I have an L28 in my 71 240Z.  It's carbureted and using points ignition.  I attached photos of the the jet all the way up and all the way down.  Also a pic of the timing scale.

Before I used Colortune:

• I plugged the vacuum advance on distributor
• I set the timing at 10 degrees on the scale.  
• I adjusted carbs for a 800 rpm idle
• I also balanced the carbs.
• I turned both Jets all the way up and then backed them down two full turns.
• I reconnected the vacuum advance.

When using the Colortune:

• I couldn't get a blue color no matter what I did with the mixture screw
• I went all the way up to a weak mixture - screw wouldn't go up any more
• It seamed to idle best when it was screwed all the way up
• All the way down didn't make much difference, but it did run rougher.

Test Drive:

• Very sluggish and no power

Any ideas as to why I can't the Gunson to turn blue?

 

Any help is appreciated.

 

Edited February 16, 20223 yr by HappyZ

 There's a few possibilities. The nozzles could be partially down due to misadjusted choke cables, misaligned choke linkage or sticking nozzles. The next possibility is the float levels are too high. The easiest method for checking the float levels I've found is to remove the domes and pistons. raise the nozzles to their top position (0 turns) and lower the nozzles 9 1/2 to 10 turns. If you want to be precise 9 5/8 turns down, that puts the fuel level 3/8" below the carb bridge. The fuel level should be close to the top of the nozzles but it doesn't have to be exact. Raise the nozzles back to 2 1/2 turns down for a near stock engine at near sea level.

Thanks Mark...I don't have have the choke cables connected.  Should they be connected or can I leave them disconnected?  I'll check the choke linkage.

This may sound lame, but when I do what you said, what should I see?

 

 The choke cables can remain disconnected unless you need the choke for starting the engine. There's a flat bar linkage that attaches to the nozzles that pulls them down into the choke position. Pull the nozzles down using the choke linkage and then raise them back up using the linkage. If the nozzles don't go back up, lube the nozzles and try it again. If they work fine and aren't sticking check the float levels. If the nozzles still don't return, check the mating surfaces of the flat bar linkage. Often times the linkage piece need to be twisted to align the ends with their mating surfaces. Unfortunately, I've not been able to check the alignment of the linkage piece or straighten it without removing the carbs.

19 hours ago, Mark Maras said:

 There's a few possibilities. The nozzles could be partially down due to misadjusted choke cables, misaligned choke linkage or sticking nozzles. 

I had a problem on a Z I had once and couldn't figure it out for anything. Turns out the nozzles were not moving freely as a result of the previous owner using regular fuel hose instead of the more pliable "J" hose that they are supposed to have. 

If the chokes are disconnected you should be able to reach under and pull them down and they should pop back up on their own.

Good luck.

I'll mention this too. If you rebuilt them with the newer needle valves and adjusted the height like the DVD shows, upside down at 5/8", your floats are off and starving the carbs of fuel. There's a bunch on here about that and I'll help find the threads but 1st off what does yellow mean on colortune? 

For the Colortune I can only get to Yellow no matter how far I turn the knob:

Yellow    Indicates a fuel / air mixture which have excessive petrol content (rich). Like a candle flame - producing more light but less heat.

Bunsen Blue    Indicates a mixture which is correctly proportioned.

Whitish Blue    Indicates a mixture which has less petrol content (lean)

 

The Pic below is what it should look like (bunsen blue).

Cliff, you're absolutely right about the nozzle hoses. They have to be very flexible. I installed the silicone hoses from Ztherapy. As far as fuel height goes, a while back I became curious about an actual measurement from the carb bridge to the desired fuel level. In the British forums I found that 3/8" was the desired distance (it doesn't have to be exact) including round top Hitachis. Using the fact that one turn of the nozzle screw equals one mm and 3/8" equals 9 5/8 turns, that (or close to it) should be the fuel height according to the Brits.

Here's my 2¢.

First, make sure your valve lash is set properly.

Second, make sure your ignition is set properly: timing, points gap, dwell, spark plug gaps.

THEN you can set up the carburetors.

I bring this up based upon the other thread where I talked about setting the dwell. Points that are not gapped properly or having too much or too little dwell time will result in poor spark. If the hydrocarbons are not being combusted during the spark, they would probably show up as a rich mixture with colortune. You may want to settle upon what you want to do about the electronic ignition before you use up all of your colortune supplies.

I notice that there does not seem to be a separate Engine Tuneup chapter in the 70 and 71 FSM's available on the site.  Not sure why.  I would download the 72 FSM, it has a full Engine Tuneup chapter.

https://www.classiczcars.com/files/category/11-240z/

1 hour ago, SteveJ said:

First, make sure your valve lash is set properly.

By 1976, maybe sooner, Nissan recommended this as the very first thing to do.

6 minutes ago, Zed Head said:

By 1976, maybe sooner, Nissan recommended this as the very first thing to do.

That's what @240260280 taught me to do.

Well you said it ran better all the way up so that explains the yellow part. I would make sure the nozzles are completely up. That flat bar Mark spoke of can get out of shape and keep them from reaching their height and lessening the fuel. 

Have you a way to meter the air flow, balance that? These are great and fairly cheap.

Well after googling it there not so cheap anymore?

https://www.amazon.com/Latest-Rage-5385730856-Carburetor-Airflow/dp/B00CMC57R0/ref=pd_aw_fbt_img_2/140-4982961-2615742?pd_rd_w=Xlzjj&pf_rd_p=76328f70-5fff-49b7-869a-94bfdaa768fc&pf_rd_r=JECHWPJ6RGT64TVVHVNZ&pd_rd_r=42368007-404a-49b9-87a3-cbbfc73c54b5&pd_rd_wg=n1J8w&pd_rd_i=B00CMC57R0&psc=1

I agree with Steve and Zed, a good tune 1st. When I got mine the best running ever I had my timing full advance  at 17 at idle then the dial back set on 34. At 2,500 rpm it sits at 0 on my light. BUT! I have a ZX electronic distributor. Love the old car but the simplicity was increased with the newer ZX dizzy.