240Z, 260Z, 280Z Performance / Technical Discussions related to performance motor enhancements, upgrades.

jumped timing chain?

Old 03-30-2015, 08:39 PM
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jumped timing chain?

So, finally got around to putting the finishing touches on my 73 240z projects motor swap. I swapped a seemingly good 260z motor with two progessive Weber 2 bbls in place of the very tired original. Bolted on a new exhaust system over the weekend. The cars starts right up and after putting in new plugs and wires seems to idle fine. Ran if until warm and set the timing at 7 degrees. Trouble is now it wont rev very far before backfiring out the carbs. When I shut it off it runs on. Tried advancing the timing, same result seems to lack power and wont rev freely. Big backfire out the carbs this time, scared the crap outta my daughter who was helping me. No run on this time when I shut it down.

what is this an indication of? Could the timing chain have jumped a tooth?

forgot to check and see if it is advancing when I try to rev it.

it has a crane electronic ignition system.

how does one check it the timing chain is the issue?


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Old 03-31-2015, 05:18 AM
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I would pull and look at your plugs.. backfiring outve carbs is usually a lean condition, backfiring outve exhaust is usually too rich.

If your nervous about the timing chain. pull valve cover and rotate the motor by hand to see what happened. I jumped timing on my built 2.8l 3 or 4 teeth last summer by reving it as hard as it could go in neutral.. ( dont do that! )

If you did indeed jump timing, ( i dont think you did, but it could be possible).. buy a new chain because they stretch too much when they jump.
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Old 03-31-2015, 05:22 AM
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Are you SURE you got a good motor?

Get the engine to TDC. Take off the valve cover and look at the cam sprocket. Reference the 74 FSM to ensure the timing marks are lined up correctly.

Check the compression on the engine and the clearances on the valves. If the intake valves aren't closing enough, that could cause the firing out the intake.

Are the plugs wet? Any black smoke out the back?
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Old 03-31-2015, 07:50 AM
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I cannot swear the motor was good, the guy I bought it from ran it for me but it was not on an engine stand so could not really rev it without it jumping off the blocks it was sitting on. Compression was good, do not recall the exact numbers but it was where it was supposed to be on all cylinders. Certainly the carbs could use some tuning. The old plugs I pulled out were on the black side no black smoke out the tailpipe though. I will check the new plugs I just put in and see what I have. Idles fine but again won't rev freely. I need to delve a little deeper I guess. Will advise, thanks for the input!
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Old 04-01-2015, 08:31 AM
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Oil Pump Maybe

You do anything with the oil pump... maybe the oil pump/distributor shaft is installed incorrectly, throwing the timing off.

If you put the engine at TDC, pull the distributor cap off, pull the rotor off, and the shaft should be at the 11:25 position.
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