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Timing Chain Question

USNA1969

Well-known member
Joined
Sep 12, 2005
Messages
93
Location
Maryland
Corvette
1972 Big Block/4-Speed
Over the course of the last 10 miles or so that I drove my car before it died, a barely noticeable miss developed into sever bucking and backfiring. The only things I haven't checked yet are the carburator and timing chain. Would either of those cause a gradual worsening of performance? The rings and valves are in good shape and, as far as I can tell, the distributor drive is functioning.
Thanks for any help.
 

craig32

Well-known member
Joined
Sep 30, 2004
Messages
482
Location
PA
Corvette
1981 repainted black coupe
NA69, I'm not that familiar with big blocks, but the basics are the same. Did the engine run when you got home, or did it quit not to start again when you shut it off? It has the sound of a clogged fuel filter. Are you SURE it is oil in the #6 and #8 cylinders and not gas? When timing chains let go, they usually go all at once. If it's a nylon toothed gear, they tend to strip and the chain jumps, backfiring profusely and then quitting shortly thereafter, but probably not ten miles. It'll be hard to tell if the chain has jumped with the top end off of the engine.

Craig
 

Vettehead Mikey

Well-known member
Joined
Aug 26, 2003
Messages
3,486
Location
Not that far from Ottawa
Corvette
1973 Coupe
If it still runs or at least turns over, put a timing light on it. If the timing marks jump back and forth, your timing chain is loose.

From the description your problem sounds more like weak ignition.
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
I had timing chain trouble 2 weeks after I got my 1982.
The first symptom was running rough.
Then it didn't start.
Then it started backfiring out of the carb.
I started checking components.
I noticed the timing was off by about 90*.
I reset the timing, and it worked for 2 days.
Then it refused to start. I had to have it towed.

I examined everything I could think of before I finally decided to open it up and look at the chain. It turns out the gears were fine, but the chain itself had stretched about 1/2 an inch.

If you already have the top of the engine off, then it probably is worth it to go ahead and replace the timing chain. They can be as cheap as $15 and as expensive as $85. Small price to pay to know it is NOT the problem. It is also the perfect time to move to a high performance cam.

Good luck.
 

Mr. Chuck

Well-known member
Joined
Jan 19, 2005
Messages
230
Location
Southern California
Corvette
1981 Two tone silver / dr grey
Timing Chain Failure

Over the course of the last 10 miles or so that I drove my car before it died, a barely noticeable miss developed into sever bucking and backfiring. The only things I haven't checked yet are the carburator and timing chain. Would either of those cause a gradual worsening of performance? The rings and valves are in good shape and, as far as I can tell, the distributor drive is functioning.
Thanks for any help.

If your baby is high milage, then I would inspect the timming chain / gears ASAP. Timming chains that fail can really cause major engine problems if you run it much once it starts to slip / jump teeth. the older chains, none metal ones have much shorter service life. My 81 original lasted about 90K and I was told that most go bad sooner that that. The newer style are metal reinforced and are good for much longer. Since you have the top off, may a well going further and pull the front off. I would also inspect: the water pump for leaks and/or bad bearings / shaft binding, roughness / end play, hoses, fan belts, thermostat, etc. I believe in the rule "Try to only do the job once", i.e. if you have if off, check it; a little extra time spent in doing a good eye ball check plays off big later.

:upthumbs
 

USNA1969

Well-known member
Joined
Sep 12, 2005
Messages
93
Location
Maryland
Corvette
1972 Big Block/4-Speed
It's got 75K miles. It died a couple of times on the way home, but it didn't want to restart - I really had to play with it. I really had to nurse it to get it home, so when I got there, I didn't even try to restart it.

It was oil in the cylinders, although there was gas in there too. I could scrape it up with a carbon scraper and it came off as a thick gum. When I drained the oil to drop the pan, the oil had the consistency of water and a very strong smell of gas.

This repair has turned into a very slippery slope. "As long as I was in there, ..." I pulled the pistons to make sure I didn't have any broken rings and then found out that the crank pin bearings don't have any of the polished surface left. The bearings are basically smooth, and there aren't any scuff marks, but I'm going to replace them. And, "while I'm in there, .. " I'm thinking I might as well replace the main bearings. And as long as I'm this far along, I guess I should go ahead and check the timing chain, water pump, etc. as some of you have recommended. Fortunately, the crank pin journals look good and are still in spec and I'm hoping the same holds for the main journals when I pull the main bearings.

The up side of all this is that I shouldn't have to worry about taking long trips once it's up and running which, hopefully, will be soon enough to enjoy the nice weather.

Thanks for all your help.
 

Mr. Chuck

Well-known member
Joined
Jan 19, 2005
Messages
230
Location
Southern California
Corvette
1981 Two tone silver / dr grey
Timing Chain Failure

It's got 75K miles. It died a couple of times on the way home, but it didn't want to restart - I really had to play with it. I really had to nurse it to get it home, so when I got there, I didn't even try to restart it.

It was oil in the cylinders, although there was gas in there too. I could scrape it up with a carbon scraper and it came off as a thick gum. When I drained the oil to drop the pan, the oil had the consistency of water and a very strong smell of gas.

This repair has turned into a very slippery slope. "As long as I was in there, ..." I pulled the pistons to make sure I didn't have any broken rings and then found out that the crank pin bearings don't have any of the polished surface left. The bearings are basically smooth, and there aren't any scuff marks, but I'm going to replace them. And, "while I'm in there, .. " I'm thinking I might as well replace the main bearings. And as long as I'm this far along, I guess I should go ahead and check the timing chain, water pump, etc. as some of you have recommended. Fortunately, the crank pin journals look good and are still in spec and I'm hoping the same holds for the main journals when I pull the main bearings.

The up side of all this is that I shouldn't have to worry about taking long trips once it's up and running which, hopefully, will be soon enough to enjoy the nice weather.

Thanks for all your help.

My understanding is that when the timing chain has slipped the valve opening / closing are no longer in sync with the piston compression / exhust cycle and the residue you found is likely. Real bad things can happen if this sequence is off enough that valves are full open when the piston reaches top dead center and strikes the valve. I would also inspect valves for any sign of impact. Since you didn't find any bad rings and other low end have been checked, the timing chain is looking more likely to be the bad guy.

Good luck! :upthumbs
 

pgtr

Well-known member
Joined
Nov 20, 2002
Messages
324
Location
CST
Corvette
96Cpe
Over the course of the last 10 miles or so that I drove my car before it died, a barely noticeable miss developed into sever bucking and backfiring. The only things I haven't checked yet are the carburator and timing chain. Would either of those cause a gradual worsening of performance? The rings and valves are in good shape and, as far as I can tell, the distributor drive is functioning.
Thanks for any help.

When my timing chain failed - it just died on the spot.

Something similar to what you described happened to me when my distributor slipped once upon a time - I'd tightened it but not nearly enough and it slipped over several worsening iterations during a short drive until it backfired severely and wouldn't run further. I presume you already checked the ignition timing?
 

USNA1969

Well-known member
Joined
Sep 12, 2005
Messages
93
Location
Maryland
Corvette
1972 Big Block/4-Speed
When my timing chain failed - it just died on the spot.

Something similar to what you described happened to me when my distributor slipped once upon a time - I'd tightened it but not nearly enough and it slipped over several worsening iterations during a short drive until it backfired severely and wouldn't run further. I presume you already checked the ignition timing?

Actually, I didn't check it. It wouldn't idle long enough for me to check it. And, since it appeared that the proverbial 800 lb gorilla tightened the holding bolt, I don't think slippage was the problem. But thanks for the tip. I'll make sure I tighten it down when I reassemble the engine.
 

6880 Mike

Well-known member
Joined
Jun 30, 2006
Messages
2,723
Location
The Bluegrass State
Corvette
1968 convertible; 1980 L-82
If you're 3/4 of the way into a rebuild (rods, mains, rings, etc.) go ahead and replace the timing set. All things cnnsidered, timing sets are cheap.

:)
 

USNA1969

Well-known member
Joined
Sep 12, 2005
Messages
93
Location
Maryland
Corvette
1972 Big Block/4-Speed
I ordered the timing set and gaskets. And when I get it off, I may just opt for a new water pump while I'm at it.

Morre of the slippery slope. I hadn't even considered changing the rings because there was no apparent wear in the cylinder walls. They were smooth and polished looking without a ridge at the top, and they were in spec, at least to the accuracy of my mic and my ability to use it. But, now that I think about it, with 75k miles on it, there probably should have been some wear.

The rings I have don't show any scars, scuffing or burning, and the outside surface is smooth and polished looking. Do you think I should change them anyway?
 

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