First three cylinders are lean,,,, but not sure why?

[70z4fun]

Ok, I have had this car for years, but ran across a new issue. I am pretty good at getting this car tuned, I have had it since 1979.......

Took it to the post office the other day and it ran bad, with backfire and hesitation. Seemed a bit better when I pulled the choke slightly, so I went back to the drawing board.

Checked timing, all good, electronic ignition fires all plugs. When I pulled the plugs, the first three cylinder plugs had white on them, so I am thinking those are lean! Ok, go to check that carb, and it seems fine. The adjustment screw is 1.5 turns out, the float chamber is full of gas, and I check to make sure the float level adjustment is correct.  

Before I go any further, I thought I would check in to see if anyone has any advice.  If you have any suggestions, please let me know before I dig deeper........

[siteunseen]

You could try dropping the nozzle another round. Most people say that 2.5 is a base setting. That's where I started. Good luck.

Added info you most likely know already. 

Setting Mixture
0. Warm up car
1. Screw both jets all the way up to the top
2. Screw the jets down 2.5 turns (default starting point)
3. Take the car for a run and listen closely for popping in the exhaust (rich), popping in the intake (lean) and note power
4. Screw both jets up or down 1/2 turn and take another run to see impact.
5. Repeat 3&4 and do 1/4 adjustments until you are happy
6. Take the car for a spirited 30min drive with plug wrench then while driving fast on a long stretch, hit clutch, kill motor then pull over to side of road, look at all plugs and compare colour. If there is a difference between the front 3 and rear 3 then the fuel jets need tweaking. Adjust the darker 3's carb by turning up the jet to lean the mixture a 1/4 turn
If all plugs are too dark then turn both jets up 1/2 a turn.
7. Repeat 6 until you are happy. All plugs should be the same colour and very light brown. Listen for popping out the front to tell if you go too lean. Don't worry, you can take your time and do this leisurely over a couple of days to fine tune and colour match your plugs.

 

 

Edited November 4, 2023 by siteunseen

[70z4fun]

Thanks for the reply. Went out this morning and started all over on the carbs from ground zero. Checked everything, pulled the domes and checked the needles, did timing, put new plugs in, and took if for a drive. Ran much BETTER! Starts to run out of power at 4K, but no back fire 🙂

So much happier..........

[AK260]

Proper power should be starting at 4k. IMHO that’s a sign of going lean / not enough fuel in the AFR equation.

What is your engine spec? Totally stock or modified in anyway?

[70z4fun]

Thanks for the reply.

It's a L28 with 1971 carbs and a early distributor with Pertronix electronic module. Also have the original nissan performance exhaust header.

Any advice is greatly appreciated.

[Mark Maras]

 When it starts to run out of power at 4k, step on the throttle, and ease the choke on. If it's lean you'll feel the improved performance. It will also verify the lack of power is a lean condition.

[70z4fun]

Finally got a chance to get the car out today. Thanks for the suggestion. I tool it out and as it was getting up to 4K, it started to cut out and I pulled the choke. It did improve. Pulled over and move the fuel mixture another 1/2 of turn on each carb, and it was better. Took it home and check the plugs on cylinder #3 and #4 and they were a both the same, but a bit "white" so maybe still lean. I will take it another 1/2 a turn.

Just don't remember the carbs being open 3 to 3.5 turns open....... Snow is coming this weekend, so doubt I get a chance to try it again until next week when it warms up........

Thanks for the advice , greatly appreciated.

[dutchzcarguy]

On 11/5/2023 at 1:29 AM, 70z4fun said:

Starts to run out of power at 4K,

I always have to watch out that my 240z is not going over the 6500rpm's! it wants to go over 7K!  And yes the carbs should be around 2,5 turns out. Also check for air leaks and do a ignition timing check.

[70z4fun]

So far, I keep turning the mixture screw down, and at about 3 1/4 turns out.

Still see a bit lean on the plugs, and it starts to lose power at a higher rpm, around 4200.

IF I pull the choke, it "feels" better. I don't think I have many turns left in the mixture screw.

So will take it back and recheck timing again, as I have always run it at about 12 degree BTDC. I recall

many years ago running it a 10 degrees BTDC. I am thinking I should try some different needles. Currently I have the SM needle.

I did switch to K&N air cleaners, but I would doubt that would be an issue as allowing to much air.......

I did have a nice drive today.....

[Mark Maras]

  Before you change to SM needles, check the float levels. The symptoms sound like low fuel levels in the bowls to me. The float levels are the primary mixture adjustment. The nozzles are the fine-tuned adjustment. 

 @siteunseen I remember you altering the ramp angle on the float tang. Could that be the problem here?

[70z4fun]

Good point, I will double check the float levels.

The car currently is running the SM needles. Will see which other ones I have, if I can read the markings.....

Is there a "better" way to check float levels with them attached to the car?
Thanks for the advice above, greatly appreciated.

[Mark Maras]

 I'm assuming you have 4 screw, round-top carbs. In the past, I removed the float chamber lids and used a 9/16" drill bit or a piece of cardboard cut to 9/16" to set the float height. The floats don't have to be exact, just close. There are more precise methods but this primitive method worked for me for over twenty years.