Burning exhaust valves - again!

[david.g.chapman]

I have been plagued with a recurring problem with my twin cam. Twice in the past I have had exhaust valves starting to leak, giving a severe loss in compression. The last time was about 10 months ago, and I had the stromberg head fitted with hardened valve seats (with a three angle cut), bronze guides and new valves to suit.

I had been trying to run-in my new piston rings, and was monitoring the compression every 250 miles. At 500 miles the compressions were all about 145, but even across the cylinders. Now, at 750 miles, number 4 has dropped to 125 psi. Taking the head off and filling the combustion chamber with petrol has revealed a leaking exhaust valve - on number 4 only! Valve clearances had been OK.

Has anyone any idea as to what could cause this persistent problem. It only seems to affect cylinders 1 and 4 - so far.

Dave Chapman.

[M100]

Are you using valves, springs and retainers of a known spec?

[david.g.chapman]

The work was done by a reputable ex Lotus engineer. He assured me that the valves, guides and valve seats were all suitable for unleaded fuel. The original springs and retainers were not replaced, and as far as I know, they are standard.
Other information: the engine has been running hot recently, with the temperature gauge climbing to 95 C under high speed running in 30 degree heat. I have no idea if this would affect the valves much. The mixture is not lean - plugs are mid brown colour, and the timing is OK. 12 degrees static before - not after! - (edited) TDC.

Dave Chapman.

[types26/36]

and the timing is OK (12 degrees static after TDC).
Dave Chapman.[/quote]

Hope that was a typo error .....should be 12 degrees Befor T.D.C. unless its a fedral and I think thats 5 degrees A.T.D.C. @ idle
Brian

[berni29]

That is a real bummer. I have no suggestions to make but I do feel for you. How could the timing be to blame? I have not checked mine since I got the car. I am sure that is is too retarded though. No pinking at all, and lack of urge. Checking that is next on the list............

Good luck getting to the bottom of it.

[type26owner]

Huh, where does any spark plug manufacturer claim the mixture can be determined by the color of the plug with absolute certainty? None that I'm aware of. There are plenty of symptoms which can be determined by reading a plug but the mixture is not one of those that can be identified as the only cause for each fault condition. If it were only that simple!

I wouldn't believe the bullshi++ers that post that type of techno crap on the web. It's wrong.

[david.g.chapman]

Thanks everyone for your replies - The only thing I can think of is that the valve was slightly bent by something - I did get the valves to touch the pistons while I was turning the engine over by hand (a spanner on the crank pully nut). This was done some weeks ago when I re-assembled the engine. Brain not in gear! There was no loss in compression at the time, but maybe a slight leak has grown....

Keith has a point about the mixture - I think the damage is being done at high speed, and any time at idle will mask the high speed plug colour. I did use a colourtune in the past to check the mixture - and it looked OK at 3000 rpm or so (a blue colour).

Can anyone re-assure me that it is NOT coolant overheating as a cause?

Dave Chapman.

[types26/36]

I know you said its converted to unleaded but just as a point of interest are you using one specific brand of unleaded or any at hand.
Brian

[david.g.chapman]

I just use any old 95 RON unleaded. The head is low compression (very low at the moment!) and I don't have any issues with pinking.

Dave Chapman

[rgh0]

Dave

Generally the exhaust valves dont bend its normally the inlet valves which have a more slender stem and bigger head that hits the piston first and also bends more easily.

If you have an exhaust valve burn out and stop sealing properly after 10 months 3 normal causes.

1. Incorrect ignition timing
2. Incorrect mixture (to lean)
3. Valve not seating reliably due to rebuild problem allowing leakage resulting in burning.

Causes 1 and 2 can also cause the engine to run hotter. Strombergs run lean typically and the Lotus Stromberg timing is aggressive with high advance at high revs so get things a little off and burning could occur.

The fact that its just no 4 could just be luck i.e. Its just the first to go and the rest will soon follow or it could be due to a specific problem relating to the mixture in that cylinder or the assembly of that valve.

Rohan