checking tillison pop off pressure

[Mel Ruffner]

doug,
a bellied diaphram will mess up your track day.
the motor will start but it'll make you nuts.
carb will be all over the place in performance as the pulse signal gets too weak to allow enough vacumn from just the wasted diaphram.
because of the weak signal and insufficient fuel the driver opens the needles up to compensate.
in my mind that ads just enough of a vacumn pulse through the adjusting needle inlets to add a touch more pop off.
pop off gets more consistant - then you flood - then you thin the needles vacumn drops and start process all over again.
i noticed this after lots of practice with mis installed carbs covering the pulse hole.
i was still was able to run the motor for short times fiddling with the needles and choking but they'd never smooth out.
finding the covered pulse hole would fix the problem.
when i went to thinner diaphrams and they got belied i noticed a similar problem.
i thought about it and realized you can provide alternative vacumn path if you open the adjusting needles enough.

anyway that's my experience.

mel,
absolutely learn how to keep your pop off consistent.
that will require constant monitoring but it tells you alot about carb condition without dis-assembly.
as long as the carb reads the same as when you last did pop off you've eliminated a possible carb issue.
bad pop off can mimic worn out diaphrams and worn out fuel doors in you second plate gasket set.
if you get a bad reading assembled my next step is pull plates and see what it reads directly.
if it's back to a good reading the seat, fulcrum arm and spring are fine and you next should check the diaphram for excessive stiffness and or wrinkles.
if the diaphram seems good inspect the second plate wet/dry diaphram for a belly and/or the little doors standing off the plate thus not sealing well.
often if a tilly carb is mis behaving and the interior pop off is good i just chuck the internal gasket set and install new.
sometimes they look fine but are junk anyway.
they aren't a huge amount of money compared to a wasted week end.

if you haven't yet do take the flat side of the tilly plates and sand to flatness.
they almost always are not up to par as found especially the older carbs.
i also campher the pulse holes on the manifolds and the carbs.
check ever new gasket for pulse hole alignment.
i have a whole batch that are terrible and cover 1/3 of the pulse if you don't recut.

another issue you may find is the fuel inlet holes to be mis-cut and covering part of the passage.
that is also an issue with the gaskets sold today from some sources.

the biggest tilly gasket issue is in the fuel inlet gasket.
they were mis cut long ago and now the bad ones seem to be the standard.
one of the two fuel doors openings is too small so the door can't open completely.
this can be seen when you put the gasket directly on the plate where it covers metal.
on the fat one the extra material becomes apparent.
if it sounds like i figured something out i didn't there was a bulletin i read long ago about the issue but it's useful to know.

finally if you have a multiple part manifold that can possibly assemble wrong and cover pulse holes it's a good idea to idiot proof them.
it's extremely easy to do with gem manifolds as i can attest.
i've now  put little match arrows on the exterior of mine so i can check for mis assembley externally at a glance.  
Thanks Dave for that great responce, such great info. could reeds cause excess fuel problems?

Save
Save