> From: jbonina@nectech.com
> To: jmwagner@greenheart.com; jonmac <jonmac@ndirect.co.uk>
> Cc: triumphs@autox.team.net
> Subject: Re: SU carbs
> Date: 14 April 1998 20:53
>
>
>
> John, I'm no expert at anything LBC (just see my previous posts),
Don't do yourself down! You've got an LBC - ergo, skill is at an emergent
to advanced level
> thought you were supposed to fill the carb dampers with a lightweight
oil; >like a 20W or even Marvel Mystery oil. To fill them with "regular
engine oil" >when you are running a 20W50 might bee too thick; no? Please
enlighten >me.
I saw the threads some days back about oil weights - but was too busy doing
other things to put in my views. I guess opinion will be varied but all the
SU's I've come across - several thousand by now - have worked quite happily
with a 10W40 or 20W50 oil in the dashpots. What many people seem to think
is that the oil should be the lightest weight possible "so that easy
vertical movement of the dashpot piston is assured." In reality, it's quite
important for there to be a meaningful level of resistance in the dashpot
and this is achieved by the viscosity of oil inside. I don't have any of my
reference books to hand to double-check, but I'll check out some different
books in the Reading room at the weekend to see what they recommend.
However, for what I guess must be getting on from the year dot, engine oil
has been a common product to safely use in this carb. For years, we've
known the SU in the UK as a Suck Up rather than a Blow Up.
You'll tend to get a 'blow' effect if the oil is too lightweight. Oil of
the engine type also prevents piston flutter which no-one needs on any SU
as it accelerates wear on the jet and the needle. If you look at an SU
on any PRE-war car, you'll notice its shape and general mode of operation
has changed hardly at all. The dashpot top is often a bit taller and the
venturi is slightly bell-mouthed to achieve a sort of ram effect. Hardly
any of them had any kind of filter. Air just went straight in!
I'm virtually 100% certain they used engine lube pre-war and oils of that
epoch certainly had nothing like the properties we know today. They were
sticky, heavy and slow to flow when warm.
Additionally, if you look at an inlet manifold pre-war, the SU of those
days was sitting way out on its own with zero benefit from water heating
and lots of (too much?) air cooling, so what chance did the oil have to
improve its flow characteristics for improved damping? The only 'rider'
which might be relevant here is that as US anti-smog regulations bit harder
and harder, it might have been seen appropriate to use a lighter oil - but
that's a total guesstimate on my part and I've got no prior knowledge as to
why.
Suffice it to say, I've used straight engine oil in dashpots all my life
since I've been working on them and never had any trouble or problems of
any sort. Its my guess that every garage mechanic worth his salt this side
of the pond has done the same. I nurse a sneaking notion as well,
that lube sold in SU shaped pots is a brilliant marketing gimmick for the
unaware or ill-informed - and just sells lots of SU shaped pots and quite a
lot (probably (?)) of nicely coloured oil "of the type you have to use for
optimum performance." I wish I'd thought of it myself.
Try the straight stuff and see what happens. You won't do it any harm and
if anyone thinks Strombergs are a case needing special treatment, the
answer is the same.
John Mac
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