Burned exhaust valve

[alfaparticle]

This is my first experience of a burned valve and it is not what I expected to see. The valve ran for less than 20,000 miles in #1 cylinder on my spider. It was installed by a well known Alfa mechanic when he did a valve job for me. The guides were replaced and he did a 3 angle cut on the seats. I lost all clearance on that valve and the bucket follower was damaged. I checked the #2 valve and it looks OK. There is a very light mottling of the seating surface that came out with lapping. The valve seats are undamaged on both cylinders.
I am running Motronic pistons, Webers on a Euro manifold, Euro cams, Shankle headers and Marelliplex ignition. The Weber jets are exactly as supplied by Shankle. The mixture was fairly lean and I regularly got 26 mpg before the valve problem.

Does ayone have ideas about the cause of this?

Thanks,
Ed Prytherch.

[Roadtrip]

Are you sure the valve clearance was adjusted correctly when the head was reworked?

[papajam]

Additionally, the valve clearences should checked no later than the first oil change after major head work since this is when most valve recession will occur.

[alfaparticle]

I checked the valve clearances 10,000 miles after the rebuild. They were Exhaust: 16, 18, 18, 18
Intake: 15, 16, 16, 15

The burning obviously happened between 10,000 and 18,000 miles.

Ed

[VeloceOne]

Ed,
Carb too lean?

[coda tronca]

One thing that seems not to have been done, at least not mentioned is the replacement of the seats. In my experience with Alfa heads, as found from having had burnt valves myself, is that it is quite common that seats are deemed serviceable by the machine shop and then...you know the rest. I am sure I am preaching to the choir at this time but in any case , renewing the seats whenever there is a shadow of doubt is still the better idea.

[tubut]

Quote:

Originally Posted by alfaparticle The mixture was fairly lean...

[PSk]

Quote:

Originally Posted by tubut ... carb synchronization issue)?

[alfaparticle]

The carbs are Weber 40DCOE139 with 32mm venturis. I am running with the standard Euro manifold, plenum and air box. The idle jets are 59F21 (bigger than the more usual 55F8). Main jets are 138 (my GTV with 40DCOE28's has 130) and air correctors are 160 (more usually 200). I think that most significant deviation from the normal set up is that the auxillary venturis are 6mm compared with the more normal 4.5mm. This set up is exactly as supplied by Shankle for a 2L motor. I have tried richer idle jets - 55F9 were suggested, but I could not get the engine to run well on them. The 59F21's allow for a good idle and smooth pick-up.

I understand the point about running on the idle circuit when cruising and the bigger auxillary venturis extend the idle circuit to higher rpms than smaller ones. But the idle screw setting is not very relevent since most of the emulsion is delivered through the progression holes under these circumstances.

The one problem that I forgot to mention was that I had intermittent fuel pump cut out due to a bad inertia switch. Sometimes the engine died completely due to no gas. It happened a few times before I found the cause. The inertia switch is now jumped out. I wonder if the damage to the valve may have started then.

What is the general opinion of the burn pattern on the valve surface? Do burned valves usually look like this.

I am thinking of retarding the ignition a bit. I am currently running 38-39 degrees.

Any more thoughts?

Thanks.
Ed

[MrT]

General physics issues for such engines: Compresssion, burn, power stroke, etc. When the burn happens right away (detonation, etc) the peak temps in the engine are higher. But not necessarily the exhaust gas temp, as this is thermodynamically more favorable for extraction of power by the engine. If the burn happens during the compression stroke, you have not only the burn but the compression raising the cylinder temps, and this can be much worse in practice. But a late burn means less energy is available for extraction by the engine, so lower power to the wheels even when the same (or more) fuel/air is made available through higher manifold pressure. Then the exiting gases _must_ have a higher temp than for optimum extraction of energy by the engine. Late/slow burns can be (I believe) caused by very lean mixtures. My understanding is that this is what causes burned valves with lean mixtures, but that may be wrong in some detail.

Having in mind a mechanism for burned valves may help when trying to identify what happened. Anyone who knows better than the above heuristic description, please chime in. If your carbs were chronically lean or maladjusted (so only certain cylinders were lean) then you might end up with the problem you found. That is also why a partially plugged injector can be so miserable. It runs only its cylinder lean and can trash it.

[VeloceOne]

Ed,
Not familiar with 40 DCOE 139 are they late emission carbs? I tried to jet up a set of 40 DCOE 32's to euro spec for a 2l. Did not work, had to go back to near the original specs for the carbs. What are the 139's from? and can you fine the original jet specs?
You mention 40 DCOE 28's, they are for a 1300 engine are they not?
Regards, Ian.

[tubut]

Ed,

I think what you need is an exhaust gas sensor that tells you when you're running lean and when not. With a carb system, you don't need it constantly (as is the case with EFI systems). Use it for setting up the carbs, then remove the sensor and plug the hole with a bolt. The following links show some examples:

http://www.autospeed.com/cms/A_0217/article.html
http://wbo2.com/2a0/2a0info.htm

[alfaparticle]

Thanks tubut. I will check out the exhaust sensor.
Ed