> I have the same problem with my 74 MGB also with HIF4s. I was about to
>purchase a Colortune to get the adjustment right, before I resorted to a carb
>rebuild. I also checked valves, ignition, etc. I thought the Colortune would
>do it since you and Scott seemed to have good luck with it initially.
We're still having good luck with the Colortune -- we're just having
bad luck with the carburetor. Killing the bearer of bad news and all
that... :-)
>I'm wondering about the worn throttle plates that Scott mentioned. Also Moss
>has a note in their catalog about those spring loaded valves (can't think of
>their real name now) in the plates themselves and recommends soldering them
>shut. Anyone done this?
Sort of -- that is, I've got cars that don't have those valves in them.
The valves are designed to lean out the intake charge when you get off
the gas suddenly. They're part of the pollution control system, designed
to avoid the burst of HC that happens when you hop off the gas and the
intake charge suddenly gets very rich when flow drops and pressure increases.
The only effect you should see on the car is that when you get off the gas,
the revs take a longer time to drop than in a car without them. They should
have no effect on the problem you mention.
>Mine actually runs real well on the road, just not at idle. It loads up,
>and the idle speed keeps dropping..
The small, still voice in the back of my head says "valve guides are worn"
but that's merely the worst-case. :-) Get a Colortune and lean out both
carbs to the correct blue, set the idle speed where you want it, and if you
still have this problem, try going one range hotter in the spark plugs.
Your car should use BP6ES plugs if you're using NGKs, I think that for them
a higher number means a hotter plug so try a BP7ES (or RES if you need
resistor plugs for your radio). Frobbing the plug heat range is an old
trick to handle fouling. A hotter plug means the tips stay hotter, which
will help burn off deposits that can foul the plugs. Watch temperature and
adjust timing if you get any problems with detonation, and use a fairly
high-octane gas to compensate for this. On a '74 with its low compression,
you shouldn't encounter any problems with detonation or dieseling but keep
an eye open for them.
Idle speed dropping could, however, mean that the bores of the carbs are
worn and change or leak more (or in this case less) at random. Note, however,
that I've experimentally observed a relationship between ignition components
and idle speed over temperature. My '71 MGB used to idle between 900 and 1100
RPM most of the time, but as it got warm the idle speed would rise as high
as 1400 RPM -- unless I pushed the clutch in, in which case it would drop to
800 or so. I replaced the entire ignition system (as previously described)
and now the idle is dead rock solid, clutch in or out, temperature notwith-
standing, at 900 RPM. I also noticed a highly unlikely but experimentally
observed fact: when I put in a weird (but newish-looking) rotor from a parts
car a few months back, the idle speed jumped 300 RPM.
What's that we're always on about, how cars are systems?
> Oh oh, I thought we were the pros. When a guy I work with says to find an
>expert MG mechanic, I keep telling him that it should be me eventually.
Sounds like I need to post the results of my visit with Charlie Rockwell...
--Scott
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