Altitude Rejeting on Zenith carbs
- Thread starter RIQUE
- Start date
decoupe
(deceased)
A starting point.
Wow, 11,000 feet. Be prepared for a significant loss of performance no matter how well you get the tuning dialed in. There is a noticeable loss of performance with increasing altitude that nothing but a change to fuel injection, or even better, forced induction will ever make up. But you can at least aim for good operation.
Bear in mind that I have down draft webers (and know nothing about Zeniths) but this will give you an indication. Engine modifacations and condition also make a difference in what you will find works best.
I'm at 4000' and drive between 2500' - 6000' as the range of operation. From sea level jetting, I went down one size on the fuel jet (145 - 140) and up 6 sizes (185 -215) on the air jets. My idle decreased one size from 55 to 50. My fuel/air ratio at 3500' is 12.5+/- so I am pretty rich at higher altitudes as the air gets thinner. All of this was done on a dyno with a wide band gas analyzer up the exhaust so I think it is pretty close.
I'm going to add a Fuel/Air mix unit to the car this summer and would strongly advise anyone going to the outer limits of carburetor operation to get one or you will never get it right. The link is below.
http://www.wmsracing.com/wmsweb/wideband.htm
Good luck and hope this helps.
Wow, 11,000 feet. Be prepared for a significant loss of performance no matter how well you get the tuning dialed in. There is a noticeable loss of performance with increasing altitude that nothing but a change to fuel injection, or even better, forced induction will ever make up. But you can at least aim for good operation.
Bear in mind that I have down draft webers (and know nothing about Zeniths) but this will give you an indication. Engine modifacations and condition also make a difference in what you will find works best.
I'm at 4000' and drive between 2500' - 6000' as the range of operation. From sea level jetting, I went down one size on the fuel jet (145 - 140) and up 6 sizes (185 -215) on the air jets. My idle decreased one size from 55 to 50. My fuel/air ratio at 3500' is 12.5+/- so I am pretty rich at higher altitudes as the air gets thinner. All of this was done on a dyno with a wide band gas analyzer up the exhaust so I think it is pretty close.
I'm going to add a Fuel/Air mix unit to the car this summer and would strongly advise anyone going to the outer limits of carburetor operation to get one or you will never get it right. The link is below.
http://www.wmsracing.com/wmsweb/wideband.htm
Good luck and hope this helps.
decoupe
(deceased)
The 38/38 downdrafts are different - you're idling on both throttles at the same so you you can't syncronize them - they are mechanically linked. You use the funky rubber gizomo to try and balence the draw in a twin carb set up so they pull equal volumes and are opening at the same rate.