Carb jetting for altitude

[Prescott]

Hi everyone,

I've got an all-original 1967 C10 with a 283/3 speed manual/rochester 2G manual choke #7027105. I live at 5500 feet of elevation, and my truck has always run rich. The previous owner had the carb professionally rebuilt 10 years ago, and it runs pretty good, aside from stinking like unburned fuel all the time (out the tail pipe, of course). I am planning to dismantle the carb, clean it out, adjust the float level, and re-jet smaller. I don't yet know what jet it has now, but it's supposed to have a .054" from the factory, so that's what I expect to find.

My question is this: What would be the right jet to order for 5500 feet? It would be nice to have a selection of jets, and try them all to see a happy spark plug (mine are black as hell now), but these buggers are $10 each, so I'd like to get it close on the first try.

[geezer#99]

Forget about the carb for now.
First is timing. Your rich running could be timing related.
Retarded timing can cause you to open the throttle at idle too far so that you're idling on the power circuit.
What's your initial timing?
Where's your vac advance hooked to?

[Prescott]

Good point. I'll admit I have not degreed the crank to be sure that the balancer has not precessed. Recently, I did install a pertronix and full ignition tune-up parts. I then checked the timing, and it was at 4 degrees. It wasn't acting like it, though, so I advanced it to show 8 degrees. This made a big improvement, and did not cause knocking or pinging, so I left it.

You are right to suggest starting with ignition. I have done that, but will admit that I am not convinced that my timing mark is telling me the truth. I guess my next step is to find true tdc by measuring the piston with a dial gauge. Then I can know the timing is correct. Do you think the 4 degree Chevy spec is best? Or should I try to advance it beyond that?

I do have vacuum advance hooked up (not during the timing process, though) to the stock location near the carburetor base.

I am, however, still looking for an approximate formula for altitude jetting, if one exists.

[geezer#99]

Easiest way to check your timing marks for accuracy is with a piston stop like this.
You can make one with an old sparkplug too.
http://www.summitracing.com/int/part...0189/overview/
At your altitude you should be able to run your initial up closer to 14 or 16 degrees or more.

Here's a link on jetting. One of many.
http://www.chevelles.com/forums/showthread.php?t=237406

[Fitz]

Originally Posted by Prescott

I am, however, still looking for an approximate formula for altitude jetting, if one exists.

I live in Tucson Arizona at 3000 ft above sea level. In the summer the density altitude is 6000 ft and in winter is about sea level so I re-jet my Holley four times a year to maintain performance at the dragstrip. For increasing density altitude I go DOWN one jet size for every 1000 feet of altitude so, if your carb was jetted for sea level and you went to 5000 ft go down 5 jet sizes. You didn't say what the humidity is in your area, but here in the Desert in summer it is non-existent, which has a definite effect on the way your engine will run. in more "normal" areas you might only drop three sizes instead of 5. Only way to be sure is buy a jet kit for your carb and start tuning it AFTER your timing and advance curve are set correctly.

[Prescott]

Thanks for all the great info. Looks like the piston stop is the way to go, just to make sure. I'll play around with timing settings between 6-12 degrees, and see where it's happy. Then I'll do some spark plug analysis. I really think I will be jetting smaller by 3 or 4 sizes, and see what that yields.

Humidity here in Prescott AZ varies between 60% in monsoon season down to 5% other times. I guess I'll be digging out a flight manual for the old density altitude chart, and see what the temps make it.