Jump to content

Sign in to follow this  
Stanley

Running lean at low RPM OK ?

Recommended Posts

After running too rich for months put new points and swapped the SM's for modded N-27's. I balanced the mix and set it lean, then set it 1/16 of a turn richer every few days. Now it goes lean when first leaving a stop sign, burbling a little except on very hot days, but runs well otherwise. I could easily richen it up but I love the gas mileage around town. I can drive for an hour and the gauge doesn't move. I've read that these engines like a rich mix but I seem to have more power when it's a little lean, especially on hot days. The N-27's are polished down at the mid-range, cruising, and top end stations so there's no lean miss or burbling when I get on it. No overheating at all.

I do have to pull the choke now to start it. I plan to richen it slightly to the point where there's no burbling, but no richer.

Does the danger to the engine from running lean still apply if it's lean at low RPM only and engine temp is normal or less ? Also, the plugs are showing leanness. Will a little light gray stuff on the plugs hurt performance ?

Share this post


Link to post
Share on other sites

 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.

Share this post


Link to post
Share on other sites

If it's lean only when you first hit the gas pulling away from a stop, methinks you need thicker oil in your dampers, or there is something wrong with your jiggly bits.

That or you just need a set of flat tops!!!  :D

Share this post


Link to post
Share on other sites
35 minutes ago, Captain Obvious said:

If it's lean only when you first hit the gas pulling away from a stop, methinks you need thicker oil in your dampers, or there is something wrong with your jiggly bits.

That or you just need a set of flat tops!!!  :D

NOOOOOOOOOOOOOOOOoooooooooo:wacko:

Share this post


Link to post
Share on other sites

Yes it's lean at idle, intentionally. Just wondered if it would hurt the engine. If I pull the choke a little the burbling instantly goes away. In fact, I've been driving with a hand on the choke when there's a lot of stop and go, until it warms up. It doesn't do it once I'm going 5 or 10 mph so I push the choke back in. Using straight 30 wt. in the carbs per book and red springs that are stiffer than stock. I could easily tune it out by dropping the nozzles a little more but it accelerates great at all other conditions and the gas mileage is incredible. The needle profile is richer than stock in the cruise and top end zones so there's no miss when I stomp on the gas like with the stock N27's (and no fouling the plugs at low end like the SM's).

Plugs look too lean though, so it's bye bye 35 mpg I guess.

Share this post


Link to post
Share on other sites

Well here's a theory then... If you hadn't messed with perfectly good needles in the first place and made them richer everywhere with the exception of at idle, then maybe you would be able to drop the nozzles a little and end up with the mixture where you want it... Even at idle.   LOL   Haha!!! Sorry. Just had to!

I agree that 35 mpg would be hard to walk away from.

So if you pull the lid off the air cleaner and lift up on the pistons:
Are the pistons hard to lift? (Are the dampers damping?)
Are the two dampers the same?

Share this post


Link to post
Share on other sites

Yes, the dampers were the same and hard to lift two weeks ago when I balanced the carbs. I had to put a few drops less oil in the front (IIRC) one to get them the same.

Share this post


Link to post
Share on other sites

As long as the dampers stay fully submerged even at full height, then the amount of oil shouldn't matter. If you're able to tune the amount of resistance to lift by adjusting the oil volume, then you don't have enough oil in there. They should be to the full mark, and at the full mark, they should both feel the same.

Share this post


Link to post
Share on other sites

Thought I had them both full to start with but not sure. Maybe one jiggly bits functions different than the other. Next time I'll try swapping the plungers with oil levels topped off and see pressure required to lift the pistons changes.

Share this post


Link to post
Share on other sites

I've seen that. One stalk damped a lot more than the other. The check valve on the one that didn't damp wasn't working properly.

IIRC, the center shaft was cocked off to the side a little and because of that misalignment, the check valve didn't work right.

Share this post


Link to post
Share on other sites

I gotta look at my book. Didn't know there was a check valve. If I take those plungers apart, wonder if I'm more likely to learn something, or mess something up, or both ?

Share this post


Link to post
Share on other sites

Seems like the difference could also be caused by one damping too much due to a blockage. I gave it a good blast of carb cleaner, though. Didn't do anything.

Share this post


Link to post
Share on other sites
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 by Chickenman

Share this post


Link to post
Share on other sites

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

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 by Chickenman

Share this post


Link to post
Share on other sites

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?

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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 by Stanley
  • Like 1

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

 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.

Share this post


Link to post
Share on other sites

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?

Share this post


Link to post
Share on other sites

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

Edited by Stanley

Share this post


Link to post
Share on other sites
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.

 

240z specs.jpg

camshaft specs.jpg

cam specs 2.jpg

ztherapy sheet.jpg

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.