[Spridgets] Rejetting carbs for high altitude
WeslakeMonza1330 at aol.com
WeslakeMonza1330 at aol.com
Mon Mar 23 12:53:10 MST 2009
Air fuel ratios are expressed as parts air to parts fuel. So 11 to 1 a/f is
richer than 14/4 or 13/1.
For an A series 11 to 1 a/f is about as rich as you go for full power. A
leaner mixture such as 12 to 1 a/f will generally produce less power.
For perspective for a car with a strangler choke like an SU the mixture
might be 5 to 1 a/f on a cold day.
Sounds like the Lotus likes a slightly leaner mixture than the A-series to
develop full power though the jetting figures you've provided sound similar to
a standard car only with bigger main venturi but without bigger jets to match
(though you don't say what the aux vent are and they can make a big
difference).
I haven't had time to research why a hotter plug would be needed and 2
grades hotter than a 6 is really hot. I'd add that if the existing plugs aren't
fouling on a rich mixture why would they foul after the mixture had been
leaned back to adjust for altitude. Too cold a plug might give you a misfire
whereas too hot a plug will melt a hole in your engine's pistons.
Weslake-Monza 1330
Your engine
In a message dated 23/03/2009 19:33:20 GMT Standard Time, breton48 at live.com
writes:
Thanks to Roan Soave and WeslakeMonza1330 for replying...
I could not remember if the ignition had to be retarded or advanced for high
altitude... Could you explain to me why it has to be advanced? I am running
at 25 degrees advanced at idle, 35 at 4000-45000 rpm. I'll try 28 and 38.
I know it's running too rich because I can see the "black soot" from the
side exhaust on the rear left fender! Did not used to do that... in fact,
the original setting by the engine rebuilder was on the lean side, and I
should have left it there. I had the car dynoed in San Diego, and it was
running somewhat rich after that, even at sea level. According to the dyno
graph, the air/fuel ratio varied from a low 11 to 1 at 3000 rpm to a high
14.4 at 2000 rpm - it was 13.1 at 6297 (max power with 115 hp at rear
wheels).
I am running NGK BCP 6ES plugs - will try two grades hooter for the same
plug.
I will buy an airflow meter... I can use it on another car too. Any
suggestions for a good one at a reasonable price?
Thanks again,
JLC
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