Need some real world advice on jetting carburetors for the mountains I'm driving my Massachusetts car to Colorado and Utah and will be spending two weeks driving in mountains. I'm thinking of using approximately 6000 feet as a baseline. I plan on going over mountain passes in excess of 10,000 ft Car is a 78 308 GTS with carburetors, USA version (mild cams) but with exhaust stuff removed. Rebuilt engine, XDI electronic ignition, gutted airbox, K & N air filter and free flow exhaust with Magnaflow mufflers. Sea level jets are as follows: idle-57 main-135 air corrector-190 Any suggestions are appreciated.
Having gone through this myself, I would say you won't able to drop down on your idles. You would go lean on the upper end of idles before the mains kick in. 2000-2500 RPM. Maybe go down one on the mains. One up one or two on the air corrector. You have one carb barrel per cylinder, so any change in jet size is quite a large change in fuel flow in the respective rpm and throttle range. Down at Denver altitudes, your jets would work, maybe just a bit rich. Climbing up, you will need to drop some fuel flow. Sounds like you are moving more air through the engine than stock. If your jets are stock, you might already be a tad lean. Only my 2 cents Bill
You'll also have to screw in (more air) the idle speed screws. Your idle RPM will come down as the air gets thinner. Bill
Disclaimer: I have no experience with 308 carbs but LOTS of experience with many different carbureted cars running from seal level to high altitude, Porsche, prewar, Brass Era, Ferrari, 50's 60' American, etc. I would not even consider playing with jets unless you live in the mountains and never go down, or you are experiencing undriveable performance. You will run a little rich, but just be sure to run it hard in case the plugs want to foul, and if they do you can always do the roadside coil arc trick...works every time, but I doubt you will need to do it. That said, the first thing you may encounter is a stumble when starting from a stop, this is typically caused by too much accelerator pump volume, this condition will be exacerbated by the warm dry conditions found in the western mountains and plateau's. Depending on ease of access, this can be a roadside adjustment, just reduce the pump volume a little at a time on each carb. until the stumble goes away. As far as the idle adjustments, Dogdish mentions your idle may be down, so you need more air, and I agree you may want to adjust your idle stop screws, and as a last resort your idle mixture screws....last resort. Consider this, these cars were designed to cope with mountainous regions (the Alps, etc) I seriously doubt you will need to adjust anything for such a short stay and climbing high passes but it can't hurt to be prepared. I regularly drive a much older, multi-carb Ferrari up and down a hot mountain from sea level to 7,500 feet, and experience almost no drivability problems except having to slow down when I see a cop. You are going to have a blast driving that car out West. Be prepared to really use the cars performance. Blue Skies, Tom in SoCal.
