View Full Version : What causes "run on"

04-27-2009, 09:29 PM
I think this is the right term. My TR4A has started doing this recently. I turn the key off, but the engine continues to sputter and burp for a few seconds before stopping. Is this a grounding problem?

04-27-2009, 09:33 PM
It's probably incandescence in one or more combustion chambers igniting the incoming charge. Sort of like a glow-plug diesel engine.
Maybe need one or more of new plugs, decarb, higher octane fuel, or you may be running a little lean. Has anything else changed recently?

04-27-2009, 10:10 PM
:whistle: :whMaybe vapor lock?.That will happen after a HOT long drive,And suddenly the engine shuts down.Maybe your missing the 'antirun on valve'I know some Triumph's have that valve to stop just whats your experiencing.But THEN i may be wrong on that assumption :wall:

04-27-2009, 10:39 PM
A 4A would not have an anti-runon solenoid; that was on later cars with vacuum retard.

High idle rpm will aggravate the problem, so make sure it idles slow. As Roger says, the basic problem is caused by combustion chamber deposits that get hot enough to light the fuel without a spark. Unfortunately, modern unleaded fuel leaves much harder deposits that generally can't be removed by all the "old time" tricks (like slowly pouring a cup of water down the carb with the engine at a high idle).

My TR3A would run on occasionally pretty much as long as I owned it. I just lived with the problem, by letting the clutch out just as the engine should come to a stop. With a little practice, it becomes smooth and automatic. And in 20 years and perhaps 200,000 miles, it never seemed to hurt anything.

04-28-2009, 04:19 PM

Totally with what Randall said, common problem, often not an easy solution, make sure car is in good tune RE timing and mixture, keep idle as low as it will go wth a steady idle, if it still happens kill it with the clutch--you can waste a lot of time and money chasing down this problem, or just take the ez solution.

04-29-2009, 08:27 AM
The British used to regularly "decoke" their engines. The buildup can also cause pinging, which can lead to burned pistons.

04-29-2009, 01:00 PM
randall is spot on, and since it has a carburator, the engine continues to suck in fuel. Add a couple of anti run on valves to cure if you must.
An anti run on valve lets fresh air directly into the intake manifold starving the engine of fuel that would otherwise be sucked in from the carb.
Ps-i think i will need these for my 10:1 engine since higher compression increases the likelyhood, we'll see.

04-29-2009, 01:20 PM
I used to have a lot of problems with that on old high compression VW engines. Usually going up to the next higher level octane and lowering my Idle would solve the issue. I'm also more familiar with this term as Dieseling, not run-on, as usually the reason it happens if for the exact reason diesel engines can run. Heat + Enough compression to continue to try and ignite anything the carb spits out. Again though, going up a notch in octane will lessen the likelyhood of dieseling, because as we all know, higher octane only means you can compress it more before it ignites without a spark.

Although, like everyone else said, if nothing has changed recently then you could be chasing your tail for a while.

05-02-2009, 01:53 PM
Back in the 60s, my TR4A did this a fair amount. I just lived with it; let the clutch out to stop it.

Even stranger, it sometimes lshowed the opposite problem: on several occasions I would turn on the ignition and the engine would start without the starter! I always assumed that it had stopped in just the right position with a charge of fuel in one cylinder, then turning on the starter created a spark, and away we went!

05-02-2009, 09:44 PM
Have fixed run on by replacing the SU carbie jets and needles, and re-setting the float levels, as rich running causes carbon build-up.

Using higher octane fuel lessens the problem.

Letting the engine idle for 30 seconds before switching off can work.

Sometimes fully opening the throttle after switching off gives a gulp of cool air that helps.