Mystery cause of sticking inlet valves on Mk II engine.

[paul.jameson]

Now that we are in to autumn I am managing to make some progress with the Healey at long last. Having cleaned out the valve guides using a combination of an adjustable reamer, cellulose thinners, a bottle brush and rag, I have fitted new inlet valves and re-assembled the head. Today I finally removed the cylinder to be surprised at what I found. The flywheels, cranks, rods, etc are sparkling clean, as you would expect but the pistons are covered in what looks like black paint! Whether the black "paint" has been in the ring grooves until I took the cylinder off I don't know, but it may well have been. The rings are all free. The cylinder bores have what appears to be a thin coating of the "laquer" as found in the valve guides on them.

I will clean pistons, rings and ring grooves and re-hone the bores before re-assembly.

[simon.holyfield]

So what is all that black goo, Paul?

[mark.prizeman]

Someone has dropped something into your tank....

Long ago i had to remove a custard cream tart from the tank of my AJS.

That's London for you!

[paul.jameson]

The "black paint" appears to be oil absolutely saturated with carbon. It comes off easily enough by wiping but there is a lot of it. I have honed the bores such that they now look ok and cleaned one piston after taking it off to make the task easier. Again, it was easy enough to clean and no worse to de-coke than you would expect of a normal piston. But the bottom half of the engine is pretty well spotless inside.

I think I had two problems, the first being the overheating of the oil leading to the gummed up valves. The second, I think, is rich mixture at minimal throttle openings, by which I mean closed to about 1/20 of full throttle. Given that the mixture on normal running showed perfection on the plugs and that the traffic in London had caused me to run at minimal throttle for some time, coupled with the fact that after starting the bike I could immediately take the choke fully off and ride up our 1 in 5 gradient drive, it seems to me that the SU needle in use is too rich at tickover. It is a standard GN needle but not shortened as I have recently discovered was specially done on the Ariel SU carbs. But the shortening would only affect the mixture at very high throttle openings. So I have purchased a copy of WinSU which contains a wealth of data on SU needles and will be trying out some alternative needles next year. Meanwhile, my aim is to get the bike running again before we get salt on the roads here which is likely to arrive in the next couple of weeks or so.

[MarkO]

I'd concur that the black residue on the pistons does look like overheated oil. Oil often goes hard, blacker and slightly sticky - a little like tar - if overheated. I suppose the obvious questions are

Have you used the correct oil grade? If so, why Is the engine running hot?
I wonder if the gummed up/sticking valves were/are the cause of the hot engine and the subsequent black residue ? In which case, a root cause could be an overly rich mix sooting them up in the guides or even the head to a point of inoperability.

[David Smith]

Paul
It might be an oil carburisation / burning problem if you are not using the oil cooler. Experience with racing Honda Cb77's in the IOM, notoriously hot running engines, was that even R40 castor oil was turned to lacquer and vinegar in 6 laps with high consumption without an oil cooler. Another lap would have killed it. Later experience with Mobil 1 Triumph10/40 oi (Not car stuff as it lacks extreme pressure additives req for racing cams at least I think and an oil cooler was that it was perfec, with no apparent wear or lacquering, with everything as clean as a whistle! The only trouble is any tired seals leak badly,

[Roger Gwynn]

In my experience, the SQ4 engine will become so hot that it will not run properly, particularly at low revs due to the fuel vaporising but by the time it is doing this the oil is still not at operating temperature as evidenced by a thermistor placed in the return line so measuring the temp of the oil as it leaves the engine. I had a reading of 90-100C. My conclusion was that the engine gets too hot and the oil not hot enough so little point in cooling the oil as it does little in the way of cooling. Of course that is not to say that a small quantity of the oil may overheat to the point of lacquering the engine.

[paul.jameson]

I finally had the Healey running again this afternoon. Having reached the point of putting the carb back on, the thought struck me that there is a known problem with the SU carb of a very rich mixture if the piston sticks in the dashpot. (Waller, 5th Edition, page 155). So I took the air filter off and found that the piston was indeed sticking. Dismantling showed something yellow, about 1/4" long and maybe 1/32" wide on the side of the dashpot. Baffled as to what it could be, I assumed it was something I had picked up riding through London. But then I found more of the same material and realised that it was yellow paint which had come off the piston return spring. This spring (new from Burlen when I built the bike in 2020) had yellow paint on one end, presumably for easy identification of which type of spring it was. The rest of the paint flaked off the spring easily enough which made me wonder whether it had been softened due to the use of fuel containing ethanol. So my advice to Mk II owners is to remove the paint before fitting the spring or if it won't come off, then put the end of the spring with the paint downwards into the piston. That way will probably minimise the chances of the paint getting into the dashpot and causing the piston to stick.

I shall test ride the bike, weather permitting, in the next day or two and then have a serious look at carburation, particularly at low throttle openings, which may well lead to a change of needle. Meanwhile, I am pretty confident that the very rich running I suffered in London was mainly down to the piston sticking in the carb.

[JohnnyBeckett]

hi are you using running in oil to bed the engine in

[paul.jameson]

Yes, having re-honed the bores seriously, I am using Running -in Oil for the next 500 miles, following which I will change to the oil recommended by Markus, which is here in stock.