purchased a 72 240z last owner apperantly had alot of work done to engine and then it sat for years. car starts and runs but not very well. checked compression and is very good on all cylinders, valve clearance is correct. pull of plug wires on 2,3,5,and makes no differance . changed wires and distibutor cap ect. timing is correct also. has triple mikunis stock fuel pump . changed front carb with back carb to see if would make diff. but didn't. any ideas what to do next. fuel pressure problem mabye? but cylinder 6 is strong ? help

That's strange since with triples 2, 3, and 5 would all be fed by a different carb so I don't think they are the problem. Just to be clear, the timing order is 1, 5, 3, 6, 2, 4 -correct? I'd first check to see if you're actually getting a spark on the problem cylinders.

Fuel pressure would be a problem under load. Are you running a Cannon manifold. I know the Cannons have a balance tube that connects all 6 runners and would make it more difficult to trouble shoot. The Mikuni manifold does not have that tube. I would look at the floats and needles and the condition of the fuel, bad fuel can gum things up.

That's strange since with triples 2, 3, and 5 would all be fed by a different carb so I don't think they are the problem. Just to be clear, the timing order is 1, 5, 3, 6, 2, 4 -correct? I'd first check to see if you're actually getting a spark on the problem cylinders.

firing order is correct, and yes getting spark on these cylinders, it's strange that one cylinder would be fine and the one next to it not when both run off the same carb. This is why I had thought it was wiring or timing or order. replaced the entire distibuter with a upgrade mallory electonic (replaced old electronic) ignition cap wires. By the numbers timing check. and then still had problem, thats when I changed the carbs around to see if it would make a differance. The carbs are just hosed together in series and then a return line back to tank.

This may sound a bit silly, but are the spark plugs good?

I was having a problem with ignition apparently mis-firing on '76 280z, thought I chased everything down (new plug wires, distributor cap, rotor, check ballast resistor, etc.) but it persisted. Was about to check the transistor ignition module when my son had the bright idea of checking the plugs. I used a timing light to verify spark on each, located two that weren't, moved plugs around and the trouble followed. Got new NGK plugs (replaced the Autolites) and the problem was solved.

Sometimes it is not as complex as we think...

Best regards.

PSB

Also use a Unisyn to check your syncronization on all cylinders. You could have bent throttle plates.

.....Was about to check the transistor ignition module........PSB

I imagine you're glad it wasn't that. Have you priced one of those lately? 300 bucks or so unless you get it from a junkyard!

This may sound a bit silly, but are the spark plugs good?

I was having a problem with ignition apparently mis-firing on '76 280z, thought I chased everything down (new plug wires, distributor cap, rotor, check ballast resistor, etc.) but it persisted. Was about to check the transistor ignition module when my son had the bright idea of checking the plugs. I used a timing light to verify spark on each, located two that weren't, moved plugs around and the trouble followed. Got new NGK plugs (replaced the Autolites) and the problem was solved.

Sometimes it is not as complex as we think...

Best regards.

PSB

Actually not a bad idea, I did put in all new plugs but you never know. When I checked for spark on al lcylinders it was just the wire I didn't check the plug. Just assumed that new ,they should be good. Will check that next.

Thanks, Jim