Ive got a 78 F-250 AT 4x4 with a comp 256xe cam and 400 stock 2v carb. When I fire it up cold it idles good but high rpm's (around 1500 rpm). When warm it surges and wont keep a steady idle then dies. When I put it in gear its even worse. It basically sounds like its gasping for breath. Ive tried 2 different carbs and the second is worse than the first. Should I be looking for vacuum leaks, rebuild the carb or is this an ignition issue?

 

always start at the cheapest route get a can of carb cleaner and spray it around the intake manifold even a small leak can cause alot of headaches, just listen for idleing changes either a drop or fluctuation in RPMs

 

It's very common for the EGR plate under the carb to warp, burn out etc. You can't tell without taking it out and warpage is still hard to see. Also the valve itself.

 

When I had the carb off I took the egr spacer off too. I noticed the gasket under the spacer had a lot of discoloration around the port where the exhaust gas flows. Perhaps the gasket is leaking exhaust gas past the spacer?

 

if the gaskets blown the spacer is probably bad too. Especially if it's aluminum instead of steel. There not very expensive.

 

you're saying it idles good until it warms up, vacuum leaks? yes of course always look for that but also your choke plate will be in more of a closed position when cold feeding the engine more fuel, as it warms up & the choke plate snaps open you are pulling a leaner fuel mixture... are you sure you don't just have to open your idle mixture screws up a little?

another thing is that your vacuum circuits or routes change when the engine comes to temp if you are using your stock thermostatic vacuum fittings... the cold vacuum path may be leak free but when the thermostatic valve warms up & routes your vacuum for warm engine, you may have a bad hose, open ended hose etc. Those vacuum fittings (I know someone will come up with the correct name) have 3 vacuum line stems, one for vacuum source which will be drawing vacuum when ever the engine is running & it will allow vacuum to pass through one of the other 2 stems depending on engine temp. (you will often have one of these right in your upper radiator hose neck)

Check that stuff out, one example for thermostatically selected vacuum is the vacuum advance. I believe vacuum will only be allowed to the vacuum advance on the distributor when engine is warm, vacuum advance diaphrams often crack & are a common vacuum leak source.