I thought this "rebuilt" engine had a hot-cam installed in it.  Outside of the original Datsun specs and there are going to be problems for day-to-day driving.  This one might be set up for WOT and will run great, but driving in traffic will probably suck.

Way back in my teens (early 80's) I installed a "slalom" cam in my 240. Super strong from 4k and up but everything else sucked. I was always adjusting timing/carb/idle and never found the sweet spot. I finally pulled it out and went back stock. Lesson learned. As Jarvo said, if its got a hot cam, best of luck, as a lot of variables must be addressed to get it right.  

My car has a schneider 274f cam installed 

I guess the variables must be really examined. The engine was sent by PO to California Datsun and was bored from 2.4L to 2.6L. It also had a Far 311 race cam which i replaced with a Schneider 274F cam for daily driver. Nevertheless, an experienced mechanic told me that if remove the fan clutch and install a dual electric fan and bigger radiator the issue could be solved during traffic jams. Do you agree?

I think the mechanic is just guessing, but that is what most of us are doing. Educated guess work. It might fix it...

But it's not going to fix your fouling on #4...

22 minutes ago, Patcon said:

I think the mechanic is just guessing, but that is what most of us are doing. Educated guess work. It might fix it...

But it's not going to fix your fouling on #4...

This is what i have inside the engine. The only thing changed is the camshaft. Now a Schneider 274F. Ill take the car for a spin today and will pull out all the plugs again. Maybe there is a better solution than throwing money on unnecesary fans and radiator

That is what Al says is in the engine. Unless you've verified all of this then it may or may not be there. Ultimately, I suspect there is an internal issue causing your fouling on #4. I'm not really sure what it could be but I think if it were induction or ignition related we would have solved it already.

You mentioned the engine has good compression. Did you do a comp test? If not you may want to just to see if #4 is off. Rich on just one cylinder seems to isolate the problem 

20 hours ago, jalexquijano said:

The engine was sent by PO to California Datsun and was bored from 2.4L to 2.6L.

I think Cal.Datsun has installed the L26 crankshaft instead of the L24.. as the bore of a 2,4 is (exactly) the same as a 2,6 liter engine.. It's the stroke that makes the 2,4 a 2,6 L.  (Later came the 2,8 and that one has a bigger bore.)

11 hours ago, emccallum said:

You mentioned the engine has good compression. Did you do a comp test? If not you may want to just to see if #4 is off. Rich on just one cylinder seems to isolate the problem 

Engine had good compression on all the 6 cylinder. Asides from that, we even replaced the intake and exhaust valve seals from that cylinder last year.  Do you want me to tear the whole engine down? 

Have you tried swapping a spark plug and wire between cylinders to see if the issue moves or remains with cylinder #4?  

1 hour ago, DatsunZGuy said:

Have you tried swapping a spark plug and wire between cylinders to see if the issue moves or remains with cylinder #4?  

That was already done.  I guess i will need to check the float levels on each carb again and try.