Moss Motors, Ltd

I have a consistent problem with my 76 Spit 1500. Turning the ignition off after the engine is hot, e.g coming back from a drive, the engine will run on. It does this consistently. Have tried a number of suggestions made by other Triumph owners, including trying out different spark plug heat ranges, varied ignition timing as this could have an influence on engine temparature, etc. etc., to no avail.

My Spit runs on 2 SU HS4 carbs.

The only 2 things I haven't tried yet is 1.) re-installing the charcoal cannister and anti run-on valve, which was part of the original setup with Stromberg carbs, or 2.) install the oil cooler I bought a year ago, but never got around to install.

I have reached the conclusion that the engine is running hot, with enough of a residual head temperature to ignite the fuel/vapour drawn in with the throttle at idle.  

Has anyone experienced this problem and found a fix for it.

HV76,

The re-installation of the anti run-on valve is the sure fire (no pun intended) way to solve your problem. However, cars were "dieseling" long before these devices were ever employed. Usually, the problem is a carbon build-up in the combustion chamber, or some other hot spot providing a glow plug effect. Generally, just plain running hot won't do it, unless your running WAY HOT.

Two things that can contribute to the problem are, too high an idle and the lack of cool off time. Make sure you have tuned your idle as low as you can get it consistant with even running. Also, once the idle is reasonable, let the engine idle for a minute or so before shutting down. This often does the trick. You have a minute for a friend, don't you? Repost if none of this helps.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

It's carbon build-up and possibly low octane fuel. When I drove my 1965 TR4 from Cleveland to Boston and back in the late 1960's, the running on disappeared. The easiest way to stop it was to let out the clutch, but the effect is rather abrupt. Also, use the highest octane gas you can find.

I do not reccomend letting the clutch out with the car in gear to stop it. First, it does nothing to solve the problem. Second, it "torque shocks" everything in the drive train. Solve the problem. Don't make it worse by clobbering the symptoms.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Thanks for the quick feedback. I guess I'll hook up the anti run-on valve first, although I have to get creative with the plumbing, as the effect created by the activated valve is to create a vacuum at the top end of the SU fuel bowles, thus counteracting the engine vacuum, which is trying to suck mixture into the combustion chamber. The Zenith carb had a separate vacuum line connection for that purpose, whereas I'll have to hook it up to the SU's overflow.

I may not do anything for the next couple of month, as winter is going to be around for a while (Toronto), unless the garage get's warm enough before that.

Assuming that there is carbon build-up in the combustion chambers, is there an easy wasy to remove it with solvent or similar, or does the head have to come off? 

 

HV7,

There are several additives which claim to reduce carbon in the combustion chamber, but I have some doubt as to their effectiveness. One simple way to keep carbon from building up is to occaisionally spray water, yes water, into the carburetter mouths while holding the idle up just a tad. There is some controversy over this as some claim that if you have a lot of buildup and you break it loose all at once, you can damage valves. I can't really be sure. I haven't tried it in years. The procedure is probably quite safe if you start doing it regularly right after installing the head on a clean engine. No doubt the best way to solve the problem is to remove the head. It's not all that much work, actually,is it?

An alternative way to plumb an anti run-on valve was used on my 1971 Rover 2000TC. When the ignition was switched off with the engine running, an air valve opened into the intake manifold, thereby depriving the engine of a mixture rich enough to burn. The engine, thus starved, stopped. This allowed the oil pressure to drop to zero of course, which opened a set of contacts which closed the air valve. This reset the system, allowing the engine to be started the next time around. The valve had its own little air filter and a pretty good sized aperture which caused a curious sound in operation, sort of like a lion purring...

 

 

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Hello,

Maggie M,

Yes, it could be done, but you shouldn't need to do it and the sourcing/expense could be more than the proper cure. That cure is to do what the Brits used to refer to as a "decoke". This involves removing the cylinder head and cleaning all the carbon deposits out of the combustion chambers and off the piston crowns. This is also a good time to check valves, guides, etc. But first let's try fuel.

Wait until the tank is almost empty (I know, you're driving an MGA and the gauge is more of a prognosticator than an indicator), and fill 'er up with high test. Remember to  check the state of tune with respect to ignition timing and idle speed. Now, let's see if the high octane fuel improves the situation. Higher octane fuels, contrary to movie and tv news sensationalism, actually are less volatile and slower burning than "lower" grade gas. They are less likely to preignite and to respond to "hot spots" in the combustion chamber. In MGA and Earlier MGB engines, the combustion chamber in the head has a spot off to one side, between the valves where the cavity was machined to a sharp point. Often, this point gets really hot. If it has a bit of carbon built up on it, it will form a "glow plug" which will detonate the mixture in the chamber and propagate the dieseling effect. For higher performance applications, one of the first things we usually do is to round off this point. We gain back the lost compression by milling the gasket surface of the head and/or by using domed pistons. The pistons are, of course, very expensive custom parts. But, when you have the head off for decoking it may be smart to grind off this point, assuming it hasn't been done before. By being fairly conservative with the grinding you will not lose much compression, and a slight resurfacing of the head may be in order anyway. First try the high test and get back to us.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Thanks Motorbill,

Hi guys, I am back with my 76 Spit run on problem.

Took the following actions since I last signed on:

1. Installed the run on valve and did the plumbing as the shop manual shows for the a car with Strombergs carbs, as there is no schematic for one with SUs.

2. Installed an Oil Cooler

3. Tries a couple tanks full of premium gas

The problem is still there, altough the car runs a bit cooler due to the oil cooler installation. Although the Anti Run on valve is wired and plumbed properly, it doesn't appear to have any effect. I don't believe it creates the vacuum in the top of the fuel bowl, to offset gas being drawn through the jets. I did change the plumbing so that the valve would let the air into the manifold to screw up any combustible mixture, but that didn't work as the hose connection on the valve is only 1/4 of an inch, not enough air getting through.

Letting the car idle for a couple of minutes does work. I guess the only option left is to take the head off and clean chambers and piston tops.

 

 

 

My first post.  Hello.

Sounds like typical current leak situation from alternator due to a removed or dodgy regulator pack.

Usual trick is to put a strong diode in the circuit to prevent leak back which allows current to reach ignition system even when key is off.

 

The problem has been solved: Engine was simply running too lean. It took a lot of 'trying things', including suggestions from this forum.

It's  a rather interesting story how I ran into this problem in the first place. I bought the car with dual webbers on it, but decided to convert to a couple of SU HS4s, as the webbers were way too rich for my type of driving, and the result was usually fouled plugs once in a while.

After I converted to SUs, the run on problems started. When that happened I asked a british car mechanic about how to trouble shoot this problem, and his suggestion was to try a cooler plug, which I did. The runon stopped for a while, but I also noticed that the plugs were way too black. I gradually adjusted mixture to get the right plug color over time. What also happend was that the run on problem reappeared occasionally, which then led me to re-install the anti-run, which had been removed by the previous owner when he installed the webbers. I also added an oil cooler. Those two actions led to some improvement, but didn't fix the problem completely.

All along I had no idea that I was running lean, other than that I had to back off on the distributer advance somewhat to prevent pinging under load. Early fall I had an opportunity to put my car on a dyno to see what hp and torque it would show. It wasn't as good as I had expected. The technician pointed out that the engine was running lean. We adjusted the mixture a full turn, and HP came up. When I turned the engine off at home, guess what, no more run on. I drove the car for a few more days without doing any further fine tuning, and then I had a look at the plugs, and....they were black again.........All of a sudden a light came on. I had sent myself down the garden path by putting in cooler plugs. Went out the next day, bought the plugs the manual calls for, and haven't had a run on problem since.