[TR] TR3 SU Tuning Problems

[Randall]

> I blew out the Grose jets with compressed air.

Possibly I am in the minority; but my first step would be to throw the Grose
jets as far as I could, and install some original-type float valves.  Even
the original, all-brass valves were much less trouble in my experience than
the "Grose Jets"; and current supply valves almost always have the Viton
tips that IMO make them the best possible solution.

> I also pulled the domes and pistons

Did you check that they are properly matched?  Mixing pistons and domes can
cause some strange problems; the side with the loose fit will run rich while
the side with the tight fit may run either rich or lean, depending on which
way the wind is blowing in Tibet.

It's also important to check that there is NO dragging between piston and
dome, which can be caused by, for example, dropping a dome while it's off
the car.  Even a .001" dent, barely enough to be visible, can cause mixture
problems.

> What really perplexes me is if I
> lift the rear piston up abruptly

"Abruptly" should be impossible, as the damper should be fighting you and
forcing the piston to rise slowly.  Are your dampers not working?

> the engine stumbles and then begins revving.

That would seem to indicate that the other (ie front) carb is way too rich;
or one of the carbs is simply dumping raw fuel into the throat regardless of
venture depression.

> If I lift the front piston up abruptly, nothing happens.

That sounds like the synchronization is way out.  If you stick your head
down near each carb (or use a length of rubber hose), do they hiss about
equally?

> It appears
> that the jet bearing body is spinning
>  in the carb housing and I can't tighten the nut up on it. Any way around
> this?

The most common cause of that is that the nut was tightened all the way, and
then forced further.  However, it might also be a sign that the big nut that
holds the entire jet assembly to the carb is loose; or that the jet is
improperly assembled.  Also possible that the threads are gummed up or
damaged.

Once it starts to turn though, I think you are stuck removing the jet
assembly to unstuck the threads, replace the copper washers that lock the
body in place and so on.  Might as well change the cork (or rubber) O-rings
that the jet moves in while you are in there, since a leaking upper seal
might well be the source of your problems.

I would also be tempted to replace the jets as well, since I have seen
several cases where the precision bore inside the jet was worn (due to
improper needle centering) which caused severe mixture problems (including
rich idle and persistent overheating at speed).  It's been a few decades
ago, but I once spent my winter vacation laying on my back in the snow,
trying to resuscitate a TR3A engine that had scattered bits all over the
road.  I believe now that "root cause" was worn carb jets.

> When the
> mixture nut is in (spring compressed) then the mixture should be lean,
> right?

Yes, assuming the jet head is tight against the nut.  It's not uncommon for
them to stick down (after using the choke), which makes the nut position
irrelevant.

Note that some new jets need to be polished before installation.  The
originals were supplied with a very smooth surface, but replacements are
being left dull (which means more friction against the seals with associated
problems).

> If I can't come up with a
> quick fix, I guess that I'll have to do a complete tune-up with ignition,
> plugs, valve clearances and and rebuilt jet assemblies.     Any ideas?

20,000 miles is actually pretty good on the original cork seals, IMO.  Mine
would generally start leaking within 2-3 years of daily driving; before I
switched to the rubber O-rings from Moss.

Valve clearances can also have a profound effect on idle mixture, so I would
check them first.  Likewise, weak ignition can masquerade as poor mixture
(and 20,000 miles is way too long for a set of points).

HTH
Randall