Howdy, Just got my 71 240z out and took it for a drive. I had replaced the spark plugs with the stock NGK plugs before the winter season. During the winter, I would start it, move it around, but never really drove it.

Today I took it out and had popping ( slight back fire) out the exhaust and no real power when I stepped on the gas. Got it home and checked the #4 and #3 spark plug, and they were black. I did turn the choke off when I was driving and it does not look like it's stuck on.

So I am assuming that the issues is mixture on the carbs, but was also thinking of going back to Bosch Platinum plugs. I had good luck with them in the past and on other cars.

I always thought that back fire out the exhaust would be a lean condition, so thought I would ask before I dive too far into it. 

Any advice is appreciated

Thanks

 

whats the carb setup? triples or twin SU? 

for 3&4 to be richer than the rest is unusual on twin SU. 

maybe the problem isnt 3&4 but 1,2,5,6 being lean. 

I'd start with working out why all the cylinders aren't the same. 

thanks for the response

The carbs are stock SU's

I just pulled #3 and #4 to see if there was a difference between the two carbs.......

I will pull the rest of them, but I expect them all to be the same.......... IF not, that will be another issue.......

ah makes sense. it can be too rich at idle and lean elsewhere. 

also worth checking ignition timing. 

 

well for starters, I went and pulled the distributor and the vacuum plate does not move, very sticky.

I sucked on the vacuum pod, and no movement. I guess I need a new one, I bet that will be fun to find.....

 If you couldn't suck any air thru the diaphragm, it's likely good and the problem lies in the bound up vacuum (base) plate. If you can get that to rotate freely I think it'll work.

Many times popping is excess fuel lighting off inside the hot exhaust system.  Poor ignition or too much full. Plug gap in the new plugs could be an issue depending on your particular ignition system

OK, Pulled the distributor and found the plate stuck, The diaphragm won't move when I took it off the distributor when I suck on it. I also found that one of the springs in the distributor is missing............

 

 

56 minutes ago, 70z4fun said:

also found that one of the springs in the distributor is missing............

That's a common thing! I found that more than once!  It's some kind of modification? My 240z has it to! i drive it now for over 20 years with just one spring!  

Have you done a compression test on your engine? i would do one.. Maybe you have worn cyl. or pistonrings or sticky valves?

We discussed the “missing dizzy spring” on a thread here some time ago. Long story short, it is by design, the spring is not missing. Concentrate on the sticking advance issue. 

I’m willing to bet one of your choke nozzles is stuck down even if the cable is trying to push it up. Give both a good finger shove upward under each carb.

Secondly check fuel deliverability. Pressure and volume.

Thanks for the reply

Yes, it's a European Distributor, since I looked up the number on it. Competition manual says one spring. Will look for the earlier thread.

Fuel system looks good. New fuel lines, fuel tank has been dropped and cleaned.

Will check chokes. I looked at spark plugs, all the same but one was a bit cleaner than the rest, So maybe some coolant getting in? Just changed plugs, will look at them again.  I did check timing, and put in my stock original distributor until I get the plate cleaned up.

Will a sticky plate cause running issues?

 

 

7 hours ago, zKars said:

We discussed the “missing dizzy spring” on a thread here some time ago. Long story short, it is by design, the spring is not missing. Concentrate on the sticking advance issue. 

I’m willing to bet one of your choke nozzles is stuck down even if the cable is trying to push it up. Give both a good finger shove upward under each carb.

Secondly check fuel deliverability. Pressure and volume.