Re: 74 bus sudden slight loss of power, #4 valves both tight
Posted: Fri Sep 11, 2015 2:25 am
I have a few questions for the original poster. These are based on my feeling that the engine has been over-heated due to hard demand.
1. Is every piece of your cooling system in place?
2. Is your thermostat system functioning properly?
3. How many miles passed since a full tune up with dwell set and valves adjusted?
4. How often do you drive that way? Most of the time, or were you in a hurry that day?
5: During that drive, were you buffeted by headwinds constantly?
I am asking these questions because I feel (based on what I have read and not having wrenched on your bus) you may have suffered collapsing seats and/or stretched valves due to cylinder overheating. There may also be scorching or cutting around the valve seats.
I wonder if the curled or bent tang of the spark plug is a result of overheating. What do the other spark plugs look like? Spark plugs don't just get bent, and it is unlikely that it would be installed bent like that.
Replacing the now defective spark plug and readjusting the valves would probably cure the symptoms. But valves don't just get tight without cause. Your first warning was a loss of power. You noted that up until you changed out that plug, you still seemed to have the symptoms when under load. Isolating fuel/air and electrical ignition questions leaves you with mechanical questions.
Now would be a great time to establish a baseline of valve tip height so you can calculate valve creep for future considerations. Remove the rocker arm, and with a machinist scale or fine steel straight edge, lay it across from one far edge of the head to the other, along the edge that the valve cover seals. You will need to elevate this straight edge to clear the tips, use hardware for this on either end.
Center this straight edge over the valve tips, and one at a time stack your feeler gauge set between a valve tip and the straight edge. Record measurement and do it again to eliminate sloppiness. Do each valve.
This is a tedious, tiring process on your forearms and wrists, but I have found it to be about as accurate an indicator as you will get without pulling the heads off in a shop environment.
1. Is every piece of your cooling system in place?
2. Is your thermostat system functioning properly?
3. How many miles passed since a full tune up with dwell set and valves adjusted?
4. How often do you drive that way? Most of the time, or were you in a hurry that day?
5: During that drive, were you buffeted by headwinds constantly?
I am asking these questions because I feel (based on what I have read and not having wrenched on your bus) you may have suffered collapsing seats and/or stretched valves due to cylinder overheating. There may also be scorching or cutting around the valve seats.
I wonder if the curled or bent tang of the spark plug is a result of overheating. What do the other spark plugs look like? Spark plugs don't just get bent, and it is unlikely that it would be installed bent like that.
Replacing the now defective spark plug and readjusting the valves would probably cure the symptoms. But valves don't just get tight without cause. Your first warning was a loss of power. You noted that up until you changed out that plug, you still seemed to have the symptoms when under load. Isolating fuel/air and electrical ignition questions leaves you with mechanical questions.
Now would be a great time to establish a baseline of valve tip height so you can calculate valve creep for future considerations. Remove the rocker arm, and with a machinist scale or fine steel straight edge, lay it across from one far edge of the head to the other, along the edge that the valve cover seals. You will need to elevate this straight edge to clear the tips, use hardware for this on either end.
Center this straight edge over the valve tips, and one at a time stack your feeler gauge set between a valve tip and the straight edge. Record measurement and do it again to eliminate sloppiness. Do each valve.
This is a tedious, tiring process on your forearms and wrists, but I have found it to be about as accurate an indicator as you will get without pulling the heads off in a shop environment.