Can a small block timing chain jump a tooth?

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ChrisRom

ChrisRom

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I have a low compression 340 in my 65 Dart. All of a sudden I had an engine surge, the engine kept running, I shut id down and would not start again. I checked everything on the outside and all seem fine, nothing obviously wrong.
Then I put a timing light on it and start playing with the distributor back and forth and got it running again. The catch is that it starts at 33 degrees initial....which cannot be right.
I changed the distributor with a spare one, same thing.
I brought the engine to TDC and the timing on the crank seem spot on. But the rotor in the distributor is way pass #1 cyl...
The engine was rebuilt 3-4 years ago, it has iron heads with larger valves, around 9.3-1 pistons, whiplash cam, nothing else fancy inside. The engine run great, no problems for the last few years.

What do you guys think may be the problem, before I start tearing into it?

Thanks,
 
Either that or something broke in the distributor AKA advance mechanism
 
A lotta the old engines, when they "ran-on", usually with a big puff of smoke outta the air cleaner, are actually running in reverse rotation, and will walk an old, stiff, stretched timing chain over a tooth, or plastic coated gears, same issue.
I know, there are going to be many say it can't rotate backwards, but thru the years I've seen it a couple times with my own eyes, and others as well.
So yes, it can jump a tooth, usually hi-milers.
 
Quote ; I've seen it a couple times with my own eyes, and others as well.

Yes I've seen it many times...
 
it could have jumped a tooth, not uncommon on the nylon timing cogs. Some cam guy could tell you where to measure split between intake and exhust rocker lifts to determine TDC, or very close. By the time you set up the dial indicator and mag base and look up cam specs, you could have taken the timing cover off and eyeballed it. Assume your gonna need a timing chain so may as well buy one before you get into it. If you find its something else, no harm, you got a spare. Consider a MP chain tensioner. Many poo-poo it but I think its a good upgrde to help cam timing stability and reduce chance of a chain 'whip' in the future that could cause another jump.
 
Simple to verify without taking it apart. Just verify tdc using a piston stop. Rotate clock wise by hand until it stops. Mark the damper. Rotate opposite direction until it stops. Mark damper. exactly split the difference between these marks. This is actual TDC. Mark the damper again but boldly. Check the damper for broken/worn elastomer band for slippage.
You can do the TDC procedure with the driver valve cover off to also verify TDC /cam watching rocker arm movement during TDC finding procedure.
 
Seen them turn 1-1 1/2 rotations in reverse, but they are not as prone to skipping a tooth when turned backward than an engine with a tensioner in the timing system. All that said, yep, they can jump time.
 
Before split duration cam profiles, and even with, - if you put #1, or #6 cyl lobes in "equal overlap", (eyeball it) - the timing mark should be very near TDC.
A jumped tooth generally show about ±12* difference.
 
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I don't think 340's had nylon cam gears.
 
To jump a tooth requires a slack chain; find out how much slack you have in your chain. remove your distributor cap, put a ratchet on the crank bolt, start with the damper on "0" slowly rotate the crank until the rotor just starts to move; remember/write down number, rotate the crank back in the opposite direction until rotor just starts to move. That will give you a good idea of how worn your chain is. You are looking for the least amount of crank movement relative to rotor movement.
 
Thank you everybody. The engine run well for few years (summer driving) with a small tick. I always suspected it was something with the chain because the valve train was checked and re-checked. It is an aftermarket single row chain, not nylon.... I'll pull the timing cover this week and see what's going on. Can a cam bearing going bad cause that?
 
JBurch said:
To jump a tooth requires a slack chain; find out how much slack you have in your chain. remove your distributor cap, put a ratchet on the crank bolt, start with the damper on "0" slowly rotate the crank until the rotor just starts to move; remember/write down number, rotate the crank back in the opposite direction until rotor just starts to move. That will give you a good idea of how worn your chain is. You are looking for the least amount of crank movement relative to rotor movement.
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Thanks, just did the test. It has slack quite a bit. I guess the mystery is solved. The crank pulley moved >15* before the dist. rotor started to move when I went in the opposite direction....
 
The "overlap" test would verify that.
With equal overlap, timing mark will be off the scale .
jmo
 
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Do you guys install chain tensioner on a LA block? I see them out there for magnums... are they the same?
 
15* is a lot on a timing chain. To answer your question, yes the chain tensioner will bolt on a LA block. It replaces the cam plate. It's a nice upgrade that keeps all of the slack out of the chain as it stretches. Some people say it's a waste . I put them in every engine I have apart. Keeps your timing rock solid.
 
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Hellrats said:
15* is a lot on a timing chain. To answer your question, yes the chain tensioner will bolt on a LA block. It replaces the cam plate. It's a nice upgrade that keeps all of the slack out of the chain as it stretches. Some people say it's a waste . I put them in every engine I have apart. Keeps your timing rock solid.
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I have always been concerned about plastic shavings from the tensioner. Maybe not such a big deal after all, I don't know.

I do know it takes a while to stretch a good double roller timing chain enough to make it a problem though.

If the block has been line bored and has excessive clearance I guess either a shorter chain setup or tensioner would be needed.
 
I have seen many 3.9 V6 engine with well over 200,000 miles that have never been apart. No plastic shavings I have ever seen when changing the oil. I'm sure the valve seals are gone on them though.
 
I won’t put a tensioner in, put one in and guide broke and the flat spring started to eats its way through the front cover. I buy the best chain (IWIS)? and sprockets I can find and get the right length and considered it a maintenance item.
 
The mystery deepens... just open the timing cover and everything looks good there. No sloppy chain, the motor is in timing.
Next suspects are the teeth on the camshaft or the distributor shaft.
 
no, it is a simple hydraulic flat tappet setup. It does have the 273 adjustable rockers.
 
ChrisRom said:
The mystery deepens... just open the timing cover and everything looks good there. No sloppy chain, the motor is in timing.
Next suspects are the teeth on the camshaft or the distributor shaft.
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Did you see if the balancer has slipped? #1 piston to TDC and check it against the balancer. There's a good chance the balancer is original.
 
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