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kully 560

4 bolt su weather affects idle?

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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 by kully 560

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 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.

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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 by kully 560

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

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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.  

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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? 

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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.

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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

IMGP0209.JPG

IMGP0210.JPG

IMGP0211.JPG

imgp0212.jpg

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 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?

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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 by kully 560

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 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?

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 Thanks. If you notice a slight lag at the onset of acceleration (possible because you leaned it out a bit) you may want to try using 20 wt. oil. It will richen the mixture when you first hit the gas.

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 That's right. Out is down and rich, In is up and lean. We'll avoid the terms clockwise and anti-clockwise. Nice that you were able to clear it up with a 1/4 turn. In post #2 I wasn't sure if it was too rich or lean. I thought it was probably too rich (I assumed the plugs were a tan color) so my suggestion at that time was to lean it "Up a half turn".The choke test confirmed it wanted to run a bit richer. Congrats on getting it idling properly.

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Mark thank for the help !! I let you know tomorrow how far out the mixture screws are . I also remember spending some time setting up the floats I think they should be good.

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hi Mark, I check the turns down on both carbs and the front was 2 3/4 turns down and the rear was 2 1/2 turns down . so I set both at 2 5/8 turns down and re balanced  both carbs and it  runs perfect now. I guess the previous setting was a 1/4 turn up lean and maybe that is why the weather was affecting the idle [some days richer when warmer and some days leaner when cooler] thanks again for the advise on checking with the choke as far as being to lean . also it looks like my floats are set perfect .kind regards

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I will share a couple of things with you that you can think about and potentially try out. I do think SU carbs are sensitive to temperature. They are also prone to fuel percolation in the float bowls. When the weather is below about 55 degrees I move the air cleaner to the winter setting which pulls air from the warm area around the exhaust manifold. If you have headers you may want to figure out how to direct the winter air duct to the area around your headers for cooler weather (assuming you have a winter/summer air cleaner housing). On 80+ degree days, I disconnect the winter duct and run a duct from the air cleaner round winter port through the hole in the radiator support so cool air gets pulled from in front of the radiator (so, in this case I set the air cleaner to winter). On fall and spring days between 60 and 80, I use the normal summer mode where air gets pulled from the under-hood area.

Also, I was having idling in traffic problems at the start of summer this year. I have an electric fuel pump and I only run ethanol free fuel. After lots of trial and error, I solved the problem by wrapping pretty much every inch of my stock (but ceramic coated) exhaust manifold with the product in the link below. I also wrapped the float bowls. I haven't had idle problems in the heat again. Here is the product I used, though there are lots of ways to accomplish with headers. 

Heatshield Armor

As mentioned above, your plugs look great and your carb float setting must be pretty spot on....key in my opinion to getting these carbs to work right.

 

 

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The car runs best at correct air/fuel ratio. High altitude has thinner air so a leaner mix is required. High temperature also has thinner air, so leaner mix required. Cold air is dense so richer mix is required. There's a graph in the carb tuning section of the manual that shows altitude and temperature vs initial number of turns down at the mix nuts. So on a hot day in Denver you'd want a lean setting. On a cold day at sea level you'd want a rich setting. Dunno if that affects your issues.

Edited by Stanley

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thank for the suggestion Jonathanrussell I will keep that in mind about the heat shield.I know the headers put out a lot of heat and that is also been on my mind. I went with the jet hot headers as it is supposed help with the heat but it still gets hot anyway . I do not  drive the z when it is to hot out as I removed the a/c 20 years ago,and with only 2 open windows as you know these cars can get hot on the inside. now my 1971 air cleaner does not have the ducts for summer or winter modes  and I all ways thought about bringing the air in from the front of the car so as to try to decrease the heat in the engine area. but I do not know if the su carbs have enough suction to pull the air from all the way from the front of the car back to them.  so now  when you drive the z  will it be like ram air and run to lean and maybe run to rich at idle? right now as of today the z is running great if not perfect but the temps today were in the mid to upper 70s. the next hot day I will take it out and see it being a 1/4 turn lean was my problem.the plugs were a little more on the white side then the pictures I posted but the tips still had a tan color to them . it ran great off line ,mid range ,top end , I never thought about it being lean at idle! maybe one day I look into the header wrap also but the jet hot headers look so nice!

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Stanley mine was the opposite ! I balanced the carbs on a cool day early summer and it idled great ran fine . then on a hot humid day it idled poorly but ran great, I have all ways balanced several time a year as weather changed .maybe it was just a little on the lean side the hole time for a  smooth idle. for the mixture adjustment I used the color tune ,I also tried to  used the lifting pins on the sides of the 4 bolt su carbs to raise the piston a little for the change in rpms but my engine never really reacted  like the ones you see on u tube so that is why I all ways went with the color tune for the readings. I was originally around 2 1/4 turns down approx. now both are 2 5/8 turns down and much better all around.

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