Michael D. Porter wrote:
>
> The headers, exhaust system, and electronic ignition should have little
> to do with the inability of the engine to idle properly. In truth, it
> has little to do with air flow "balance," as you say. The amount of
> air/fuel mixture entering the engine isn't affected by freedom of egress
> from the engine.
Right.
>
> As a means of tracking down the problem, I would suggest the following
> steps:
>
> 1) First, check the ignition advance and timing (dynamically, if
> possible). If the advance is too great, the idle will be too high.
Well, the timing could be high. I backed it off to about 12 BTDC or
so after I installed the electronic ignition in order to get the car
to run correctly. But isn't this irrelevant?
Shouldn't I be able to *shut down* the car with the
throttle stops, if there are no air leaks?
> 2) Visually inspect both carburetors to make sure that the choke is
> fully released on both--if the chokes are both completely shut, the
> choke stops will be completely free of the choke levers (about
> 3/32"-1/8" gap).
Done that.
If this is the case, disconnect the throttle shafts
> from one another. If the engine speed drops suddenly, this indicates
> that one throttle plate is hanging, and keeping the other open. Make
> sure that the throttle cable is loose, as well.
You know, you might be on to something here. On the front carb,
I had to attach another spring to the throttle arm -- I presumed that
it's own spring had weakened over time, but maybe it's hanging up.
>
> 3) With the throttle shafts completely loose from one another, push one
> closed, and hold it for a few seconds. If the engine speed drops, it's
> hanging up. If engine speed slowly picks up after it is released, then
> it's sprung in some way (shaft distorted, butterfly misaligned, etc.)
> and needs to be repaired. Do the same for the other shaft. Given the
> crank breather arrangement, it's very possible that there's considerable
> carbon deposition near the throttle plates from crankcase vapors, and
> that can cause sticky throttle plates.
I'll give this a try. What if the throttle on the front carb sticks? Then
what?
>
> 4) If these checks don't have any effect, check for vacuum leaks. There
> are a variety of ways to check--a little spray oil around likely places
> (throttle shaft openings in the carb bodies, around the manifold-to-carb
> and manifold-to-block joints),
I've done most of this, without finding a problem ...
or one can use an unlit propane torch,
> just cracked open (a vacuum leak will suck in the propane gas and raise
> the idle). Since you've replaced the exhaust manifold with a header,
> it's possible that the intake seal was disturbed when doing so.
Of course, I renewed
the manifold gasket when installing the header, and there is no leaking
at all from the header (thanks, Triumphtune), but this doesn't rule out
the intake seal. Also, do you mean I should crack open the propane torch,
and pass it by suspected points around the carbs
and manifold? BTW, is there any danger in this procedure? I suppose
not ...
>
> 5) Last, there can be other carburetor problems. A stuck temperature
> compensator can cause idle problems. A torn or tired bypass valve
> diaphragm can leak air, and that, combined with a worn jet or bent
> needle, can keep the idle too high, no matter the position of the
> throttle plates. A slightly bent needle can keep the piston from
> seating, and richen the mixture enough so that other air leakage behind
> the throttle plate can keep the idle too high.
Well, I did a minor rebuild on the carbs a month ago. New piston
diaphragms, decel valves and Grose jets. The needles are the same,
though ...
>
> Unfortunately, there are just enough things that can go wrong with the
> intake system, which can cause similar problems, that finding the actual
> cause can be a bit maddening. Checking for the above will eventually
> narrow down the cause(s) a bit. As with any older engine, the ultimate
> cause will likely be the sum of several small things, rather than one
> big thing.
>
> One thing to remember in all this--if all aspects of the fuel delivery
> system are good, air leaking behind the throttle plates leans the
> mixture, and causes stumbling at idle, for which one usually compensates
> by raising the idle speed if the cause is not detected. If the idle is
> still high after all the mechanical items have been checked and are
> found to be okay, then there is a combination of problems relating to
> excess fuel metering _and_ air leaks at or after the throttle plates.
>
> Hope that helps.
>
Boy, it sure does. That's a great assessment, Michael.
And since it might help someone else, I'm copying
this back to the list. That's an interesting point you make about excess
air making the idle stumble (which is not my problem). Thanks again.
--
Martin Secrest
73 GT6
72 TR6
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