Running lean at low RPM OK ?

[Chickenman]

Quote

Using straight 30 wt. in the carbs per book and red springs that are stiffer than stock. 

The Red springs that are stiffer than stock are another variable thrown into the mix. You've reduced  the diameter of the needles  at Cruise and Top end which would would richen the mixture slightly. But then you've increased the spring tension which should Lean out the whole curve.. That seems counter intuitive.

Edit: The stiffer Red springs are also going to cause a leaner transition when accelerating off idle. This can cause a Lean stumble. 

IMHO, you would have been better off  working with the needles only with just some profiling. Playing with spring Tension is a pretty large correction factor and may have complicated matters.  

  

Edited July 16, 2017 by Chickenman

[Chickenman]

Do you have an AFR gauge. If not may be time to consider one. Helps with the really fine tuning. 35 MPG is impressive. 

If you can get it to idle at near Stoich you are doing well. Most cars other than modern EFI will prefer slightly richer  than Stoich at idle. 

Note: Running those Lean of mixtures at cruise requires a good Ignition system and a wider plug gap .042" to .045" The match box dizzy can handle it ans well as An MSD or Crane CDI bos. Pertronix.... uhmmm not so much. Not enough current capability IMHO. 

 

Edited July 16, 2017 by Chickenman

[Captain Obvious]

Yeah, refresh my memory... Why did you change the springs? I remember having discussions about springs with someone a long while ago, but don't remember the details... Was that you?

[Stanley]

Yes I was on a thread about springs. I had a lean miss when accelerating from cruise at about 4500 rpm. Couldn't tune it out without being too rich at low end. Installed red springs and the miss went away, maybe I changed the mix too, maybe needles, at that time. I got the idea from reading the "Just SU's" book. It says using slightly stiffer springs will increase performance but reduce gas mileage. Counter-intuitive I know. It also says using very stiff springs will reduce performance but increase gas mileage. Blue, red, yellow and green springs are available, with stiffness increasing in that order. Called APT and they recommended red springs for the 240Z.

Car is running good, except as described from stop to 5 or 10 mph. I don't think it's too lean at cruise as is. I'm experiencing better acceleration than before when it was running well but on the rich side. The head is higher compression than stock which might have something to do with the best A/F ratio. I will set the nozzles a bit lower lower to tune out the burbling at the stop signs though.

Last week we had a very hot day, a record-breaker. I had it set a 1/12 turn leaner than it is now and there was no burbling. Next day it was foggy and cool and the burbling was back. Previously I've often noticed better performance on cool days, indicating to me that it was too rich on average days. If I leaned it out, the top end miss came back. Installing SM's eliminated high end miss but due to the profile at the first couple of stations I had to set it extra rich to run at idle, which resulted in fouled plugs. I don't miss the SM's, don't think they're right for a stock-displacement 240Z, except maybe if it's snowing at the coast when 3 1/2 turns down is correct per FSM.

[Captain Obvious]

Do you still have your old springs? Maybe just throw them in there to see what happens?

Also, if you're interested, I can walk you through how to measure the spring constant so you can compare the two.

[Stanley]

I've got them, also have a new set of yellow springs to try when I feel like experimenting, so maybe. It's running the best it has in a long time though. It would cream its former self in a drag race. It ran mid-16's a few years ago at S.I.R. (with a lucky perfect light) which beat Car & Drivers test of a 240Z automatic by 2 secs, and it's not a race car by any means. Now I've got better size tires for the quarter mile and it's running much better so I think I can get to the mid-15's if I have a good light. That's what C&D got with a 4-speed and they may have burned up a clutch on the borrowed car to do it. Maybe next spring in Tucson. If it goes much faster they'll make me wear a helmet. Now I've got to replace the alternator before it eats a bearing, and get a new radio installed. Oh yeah, and work.

Next time I have the air cleaners off I want to test those plungers. I expect there's something going on with at least one of them.

Edited July 17, 2017 by Stanley

[Captain Obvious]

Haha! I understand completely! It's working so much better than it used to that it's changed position on the priority list.

Glad you've made such an improvement!

[Stanley]

I first noticed the difference in pressure required to push the pistons up a few years ago. I've been "fixing" it by putting less oil in the front one for a while. I also noticed that pressure required seemed to vary by how tightly the plunger caps are screwed down. Maybe, maybe not, but I want to test that again too. Maybe with both full of oil it wouldn't make a difference. With no oil they're both the same, very little resistance with just the springs. Don't remember if the plungers were installed and screwed down when I checked that, so that's another thing to test.

Another question is how fast does the oil go down, and so how often should it be refilled. I have trouble seeing the oil level, both down in the hole and on the plunger rod which makes getting an accurate measurement difficult. I recently tried using a rolled-up piece of paper as a dipstick. The oil soaks in making in easy to see. This ain't particle theory but "uncertainty principle" applies: when you wipe off the plunger rod to re-measure and see the oil level, the level is reduced a little. That's how I reduce oil level in the front carb, by wiping down the front rod and plunger a few times until both pistons feel the same.

Crane ignition on my do list BTW.

[Mark Maras]

 IMO, it's better to experiment with different wt. oils in each carb to achieve equality than reducing the oil level. Post #5 you mention using 30 wt. oil. I believe the F.S.M. calls for 20 wt. I use ATF. The red color makes it a little easier to see. The tops with the plunger rods should be snug.

[Captain Obvious]

So let me poke around this a little more. When you noticed a difference between the two, and "fixed" it by putting less oil in one of them... I'm assuming you were putting less oil in the one that was harder to lift, right? And by doing that, the two of them were closer to the same.

If that's the case, then I claim that the one that was harder to lift was the one that was actually working CORRECTLY, and the check valve on the other one was screwed up. So by starving the good one for oil to make them equal, your broke BOTH of them. 

In theory, the check valve damper should still work even if the caps are a little loose, but they should be snug. They are, however, sensitive to alignment, and that was the problem I was having. I think one of my stalks was tilted to the side a little. Not enough to prevent the whole thing from fitting down into the carb, but enough that it was preventing the damper from working. That should be identifiable by swapping the two dampers between the two carbs. A check valve issue should follow the stalk.

About oil level: You can fill the tubes all the way to the top. Worst thing that will happen is that a little oil will spill over the edge and run down into the carb for the engine to burn. In short, you cant really use too much. It will find it's own level if you put in more than necessary.

About how fast it goes down: There was some discussion a long time ago about such things, and my answer is that it should never go down. Although there are documented cases where some people are positive they need to refill theirs every now and then, I'm not sure why. But with that in mind, you should check it every now and then. Monthly maybe?

[Stanley]

Sounds right. The weak one was only slightly weaker though. Test will have to wait.

Edited July 18, 2017 by Stanley

[jalexquijano]

On ‎07‎/‎15‎/‎2017 at 3:56 PM, Mark Maras said:

 Curious. What wt. oil are you using in the carbs? Neither too rich or too lean is good for the engine. Keep going richer with the screws until you get the light tan color on the plugs. I doubt that a little light gray stuff on the plugs will hurt anything although new are always best. When starting out from a stop sign, are you sure the mixture is lean? You can check to be sure by adding a little choke during acceleration. If it is lean, you'll feel the additional power as you add a little choke.

Well i guess i´m also too lean at 2.5 turns clockwise. I will need to check my plugs again and post some pictures. the car is taking 7 to 10 minutes to warm up once cranked, so during that period i need to drive with the choke on and slowly regulate it until it reaches the desirable temperatura to close the choke.   Once accelerating from a stop sign it still lacks power at low rpms.

I did had a flaw on both of my su carb float levels and had to set them both to .055 measured with a caliper from the top of the lid to the float.

What do you recommend? Also my engine was modified to 2.6L with a Schneider 274F camshaft and i was wondering if i should also swap to better needles?? When both of my su carbs were rebuilt, ztherapy installed SM Needles.