I have a 79' with a 351 modified that has a vibration at any rpm. It feels and sounds like the same thing as a bad u join, but does it at idle as well. It also is smoking a bluish color, any ideas. It has 82,000 original stock miles on it, but would like to do a 400 build on it with a decent carb and intake down the road, if I can find a place that isn't wanting to get too rich off me.

 

It could be a number of things, but the blue smoke means it is burning oil and that probably means the rings are badly worn. We could go through the whole testing process to determine why it is shaking, but it is probably due to one cylinder or more not running at all. However, given the oil burning why bother to determine why it is missing as it isn't likely you are going to drive it that way.

As for rebuilding it, there is no cheap way on these engines if you want power. You can't find pistons that will give you any compression for a 351M, so need to go to a 400. But that means you need a new crank, which will probably set you back a couple of hundred. Then you need pistons, and those aren't as inexpensive for a 400 as some engines, but Tim carries a good selection at a reasonable price. And from there the machining and other costs are the same as for any engine.

 

Thanks Gary, I picked the truck up cheap knowing it had engine issues, but was hoping it would last a little bit. I'll just make it my winter project doing up a 400 motor.