I have a misfire on one of my cylinders(4th from front). I used to get a misfire only when the engine was warming up but now it never fires.

I checked and I do have spark at the plug. I even switch plugs and wires and still misfired while the switched plug and wire fired.

I don't have a compression tester, should I pick one up and test the compression? The Plug in the misfiring cylinder is wet with gas so I can assume that the fuel/air is not the problem.

My guess is it is a bad valve. Should I take off the valve cover and look for a bad spring or something unusual? Thanks, Jeff

EDIT: 260z L26

 

new dizzy cap. clean that fuel injector. could be stuck open. then if that doesn't work, check the compression

 

It is Carbed. No injectors.

If it was the dizzy cap wouldn't it be effected on all cylinders?

 

oops. forgot to look. that narrows it down. no. the inside of your dizzy cap has 6 contacts, and it's really impossible for it to be made with them all ex****inzactly the same, so one is likely to wear down a little before the rest will follow. now the plug is getting some spark, but not enough because the contact is worn. possibly. or.... you are right, and there is a valve or cylinder problem.

 

would it be possible to take the cap off and make a visual assessment of whether or not the cap needs to be replaced (I am kinda poor right now.)

 

not really. the problematic contact MAY be a little burnt, but you won't be able to tell for sure. the cap is less then $10 i bet. might as well go ahead and get a rotor too. if you are that poor though, i would check the compression first so that you don't spend money on something you don't need. just borrow or rent a guage

 

ya I have been trying to rent one but can't find where (autozone does not lend them)

 

Exactly...

Any neighbors? Local mechanics? Other local parts stores?

 

Pull all the plugs and use your thumb to compare the pufftt on each cyclinder. Check the dizzy shaft for side play.

 

second the dizzy shaft thing, but since this is a carb motor i doubt you loss compression in only one cylinder ive never had a datsun with anything other than +/- 9psi from factory specs in my possession these motors seem very resilient also get the timing checked and do a full tune up you cant be that poor

 

No dizzy shaft movement.

The poor newbs compression test worked. a good cylinder pushed my thumb off the spark plug hole and the bad one did not.

What is next? I guess it is either a valve or rings right? Most likely valve because all the rings would go not just one right?


Thanks, Jeff

 

i would guess a valve. to be sure, so you don't waste any money, you will have to do a leak down test. it requires a tool.

 

leak down test wont render anything if its lost compression. theres two ways to tell, one you need a compression tester and some motor oil, the other you need to tear open the motor.

 

ya I know the test for rings I just need the compression tester. Thanks

I will try and pick one up soon.

 

why won't a leak down test work? but true, i forgot about the easy way to find the problem in this area. but a leak down test would show weather the pressure loss was through the rings or the valves would it not?

 

no... leakdown test from my understanding is to see if the cylinder holds pressure over time, if it doesn't right off the bat theres nothing to leak down... test... or... something...

 

Alright I got a compression tester.

I tested the bad cylinder and got just under 30psi
I tested a good cylinder and got 160psi
I put two capfuls of motor oil in the bad cylinder and got the exact same reading.

The choke was closed but that was constant for all the tests so I think I still had accurate results.

The Results say it is a bad valve right? What do I do next?

 

fix the valve. durrrr. haha. take off the valve cover see if its stuck open or if it blew apart or something single cam heads of this age are pretty simple.

 

I took the valve cover off and found the rocker arm is off the valve spring. And the little metal piece(Lash Pad?) that connects the rocker arm to the valve spring is off. How do I go about getting the arm back on the spring?

 

Anyone?

I tried a valve spring compressor but I could not compress it enough to put it back on with out the compressor slipping off.

 

Rotate the crank / cam so the cam lobe is straight up, then get up on the strut tower so you have more body weight to push down. It's a lot easier when you have a second person who can slide it back under.... otherwise it's super easy to slip off the spring and shove some nice metal into your knuckles.

Then check and re-set the valve clearance.

 

I can't push down the valve far enough. I am not weak or a small, is there something wrong with the valve?

How do I adjust the valve clearance?

 

Can you push any of the other valve springs down?

As for adjusting valves... the Haynes manual has detailed steps... if you don't have a Haynes manual, I suggest you stop work on the car until you have one.

A good read: http://atlanticz.ca/zclub/techtips/v...ust/index.html

And here's a personal scan from the Haynes manual:

 

Thanks for the scan. I have the FSM and I will try and get the haynes manual.

As for pushing the valve springs down, I can push all of them down only slightly(not enough to slip the arm over it) and definitely cannot hold them down.

Possibly could the valve springs not be stock and be firmer?

 

The FSM has the same info as the Haynes for adjusting valves.

Are you just pushing down on the spring w/ the force of your arm? Kneel on top of the strut tower / fender & push down w/ your weight.