On Fri, 30 Sep 1994, Zahid Ahsanullah wrote:
> My 1980 Spitfire 1500 still has blowby after the rebuild. It could be
> because I lapped the valves instead of machining it.
Leaking valves don't cause blowbye. Blowbye is combustion gasses escaping
into the crankcase past the piston rings. If you still have blowbye after
a rebuild, then the rings are not seating correctly against the cylinder
walls. There can be many reasons for this, all of which point to a poorly
done rebuild.
> Usually cars have
> some sort of crankcase venting that burns the secondary mess through
> the carburettor, one comes from the valve cover and the other through
> the bottom of the engine block and feed through the carb. But mine
> has only the valve cover venting to relieve the backpressure.
That's not an unusual arrangement.
> If I
> put the oil cap on, the pressure through the crankcase causes oil to
> spew out of every nook and cranny including the dadgum dipstick hole
> placed next to the exhaust causing a smoky mess at high rpm.
> You usually figure out your rpm is high when people start honking behind
> because the smoke is causing them to gag.
Sounds like you have a _serious_ crankcase pressure problem!
> One of the main causes of this
> backpressure is that I have Webers and the large venturi is, as far
> as the engine is concerned, uncool.
Sorry, the carburetters have nothing to do with excessive crankcase pressure.
> I do have the valve cover vented
> to the carburettor but it is not enough to relieve the pressure.
> So my question is, how do get rid of this backpressure and how come
> the bottom of the block does not have some form of venting?
The kind of oil spewing you've described indicates crankcase pressures
that _no_ reasonable amount of venting can handle. You need to stop
worrying about whether the Triumph designers left out a vent and start
worrying about how to get your money back from who ever rebuilt the
engine. Your problem isn't bad vents, lapped valves, or oversized carbs,
it's combustion gasses escaping into the crankcase. You need to have the
engine torn down and rebuilt again. I'm sure that's not what you want to
hear, but there it is. It always takes new piston rings a while to seat
in, but even so you should not be getting that much blowbye. Someting is
wrong inside your engine, and the only way to find out what is to
disassemble the engine again.
> The pistons
> do create some pressure in the crankcase because of the downward motion
> that must be vented.
That's true in a single cylinder engine or in a multi-cylinder inline
engine with an odd number of cylinders (3, 5, 7, or whatever). It's also
true in some V-block engines, depending on how the cranksaft throws are
arranged. But in an inline engine with an even number of cylinders (2, 4,
6, etc.) like your Spitfire's, it's not a problem because for every
piston moving up there is another one moving down. They balance out.
Stop looking for design flaws and external causes. They aren't causing
your problem.
-----------------------------------------------------------------------
Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO (daily transportation)
feold@umd5.umd.edu
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