Thanks, Dattodude, Phunkdoktaspok and Maztec.
The basic problem is basically, right after starting
up, the idle speed seems OK. If I drive off the car at
speed, and I don't get held up much by traffic, it will
run fairly well too. However, if I leave it at idle right
after starting, or if I get held up by traffic after some
running, and the temperature gauge starts climbing
towards to the 2/3 or middle position, the erratic idle
starts to come in, with the engine sputtering a bit,
close to dying. It did die off once, but I managed to
start it back, though with the engine sputtering and
struggling to hold idle. I only managed to keep it from
dying again by pulling out the manual choke as a last
resort. I can't really figure out what's wrong, but think
it's the idle circuit in the carburetor.
Yes, I did have the mechanic tighten the manifold
nuts, suspecting a manifold leak too. The head
gasket has just been changed, valves decarbonized,
valve seals replaced and rocker clearances adjusted.
The mechanic is really scratching his head too.
(The mechanics over here aren't particularly skillful,
though.)
The ignition system was the first to be suspected.
I'm using a contact points-triggered electronic ignition
system made from a kit by Dick Smith Electronics,
Australia, originally designed by Silicon Chip magazine.
I tried switching back to the standard points ignition,
but it was the same. The contact points and condenser
have just been changed too.
Initially the vacuum advance pump at the distributor was
causing this rough idle problem too. What happened was
the original pump had given way, and the only replacements available here were generic "Century" brands made in Japan (or so the label said, but I suspect more likely from Taiwan). Well, this pump has a plastic adjuster at the rear for fine timing adjustments. The stock Nippondenso one was very well made and heat durable, but this replacement's adjuster was not, and virtually disintegrated from the heat after a few months. Once this happened, the timing went haywire, since this adjuster was at the end of a tensioner spring indenting against the pump's arm connected to the distributor's plate.
I had to buy another pump, same thing happened after
a few months again, before my brother-in-law finally fashioned
a copper adjuster and screwed it in. The ignition timing
at idle appears to be holding well since then.
The spark plugs, plug leads, ignition coil have all been
replaced. Fuel pump and filter have been changed too.
After the second carburetor change, the problem
appeared to go away after I replaced the mechanical fuel
pump, suspecting it didn't have enough pumping
capacity anymore. It's just over a year since, but I'm
wondering whether this pump is starting to give way too ?
There are lots of Taiwanese parts being passed off as
Japanese ones in Malaysia.
There is a quite a bit of discussion in the forum in the
Toyota Starlet website (
www.kp61.net), that the stock
Aisan carburetor is not very well designed and prone
to all kind of rough idle problems, virtually impossible
to correct with the idle mixture and idle speed screws.
The Starlet in the US has the same 1,290 cc, 4K engine
as my Corolla, and basically the same Aisan carburetor.
One owner complained his idle speed varied anywhere
from 700 to 1,000 rpm !
I had also read in the forum of a Suzuki jeep website
(can't really remember the URL), which also comes with
Aisan carburetors, that this is the case too. The owner who
made the post mentioned that the source of this rough
idle problem appeared to be a "power piston" in the idle
circuit. The function of this piston is to enrichen the
mixture during low vacuum conditions, such as idle.
If there are even micro-abrasions in its chamber,
possibly from wear and tear, the piston will behave
erratically, the mixture will become way to rich, and
the engine will likely sputter and die. This owner was
quite innovative - he managed to rebore the chamber
and replate the piston or something, and the idle was
fine after that.
This is one jinxed car, but I'll try checking out for things
like manifold leaks, igntion and fuel pump problems again.
Transfer
