I had the opportunity to take my vette on a small adventure last weekend. I had my pack strapped to the car and was on my way.



After about 3 minutes on the road, and less than a 1/4 mile from the house, I had a sudden loss of power. No weird noises like when it used to bog, just very little power, and almost no response from the pedal. A second later, and I get one VERY loud backfire from the tailpipe, followed soon after with an engine stall. There may have been a little knocking sound after the backfire, but my ears were ringing and I'm not sure.

I tried to restart it on the side of the road, and it would turn over and over, but only catch for a second, and only on occasion. I had to make use of my Hagerty policy, and have a flatbed come and rescue me, and drive me the very short distance back to my house.



So, now I'm once again faced with doing diagnostic work without a lot of mechanical experience. This car has already taught me about cooling systems and braking systems, along with a host of minor things, and I'm wondering what I'm about to learn next.

I don't really know where to start with a problem like this. With the backfire, I'm guessing it's either the distributor, a valve, the cylinder heads, or the head gasket, but I really am just guessing. Prior to this, the car was running the best it has in the 10 years I've owned it. With the business of my life right now, I don't know when I'll get to the repair, but I'd at least like to get your cumulative advice on how I should go about figuring out what's wrong.

As always guys, thanks for the help!

 

Remove the distributor cap and while turning the key to start see if the rotor turns. If not, good chance your timing chain snapped. If that's ok check if you see have spark at the plugs, if not you have an issue with some component in the distributor and can find on the internet a sequence of diagnostic tests for HEI type distributors. The other possibility is no gas, hold the choke open and move the throttle by hand, you should see two healthy shots of gas on the primary side, front, of the carburetor. Let us know how these steps work for you.

 

timing chain or distributor

 

I don't think it's your timing chain. If it broke the camshaft wouldn't be turning and cranking the engine would have driven the pistons into the valves that happened to be open. I think you would be telling us about the awful noise it made when you cranked it.

So, I would look for possible causes of a backfire. A backfire happens when you get raw gas into the exhaust system and it ignites. So how can that happen? Either you weren't getting spark and then you got one, or the carb was pumping too much raw gas (literally flooding the engine after it was started) and the catalytic converter or something else was hot enough to light up the gas once it got that far.

Either way, you need to see if you are getting spark and a proper mixture of gas and air.

 

Possibly the ( timing chain ) cam chain has jumped a tooh .

Bill

 

Fuel pump is a suspect IMO.

 

My neighbor came over and we ran some diagnostic tests on the '74 this afternoon, and I wanted to post the results to get your opinions, and so I don't forget what happened.

1. We pulled the coil wire off the distributor and checked spark. The coil is good.

2. We pulled one of the plugs and grounded it against the block. There's spark at the plug.

3. We checked the throttle, and the carb is pumping gas into the primaries.

4. We put the plug back and just tried starting the engine. It wouldn't catch until my neighbor cupped his hand fairly tightly over the carb primaries. The engine ran for a few seconds, but it was slow and sluggish and quit fairly quickly. It didn't seem to be missing on any cylinders, it just seemed, well... lazy.

5. We tried starting it a few more times just to see what would happen. After a couple tries with no catch, it caught for a second, and a decent cloud of haze ejected out the top of the carb with a chuff sound.

6. We loosened the distributor and tried moving it to see if we could get the engine to catch. Turning it clockwise did nothing, but turning it counterclockwise quite a bit caused the engine to catch. Again, it sounded steady, but weak and not right, and it quit fairly quickly.

7. We tried starting it up again that way, and it might have caught for a second, but we heard a couple loud knocks in a row and shut it back down.

I put the distributor back where it was when we started, and closed up the shop for the night.

So there was my adventure today. Does that help reign in the diagnosis? My neighbor thinks slipped timing chain, since the engine is trying to fire, and rotating the distributor sounds to have helped a little. I'm still kind of at a loss. I understand the principles of engines, but trying to diagnose something with these symptoms seems beyond me. If I don't get any good ideas, I guess I'll start pulling everything off from in front of the timing cover. I JUST finished doing a radiator flush and an oil change, too... sigh.

 

how many miles are on that engine.? it does sound like the timing chain has slipped.

 

Did you use ethnol gass and melt your fuel float?

 

Sounds like it's starving for fuel.. checked the fuel filter?

 

if a person cups his hands over a carb what he has done is cut off the supply of air to the engine.if the engine attempts to start then it tells me the engine is getting air from somewhere else.sounds like a major vacuum leak to me.carb gasket,line,intake etc. Just start checking.

 

 

I agree with checking for a vacuum leak next.

That could be a very lucky thing considering your first symptoms.

I just went through sucking in a large portion of my carb gasket, creating a major leak with the exact same symptoms.

I'd say plug all the hose ports and see if it starts.

 

Yep. That gets my vote.

 

Well, I spent several hours under the car today. I got the shroud, fan, water pump, etc. out. I went to AutoZone and picked up a rental harmonic balancer puller (harmonica bouncer according to my son), and got that removed. Took most of the screws off the oil pan and got it out of the way enough to pull the timing cover. Here is what I found:

Timing Chain Video

That video shows that I do not have nylon gears, it's all steel, but it also shows what I consider to be an excessive amount of play in the timing chain. Now, when I turned the engine over by hand, the gears are still lined up dot-to-dot, but I could turn the crank shaft a good 10 - 15º before the chain even started moving the cam shaft. It was about a "tooth's worth" of travel in the crank gear.

So my question is, could this be the cause of the engine just not working as discussed in my first post? One way or another, I know I need to get this replaced, but can I expect the engine to work once it's replaced and timing is reset?

 

Are you sure thats a steel gear,it almost looks like stock gear with no plastic.
Thats way too much play.

 

Sorry, I misspoke. I meant to say metal, not steel.

 

Looks like you need a new timing chain set.
Your going to have to remove the pan and get the plastic out.

 

Oh, you were implying it looked like a nylon toothed gear that had all the nylon stripped off. I guess that would explain all the slack in the chain...

 

Yes ,that's what it looks like to me ,that's a lot of slop.