>
> Marcus,
> I forgot which year 6 you have, but the vaccuum line should cause the timing
> to RETARD at idle. The other vaccuum line starts pulling vaccuum at
>increasing
> throttle loads, and supplies vaccuum to the advance unit. Sounds like you got
> the two lines crossed, so you're advancing at idle. But I think you need to
> fix the carbs first.
Umm, Marcus has a 74 MG Midget with a 1275 BMC A motor. All BMC engines
run a manifold vacuum driven advance unit. However, there is also a TR6
misfire discussion going on (have to dig into my notes to find who again).
> As you face the carbs from the RF wheel, on the right side of each carb
> close to the engine are the vaccuum bypass valves. They look like little
> models of Devil's Tower with 3 pozidriv and 3 flathead screws going into
> the carburettor. The function of these valves is to deliver
> more air to the engine under conditions of highest manifold vaccuum. That
> occurrs when you are at say, 6000 rpm and you let your foot completely off the
> gas. Since there's a lot of gas still in the manifold but little air flowing,
> the mixture becomes rich and you get backfiring. So this little valve senses
> high manifold vaccuum, and bypasses a little air around the throttle plate
> until things settle down a little. Sounds to me like you have a ripped or
> maladjusted diaphragm inside this valve, so you're letting air get around
> the throttle plate all the time. This would cause the idling and piston
> rise you describe. The diaphragms for these valves don't come in the
> rebuild kit, so call your favorite supplier. They're like 6 bucks apiece or
> so.
This applies to TR6 ZS carbs. Marcus' SU's have a spring loaded poppet valve
built into the butterfly that preforms the same function. This function is
to ensure that there is enough airflow to keep the cylinders firing. High
manifold vacuum means not enough air movement, which means not enough cylinder
filling to get combustion, which means that the air/fuel that did not burn in
the cylinder explodes when it gets to the hot exhaust manifold. So, yes, the
valve is to eliminate overrun backfiring. But it does not affect the mixture.
> Now would be a good time to adjust you Idle air compensator valves as well.
> These are located just outward from the vaccuum bypass valves, and look like
> pale yellow banannas? at least that's what I think they look like. Anyway,
> take them off the carb and remove the pale yellow cover. Inside you see a
> Nyloc nut holding down a bimetal flat spring. The idea here is that the
> bimetal spring heats up and lets a little air bypass the piston (and therefore
> the jet). This is when the car is at operating temperature. When the carbs
> are colder, the car runs just a little richer by itself, and this is what
> allows the TR-6 to idle somewhat normally even when the choke's in and the
> engine is not totally warm. Turn the valve over so that the black cone
> is protruding towards you from the aluminum valve body orifice. Now put your
> lips just over the top part of the aluminum orifice, and adjust the Nyloc nut
> on the other side until just the point when you can't blow air into the valve.
> When you reinstall the valve, make sure that the 2 o-rings that seal the valve
> body to the carb are in place. One usually stays inside the carb, and the
> other usually stays around the valve protrusion. Just make sure the one in
> the carb isn't cock-eyed.
These compensators are a ZS only invention, and are only used on cars where
the close proximity of HOT things (such as the TR6's unshielded exhaust
manifold) can cause abnormally high float bowl temperatures. The object is
to maintain a constant fuel mixture with the varying fuel viscosity caused by
the heat. Their adjustment is critical. I strongly recommend not messing with
them unless you're *sure* they're already screwed up (i.e. PO disaster)
BTW, on a properly engineered intake system, you should be able to totally
drop the choke by the time the engine temp hits 100 F. Contrary to normal
wisdom, an engine does not need a richer mixture when cold. You want about the
same mixture, so have to add a bunch of fuel to cover for the fuel that
condenses out of the air stream on the cold intake and cylinder walls. Todays
engines with port injection (and heated O2 sensors) are running at normal
mixture within 15 seconds of starting.
> If you have two vaccuum units on your dist, and two vaccuum lines coming from
> the carbs, you can determine which vaccum unit is which by turning the car off
> and removing the dist cap. Connect a small tube to a vac. unit, and suck on
> it. If the distributor plate moves counterclockwise, it's a retard unit.
> Connect the vaccuum lines appropriately and fire the car up. Again set your
> idle and time the car to 4 ATDC with the vaccuum retard line connected.
> Set your idle and sync the carbs again.
On a TR6, the advance can points forward, and the retard at the back.
Randy
randy@taylor.wyvern.com
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