[Spridgets] Rejetting carbs for high altitude

[WeslakeMonza1330 at aol.com]

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