4 bolt su weather affects idle?

[kully 560]

I have a question for the carb guys, when I adjust / sink my carbs  it idles smooth. then on  another day when it is humid out and the temps change it idles rough. I can not seem to find the sweet spot with weather, I think it is weather related ? off line no stumble, mid range and top end pulls great it is just at idle?  my idle rpm is 730 rpms in gear and it does not matter in gear or out or gear it does not idle smooth. plugs are perfect color , my timing is 15btdc with vacuum plugged. any thoughts would help just like to have a smoother idle. thanks

Edited September 1, 2019 by kully 560

[Mark Maras]

 More info please. Stock cam? How well does it idle when the engine is cold with the choke on? Describe "perfect color" plugs or better yet, pics. Are the metering needles stock? My first guess is it's running rich at idle. That's assuming that as the humidity is rising, so is the temperature. If the plugs are a nice tan color like we used to try to attain in the good old days the mixture is likely rich. Plugs run nearly white with today's fuel. Assuming it's had a recent tune-up, including a valve adjustment, I'd put in a new set of BP6ES plugs, and lean the mixture adjustment nuts a half turn up and see if it improves.

[kully 560]

the plugs are on the white side with very little tan color on the tip of the white element. stock cam , as I think now it does idle a little better when cold , very really use the choke to start it . I can not tell you about the metering needles never pulled them out . bought the set of rebuilt carbs back in 1997 from motorsport auto and never played with them except to set up and adjust them . I adjusted the valves about 2000 miles ago and the bp6es I put in about 500-700 miles ago and gap them at 45 thousand for the high output ignition [ msd 6al]. this problem has all ways been there but never really chased it as most of the little time I take it out is on really nice cool days and it idles great . I drove it today with the temps up in the low 80s here with the humidity also up in the 70s and that is when it acts up. so when I drove it today I would stop at a traffic light I could feel the motor shaking a little, not going to stall at all but not smooth either.  the last time I sink the carbs were back in the spring when the temps were in the 60s and low humidity  it ran great and idle very smooth. so I started thing today about the temp changes  if this would affect the carb function .

Edited September 2, 2019 by kully 560

[jonbill]

Do you have stock air intake and filter? Stock exhaust manifold? Heatshields between exhaust and inlet?

[siteunseen]

It gets about as humid here as anywhere and I haven't experienced any idle issues on my '72.  I've drive it early in the mornings too when the humidity is at the highest.

I have the water heat up tube deleted but your 4 screws won't have that.

But my snorkel has the WINTER/SUMMER flap so Nissan thought enough about temperature to add that.  

[kully 560]

yes I have the stock air intake and filter ,has a jet hot header with a heat shield installed. siteunseen do you feel any king of rumble at a traffic light? 

[siteunseen]

I have your setup, ceramic headers and heat shield but I set my idle at 800rpm.

I removed my heat riser so mine gets plenty of cool air using a Nissan air filter.

No stumbles, sorry.

Remove the filter and see if it smoothes out. Also I think adjusting the valves would be a good idea too.

Make sure your nozzles are full up off choke. The choke wires slip sometimes and won't let them pull the nozzles all the way up.

[kully 560]

well I tried marks suggestion checking the choke and when I pulled the choke on a little it was idling better so I richen both carbs a 1/4 turn down and rebalanced the carbs and it is a lot better . must have been a little lean for the L28 motor. also the temps are a lot cooler here now but still have the humidity .I so she runs  a lot better now with the rpms idling in gear at 675 and pretty steady with barely any shacking of the motor. I  put 20 miles on the car today and pulled the plugs to see what is going on . here are the pictures you asked for mark the  plugs from left to right are 6,5,4,3,2,1 thanks

[Mark Maras]

 Plugs are looking good. I actually thought it might be running a bit rich. The SU chokes are a great way to check for a lean mixture. How many turns down are the mixture screws now?

[kully 560]

I was about 2 1/2 down, but  I would have to check again when I pull the air box off for easy access to them . but I think about 2 3/4 down now but I am guessing. great idea never thought about checking with the choke!! l will let you know tomorrow on the turns down kully

Edited September 2, 2019 by kully 560

[Mark Maras]

 If the mixture screws are within 2 1/4 - 2 3/4 turns down it indicates the float levels are close if not spot on. Great news. Random question. What weight oil are you using in the carbs?

[kully 560]

I use the 3-1 oil from any hardware store I believe mark it is a 10w oil

Edited September 2, 2019 by kully 560