For those of you have taken an interest in my rich running
spitfire, I think I have found the source of the problems. The
engine was modified with higher compression pistons and a header.
I was tipped off by another sol'er who said his carb started to
run rich when he installed a header. So I pulled the needle out
of the carb and measured it with micrometer. For comparison sake,
I took a needle out of a 76 spitfire cd carb (which had high
compression pistons) and measured it too. Sure enough, the profile
was fatter in general. I installed the fatter one and I was able
to set the mixture properly at idle. I'm still not convinced the
mixture is right all through the engine's operating range since
I haven't got a nice grey color to the tailpipe. I haven't driven
out on the highway yet, so I can't say for sure. Around town
driving with all its starts and stops tends to blacken up the
tailpipe.
We often discuss the need to change the needle profile in
the carbs, esp. on the su's. I have often wondered why this is
necessary. Strictly speaking, a carburetor is mass flow measuring
device. Given a certain amount of air entering the carb, it mixes
in the right amount of fuel. Now that shouldn't change from engine
to engine. In reality there are some factors which do change this.
I really like to know what they are. I suspect its all related to
the magnitude of the total pressures and pressure ratio across the
carb. Any ideas out there? I think I might spend some time
looking into this question.
woodruff@caen.engin.umich.edu
|