TR4/4A Pre Ignition Run On

[KVH]

Can I ask again: Assuming timing and points are correct, what could be wrong at the carbs to cause this? Too rich? Too lien?

Also, I'm drinking gas like crazy in this car. I seriously doubt I'm getting 16 miles a gallon. The gauge just sinks as I drive. The car has a performance cam from the PO, but nothing radical. It also has LSD differential and CV axles. Runs like a charm. Drinks gas and won't shut off until it rattles and chucks for a good spell (unless I pop the clutch which I hate doing).

Any thoughts or ideas?

 

[JerryVV]

Check your oil for a gas smell indicating a fuel pump leak into the crankcase, that can cause poor gas milage and will ruin your engine in no time.
Run-On can be caused by the carb throttle plates not fully closing. Also a hot spot from a gasket that is protruding into the combustion chamber. What bore is your engine? Do you know what gasket was used when the cam was installed? Too rich mixture will use a lot of gas but plugs should show to be very black and would foul quickly.

 

[TR3driver]

How's your idle rpm? My experience has been that just a fast idle can make them run-on at least part of the time. Carbon build-up can do it too.

My suggestion would be to focus on the poor fuel mileage first, then see what effect that has the run-on. Also, killing the engine with the clutch is a Good Thing, IMO; rather than letting it "rattle and chunk". The run-on is basically knock that is happening very close to TDC, which puts a tremendous strain on the pistons and bearings (kind of like lugging the engine in high gear). No need to "pop" the clutch, just let it out promptly. With a bit of practice, it becomes smooth and automatic.

As Jerry said, if the mixture was that far off, you should be noticing black smoke and soot on the plugs. Assuming that is not the case, (and the fuel diaphragm isn't leaking, which should dump fuel on the ground instead of into the crankcase), then I'd be looking at other things besides the carbs.

When you say "timing is correct", I assume you mean initial timing. Have you checked that both timing advance mechanisms (vacuum and centrifugal) are working?

You mention a performance cam; are you using the lash specifications for that cam? Setting the valves too tight can increase overlap, which causes the mixture (and idle quality) to do bad things near idle. Is the cam timing per manufacturer spec? Having it off by a tooth can produce an engine that seems to run fine, but always acts rich (or lean). Last time I worked on an engine with the cam timing off, it would literally foul the plugs in 40-50 miles, even with the carbs as lean as they would go.

Come to think of it, the last engine I had with a performance cam also had a pronounced tendency to run-on. But it wouldn't run at all below about 1000 rpm indicated and had a definite lope at 12-1400.

One last thought : What are you using for ignition? I ran an MSD box for awhile that would keep the engine running just on the current through the "ignition" light. They supplied a diode with the unit, that had to be wired into the light.