67 gt350 running too rich

[shelbyhertz66]

Hi All,
So my 67 came from the mid west and is now at 5000 ft. elevation
seems to be running very rich. I have not taken the carb apart yet so I don't
know what jets are in it but probably too big. Would anyone have a good
recommendation for primary jets as a starting point for this altitude?
thanks much!

[Bigfoot]

Your timing may be off as well due to the altitude.
Most probably

[Greg]

I would definitely look at the timing first.

[shelbydoug]

Verify that your air bleeds, both the idle and the high speed are clear.

I think for high altitude you need bigger air jets, i.e., the air bleed holes need to be enlarged?

I don't know how much BUT keep in mind that will lean you out at sea level.

I think I saw a service bulletin on that but I don't remember the specifics of the recommendation?

Which carb are you using?

[shlby66]

Hi All,
So my 67 came from the mid west and is now at 5000 ft. elevation
seems to be running very rich.

    I bought my '67 GT350, new, in Albuquerque, NM.  Elevation, there is 5200' above sea level.

    The Shelby ran poorly, and after a visit, to the Ford dealer, where they went DOWN, 2 Sizes, in the jets. The Shelby
    was happy.

    Doug

[rmarble57]

Actually at altitude, the air is thinner, therefore less available and you would "Lean" out the fuel available with smaller jets.
With the old school parts we work with, there just isn't much available to help aside from experimentation and reading the plugs.

The only true way to get better results is to weld oxygen sensor bungs into the exhaust and put a wide-band sensor in while driving at the various types of altitude, speed, load etc.

[texas swede]

When I moved from Texas to Evergreen, CO (7000 elevation) in 1990 my 65 GT350 was running very rich so I took the car to
Walt Hane's shop for an adjustment. His son Chip used the Dyno and put a copper wire restriction in the power valve channel.
The car was running like it never had before, throttle response was like a 4-banger and after moving
back to Texas in 1993 I removed the copper wire and the car never ran as good as it did before.
Texas Swede

[shelbydoug]

It depends on which circuit, idle, main or power is rich.

I'd bet that it is the idle circuit rather then the mains?

I was never a believer of a 715 carb on a 289. That carb originated on a MR 427.

Having worked on these engines and experienced the difference even stepping down to a 600 cfm 1850 with 63 main jets makes, I'd say take your pick on which circuit the 715 needs correction on?

It actually isn't the main jets or even the idle "jets", it's the combination of the air corrector (air bleeds) and the fuel jets.

Once the proportion of a/f is established, you stay in that proportion. If you reduce the size of the fuel jets, you also reduce the size of the air bleeds.

Now if it is the proportion of a/f that you need to change, that's unusual.


On Holleys, sometimes that's not clear to see. With Weber 48ida's the idea is much clearer.

The emulsion tube generally stays the say, and you reduce the a/f with the same ratio.

By reducing the size of the fuel mains in proportion with the air correctors, you also lean out the engine even though you are just reducing the volume of fuel to the engine.

Holley's system is just to change the main fuel jets and explains why going over a certain size has a disproportionate effect on the "richness of the engine".

What complicates this even more on a Holley, is that they do have emulsion tubes which COULD be involved in a mixture problem some where in between the air and the fuel jets BUT are not accessible or changeable generally speaking.


IF you can clean the Holley up by reducing the pcv restriction, consider yourself lucky. Even with just a main fuel jet change as well but the culprit here is the air jet, i.e., air bleeds are wrong.

New tunable aftermarket Holleys are using changeable air bleeds as screw in jets. That should give you some suggestions as to where you might look?

[shelbyhertz66]

Thanks all for generously sharing you tech expertise and time so far!
I did forget to mention that yes it is the original 715 carb.   It is
making me think that to really get the best performance from the stock
engine it may take more in depth tuning and adjustments for this altitude.

[oldcanuck]

Doug,

You mention a 600 cfm Holley swap for the 289.  Was this a common thing, or why ?   The reason I ask is that the original owner of my GT350 pulled the 715LM in mid - 1975 at the recommendation of his local Ford dealer in FL. and they installed a 600 double pumper. That Holley dated coded May of '75 is still on my car today.  It was rebuilt for the first time last month.  My plan was to have the orig. 715 rebuilt and put it back on. I'm wondering if I should just put it back in the box for safe keeping ?

Thanks,
Bob

[Royce Peterson]

The cars were built with the 715 CFM carburetor in order to homologate that part for FIA and SCCA rules. With stock camshaft it may well run better for daily driving with a 600 CFM Holley. I don't care for the double pumper on the street, if you want best gas mileage and best manners the 0-1850 carb is hard to beat on a 289.

If on the other hand you have good heads, bigger valves, a rumpity camshaft and a bottom end that can take some RPM the stock 715 will make more power above perhaps 6000 RPM.

Doug,

You mention a 600 cfm Holley swap for the 289.  Was this a common thing, or why ?   The reason I ask is that the original owner of my GT350 pulled the 715LM in mid - 1975 at the recommendation of his local Ford dealer in FL. and they installed a 600 double pumper. That Holley dated coded May of '75 is still on my car today.  It was rebuilt for the first time last month.  My plan was to have the orig. 715 rebuilt and put it back on. I'm wondering if I should just put it back in the box for safe keeping ?

Thanks,
Bob