I put in a rebuilt '78 300 and could never get it too run right. I jumped through a lot of hoops with the carb and distributor to no avail. Compression test cylinders 1-2-4-5-6 all read 110 but 3 reads 80. Finally did a leakdown test and was hearing airflow through the dipstick tube on both 2 and 3 (only 60 psi applied). By now it's cold, so I put everything back together to warm it up again to test the other cylinders --- it will barely start now and pops and backfires and stutters horribly --- as though the leakdown test caused a big problem. I'm about ready to pull the head for machine shop inspection (hoping I don't find a holed piston) Any other ideas?

 

There is a lot more pressure from compression when the engine is running than there is from a leak down test. I would think it's more likely you knocked a vacuum line or wire connector loose when getting in there to do the leak down test.

 

Thanks - that was my thinking too, but I can't find anything, so I'm going to pull the valve cover first and turn it over a few times to check the valvetrain. I hate the idea of pulling the head, but ya gotta do what ya gotta do.

 

Check to make sure are the rockers are moving as they should. On a new engine cam damage is all too often occuring.

 

THANKS. Yes, they're all moving. A local motorhead at the parts store has suggested that the cam drive gear could be loose (which could explain a lot of the issues I've been having). Another suggested a distributor drive gear having a sheared set pin. Both ideas make perfect sense to me.

 

What oil are you using and what oil did you use to break in the cam?

 

I got the head off yesterday and found the chambers were odd. #1 was very clean and going towards 6 they got progressively dirtier to the point that 6 was real real sooty. I would have expected them all to look about the same.

 

Yardbird - I used the grease that came with the cam and there was Castrol engine break in oil in the pan. The valves are all moving when I turn the engine over.

 

The main thing is they all move the same amount. I only run diesel oil in my 300. There just is not enough anti-wear additives in today's oil for a flat tappet cam. Right now I am running Rotella T5 15/40 in both my trucks. Been running diesel oil in them, especially the Ford that I've had it for close to 30 years, since the early 2000's.

 

They seemed to be all the same, but I have the head off now so I can't recheck it. I'm going to check the 2 drive gears (camshaft and distributor) and consider swapping out the cam again too. PITA changing the cam though. I've got 15 gallons of Royal Purple diesel oil left over from when I drove semi and was planning to use it once a get this thing running right.

 

When doing a leakdown test you must be sure each cylinder is at top dead center with both valves closed. Caution 100psi can cause the piston to start down suddenly and turn the crank and your arm if it's in the way.