Incidentally to all these discussion about modifying carburetors, I have
noticed that many people drill out jets using standard number-sized drills.
I used to do a lot of motorcycle tuning, and I found that it was much better
to use a reamer. I have a set of very small, graduated size, very slowly
tapering reamers that I bought in the dim past--as I recall they were sold
by Sudco (the US Mikuni importer) for the purpose of fine-tuning jets to
intermediate sizes. These leave holes with smooth sides vs. the inevitable
grooving that happens with drills. I found that drilled jets gave
inconsistent mixtures that were easy to isolate to the jets on
multi-cylinder bikes. If I drilled the jets for all four carbs of a Honda 4,
the variation in mixture cylinder-to-cylinder was extreme. Swapping the jets
from one cylinder to another caused the mixture variation to move as
well--even when all four jets were carefully drilled with the same drill.
With the tapered reamer, I would mark the depth I reamed the first jet to,
and then ream all the others to the same point. Then go back for a fine cut
in reverse order just to be tweaky. That way yielded no variation cylinder
to cylinder.
Perhaps cars, with their long intake manifolds and much larger jets, are not
as sensitive.
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