Carburetion at altitude

[67_6cyl]

My engine should be back from the shop any day now, and the last components for startup are starting to arrive.

Several questions:

Any tips on breaking it in, other than what I've read in the various manuals?

Is it advisable to have the rebuild shop do the initial run in? In an ideal world, it would be, but in this one it costs more money.

I've purchased a 32/36 from Stovebolt - I'm quite confident I can get it running - I've read all the threads on these and I haven't seen too much that scares me, but I'll play that by ear. The main concern I have with running a Carburetor in Denver is that it is at altitude (5300ft) and the air is quite a bit thinner up here. Dyno tests show ~26% drop in horsepower compared to being at sea-level so it's not insignificant.
I will need to rejet the carburetor to lean out the mixture, but by how many sizes? From what I've read, the A/F ratio will be around 11:1 at this altitude which is too rich for day-day driving. Any thoughts??? Stovebolt Tom figures a 0.1mm decrease in primary jet size will do it. What about the secondary jet size?

 

[Lazy JW]

... Stovebolt Tom figures a 0.1mm decrease in primary jet size will do it. What about the secondary jet size?

Go with his recommendation to begin with, then work from there. The secondary will need changed by a proportional amount.

You can run more compression at high altitudes but it's probably too late to change anything now.
Have fun,
Joe

 

[Harte3]

Some shops routinely do a run in as part of the rebuild. Unless it is an expensive option it might not be a bad idea.

 

[67_6cyl]

Thanks.

I think I'll stick with the stock jetting. I found a rule of thumb on the web that you need to decrease jet size by 0.05mm on the Weber carbs for every 5000' of altitude. In the Weber jetting thread on this site, it indicates that the jet sizes should be increased somewhat for our larger engines - this is similar to other articles I read on optimizing power. The stock jets put that size right in the middle so if it's off, it won't be by much.

I think I will get the shop to do the run in. All this stuff is new to me and there's certain things I don't want to learn the hard way.

Cheers