I have a 73 240z with flat top carbs. It feels as if the car is only running on 5 cylinders. It idles really rough. I took the spark plugs out today and the plugs in cylinders 1-4 looked normal, but in 5 and 6 they were really black. Is it running rich? The manual says to turn the idle mixture screw counter clockwise to increase the mixture. Is that correct? Also I have replaced all spark plug wires, and sparkplugs, should I check the distributor cap?

For the price of a cap & rotor .... replace them if you haven't or don't remember when it was done. Check the throttle linkages for any stiffness

on 'return to closed' position. Check & see if the chokes ( on the bottom of the carbs) are closing fully with the choke lever ( will run rich if one or both do not close fully). check / replace the air cleaner if you can not see through it from the inside to outside. Is the black on #5 & #6 plugs wet or dry?

- Jeff

Also check the points if you are still using a points type of ignition, I would refrain from messing with the carb settings til you eliminate other possibilities, also was this a all of a sudden thing ? did it run fine and slowly went to crap and a bit of info on the motor would be helpful , stock mods or any new parts put in , good luck

1st thing dump the flat top carbs. I will bet a dollar against a hole in a donut , round top SUs will cure the problem . The flat tops is one of the reasons that Nissan went to injection . These are emissions carbs and they were not good when new. Untill you replace them I am afraid that you will be going in circles trying to tune the engine. my 2c

check that you haven't blown a headgasket

1st thing dump the flat top carbs. my 2c

Do the other things, cap, rotor, points, etc. first. There are a few of us that don't believe that 'lose the flat tops' theory the minute you have problems. Many times the problems aren't even carb related. I sometimes wonder if Z Therapy themselves began this train of thought. An answer to seerex's question would be helpful here.

sblake, I have a question of you . What type of carbs are you running now? What type were you running on the past 240s ? I stand on my openion. Gary

sblake, I have a question of you . What type of carbs are you running now? What type were you running on the past 240s ? I stand on my openion. Gary

beandip, we have had this discussion many times. It's time to accept the fact that flat tops can be made to perform as good [and in my case] better than round tops. I for one run both [240 & 260] and can verify that once properly overhauled they do in fact perform better than roundtops. sblake's conspiracy

theory may be valid, but who really cares, it's all been consigned to history.

Why where they dumped? For the same reason roundtops where...outdated and superseded by FI.

It's purely a personal view and I respect your opinion, however, you should also reciprecate and respect ours...

Off my soapbox...

MOM

The plugs in 5 and 6 were wet black. I have brand new Omni spark plug wires, month and a half old air filter, modified down pipe and free flowing Monza exhaust system.

sblake, I have a question of you . What type of carbs are you running now? What type were you running on the past 240s ? I stand on my opinion. Gary

And I also stand on mine. I don't personally have a car with flat tops but I have a freind who has two. He also owns and operates a Z Car repair shop and has for over 20 years. His cars run fine with them, he knows how to work on them and scoffs at the idea that you need to toss them the minute your car sputters and misses. I've driven both of his cars and they both seem to back up his opinion. That's good enough for me. We talked about this in another thread a while back. There were others who agree with me and those who don't. That's fine. I'm not knocking your opinion. But I am entitled to my opinion as are you and anyone else.