> My brothers TR3 is all back together now and he needs
> some help tuning his Webers. This is what he has
> TR4A motor (that's right it's a motor from a TR4)
> Big bore kit 2187cc
> headers
> Mallory dual point dist
> 40 DCOE Webers with
> F11 emulsion
> 155 air corrector
> 140 main
>
> The motor seems to run out of power at about 4200 rpm.
>My first suggestion is that you visit the library or call Classic
Motorbooks and get both copies of John Passini's Weber carb books. The
first is devoted to theory of operation, and the second is practical
application (My local library has an edition which combines both volumes
into one). He was the official guru of tuning Weber carbs on British
engines, and most people who know even a little about the beasties
learned it from his books--given the cost of jets, the book's well worth
the price.
> What's the problem?? Need different jets?Despite the fact that 45DCOEs are
>recommended for the TR4 engine, I had
an old set of 40DCOEs scrounged from an Alfa on my TR4 (in fact, these
carburetors are on the Lancia twin-cam engine I installed in my VW bus),
which also had a big-bore kit in it, and mine would rev to about 6600 rpm
fairly easily.
So the first question to ask (one which Passini would ask), is everything
else about the engine right? Have you checked the timing and verified
that the advance is working properly? Is the coil condition okay? Are
the points set correctly? Is the fuel pump getting fuel to the
carburetors at all speeds in adequate amounts?
Now, as for jetting, that depends upon your altitude, and I don't know
where you are, but you'll have to make corrections for altitude from what
I'm suggesting here, and remember that you don't mention the size of the
auxiliary venturii, so that's an issue, as well.
>From the setting you mention above, I'd say this engine is probably going
dead rich around the speed you suggest. Have you looked at the exhaust
when the engine went flat? Any black smoke? Given the modifications you
mention, if that's all the changes in the engine, over-richness on the
upper end is the problem, I think. I had much the same equipment on my
TR4 (but for single-point transistor electronic ignition) and for
ordinary day-to-day commuting, I used F16 emulsion tubes (more high
holes, which encourages air emulsifying in the upper range, I believe),
112 mains and 200 air correctors for normal driving (120 mains and 180
air correctors for brisker driving), and I think 35 pump jets.
Remember that the idle jets act as a high-speed fuel bleeds to prevent an
over-lean condition at the top-end when the settings for the rest of the
rpm range are correct, so big idle jets will over-richen the high-end
mixture, as well.
The general rule of thumb about mains and air correctors is: mains
affect the mixture throughout the rpm range. The air correctors have
their most significant effect at the upper end. Generally, emulsion
tubes with low holes richen the upper end mixture, and in combination
with too-small air correctors, will richen the upper end significantly.
And none of this will apply if the fuel pressure isn't correct, or the
float level is wrong.
> Any suggestions would be appreciated.
>
> Thanks
> You're welcome, Tommy.
"It's always, 'It's the carburettor wots wrong, guv.'" --John Passini
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