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In the process of getting my 260 (with round-top carbs) smog-ready, I did the following: Replaced wires (in proper order), rotor, distributor cap; check valve, pcv valve, and oil and filter; Used vacuum cleaner to vacuum through outlet port where the smog pump outlet hose was to be installed; Blew through outlet port through a hose;Installed a used smog pump (pumps strongly) and a used anti-backfire valve (took this valve back off); Cleaned and gapped NKG plugs to 35 that have few miles and reinstalled, in order to avoid blackening my new set until I run it some and adjust the valves, etc.; Cleaned carb's pistons, inside of domes and visible areas inside of carbs; Used 10W-30 oil in dampers; Richen carbs a fourth of a turn and then another fourth to compensate for the extra air from the pump; has new k&^G air filter in stock box; Reset back to original setting when no progress was evident; Choke works smoothly and evenly on both carbs.

Prior to the above, there was no back firing. After the above, It runs very rough, RPMs dropped, backfires while pressing the accelerator; Checked each new plug wire and plug for fire (each fires despite all except cylinder #1 plug had blackened #6 was pretty clean); Cylinders 1,2 and 5 dropped affected performance greatly, backfired and stalled when I took a wire off one at a time; #1 blew a little smoke out of the plug hole; Taking the wire off of each of the other cylinders made little or no difference in performance. Help, What seems to be going on? Thanking you in advance for any help/suggestions/opinions. Ernest, Las Vegas
 

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hello last time that happen to my 71 240z...(pulling the plug wire and no respnse from the engine ) it was the plugs,,, yes brand new NGK' two where bad....i did everything you had done as stated above and it ran bad.....i double checked everything and it was ok....so even though i got some spark from each plug ..two where not a nice blue color....try putting in your old plugs or just replacing the plugs on the cylinders that have little or no effect on the engine when you pull the plugs.wire..also be sure you did not pull the plug end (inside boot) off the wire, sometimes they will diconect and you dont even no it till the car runs rough...also N.G.K's need to be torqued to specs on the box...i think like 17-23lbs...somthing like that..its written on the box of plugs...good luck michele
 

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You just described what my car was doing prior to it's last visit to Dr. Mechanic.

His finding: The vacuum advance to the distributor actuates this little plastic thingie. I don't recall what he called it, but it essentially allows the distributor to be advanced by the Vacuum Advance Solenoid pot. Without it, even though you set your timing correctly, the pull or push of the vac pot will actually throw your timing off, and since there is no spring return or easy action for the vac advance it goes crazy, one time back firing another time poor acceleration, and generally doing all sorts of crazy things. It is a two disk black plastic thing with ball bearings and a spring on it. He showed the new one to me, but I will admit that this is his area of expertise and I just nodded my head and told him to go ahead with the procedure.

There was a post earlier on this item, and the part is still available from Nissan.

I admit that I have all my engine work done by this mechanic, but he is a specialist in the engine and mechanicals, while I am at best a General Practitioner on Body Work and electrical. When I got the car back, what a difference, I have power, acceleration, fast response, the difference could be equated to night and day.

FWIW
 
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