77 280z jumping time

[ronroyboy]

I have a 77 280z all stock original 78000 miles that jumped the timing chain bending the valves. I replaced the head and timing chain and it did it again but luckly the valves didn't hit the pistons this time. I reset the timing chain and tightened the tensioners and it ran perfect. This morning I went to start it and it turned over very fast like it did it a third time. Can anyone give any suggestions what I am doing wrong? maybe a cheap ebay timing set that I used?

[Jeff G 78]

That makes no sense. When you tore it down, was there any sign of damage to the chain or guides? How many teeth did it jump? When you replaced the chain, did you also replace the gears, guides and tensioner? I have never had a Z chain jump regardless of the age or condition of the engine, so for it to happen 2 or 3 times means something else is going on. Please give us more details about exactly what happened, the driving conditions, assembly method, etc.

[ronroyboy]

That makes no sense. When you tore it down, was there any sign of damage to the chain or guides? How many teeth did it jump? When you replaced the chain, did you also replace the gears, guides and tensioner? I have never had a Z chain jump regardless of the age or condition of the engine, so for it to happen 2 or 3 times means something else is going on. Please give us more details about exactly what happened, the driving conditions, assembly method, etc.

The first time it happened when the valves hit the pistons I didn't check to see how far the chain was off.

The second time It was one link off. The third time I havent tore into into it yet. I did replace the complete timing set with tensioner and guides. I also replaced the head with one from a salvage yard and installed new rings and bearings since the motor was out. I thought it might be easier to replace head and timing with engine on stand. All three times it happend it did it as I was starting the car. the car was running perfect when I parked it last night and the problem came back this morning as soon as i turned the key. that is the same thing that happened the first and second time. maybe a tight sticking valve preventing the cam from turning? but on two heads. Also when installing the crank gear it just slides onto the crank. I beleive the haynes manual said to use a puller. I am thinking about buying a rebuilt head before doing anything else but I will be sick if I put on a $450 head and the chain jumps again and bends the valves. There was no signs of damage to the original chain. The original guides did have a little wear. The new guide on the tensioner side had two sets of holes the first time is used the one that would make the chain less tight the second time I used the other hole that held more pressure on the chain. I think the original guide only had one hole.

[Jeff G 78]

My only thought on the root cause of the second failure was if the used head you bought had been shaved before you bought it. That would make the distance between gears to be short and cause chain slack. Did you check the notch and mark per the manual after you put it together?

[Zed Head]

I would confirm that you have a problem first. "Engine spinning fast" doesn't tell much. Check cylinder pressure, or pull the valve cover and check your valves and keepers. Check the cam position at TDC via the notch and groove. Might not be a chain issue at all.