Dr. Crankshaft & the Skipping Chain
A while back Dabney had just gotten delivery of his "new" 1955 Chevy Nomad. Dabney almost enjoys buying and selling classics as much as he does owning them. This was his first Nomad, and he was particularly pleased. So he decided to go over to Doctor Crankshaft's place to show off his new prize.
Doc was in his garage tinkering with his 1947 GMC wrecker. His dad bought the truck new for Second Chance Garage, and there was some sentimental attachments to the truck for the Doctor.
Dabney pulled into Doc's driveway. "What do you think?" he said as he was getting out of the Nomad. "I just got it. I bought it from the children of the original owners. Its got 125,000 miles on it, and the engine hasn't been rebuilt. I think it's one of those that'll run forever."
"Want to try her out?" Dabney said handing the keys to The Doctor.
"Sure." Doc wiped his hands, turned and pulled down his garage door.
The Doctor got in and started the Chevy up and they drove off to enjoy a pleasant sojourn. It wasn't long before the Nomad's engine balked and hesitated as it moved off from a stoplight.
"Runs a little rough, doesn't it," remarked the Doctor. "Odd, though, that it idles quite smoothly."
"The folks I bought it from said car ran well," Dabney said. "But they couldn't keep the engine properly timed. The distributor wears out the points quite quickly and the engine stumbles and balks as it's revved up. It takes a gentle touch on the throttle."
"I think the timing chain is skipping and that's why the engine can't be kept in tune, Dabney said."
"Sufferin slushboxes, Dabney! What makes you think that?" demanded the Doctor.
"Everybody knows these old engines' timing chains could loosen up over time and then skip a tooth on the camshaft sprocket," said Dabney trying to bolster his position.
"Sure, sure, everybody knows that, except it just ain't true," replied the Doctor. "Do you hear any noise from the engine?"
"No, other than the usual mechanical noises. Why?" replied Dabney.
"Don't you think if a timing chain skipped a tooth on the camshaft sprocket that you'd hear some rattling? How could that happen without noise? Besides, how many spocket teeth could a chain skip before the valve timing got so far off that the engine either wouldn't run or the pistons would slam into the valves? Let's go back to your garage and see if we can figure this one out." said Doc.
As the Doctor drove the Nomad back to Dabney's garage, they talked about everything except the car. Dabney was thinking to himself that he wanted to prove to the Doctor that the timing chain was causing the engine problems, and was trying to figure out how to do it.
They arrived at Dabney's place. Dabney opened the garage door and Doc drove the Nomad in. He left the engine running, got out and opened the hood.
"Get me a dwell meter, vacuum gauge and a stethoscope," said the Doctor. "I'm going to demonstrate that the timing chain isn't your problem. It might be a minor contributor, but it isn't the cause."
Dabney looked around a little and found the tools. "I'm not sure why you've ruled out the chain," he said as he handed Doc the tools.
The Doctor put the tip of the stethoscope onto the timing chain cover at the front of the engine. "I ruled it out because there's no noise. Listen for yourself and tell me if you hear any rattling sounds from a loose chain," said the Doctor.
Dabney took the stethoscope. "I don't hear anything but the overall whirring sounds," said Dabney. "I guess that means the chain isn't too loose, but doesn't explain the changing rpm at idle."
"OK, let's try something else. Let's put the vacuum gauge on the engine and see what we've got," said Doc.
They connected the vacuum gauge and saw the needle sitting at 16 inches of vacuum , but wavering up and down an inch or so as the engine ran. "See that?" said the Doctor, "That wavering needle might indicate a little wear or stretching of the chain, resulting in a tiny amount of valve timing change. It's nothing much to worry about in an engine this old, with so many miles," stated the Doctor. "Since we can rule out the valve train for the moment, let's hook up the dwell meter."
Dwell readings constantly rose and fell from 20 to 38 degrees at idle, with the engine's rpm rising and falling as well. Revving the engine cause the dwell to drop low and stay there.
"We've got ourselves a distributor problem, Dab old boy," said the Doctor. "Let's shut 'er down and have a look."
Dabney shut off the ignition and took off the distributor cap. He looked at the points, noting that they were new, and then said, "Well, Doc, unless these points are bad these aren't the problem."
"Right!" said the Doctor, "Try jiggling the distributor shaft and see if there's side-to-side play."
Dabney did so. "Wow! This thing moves side-to-side a lot, Doc. I guess that explains the changing dwell readings. The point gap would be constantly changing because the shaft is wobbling as it spins."
"Right again, Dabney," said the Doctor. "Let's pull out the distributor and see how much of it we can save."
Once the distributor was out they carefully verified that the condition of the drive gear was good and that the only problem was worn bushings in the distributor housing. They decided to get a rebuilt replacement from the parts store since prices are so low that it wouldn't be worth the time to do the repairs themselves.
"Well, Dabney, what have you learned today?" asked the Doctor.
"Okay, Doc, okay. I learned that if you want a ride home, you don't tear apart the car you came in." Dabney cracked up laughing. The Doc wasn't amused.