Jump time?
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Joined: Dec 2018
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Jump time?
How common is this? 76 l82. Installed EFI. Ran pretty well. Alaways has some weird valve noise and low vacuum (8 with carb). After several days engine running worse and worse. Vacuum down to 2-3in. Won’t run well enough to drive. I threw a light on it tonight and with it idling, the timing Mark is no were to be found. Pulled cap and it was 4 -5 cylinders off. I pulled engine back to 0 TDC using mark on crank, resest distributor to point rotor to 1, and now no start at all.
Pretty sure chain jumped. Sound right?
Pretty sure chain jumped. Sound right?
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Vacuum dropped a lot. Most likely. Pull either valve cover. Bump motor over. You are watching for both intake and exhaust moving on either 1 or 6 as timing mark passes pointer at zero tdc. If both valves moving, exh just closing and intake just opening at tdc, you are in time. Sounds like many teeth off.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
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From: Unreconstructed, South Carolina
BR-OP
You may find a failure-prone OE Nylon-jacketed, aluminum-based Cam Gear/Sprocket.
With heat and age the plastic-capped nubs becomes brittle and break away: bits fall into oil pan; leaving little-to-nothing for chain to mesh with.
When that happen, timing chain slips aka "jumps time."
If you find partially-denuded Cam gear; you really should pull pan AND oil pump pickup/screen; clean both if not also replace screen.
Plastic bits can & do get into oil pump causing pump failure.
You may find a failure-prone OE Nylon-jacketed, aluminum-based Cam Gear/Sprocket.
With heat and age the plastic-capped nubs becomes brittle and break away: bits fall into oil pan; leaving little-to-nothing for chain to mesh with.
When that happen, timing chain slips aka "jumps time."
If you find partially-denuded Cam gear; you really should pull pan AND oil pump pickup/screen; clean both if not also replace screen.
Plastic bits can & do get into oil pump causing pump failure.
Burning Brakes
Joined: May 2013
Posts: 789
Likes: 129
From: Los Banos CA
As Derekderek mention - you can't just use the timing mark to find TDC. The mark will be lined up on TDC on the compression stroke and TDC on the exhaust stroke. However, I think that you want both valves on cylinder 1 to be closed at TDC for the compression stroke?
Last edited by bradleyb66; Feb 13, 2019 at 10:45 AM.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
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From: Cool Northern Michigan
I really doubt it jumped time. For one thing, I have yet to see a SBC chain jump a tooth. Stretch, yes. Jump, no. There is no room in the timing cover for that to happen. I have seen it on SB Fords. And if it truly jumped a tooth, it is now firing almost on the exhaust stroke. The headers or exhaust manifolds will be cherry red.
So, having said that, what's the story on your dizzy clamp? I have had brand new clamps (chrome) allow the dizzy to retard / slip. Its not something that happens all at once, but very slowly over time. There is a lot of force trying to twist that dizzy.
When you put your timing light on it, what was your vacuum line doing that goes to the dizzy? Hooked up? Unhooked without being plugged off?
So, having said that, what's the story on your dizzy clamp? I have had brand new clamps (chrome) allow the dizzy to retard / slip. Its not something that happens all at once, but very slowly over time. There is a lot of force trying to twist that dizzy.
When you put your timing light on it, what was your vacuum line doing that goes to the dizzy? Hooked up? Unhooked without being plugged off?
Last edited by HeadsU.P.; Feb 13, 2019 at 08:08 PM.
Thread Starter
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Joined: Dec 2018
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Dizzy? Distributor? It’s an electronic distributor as part of the EFI system. No vacuum to it. Engine only making 3” of vacuum idling. Backfiring, etc in exhaust.
Heres the thing. Timing was set at 12 when system installed. 4 days later, it’s making 3 “vacuum, barely will start and idle, and timing mark not even viewable when running.
Some say you can’t jump time, some say nylon coated teeth break. I have no other explanation for what happened.
Sat, it gets torn down and I’ll see for sure.
Heres the thing. Timing was set at 12 when system installed. 4 days later, it’s making 3 “vacuum, barely will start and idle, and timing mark not even viewable when running.
Some say you can’t jump time, some say nylon coated teeth break. I have no other explanation for what happened.
Sat, it gets torn down and I’ll see for sure.
Last edited by BobRiley; Feb 16, 2019 at 12:25 AM.
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Race Director
Joined: Jul 2015
Posts: 13,082
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From: SW Florida.
put it on TDC mark. look at the rockers on 1 or 6. work balancer back and forth and see if rockers are both moving at that point. that will tell you if your cam is in time. also pull your distributor. see what the gear looks like. it could have had a brass gear put on a couple decades ago and has shed teeth. if so, pan and oil pump has to come out anyway to get the brass out of the engine, and virtually all 40 year old chains could be replaced, but it is better to know what is going on ahead of time.