[TR] Good tri-carb manifold?

Anthony Rhodes spamiam at comcast.net
Sun Apr 26 06:08:41 MDT 2009


Gary,

A friend of mine re-worked his engine and also installed the GP header.  He
had been told that 3 TR4-type early ZS carbs would be better than later ones.
He got them and installed them but had trouble getting a good tune for some
unknown reason.  He went with 3 later TR6 type ZS carbs with a "Jag" needle
and that has made him happy for several years.  He has done no advanced tuning
to improve the needle selection.  I think it runs a little rich.  He has been
using the stock springs in the carbs.

In my experience with carbs, I have 2 comments.  The first is about the
springs.  These stock springs are rated to  achieve full piston lift at the
maximum flow of a stock engine.  I.E. half of the full flow per carb.  The
flow of the engine is roughly proportional to HP and RPM.

So, lets say that you have a re-worked engine and achieve about 130% of the
original HP.  This means that you have about 130% of the flow.  Now divide
that by 3 carbs, and you get about 43% per carb.  The springs are rated for
50%, so each carb never sees the full amount of flow that its spring is rated
for.  It will not achieve full lift.  Since the carbs have a variable choke,
you will not achieve the goal of having that 150% of the original carb choke
area.

You need to use a different spring, and the exact spring strength will be
strongly influenced by the exact specifications of the engine.  Most likely it
needs to be about 43/50ths weaker, but if you have increased the engine output
by more than 50%, then maybe you need stronger springs!

Once you get the springs tuned, then you can work on the needle profile.  The
best way to do this is in a dyno shop with a good exhaust gas analyzer.
Preferably at the same time you can measure the carb piston lift so you can
determine exactly what part of the needle needs to be adjusted.

-Tony


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