Nailhead high altitude jet size?

[65nailhead]

Hello All,

I’m new here so this may have been covered. I have 65 Riviera with a 401.

I am currently rebuilding the original carburetor. It is a Rochester 4GC model 7025040. I recently moved to Wyoming and live at 5000’. The jet sizes are currently 52 primary and 80 secondary. I’m not sure if this is stock. The car came from the Central Valley in California. Maybe a few hundred feet above sea level. I know I need to drop jet sizes with altitude. Any help would be appreciated.

[JACK M]

I cannot help you with sizes, But....

I used to re-jet outboard motor carbs so boats would run better in the high lakes.

The most important thing is to remember to fatten it back up when you come to lesser altitudes.

Since the air is thinner at altitude you need to lean out the carburetor (smaller jets as you suggest).

If you try and drive down the mountain so to speak you will be running lean which is not good.

If you are NEVER going to leave that altitude, go down a number or two and read the spark plugs.

[joe_padavano]

52 primary and 80 secondary are the correct original jet sizes for your carb. I concur with Jack M. If you plan to drive at altitude only, I'd drop down one jet size on the primary side. Personally I'd leave the secondary jets untouched for now as a precaution. It's safer to be too rich than too lean. Read the plugs or get an A/F meter for finer tuning. Also be aware that if you use ethanol-laced gasoline, it's already leaner than the car was originally designed for, so you may not need to down-jet.