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Dick;
Flattered, I'm sure.
The experience I have had with the D-jet is in the late 80's version installed in the VW Vanagon (wasserboxer) and they had Bosch mess with it so it is not exactly like any other applicaiton I have seen (for one, it has a lambda sensor feedback and an air-mass sensor rather than MAP and throttle position).
I'd start with the various harness connections and make certain they are all in good condition, especially at the ECU, with no pins visibly damaged (like shoved in compared to the rest) as intermittant contact could be the issue.
Another poster noted that his water temp sensors (two in a row) failed and gave an infinite resistance at some temp in the mid-range which led to an overrich condition and stalled his rig. I suppose a similar fault in the air temp sensor could lead to a similar condition.
Inspiring confidence everywhere, I have a similar issue with the Vanagon. It will start and run properly until it warms up and then the idle will drop off and occasionally stall. The idle will hunt around in the 150-900 range but it will usually stabilise if I blip the throttle to about 1500 and back off... I think my issue is related to the MAF sensor so yours may be the TP sensor. I understand they can be "tweaked" to give the rub-block a fresh area to rub on.
Insofar as the head is concerned... you are right, warped head gives entirely different symptoms. However, if you have never checked the head torque you may wish to re-torque anyway (heck, I forget too). To clarify, the exhaust seat area was induction hardened, not the whole head. That is why exhaust seat recession has been such an issue on older cars when unleaded fuel became the law of the land.
The only issue I could imagine at the head to give these symptoms would be a poor seal at the manifold gasket giving you false air into the system and (therefore) lean running. Cracked vaccum hoses or brake booster leaks will also cause terrible idle issues.
Is it possible that the exhaust and intake manifolds are not the same thickness at the flange? This could definitely lead to poor clamping action of the gasket and subsequent leaks.
Siezed valves are a function of guide to stem clearance over the operating temp range of the head, not wether or not the fuel is unleaded. Nearly any modern valve will be OK on unleaded (although stainless steel is the most common). It is the exhaust seat that takes special attention by installing an insert and grinding it for the valve. Pretty much all seat inserts are unleaded compatible for two reasons: 1) why stock anything else? 2) they are not enough more expensive than the "non-lead" inserts especially compared to the labor to install them to bother.
Galling and sticky valves are often an issue with SS valves and some types of guide material. Manganese-bronze has gottne a bad rap due to this in some circles. The cure is to ream the guide with a slightly larger initial clearance or to hone in place the valve to the guide (dry, "naturally ocurring" oxides provide the cutting medium. Then the parts are cleaned carefully and you are good to go).
My point is that the shop may not have ripped you off so much as they failed to take this into account.
Silicone bronze and iron guides don't have this problem due to expansion rates closer to the SS valve.
Mike!
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