Side of the Road Again

Hi, this is the 2nd time I have tried to get this on. Maybe it will work this time. Anyway, I have a Merc with a stock 390 2v in it and it runs great until you try to go over 55mph. As soon as you hit that it stalls like crazy, and becomes totally undrivable. At all other speeds it runs like a top, and in the driveway it runs great when you stomp it. It has all new plugs, wires, coil, dis. cap, a pertronix module, rebuilt carb, fuel pump etc. I just timed it to stock, and changed the fuel pump but this did not help. I had it into 2 different shops and all the primary ignition has been checked out and OKed, but to no avail. I am totally stumped. Some friends of mine think it is starving of fuel, but I don't think that is the case. Any thoughts would be great! Thanks

 

I had a vacuum modulator go bad, ****ed trans fluid under load at higher rpms, made it act like it was way rich.

 

Is you gas cap vented?

 

*could* be flattened lobe on the cam, or bent valve, or broken valve spring. I have a chipped valve on my truck, it'll run great in neutral and low speeds, but anything in gear under load it bucks and spits like a mo-fo! It's an intake valve and it pops through the carb, under 1,000 and after 2,500 RPM, she runs very strong on the road

 

No, I don't think it is. It is the stock model

 

Didn't see fuel filter in your list. I've seen that a few times.

 

I don't want to sound like a newbie dumb***, but what exactly is the vacuum modulator and where is it found? Thanks

 

Yeah it has a new fuel filter as well. I pulled it and it was clean and clear. I thought maybe I was getting **** from the old tank, I don't think that is it. I pulled the needle seat in the carb, and checked the screen and it is clean as well so...

 

it "modulates" the trans shift point based on the "vacuum" being pulled at the intake manifold.

 

My buddy had a GTO that had it's gas tank pushed up from the bottom (mechanic dropped it down on the lift onto something) and it crushed the pick up tube in the tank. Ran fine except under load. Took a long time to figure that one out. Some pick up tubes also have a filter on them. Just a thought.

Spike

 

Do you have a fuel pressure guage ? It could be low OR high fuel pressure. Low pressure and you run out of fuel at a certain RPM, High pressure, and it ****s fuel past the needle and seat at a cruising speed. I had a similar problem with my flathead, and a pressure guage and regulator solved 90% of my driveability issues.

 

I had a 390 in an F100 that acted just like that.

Turned out to be a head gasket going bad. Do a dry and wet compression check; that's how I found it.