Joe was a very meticulous mechanic. His productivity level
was on the low side but when he finished a job, it never came back. That was
until he met the Dodge van with the V-8. It came in for a timing chain and
waterpump. "Piece of cake" Joe thought to himself. Instead, the cake turned out
to be a bitter pill he would never forget.
The job went smoothly; too smoothly. For good measure, Joe
ran a compression test as soon as he had the new timing gears in place. It was
the thorough thing to do. The readings weren't bad, but they weren't good
either. He suspected the valves weren't in great shape and suggested the heads
be pulled so the engine could get a valve job. The owner was perfectly happy to
comply.
The rest of the job passed smoothly, at least until the
owner came roaring back into the shop yelling "What have you done to my engine!"
He was steaming mad and wanted an immediate explanation. Joe asked what the
problem was and the owner threw his keys at him and said, "Just drive it. It's
pretty obvious!"
A test drive revealed a total lack of power. Ironically,
the engine didn't idle rough. But under power, it was a real dog. The first
thing he did was take compression readings. They were all normal. Next he warmed
up the gas analyzer.
The exhaust gas analysis readings were so high they went
right off the scale. "It must be the carburetor. Nothing could make the exhaust
readings that high except a carburetor problem. It's flooding," he thought.
Rebuilding the carburetor made no difference. Reasoning
that it may have an internal flaw, he installed another carburetor. No
difference. Maybe the timing isn't advancing or is wrong. Joe began to suspect
the timing chain. Could he have installed it wrong? It wasn't a happy thought.
The entire front of the van would have to be pulled apart in order to get to the
chain. But, doggedly, Joe took it back apart again. The timing chain was on
correctly.
He took the gears off and matched them up with the gears
from a different manufacturer in hopes that the gears were mis-boxed and for a
different application. No such luck. They matched perfectly. What about the
distributor?
Despite the fact that he never touched it, he would try
another one just to see.
Once again, no difference. It still ran very badly.
Joe was at his wits end. He went over every thing he had
done again and again without so much as a clue to the new problem. He feared he
might have gotten back the wrong cylinder heads from the machine shop, but the
casting numbers were found to be correct. He checked for exhaust restrictions,
emission control problems, secondary ignition problems. In short, Joe was fresh
out of ideas.
The van sat, like a dejected child, over in a corner of
the shop for a week. The owner was becoming more and more outraged each day.
Rightfully so! Joe did not know what to do. In desperation, he decided to visit
an old mechanic friend for some advice. As Joe explained the gory details to his
friend, he looked into his eyes for some glimmer of hope.
When he got to the part about the valve job, his friends
eyes widened and he exclaimed, "That happened to me too!" "Really?" said Joe.
"Yep. and I know how to fix it." Joe's heart sang. "What! What! You gotta tell
me," he exclaimed.
Instead of talking, the old mechanic slowly walked over to
his tool box and reached into the back of one of the bottom drawers, pulling out
a thin strip of brass sheetmetal.
"That's shim stock. What's that got to do with it?" The
old man told him to cut the metal into thin strips and place one strip between
the rocker arm shaft and the cylinder head. "What will that do?" Joe asked.
"Just do it and call me back to tell me the problem is fixed." The trick did the
trick. The engine ran with full power and the emission levels were now back to
normal.
Joe could hardly wait to call. "What did I do? Why did it
work?" His old friend explained that he had the same predicament once upon a
time. The problem was caused by the machine shop not cutting the valve stems
after doing the valve job. "It's a problem with what is called installed valve
stem height. Your machine shop is sloppy."
Joe couldn't wait to rip the heads off and take them to a
different machine shop to be checked. Sure enough, the valve stems were
protruding much too far into the cylinder head. The valves were burned where
they had been held open by the lifters. "But what about the normal compression
tests? Why were the compression readings good if the valves were held open?" He
asked the old man. "Because the oil pressure had not come up when the engine was
cranking over. After it started, the oil pressure was enough to pump up the
lifters and hold open the valves."
