weird backfire on intake and misfire on my 72 240z

[jalexquijano]

Yesterday i cranked my 240z and drove with the choke pulled for 5 minutes until i reached to normal temperature as otherwise the car misfires and stumbles. Once on the speedway i pushed the choke lever off and when accelerating the car started to pop or backfire at the intake. In order to avoid this i had to add some choke so i could reach back home. Is this a symptom proving that the Airtex 8012s electric fuel pump is failing? I had this problem 2 years ago, replaced the pump and never had this again.  What is the mínimum PSI that i need in order to avoid this problem? Both of my carbs are set to 2.65 turns clockwise and have been balanced.

https://www.amazon.com/Airtex-E8012S-Universal-Electric-Fuel/dp/B000DT7Y7A

 

[Mark Maras]

 If the outside temperature has dropped, it's likely that the mixture is a bit on the lean side. In that case, you just need to richen up the mixture screws a bit. I'd try an 1/8 turn, drive it, and see if it improves. IMO, this is good news. You've finally got it leaned out and now just have to fine tune the mixture. I'd pull the plugs and take a look at them before and after the mixture adjustment. I doubt that this new problem has anything to do with the pump. 3-3 1/2 PSI seems to be a good pressure to run. I wouldn't go past 4 PSI. Don't know the minimum PSI, It depends on the engine load but I'm betting that the engine would idle with as little as 2 1/2 PSI. If your pump wasn't putting out enough fuel, adding choke wouldn't affect the mixture. 

[Patcon]

Did you check to see if you had a sticking choke?

[jalexquijano]

No sticking choke! 

[Patcon]

Did you reach under the carb to confirm that or just push the handle down?

[jalexquijano]

i reached under the carbs. I have them dialed at 2.5 turns and will start making 1/8 increments but avoiding to reach to 3 turns clockwise. How many minutes should i leave the choke open once i crank the car?

[Mark Maras]

 The hesitation (engine lacking power) that you experienced before you corrected the mixture with the choke is a good lean mixture indicator. You can gradually close the choke as the engine warms up.  If you close the choke too far before the engine is warm enough, the engine will lack pulling power. IMO, the exact symptom that you described and cured with the choke in post #1. Remember to pull the plugs when you think you're close to the right mixture. I'm sure you're very good at this by now. Watch for plug #s 1, 2 & 3 being a different shade of brown than 4, 5 & 6. That would indicate that 1 carb is running richer or leaner than the other. We can deal with that if it occurs. Just part of the fine tuning process. Let us know what effect the 1/8th turn has.

[jalexquijano]

im starting at 2.5 turns again with Brand new ngk bp6es plugs.

[siteunseen]

On 11/13/2017 at 12:34 PM, Patcon said:

Did you check to see if you had a sticking choke?

My cables had slipped over a year or so.  Once I got those adjusted it fired right up.  If I remember right 2" down with choke,  1 1/4" without. Measured from the clamp down on the cable to the bottom of the wires.  That's my goofy arse measurements.

Good luck Mark,  hope you get it figured out.