I would like to know if these Vacu-jet choke slide carburetor are prone to hard starting?
G'day
Beantin,
AVB and
GadgeI don't have too much to offer here, but recent posts have made me think.
The
Vacu-Jet was Brigg's most
simple carby design.
It did not use a diaphragm fuel pump (Pulsa-Jet) or a small secondary fuel tank
as in an automotive style job with a small fuel bowl (Flo-Jet).
It did not have gravity feed - instead, the tank was located
below the carby!
This would always be a challenge. Even atmospheric pressure at a location
played a significant role.
Vacu-Jets relied on a sound mechanical pump to do the job.
That pump, here, was the cylinder and pistons and rings. It also required good
condition of valves and valve clearances.
The challenge was not to create too rich or too lean a mixture, and ensure
fresh fuel was used.
In terms of starting, the design was compromised. I guess that's why the
designed was abandoned on all but the smallest of Briggs engines.
It's not used on modern Briggs engines of any size.
In service, I noted that we sold quite a few of these carbies.
The main reason was wear on the throttle body and carby housing,
brought about by poor air-cleaner service. More common, but less
expensive, were blocked emulsion tubes.
In early autos, vacu-jet designs were common, but assistance was
given by mechanical fuel tank pressurisation (given the tank sat lower
than the carby height in the chassis.
A similar system was offered on the Villiers used on the Qualcast P1 ...
But that primer was never up-to-the-job and a calibrated mixture
adjustment was used for different weather conditions.
Ultimately, Villiers abandoned the idea.
Briggs ran with the idea - but only on their smallest of engines.
A small fuel tank was obligatory - given it sat below the carby.
Why did they do this? ANS: the tank had to be shallow for vacuum
to suck up fuel, and to make for a compact engine.
I would persevere with the original engine provided it is in sound condition -
cylinder, piston, rings and valves.
Ensure no leakages that would create lean start-up mixtures.
The obvious ones - gaskets and throttle butterfly wear.
Turn the aircleaner around to its
correct position.
This avoids exhaust gas interference with the air-intake.
Consider fitting Magnetron electron ignition - the best for
ignition of lean mixtures.
Vacu-Jets should not be considered 'hard-to start'.
But they're not a first-pull design.
Hope this helps.------------------------------
Jackp.s. interesting question. Got me thinking
Sorry for long-winded reply.
