RossABQ
Mechanical
- Dec 20, 2006
- 942
I have a '52 Ford pickup with a flathead V8 engine, rebuilt last winter. I put about 500 miles on it around town to break it in, with occasional runs up to 55 mph (2300 RPM). It ran great. So I took it on a 3500-mile cross-country trip to meet up with some fellow enthusiasts.
My route took me across the panhandle of TX/OK while ambient temps were up to 95 deg F. No problems, cruising along at 55, 2300 RPM, a self-imposed limit. But by the time I got to Missouri, it would regularly die when I came to a stop for gas or whatever. It would restart but not easily. It got worse and worse.
I figured it for a fuel problem, because a brief cooldown would help, so I did some things to reduce heat on the fuel lines, to no avail. It has an electric fuel pump with regulator, both appeared to function perfectly. Disassembled the carb (in the parking lot of an Arby's), clean as a whistle.
Checked the timing, no problem there. The engine has been updated with an MSD distributor that is essentially a GM HEI style, with separate mechanical and vacuum advance, both of which can be (and were) tailored to this application.
The engine ran at normal temperatures the whole way, with only brief excursions a half-a-needle width upwards on long or steep hills, even in 95 deg weather (engine ran at the same temperatures whether it was 95 or 75 outside).
I barely made it home, and first thing I did was run a compression test. Only 1 or 2 cylinders had anywhere near expected pressure (90-100 psi at my altitude). One was only 50. So out it came, pulled the heads off. All of the exhaust valve seats had receded, enough that there was no clearance between the lifter and valve when cold -- clearly once hot, the valves were being held off the seat. Clearance is spec'd at .017" - .019" cold, so the valves sunk that much. This model engine does not have hardened valve seats, instead it has valve rotators.
The machine shop examined the engine and found that the margins on the seats had gone completely away, and the valves had sunk. The valves' surfaces were in great shape, no deformation.
So here's my question. Which of these (or another?) could have caused this in such a short time, under relatively modest loads?
1. The machinist believes there was too much ignition advance, due to the vacuum advance. The vacuum advance is limited to 8 degrees, and is run off the manifold vacuum. He believes at cruising speeds, that would provide too much advance. There was never any pinging at any time, even on long grades or hills in hot weather. Flatheads generally run hot if there is not enough advance, I am not familiar with problems with too much.
2. The stock road draft tube crankcase ventilation system has been changed to a PCV setup, using a PCV valve from an engine of comparable displacement (a Buick 3.8L). I've had that system on the engine for 7 yrs, but admittedly never for extended highway use. The thought is that the PCV addition requires additional fuel be added in the carb to compensate, and that there wasn't enough added. I went one jet size up to account for it when I added PCV. Plugs were gorgeous, light tan, throughout the trip.
3. For this kind of operation on unleaded gasohol (E-10), hardened seats are needed for durability.
4. The shop did a poor job on the valves during the rebuild; inadequate margins, poor stone quality/rough seats, leaving a surface that wasn't durable.
I would appreciate your thoughts! I have no pre-conceived notions, all four ideas make some sense to me, but to do this much damage in this short of a time is shocking.
My route took me across the panhandle of TX/OK while ambient temps were up to 95 deg F. No problems, cruising along at 55, 2300 RPM, a self-imposed limit. But by the time I got to Missouri, it would regularly die when I came to a stop for gas or whatever. It would restart but not easily. It got worse and worse.
I figured it for a fuel problem, because a brief cooldown would help, so I did some things to reduce heat on the fuel lines, to no avail. It has an electric fuel pump with regulator, both appeared to function perfectly. Disassembled the carb (in the parking lot of an Arby's), clean as a whistle.
Checked the timing, no problem there. The engine has been updated with an MSD distributor that is essentially a GM HEI style, with separate mechanical and vacuum advance, both of which can be (and were) tailored to this application.
The engine ran at normal temperatures the whole way, with only brief excursions a half-a-needle width upwards on long or steep hills, even in 95 deg weather (engine ran at the same temperatures whether it was 95 or 75 outside).
I barely made it home, and first thing I did was run a compression test. Only 1 or 2 cylinders had anywhere near expected pressure (90-100 psi at my altitude). One was only 50. So out it came, pulled the heads off. All of the exhaust valve seats had receded, enough that there was no clearance between the lifter and valve when cold -- clearly once hot, the valves were being held off the seat. Clearance is spec'd at .017" - .019" cold, so the valves sunk that much. This model engine does not have hardened valve seats, instead it has valve rotators.
The machine shop examined the engine and found that the margins on the seats had gone completely away, and the valves had sunk. The valves' surfaces were in great shape, no deformation.
So here's my question. Which of these (or another?) could have caused this in such a short time, under relatively modest loads?
1. The machinist believes there was too much ignition advance, due to the vacuum advance. The vacuum advance is limited to 8 degrees, and is run off the manifold vacuum. He believes at cruising speeds, that would provide too much advance. There was never any pinging at any time, even on long grades or hills in hot weather. Flatheads generally run hot if there is not enough advance, I am not familiar with problems with too much.
2. The stock road draft tube crankcase ventilation system has been changed to a PCV setup, using a PCV valve from an engine of comparable displacement (a Buick 3.8L). I've had that system on the engine for 7 yrs, but admittedly never for extended highway use. The thought is that the PCV addition requires additional fuel be added in the carb to compensate, and that there wasn't enough added. I went one jet size up to account for it when I added PCV. Plugs were gorgeous, light tan, throughout the trip.
3. For this kind of operation on unleaded gasohol (E-10), hardened seats are needed for durability.
4. The shop did a poor job on the valves during the rebuild; inadequate margins, poor stone quality/rough seats, leaving a surface that wasn't durable.
I would appreciate your thoughts! I have no pre-conceived notions, all four ideas make some sense to me, but to do this much damage in this short of a time is shocking.
