1986 E250 I6 300
I rebuilt this engine several years back, 40k ago, it ran great for a while, then began to stutter at idle, and started losing power, the engine shakes at idle, as the rpm's increase it gets much smoother but there is still a slight miss even at higher rpms. it slowly got worse over time. So began the chase. I replaced near everything around the engine, yet nothing sorted the problem.
I pulled the motor and put it in a 1990 E350, got a DSII dizzy out of a 1984 F150, Did a chev H.E.I. module and (NON FEEDBACK) carb out of a 1976 E150 which I rebuilt. premium wire set, EFI plugs. Made a new wiring loom, As well got rid of all emissions.
The only stuf that came from the old van besides the engine was the manifolds ( which were new a year ago) and the exhaust system which is clear with a new catalydic on it. (both replaced trying to sort the problem)
It seems to be running better, yet still the same stutter or miss, I am baffled.
Compression is at 170 across the board, no excessive valve noise. Timed it with a light and have played with the carb settings but can not get it to smooth out.
At a Loss, Thanks for any help...

 

It sounds like a burnt valve or a head gasket blown between two cylinders, until I read where you checked the compression already.

 

Check the vacuum...may have a leak somewhere including the vac can on the dizzy.

 

Thanks for the replies,
I think have a valve that is not seating properly, did another compression check and it is low, at 60lbs on #3.
Strange, I swear the compression test I did a few months ago (before the swap) was solid...
Hope there is not something stuck in my intake port. Pulled the manifolds, but could not see anything.
Had a shop Mill and Valve the head when I rebuilt it. Looks like I will do it myself this time...
All the best...