The pistons provide 2 functions; they allow more air into the engine on an on
demand basis, and they also allow more fuel in by raising the needle at the
same time. When they work properly, they work great. The oil in the dashpot
is there merely for damping the piston, otherwise when the car hits a pothole
the dashpot pistons bounce all over changing engine speed when you don't want
it to. For example, if you are taking a fast corner and hit a bump with no
oil in the dashpots, it is possible that the dashpot would bounce open, and
increase engine rpm at the exact moment when you don't want that to happen.
This is why I say that a thick oil is not necessary. The thick oil
provides great damping, but it gets too thick when cold. On a cold morning
when you start the car, step on the gas and if there is that hollow sound and
no instant increase in rpm, it is because the oil in the dashpots is so thick
that the pistons can't move quickly enough. That's why I use a thinner oil.
The thin oil has enough damping and allows quicker throttle response, by
allowing the piston to rise more quickly. But, this is just my
experience...all British cars are different.
Cheers,
Chris
BJ8...many,many pieces
In a message dated 98-12-17 07:29:03 EST, schooler@erols.com writes:
<< Chris,
Could you explain a bit more about how and why the viscosity of the oil
used in the dampers has anything whatever to do with "throttle
response"? My understanding (perhaps mistaken?) is that the function of
the oil in the carb dampers is to provide for mixture enrichment upon
throttle opening (an accelerator pump function) and that the the height
of the piston in the SU carb affects only the mixture, not the amount of
air/fuel getting into the engine.
(I did switch to 5W-30 when operating an MGB in Montana and North Dakota
winters, but have never sensed the need to use anything of lesser
viscosity in the dampers.)
>>
|