Help Diagnosing Misfire 240Z L24

[Miles]

I installed rebuilt SUs from Z Therapy about 2 months ago to fix a rough, misfiring L24. It ran well for about 2 1/2 months and then started running rough like a bad plug or ignition problem. It also seemed to be losing power. I checked the ignition for misfiring clues even though all of the parts are new. The ignition seems to be working as it should. Then I checked for intake leaks. The carb gaskets leaked when I shot them with carb cleaner so I replaced them. The car still misses and idles rough especially when it's cold. The misfire improves as the car warms up.

 

The engine does not have the power it used to have. Up until now it pulled hard and I could spin the tires in 1st and 2ed gear.

 

The car sounds like: rrrrrrrrrrrrr blump rrrr blump blump rrrrrrrrrrr blump blump blump rrrrrr blump ...................

 

I did have the valves adjusted just before I installed the rebuilt carbs.

 

Could this be a burnt valve? What would the symptoms be if a valve is bad?

 

Thanks

 

Miles

[Miles]

I pulled each plug wire off at the distributor cap and there was no rpm drop on number 3 cylinder. Then I pulled the plug wires off at the the plugs and there was no rpm drop at the number 3 cylinder. On all of the cylinders other then number 3 there was a rpm drop when a plug wire was removed. Could this be a burnt or out of adjustment valve?

 

Thanks

 

Miles

[DCZ]

Miles,

 

Try a new spark plug in cyl #3, or switch spark plugs with another cylinder and see if the miss follows the plug. You can do the same thing with the plug wire. Pull the distributor cap and inspect the contact for the #3 cylinder. Also, pull off the valve cover and re-check your valve clearances on cyl #3. If the keeper nut wasn't torqued enough they may have wandered.

 

If those checks don't reveal anything, I would do a compression test or leakdown test to see if cyl #3 is out of whack.

 

Good Luck

[Tim240z]

Since it gets better as it warms up, check for vacuum leaks again. It could be that the expansion of the metals when warm is closing the leak.

Tim

[Miles]

I ran a compression test, adjusted the valves and hooked up a vacume guage.

 

Compression:

 

Cylinder/PSI

 

1/150

2/150

3/149

4/145

5/145

6/ 125 (190 with shot of oil)

 

 

All cylinders held pressure.

 

 

Vacume guage connected to the master vac connection on the balance tube shows a vibrating guage reading. The needle vibrates vigorously between 15 in. hg and 16.5 in. hg.

 

I have noted that there is one tapet clacking and had a shop adjust the valves about 3 months ago. The valve continued to clack and there was a noticable drop in power. Yesterday I checked the valve clearances hot and cold using the appropriate valve clearances. They all were adjusted so tight that a feeler guage would not fit between the valve stem and the tappet. I readjusted the valves cold at 0.008 in. (intake) and 0.010 exhaust so that the gauge made a stiff sliding fit between the valve stem and the tappet. The clacking valve is still present and the engine still runs rough through all speeds. The misfire is most noticable at idle especially when cold.

 

All of the ignition parts are new except for the coil. I am going to swap in a spare coil today to see if that helps.

 

Since the compression on cylinder 6 was lower then the rest of the cylinders, could the head gasket have sprung a leak near cylinder 6?

 

I have checked the usual places for vacum leaks using carb spray and water. The carb gaskets seemed to leak so I replaced them.

 

The carbs are new rebuilt from Ztherapy. Anything in a new rebuilt carb that would cause the engine to misfire?

 

What could cause the clacking valve if the valve clearances are ok?

 

 

Thanks

 

Miles

[Miles]

Problem solved

 

Number 6 exhaust valve was way out of adjustment. Also got rid of all old cheap ignition parts and installed a new OEM distributor, NGK plugs and wires followed by fine tuning the new Ztherapy carbs. Car now runs like it came out of the factory.

 

Miles