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71zman

71 240Z Valve Problem

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My 71 has been running really poor lately. Well, I took a compression test and WOW, no compression on 2,3 and 4 - no wonder. It appears that the exhaust valves on all three (2,3,4) are really tight. Even when I adjust the the screew all the way down, I cannot get a feeler gauge under it. All the other valves are all about right. Suggestions short of ripping the head off? :cry::cry:

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Someone must have told you to get bent, but it appears your valves already did instead...

Sorry man,

You could remove a manifold for a look through the ports.

Will

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Even when I adjust the the screew all the way down, I cannot get a feeler gauge under it.

Something is wrong. Inspect the entire valve train. There should be plenty of slack when the adjuster is all the way down. Is the cam in the correct position to check that particular valve?

Remove all the spark plugs, and try turning the engine over by hand (big socket on the crankshaft bolt). Pay attention to those #1-3 exhaust valves when it's there time to open. Is there any resistance? The valve should open and AND close freely. When both intake and exhaust cam lobes are pointing up for that particular valve is when you check clearance.

Do not start the engine until you figure out what is wrong. You could be damaging it more every time.

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Yes, I have been checking adjustment when the cam lobe is at the top. I am pretty expereinced at these motors, this is the 4th 240Z I have rebuilt, I have always done all the work myself.

A little more of the story - I rebuilt this engine about 2 years ago and it never did run right - it always had a slight miss at low rpm's. I tinkered with it for about a year before giving up and taking it to a mechanic. I thought the problem was the carb's and timing because I upgraded this time to a petronix ignition and weber carbs

Pierre's in Hawthorne CA told me that one of the valves was adjusted too tight and caused that cylinder to miss. He adjusted it and compression went back up but the engine did still not run right - still a slight miss at low rpm's. I drove the car for about another year and got to this point where it was running worse and worse.

I suspect that maybe what happened is that when I rebuilt this engine, the machine shop replaced some of the valves - I wonder if they used the wrong valves that might be a little longer than the original 71?? Or maybe he did something wrong in the grind??

Could another possibility be that when he put the cam back in the head, it sits a little higher than it should creating a tolerance problem?

One clue to this is that it is all exhaust valves in the middle of the head (2,3,4)???? When a valve burns, will it sit a little higher in the head??

Thanks for the replies - it is great to have nice folks who willing to help.

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Hey Carl,

You might be on to something here.

My initial thoughts where bent valves, cam towers need shimming [correct valve geometry] or WTF!

In reality, the only why forward now is to remove the head, there must be something, someone forgot about.

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I actually like the shim theroy, if the cam assembly started out lower than specified, "mechanical settling" might account for the changes over time.

Regardless, I bought the new gaskets yesterday and will start taking it apart. Luckily I have a couple of Z engines in the back yard that I can salvage a head from and get it reworked. Although I don't think I will go back to that same machinist. I will re-post when I find something.

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