High altitude carb adjustments - replace jets?
 

I just moved from Anchorage Alaska (elevation 268 feet) to Colorado Springs, Colorado (elevation at my house is 7,100 feet). I can't even get my car to start, and it reeks of fuel something terrible. It was running rich before I moved, and now I suspect that it is so rich that it won't fire up, but I have to admit that I've been too busy with the house move to give it any attention at all.

I am running a tri-power on it. Anyone out there have recommendations on what size jets I should get (and recommendations on where to get them) to replace what is in mine now? I don't know the size that is in there now.

Thanks, Marc

Cliff R may have the jets.
(Summit may even have them)

I'd start with a jet that is 2 sizes smaller.

:)

When Ilived in CA and had rejetted my TA for San Jose with an HO Racing kit we drove to Tahoe to ski so it was also cold. Barely would start black smoke out the exhaust. It is amazing what some altitude can do to a set up.

In simple terms, at 7000 ft atmosphere you basically have about 22% less pressure forcing air into your engine vs at sea level. 22% less air vs the air/fuel ratio you had at seal level says that you need to remove 22% of the fuel coming thru the main jets.

http://www.denysschen.com/catalogue/density.aspx


Density at 268 feet = .0738 lbs air mass per cubic foot.

Density at 7000 feet = .0575 lbs air mass per cubic foot.

.0163 lbs air mass difference.

22.0867% difference. So you need to remove ~ 22% of the fuel presently in the main jet area you currently have for both the center carb and the end carbs.

So I would recommend a #56 or #57 Rochester Jet for the Center Carb
and #63 or #64 jets for the outboard carbs.

Tom Vaught

Thanks Tom. I appreciate the education and advice.

Marc

Yup, 22% is a good number. Sea Level averages around 101kPa atmospheric pressure, and 7100 ft averages around 78kPa atmospheric pressure.

One of the significant reasons I converted to EFI is I live and work between 6,500-7,300 feet elevation and make regular drives to around sea level.

While I agree with Tom's thinking, I would hate to jet it lean.

It would be nice to know some additional parameters: ie engine size, engine mods (especially camshaft and or compression), and what jets would have been correct for your 268 foot elavation.

Also would want to know if you are using dinosaur fuel or a corn mixture. Basically, the reduced energy content in E-10 seems to work out very well to 1 size lean (4000~6000 feet). But you are at 7000 which works out to 2 sizes lean.

So, I would put in 1 size lean (0.002) for E-10 fuel or 2 sizes lean (0.004) for gasoline.

And once you change the jets, change the spark plugs.

Jon.

Additional Parameters:
455 + .040, SD Performance heads (250 CFM), Crower 60919 cam, 1.65 rolling rockers, 9.25:1 compression, fed by a 1965 Tripower, no mods to that or the intake.

No clue what the jet size is right now. Was running rich at sea level though.

Dinosaur fuel, 92 octane.

Best reason I know to use EFI (other than smog emissions).

But you can have your cake and eat it also. Carter AFB and TQ 4-barrel carbs use metering rod technology like the Q-Jet. Unlike the Q-Jet, the Carter metering rods may be changed without taking the carb apart. To prove the point, I have changed rods on Carter AFB's with the engine running!

Carry a set for altitude and a set for sea level in the glove box and change as needed. You can use the same jets for both and modify the rod calibrations to give the same A/F ratio as changing jets in the tripower.

Of course, if the ambient is zero, the automatic change of the EFI is nice!

Jon.

For the ends, we would have suggested 0.076 at sea level. So 2 sizes lean would be 0.072, maybe a 0.071.

Also, do you have enough vacuum to keep the power valve closed other than WOT. An open power valve would cause the unit to be rich, and could confuse the selection of the jet size.

Since we don't suggest the small center of the 1965 on a 455 period, I don't have a suggestion for it.

In addition to changing spark plugs, I would go for a good ignition tune-up.

Jon.