Protecting exhaust valves

[david.g.chapman]

My +2 stromberg engine has had a history of burning its stainless steel exhaust valves - I had to take the head off last winter and replace valves 2 and 3 after 12000-15000 miles of use.

I run the engine as below:

Static timing is 10 degrees BTDC (increases to 20 degrees at 2000 rpm and 25 degrees at 3000 rpm)
Mixture is as rich as possible without fouling plugs 1 and 4 (2 and 3 run hotter). All plugs are NGK BPR6E.

Engine temp is levelled at 85-90 degrees by the fan when hot.

Valve clearances are nominal.

Valve timing should be OK - I use the Burton cam sprockets with both inlet and exhaust timing marks, and the marks line up well with the top of the head at TDC.

I run RON 95 unleaded fuel.

I rarely go over 5000 rpm.

Any ideas about what I can do to extend the life of the valves?

Cheers,

Dave Chapman

[worzel]

Hi

Get hold of Tetraboost- basically this is genuine Tetra Ethyl Lead so if you want you can make 5 star. Just google the name. Last time I bought some it was sold in minimum quantities of 8 bottles.
Dangerous stuff- it's carcinogenic so needs careful handling. Cost- about ?120 but this will cover 3-4000 miles in an elan based on average fuel consumption. I've used it for the past 9 years.

regards

John

[david.g.chapman]

Yes John - that might be a way forward. it would be nice to have that light grey exhaust again.

Shame it's so pricey, but over 3000 miles it is not so bad. You could miss out every other tankful as well, perhaps.

Any other thoughts?

Dave.

[rgh0]

The lead additive will assist in preventing pre-ignition and reduce valve and seat wear but this does not appear to be the major contributor to the problem at least from the description. If the exhaust valves are good quality stainless steel and actually suffering from burning / Oxidation then the exhaust has sufficient free oxygen in it to cause it and the carbs are to lean. I would run the carbs richer until the plugs 2 and 3 show some slight signs of carbon deposits. Plugs 1 and 4 may have more fouling but still should be OK

cheers
Rohan

[david.g.chapman]

Thanks for that Rohan.

I have been running valves from Burton marked REC 222 on the stem. I am assuming that the quality is OK.

My plugs when rich show a black sooty deposit. As I lean out the mixture the plugs start to self clean and the insulator turns clean and white. There is a brown stage with slight carbon deposition as the classic plug photos show, but it is over a narrow mixture range.

I think it is as Rohan writes, and I will have to put up with some fouling on 1 and 4 to get 2 and 3 into that brown range.

From other posts I believe the 2-3 to 1-4 mixture inbalance is typical on Stromberg engines - but please correct me if you think I am wrong.

I might toy with the idea of putting NGK heat range 5 into 1 and 4 to beat the fouling and run plenty rich!

Dave.

[gus]

I have never burnt a valve and leaded gas was phased out here in the 1980s

Either your timing is not what you think, or your mixture is not as rich as you think

[gus]

oh. or perhaps your clearances are not right

[rgh0]

Bronze guides help keep valves cooler. If you have original cast iron guides a change to bronze may also help

cheers
Rohan

[alan]

don't forget that n?1 on baby elan has vacuum connection for headlamps and servo and for +2 ?lans it is n?1 and n?4 that are effected with vacuum connections. Weaker mix Under certain conditions

[69S4]

Thanks for that Rohan.

From other posts I believe the 2-3 to 1-4 mixture inbalance is typical on Stromberg engines - but please correct me if you think I am wrong.

Dave.

I've not found that to any great extent on my engine- certainly not the amount of imbalance you seem to be getting. There is an issue with the vacuum take-off affecting the mixture but only really at idle. In normal running it seems to be too small to make much of a difference and all four of my plugs come out more or less the same. I've also never had any valve issues with the car (other than a badly fitted valve seat coming loose about 25yrs ago)

[david.g.chapman]

Thanks everyone.

I have bronze guides, and they have not worn much since they were fitted a few years ago now. One thing I forgot to mention was that the valve springs are NOS Sue Miller items with the correct free length. There is a 1.5mm grey ring on all of the valves after grinding in.

The engine runs fine with no misfiring or hesitation. It does not smoke or burn any oil.

Gus - the clearances are OK but it might be worth checking the crankshaft pulley against TDC. If that is wrong my timing will be out. I will let you know if this turns out to be the case.

Otherwise it's go for a richer mixture. I like Rohan's thoughts on free oxygen.

Dave.

[promotor]

Do you have the link tube that joins the two carbs installed?

[david.g.chapman]

Yes, I do have the connecting tube mounted in the lower position.

I have just checked the crankshaft pulley mark with a dial gauge down no 1 cylinder, and it is within 1 degree of TDC or better - OK for my purposes.

Dave.

[RogerFrench]

I'd try a colder plug, say NGK 7s. A plug burning too hot can encourage valves to do likewise.

[BobP]

In addition to my Lotus S4, I fly a home built (by others) aircraft. Piston aircraft engines need to produce high maximum power for takeoff but use minimum fuel consumption in cruise, and all at minimum weight. To do this many engines have a control to adjust the fuel/air ratio in-flight. At cruise power (up to 75% of full power) the mixture is run lean, but at full power the mixture is run rich to avoid exhaust valve damage. Aircraft with this facility (and many without the facility) have an exhaust gas temperature sensor so the pilot can monitor and adjust the fuel/air ratio.

I wonder whether your carb main jet might be restricted or too small so the mixture would be normal/rich during normal running but too weak at wide open throttle. Temporarily fitting an EGT sensor (needs a small hole in the exhaust manifold) would tell you for sure. Or perhaps you could fit an oxygen sensor in the exhaust?

just ideas
BobP