OK - got it running - well even!
A quick recap - third gear in the transmission failed last November, but
the car had otherwise been running pretty well, except for some vacuum
leaks on the carbs. Worked on the gearbox over the winter and replaced
the layshaft and bearings. While the engine was out, I repainted it
maroon (RustOleum Burgundy). I also tore down the carbs, re-bushed,
re-shafted and rebuilt them. The goal was to get the car running really
sweet for this driving season.
It was frustrating, then, to put the engine and gearbox back in the car
and have some teething problems - rough idle, cutting out, leaking
heater valve, sooty plugs (rich) - all things that I wasn't really
experiencing before (at least not this acutely). Especially
disheartening was the lumpy idle. I was counting on the new
damn-near-perfect carbs to idle beautifully - no air leaks now!
So - I spent some late night hours reading the archives here and on
mgcars.org.uk and went out to the garage today with a plan!
The biggest issue was the lumpy idle. Not a high idle like I was
experiencing last year - fixing the carb shafts - no leaks - seems to
have fixed that finally. Originally I set the carbs up on the bench by
turning the mixture nuts down 12 flats on each carb as I have been
taught. As it turns out, this is NOT the best way to begin. Today, I
used my digital vernier caliper to actually measure the distance from
the bridge to the top of the jet when the jet was fully home (up) in the
jet bearing. I set both sides to 0.10, ignoring the number of flats. I
cycled the choke many times and then rechecked to make sure they were
still the same - all good, so I put the dashpots back on and filled them
with MMO. (I had earlier checked the jets to be sure they were
centered, and they still are). I then went back and much more carefully
set the float arm fork heights to 7/16" using a drill bit, making very
sure that the flat part of the arms were flat and straight, and the fork
arms were evenly contoured. The end result (once I solved the ignition
problems) was that the car now idles well even at 800 rpm's and the
idle not is much smoother and sweeter - no loping. The plugs are all an
even tan color and they are now the same across the board whereas
before the fronts were pretty good and the rear plugs were very sooty
(even though they had been set "correctly" to twelve flats). I checked
the sync with the SU tuning bars, and it was spot on.
The issue here I think is that what you really, really want is for the
jets to be metering the same amount of fuel to each carb so they are
nice and balanced. Counting flats is very accurate (turning a measured
distance around a fine thread of a known pitch is how micrometers work
after all), but other things come into play - the accuracy of the carb
body casting, the compression of the cork gland nut, the length of the
jet itself, the thickness of the upper and lower jet bearing washers,
etc. If all these are identical, then counting flats gives you an
identical result on each carb. Measuring it directly and setting
accordingly seems like a much better method, though. I had read that
you are shooting for a distance "down" of between 0.062 and 0.120" -
depending on whom was doing the posting. Most often cited, though, was
0.072 to 0.075 which I think is actually a T series spec. I knew my
carbs were rich, though, and I measured them at about 0.12, so I
arbitrarily picked 0.10 as my target, and it seems to have worked out
great. Now that they are the same, I could adjust by counting the same
number of flats to enrichen or lean them out without having to take the
dash pots off and re-measure, provided I am careful to keep them the
same. If you don't have a caliper, cut and file a 1/16" notch in the
end of your SU carb wrench and you will have a handy special tool for
checking this setting.
The ignition problem was caused by the heater valve leak. The rotor and
cap were covered with glycol, and there was some glycol in the
distributor itself. Boy is that stuff hard to get off! I ended up
having to dunk the parts into my parts washer, scrub them, and then dry
them off. They still didn't work and the car would not start! After
letting them dry out for a while, though, the car finally fired and I
was able to set the timing. Odd thing - I set the timing to 10*BTDC
statically, and it was too advanced - I retarded the timing to more or
less where I had set it the last time and it was much better, so I think
I probably just did a bad job of lining up the mark on the crank pulley
- but I didn't go back under the car to check since setting it "by ear"
seemed to work fine. BTW - I repaired the heater valve by lopping it
off and tapping the base to accept a stanard 1/4" ball valve (see
Manchusen's Garage site for more on this). I had to disassemble it
again to add the teflon tape I had forgotten, but now the leaking is
banished.
The test drive with the wife was glorious...just around the neighborhood
and to the gas station to fill up - but what a huge difference! It is
so smooth now, it sounds great, shifts well, etc. etc. - very relieved
that I was able to get it sorted! It will be interesting next week when I
get my vacuum venturi spacers back to see if they actually are the same
by directly measuring vacuum! I bet they're damn close.
I did figure out that we had put the speedo cable on the transmission,
but never actually tightened up the fitting, so it was sort of just
hanging there in place. I just did that, so haven't actually checked
yet that the speedo now works - but it should.
Now I can go back to my list of little things to fix, since the big things seem to be done! I'll update them at my blog - http://mgarestoration.blogspot.com. I need to also update my "carb rebuilding" article!
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