This question was recently asked by a visitor to the site.
Because of the potential causes, and my lengthy response, I thought it might help others to have it posted here.
The condition is that the engine in a 1969 Austin America with Automatic transmission
will not idle at below 1,500rpm. In fact, it needs to have the choke pulled out to even run at that speed.
Additional information is that the carburetor has just been rebuilt, and the emissions
gear, ie., Smog pump, gulp valve, cylinder head air pipes and check valves, are all in place and believed to be working.
As indicated by the need for choke, ie., more fuel at idle,
there is infact "un-metered" air entering the engine.
There are several sources to look at for the cause of this
extra air that the carb doesn't know about.
Before we get into those sources I'll share my method for
finding "external" vacuum leaks. It's important to search for external vacuum leaks first because we want to rule
out as many "simple" causes as possible before continuing on to look at more complex causes of this running problme.
With the engine idling I spray the intake areas with starting
fluid. I spray one specific area at a time, then I wait for a few seconds to see what the engine does. Any change
in the running indicates the location of a leak has just been sprayed.
One more basic thing to look at before moving on is the fact
that the other common cause for an engine that won't idle is having the timing too advanced. If you have an engine that only
idles at 1,500rpm, it's pretty difficult to set the timing correctly, since the timing needs to be set at about 600-700rpm.
In this specific case, where the choke also needs to be operated, I'd say we can safely rule out a timing issue right off
the bat. However, timing is basic to engine running, and we need to cover the basics first.
Now on to the more substantial items and diagnostics:
The emissions system is having internal problems.
The way to test is to disconnect the pipe from the gulp valve
to the intake manifold and plug the port on the intake. This includes disconnecting the vacuum line from the intake
to the diaphram on the gulp valve. If the running changes, you know where your problem is.
Next on the list is the crankcase ventilation system.
It operates under some fairly tight constraints and if these
are not operating correctly, you will get alot of unmetered air into the intake manifold.
Again, disconnecting the pipe from the Smith's PCV Valve
to the intake, and plugging the port on the intake will show where the problem is. The Smith's valve requires a good
and flexible diaphram,(these are still available for about $5 at various British parts places like http://www.mossmotors.com/ )and the correct "tension" spring, to function. It also must be clean inside, and they are typically not after
all these years. A metering valve on the inside of the Smith's Valve must be free to pulse back and forth against the
spring and the diaphram. Also, the oil filler cap must be of the vented style and it must be working properly.
It should have a 9/64th oriface to control the amount of air that's leaked into the engine.
Next item on the list is the "poppet" valve placed in the
top side of the carburetor's throttle plate. This spring loaded valve is designed to open during sudden deceleration
to prevent a huge discharge of hydrocarbons. The valves leak air when they get old. The fix here is to remove
the carb, undo the 2 screws on the throttle plate and slide the throttle plate out. Either replace the throttle plate
with a readily available solid one, or solder the poppet valve closed and trim off it's stem and spring....or remove it completely,
lay the throttle plate on a piece of wood, and fill the opening with solder. I've done all 3. It's actually a
performance increase to do this, especially if you put in a solid plate...because you are getting that valve and its guts
out of the air flow.
Hopefully this will be of some help.
I've just heard back about the resolution to this problem. The owner of the car
found the following things:
- Smiths PCV Valve had a torn/leaking rubber diaphram
- Ignition timing was very retarded
- Distributor vacuum advance diaphram was torn and wasn't working
- Wrong jet tube fitted to the carburetor
- Removall of the smog equipment improved running
The car now idles smoothly at 850rpm and runs like a top.