Loose timing chain.
Thread Starter
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
Loose timing chain.
Hi, I was checking my chain earlier, and found that I can rotate the crankshaft by about 7* before any any movement can the seen at the distributor rotor (or valves). I assume this is too much and the chain should be replaced (correct me if I am wrong). Anything else I should replace at the same time? I replaced the balancer not long ago so that should be fine.
Could this also cause a lower than normal manifold vacuum? I'm getting a stable 15-16"
Could this also cause a lower than normal manifold vacuum? I'm getting a stable 15-16"
Instructor
Joined: Jul 2016
Posts: 195
Likes: 37
From: Poway CA
Antz-
I am no expert, but I think this play would be normal because of all the spacing between gears and the chain and sprocket itself. Once the motor is in motion, however, it is not bouncing around because of the forward angular momentum of the engine, kind of like rotating pedals on a bicycle... as long as you keep pushing on the pedals in the direction of rotation, you will have constant contact as needed. However, your chain should not be sloppy, due to stretch. If it has over 100,000 miles on the chain, I would replace it as long as you're in there- they are not that much.
Kirk
I am no expert, but I think this play would be normal because of all the spacing between gears and the chain and sprocket itself. Once the motor is in motion, however, it is not bouncing around because of the forward angular momentum of the engine, kind of like rotating pedals on a bicycle... as long as you keep pushing on the pedals in the direction of rotation, you will have constant contact as needed. However, your chain should not be sloppy, due to stretch. If it has over 100,000 miles on the chain, I would replace it as long as you're in there- they are not that much.
Kirk
Team Owner
Joined: May 2001
Posts: 87,301
Likes: 1,583
From: Western NY
If you go with a Cloyes "True Roller" timing set, there should be VERY little slack in the assembly, therefore very little movement in the crank gear, before the distributor starts to rotate.
FWIW, a number of years ago, I replaced the OE timing set on a 1993, 50,000 mile, 350 cu.in. small block Chevy, with a Cloyes TR set. After doing the job, I put a timing light on the engine, and the timing had changed by 3*. So, yes, I think you'd see a difference with a new timing set, but I don't know if it would affect your vacuum.
FWIW, a number of years ago, I replaced the OE timing set on a 1993, 50,000 mile, 350 cu.in. small block Chevy, with a Cloyes TR set. After doing the job, I put a timing light on the engine, and the timing had changed by 3*. So, yes, I think you'd see a difference with a new timing set, but I don't know if it would affect your vacuum.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,444
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Any c3 motor with a stock timing chain needs to be replaced after 50K miles or any signs of slop no it is not like a bicycle in the above post
When the chain becomes longer it is retarding the valve events and timing
Replace with quality double roller timing chains
When the chain becomes longer it is retarding the valve events and timing
Replace with quality double roller timing chains
Racer
Joined: Apr 2016
Posts: 493
Likes: 65
This is the only fix
Thread Starter
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
I've got it all open now. The chain appears loose just looking at it. Not sure but may be the original chain. It certainly was the original water pump. I'm going to go with a cloyes double roller. I'll also replace the pump since it's off.
Safety Car
Joined: Apr 2006
Posts: 4,057
Likes: 421
From: BC
Hi, I was checking my chain earlier, and found that I can rotate the crankshaft by about 7* before any any movement can the seen at the distributor rotor (or valves). I assume this is too much and the chain should be replaced (correct me if I am wrong). Anything else I should replace at the same time? I replaced the balancer not long ago so that should be fine.
Could this also cause a lower than normal manifold vacuum? I'm getting a stable 15-16"
Could this also cause a lower than normal manifold vacuum? I'm getting a stable 15-16"
Last edited by CanadaGrant; Feb 13, 2017 at 12:49 PM.
Team Owner
Joined: May 2001
Posts: 87,301
Likes: 1,583
From: Western NY
One other thing besides the loose timing chain is distributor shaft end play. Because of the helical gears on the cam and distributor, shaft end play can cause large variations in timing like you describe. I don't doubt your chain is worn but correct shimming of the distributor shaft will also cut down on large timing "movement" under acceleration and deceleration. For roughly every .013" of end play it's one degree of timing slack or movement and .050" shaft end play or more on some of this old stuff is not uncommon.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
Thread Starter
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
One other thing besides the loose timing chain is distributor shaft end play. Because of the helical gears on the cam and distributor, shaft end play can cause large variations in timing like you describe. I don't doubt your chain is worn but correct shimming of the distributor shaft will also cut down on large timing "movement" under acceleration and deceleration. For roughly every .013" of end play it's one degree of timing slack or movement and .050" shaft end play or more on some of this old stuff is not uncommon.
Thread Starter
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
Slowly getting things back together. New timing chain is on, and is alot tighter than the old one. New water pump fitted. I just need to get a 14mm hex bit to tighten the bung on top. I had play with the old fan clutch and decided I didn't like the way the bearing felt so I should have a new one arrive tomorrow.
I checked the distributor end play. I will need to address this (I got 0.066"). No one locally sells the shim kit, so I will have a look at making a shim or shims at work later this week. (I would use 316 Stainless or maybe 4140 steel, not mild steel) But I may end up ordering some.
I checked the distributor end play. I will need to address this (I got 0.066"). No one locally sells the shim kit, so I will have a look at making a shim or shims at work later this week. (I would use 316 Stainless or maybe 4140 steel, not mild steel) But I may end up ordering some.
Team Owner
Joined: May 2001
Posts: 87,301
Likes: 1,583
From: Western NY
Slowly getting things back together. New timing chain is on, and is alot tighter than the old one. New water pump fitted. I just need to get a 14mm hex bit to tighten the bung on top. I had play with the old fan clutch and decided I didn't like the way the bearing felt so I should have a new one arrive tomorrow.
I checked the distributor end play. I will need to address this (I got 0.066"). No one locally sells the shim kit, so I will have a look at making a shim or shims at work later this week. (I would use 316 Stainless or maybe 4140 steel, not mild steel) But I may end up ordering some.
I checked the distributor end play. I will need to address this (I got 0.066"). No one locally sells the shim kit, so I will have a look at making a shim or shims at work later this week. (I would use 316 Stainless or maybe 4140 steel, not mild steel) But I may end up ordering some.
Safety Car
Joined: Jan 2008
Posts: 3,791
Likes: 428
From: Aarhus, Denmark
I replaced chain setup last week, on a 71
I needed it a "bit"
Thread Starter
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
From what I can tell they no longer sell the shims. I did a Google search for them, found summit and Amazon still have them on their sites, but not available.
Safety Car
Joined: Apr 2006
Posts: 4,057
Likes: 421
From: BC
Glad you checked for end play. At .066" play that's 5 degrees of timing slack right there. I think Moroso also makes a shim kit but not sure of the size selection in the kit. There are some listed on fleabay under "distributor gear shim kit" I think Corvette Central also sells them. Between that and the loose chain, looks like your timing problem will be cured.
Last edited by CanadaGrant; Feb 21, 2017 at 06:09 PM.
Racer
Joined: Apr 2016
Posts: 493
Likes: 65
If I can help you get parts, and ship them to you, let me know. I know you can ship them from vendors with no issues, but if you run into something, let me know. I've lived in other countries and trying to get parts can be a tough one. If I can help, let me know!!!
Team Owner
Joined: May 2001
Posts: 87,301
Likes: 1,583
From: Western NY
Thread Starter
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
thanks for the offer. Most places will ship straight to NZ but I also have a US shipping address that I can use for larger items or items that the vendor won't ship to nz. (It just takes a bit of time as they come via ship)
Thread Starter
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
All back together and running. Timing moves around a bit less when setting it, not that it ever moved a lot. I will still get some proper shims for the distributor.
Found a good tip for installing the distributor, move the engine to TDC on #1 compression, then fit the distributor with the rotor roughly at #1. if to doesn't line up the the oil pump turn the engine over by hand and the distributor will drop down when it does line up, but will stay in time with the engine. This made the job so easy.
After all this no change to the vacuum readings. (but at least I know this was not the problem) Any ideas what could be the cause for this? I have done a thorough check for vacuum leaks and found nothing. Timing is set at 6*BTDC. Idles is 700RPM in Neutral. I'm not 100% sure but I believe it is still the factory cam. I know ignition modules normally just fail completely, but can they fail so that they don't advance the timing correctly? I'm sure mine is still the factory module (With the yellow dot on it) and do have a spare module anyway.
Found a good tip for installing the distributor, move the engine to TDC on #1 compression, then fit the distributor with the rotor roughly at #1. if to doesn't line up the the oil pump turn the engine over by hand and the distributor will drop down when it does line up, but will stay in time with the engine. This made the job so easy.
After all this no change to the vacuum readings. (but at least I know this was not the problem) Any ideas what could be the cause for this? I have done a thorough check for vacuum leaks and found nothing. Timing is set at 6*BTDC. Idles is 700RPM in Neutral. I'm not 100% sure but I believe it is still the factory cam. I know ignition modules normally just fail completely, but can they fail so that they don't advance the timing correctly? I'm sure mine is still the factory module (With the yellow dot on it) and do have a spare module anyway.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,108
Likes: 9,236
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I replaced chain setup last week, on a 71
I needed it a "bit"
https://www.youtube.com/watch?v=pf_zJlf3y7k
thats amazing how did it run before and after the replacement
Safety Car
Joined: Jan 2008
Posts: 3,791
Likes: 428
From: Aarhus, Denmark