to that number too?

Sorry I was not clear on my setup in OP as I should have been.

My setup, removes the need for the factory Duraspark 2 ignition system and replaces that with a new distributor that runs the motor based off a timing table. All OEM wiring is still intact and unmolested. Ignition box is still on vehicle and being used somehow for battery charging. If is unplug the ignition box my alternator stops charging my battery so the setup "looks" factory but not used for ignition.

 

I don't think that I believe it these rule of thumb ideas about timing for elevation. I've never found that my 2 barrel carburetor circle track engines require any more timing than if they had unrestricted induction. if 28 degrees works the best at 4500 rpm it is also the best at 7200 even though it has more than 5 inches of manifold vacuum. That has to be close to the same amount of change that you're experiencing from the altitude vs sea level.

 

Interesting, maybe its the 85 octane fuel we have up here affecting burn rates. High elevations let motors run pretty low octane fuel. Using a 87 or 89 octane fuel which is available would only allow for even more timing though, right?

Using 85 octane and setting timing to factory specs is just plan disappointing. I have to push gas pedal so far to floor to actually move vehicle the vacuum goes to "0" and bronco barely is able to pull itself out of driveway. Setting base timing to 24 degrees at idle will spin the back tires as I pull out of driveway.....if I push pedal to reach "0" vacuum.

Do you think something else is wrong?

 

I'd say that the elevation reducing the air density and cylinder pressures that reduces the octane requirement. 85 octane probably works fine and does not detonate with normal timing settings. 87, 89 or 110 racing fuel would allow you to run more timing but running more timing isn't a goal and isn't always better. Just last Sunday I was running a 427 inch small block Chevy on my dyno. The owner had been running 35 degrees of timing yet after a few pulls we had that down to 31 degrees which is what ran the best. It could have probably tolerated more than 40 without detonation with the leaded racing fuel but it wouldn't have made as much torque and horsepower.

As you said I also wonder if something else is wrong with the engine. Did you ever try just loosening up the exhaust pipes at the manifolds to give it a big exhaust leak as a test? Does it run better?

 

I got the bronco with headers on the motor that were completely rusted out and there was no exhaust hooked to the headers. Motor was LOUD but ran fine. Someone in the past ownership missed a PCV valve that was so bad it was a 100% vacuum leak. They "adjusted" the carb and timing to compensate for the huge vacuum leak. So, once I fixed the vacuum leaks the motor ran STRONG but timing was really advanced, like over 30 degrees at idle.

I have OEM exhaust manifolds on it now running thru new 2.5" exhaust pipes with no mufflers yet.

Leak down test was pristine and compression tests show about 125 to 130 psi on all 8 cylinders. Motor starts so fast and easy the starter does not have time to disengage before motor is running.

On a side note I get the impression I am failing to convey my data. 400 motor runs strong at up to 30 degrees advanced AT IDLE. I have no clue what the max rpm timing number was as the marks on the damper only go to 30 degrees BTDC. I assume it was in the 50s at full mech and vac advance with no detonation I could hear from a LOUD motor without exhaust. I realize this is too much timing....Hince my question here.

You have any suggestion on what the starting point would be for base timing, high vacuum cruise timing, and WOT timing values I should start at?
Once I have those I can modify ignition timing as motor is running to optimize via my smart phone.

 

I once drove my truck from home at sea level to see friends at 6,000 feet.

I made no timing or carb adjustments and the truck ran like a dog when I got there.

So much so that an old lady in an electric wheel chair bet me $10 that she could beat my truck.

I lost $10 that day.

The next time I drove I advanced the timing by 2 degrees at regular intervals, with no carb adjustments.

I won my $10 back.

Basically, if your truck likes 20 - 25 degrees at idle, so be it.

6 degrees on the sticker is advised for sea level.

Try your total timing at 34, then 36, then 38 etc. for WOT.

Adding 15* to those numbers for cruising could be a good start.

Drive around and ''feel'' which setting works best. Your engine will tell you what it likes.

The 14'' vacuum sounds about right, but more importantly, the needle is steady, and your compression numbers are very acceptable.

 

I wonder if the balancer is wrong and 30 degrees is more like 10-15 degrees at idle.

 

What Dave said about the right timing being right all the time is what I have experienced. It is the fuel mixture that needs to change. Piston aircraft engines use magnetos. The timing is fixed and never changes, and they go from sea level to 10,000 ft. all the time. What changes is the pilot has a mixture control and leans the engine as altitude increases. If you are going to drive around in the clouds, you need fuel injection with enough sensors to make those adjustments on its own.

 

That takes care of the question about needing to change timing. (I bet the engine vacuum increased with the increase in advance.)

Agreed about the need to change the gas mixture.

My carb at 6,000 feet with no adjustment was far too rich.

 

The thing to remember too is that it isn't just the main jet mixture it is the entire calibration that needs to be adjusted leaner. idle feed restriction, jet and power valve channel restrictions would all be different. Having the mixture way off would probably increase the timing requirement and increasing the advance would probably help to a point but only because the mixture is out to lunch.

One of my high school teachers had a '72 Olds Vista Cruiser wagon with a 350 Olds and a 3 speed column shift(had to be really rare in an Oldsmobile). He told me that one time on vacation they went to Colorado where at high altitude in the mountains he held it flat on the floor in low gear for over 8 miles. Knowing him I'm not surprised..

My father-in-law told me one time that he had a Chevy with a 235 and a Powerglide. It towed a little camper around great when they were in Los Angeles but one time they went camping in the Sierra mountains and he didn't think that it was going to make it to the top of the hill and there wasn't any place to turn around. I think he said that he got down to about 10 mph but made it.

 

Once I have the timing figured out I will deal with carb.

stickers all over engine bay stating it's a high elevation motor. No special ignition control box for elevation like ford manual shows though. Just high elevation carb according to stickers.

Ideally I could get the specs on the special ignition control boxe Ford used for high elevations. Supposidly that box has 3 connectors on it and a barometer switch.

just took a look at them and it says idle screws are more in. And TIMING SET AT 12 INSTEAD OF 6!!!