Starts running rough after about 20 minutes

Hey Everyone,
New issue I wanted some input on. Got the van completely rewired and everything seems to be working well. We've been going on some longer runs with it (running errands, multiple stops) and now we have a new issue. After about 20 minutes of run time at highway speed it begins to sputter a bit. Mostly noticable when at a stoplight or pulling away from a stoplight. Feels like a misfire or fuel starved? It hasnt left us stranded yet but today we cruised for about an hour and I was wondering if we were going to make it home. Since its only happening when the engine warms up im planning on replacing the coil first. I know in motorcycles this issue is a sign that a coil may be going bad. Anything else to check?  Ill probably replace all my fuel filters and fuel pump as well this weekend.  
Any advice is much appreciated.

Thanks!

Here's a picture from today.

If you turn it off and wait 2 or 3 minutes and then restart it and it runs good, it is probably not the coil. It probably is the fuel filter. The filter will get jammed up and then when you shut down it back flows enough to free it up momentarily. I don't know the history of your van or what mods you have made. What are you running for ignition?

points and a 6v coil right now. Going to upgrade to the pertronix system soon. If I start it up just a few minutes later its better but still sputters and gets progressively worse much quicker than a cold start.

Check the fuel filter. If it is the filter, the time to rough running and then stalling will get shorter and shorter. It might not ever kill it, but it will be so fuel starved that it runs like crap.

Put a piece of clear tubing or fuel filter between the fuel pump and carburetor,,,,,,,see if the fuel flow starts acting up as you have problems,,,,,,,,also,,,,,shut it off when it acts up,,,,see if you got a puddle of gas down in the carb from flooding out,,,,,,, you need to find some clues,,,,,

I'll put the clear fuel filter between the pump and the carb back on it tomorrow so i can watch the flow and I'll also change the main fuel filter. Im getting closer to getting this van running really well. I cant wait until this thing becomes a reliable runner.

i can not speak for the econo's but the dodge's have a sock on the fuel line in the tank i't may be there as well if ford is the same ??

Sounds like it could be vapor lock.  Do you have metal fuel line from the fuel pump to the carb? Do what Donivan said. Get some clear braided fuel line and install it from the pump to the carb. Any pvc line like this will work. http://www.dudadiesel.com/choose_item.php?id=HPpvc025-100ft&kpid=HPpvc025-100ft&gclid=CObhzKLUm8UCFVg8gQodGhQAaw

I've got a metal line from the pump to the carb.there is a short piece of rubber line which I'll replace with some clear. I see a lot of talk about clothespins on the metal lines between the pump and the carb somehow helping. Might try this as well.

Another question. Can the coil be mounted outside of the doghouse near the battery so that it avoids heat. Mine is currently held on the bracket directly to the engine. It seems like moving it somewhere cooler would help.

Clothes pins on the metal fuel line??? Disconnect the metal line from the carb and put a length of flexible line on the metal line and into a coffee can or suitable container. crank over the engine and see how much and how strong the gas is flowing. Also check if the fuel looks dirty or milky. If everything looks good bypass the metal line with the flexible line and a clear fuel filter. Get everything up to operating temperature and check fuel flow again. Make sure that you are getting fuel. If you are then start checking the ignition system. I don't know if you have a ballast resistor or a resistance wire to your coil. It could be that. Coils are usually good or bad, no in-between.

Currently I have a resistance wire. I have the correct ballast resistor if needed.
Clothes pins seem to be some sort of fix for vapor lock. Apparently it's been used for years with success, it's even recommended in a few places on this forum.

Clothes pins act as a heat sink and help pull heat out of the fuel line.

You guys willlearn!! The old guys remember stuff that worked =long ago! Also - when you use those clothes pins - you can hang those damp undies on the clothes pins to =DRY!! see== 2 problems solved @ once!!

I'm trying to soak up as much info as I can from the old timers. Theres is some real valuable information floating around on this site. I appreciate all the help i've been given and hopefully one day I can help someone else out here.

just from me =Thanx.

Well I havent had a chance to run her in a few days but I have made some progress. I replaced both fuel filters and all rubber fuel lines on the van. The filter between the fuel pump and the tank had a bit of fine rusty metal in it. I rerouted the metal fuel line as far away from the engine as possible and put a clear fuel filter and a fuel pressure gauge inline to the carb...What fuel pressures should I be looking for? Engine is a 200ci inline 6.  I also replaced the radiator, water pump, thermostat and radiator lines. Flushed the engine out until everything ran clear. Hoping to get my highway temp down, gets as high as 210-215 when Im doing 60 for a while on an 80 degree day. Also going to use some of that water wetter stuff and see if that helps. Hopefully I'll get a chance to run her this weekend.

fuelo pressure - 4-8 psi - now -Good advice!!+ GET RID OF THAT CLEAR FILTER!!!!!!!!! NOW!! Those are only FIRE HAZZARDS!! weve herein had a few people that lost vans due to those clear filters!! The fuel tanks are generally the reason for fine rust!! Forget that water wetter stuff!! waste of $$ !! Remember - when these vans were new -no one had cooling problems - so look @ radiator or replacement! & add a shroud! Install a cooling fan in the doghouse to remove the heat =Helps!! Good to have done the work so far on hoses & lines & such!!

I had a metal filter in there but a few people suggested putting a clear one in to watch what the fuel flow was doing since that may be causing the problems im having. It is a new 3 core aluminum radiator and I bought an electric fan but it doesnt fit. Ill have to move the radiator forward, but if i were to use it to remove heat from the doghouse instead where would be the best place to mount it?

Also Is fine dust anything to be worried about?

The fine dust is the worst. It gets past the filter into the jets and emulsion tubes in the carb. It can also plug the filter up without it looking plugged.