SU carbs on L24 stioc at idle and TO lean under load

[rayaapp2]

So Ive been running the car on the smog machine all day... 5 times 3 asm and 2 tsi test and used the gas analyzer multiple times. I started out with pretty stioc numbers at idle 14.6:1 or so and ran my first ASM test to find that 15/50 test shows 16.98:1 and the 25/25 17.97:1 using Lambda calculations.

I started playing with the jet adjustment and was able to get the ASM test to 15/50 @ 15.43:1 and 25/25 16.07:1

Then I ran the TSI test and found that idle was now at 11.08:1 and unloaded 2500 rpm at 17.1(for what ever unloaded is worth to anyone).

 

So i reset the adjustments back and was able to get my idle to 14.28:1.

 

All I can think is that my needles are wrong or the spring over the slide is wrong. Am I way off? I could just keep going with the jet until I get to a good AFR, but I have noticed the higher the RPMs the leaner it gets under all the ASM loaded tests.

 

 

ASM is the loaded test 50% load at 15 mph and 25% load at 25 mgh for those of you who dont know. TSI is the two speed idle test. Idle and 2500 rpm in Neutral or Park.

All my AFR's were calculated using 5 and 4 gas readings and a Lambda Calculator

Smogsite Lambda calc

 

My head hurts so I may have missed an obvious answer.

 

Thanks Guys for any direction you can point me in.

Edited June 21, 2010 by rayaapp2

[johnc]

So Ive been running the car on the smog machine all day... 5 times 3 asm and 2 tsi test and used the gas analyzer multiple times. I started out with pretty stioc numbers at idle 14.6:1 or so and ran my first ASM test to find that 15/50 test shows 16.98:1 and the 25/25 17.97:1 using Lambda calculations.

I started playing with the jet adjustment and was able to get the ASM test to 15/50 @ 15.43:1 and 25/25 16.07:1

Then I ran the TSI test and found that idle was now at 11.08:1 and unloaded 2500 rpm at 17.1(for what ever unloaded is worth to anyone).

 

So i reset the adjustments back and was able to get my idle to 14.28:1.

 

All I can think is that my needles are wrong or the spring over the slide is wrong. Am I way off? I could just keep going with the jet until I get to a good AFR, but I have noticed the higher the RPMs the leaner it gets under all the ASM loaded tests.

 

 

ASM is the loaded test 50% load at 15 mph and 25% load at 25 mgh for those of you who dont know. TSI is the two speed idle test. Idle and 2500 rpm in Neutral or Park.

All my AFR's were calculated using 5 and 4 gas readings and a Lambda Calculator

Smogsite Lambda calc

 

My head hurts so I may have missed an obvious answer.

 

Thanks Guys for any direction you can point me in.

 

IMHO, those are kinda normal numbers for part throttle operation at least for how I tuned the SUs on my track car.

[Xnke]

Check your damper oil.

[Tony D]

Try a set of FLAT TOPS.

 

They idle on a SEPARATE CIRCUIT from the needles, so the IDLE mixture can be adjusted SEPARATELY AND INDEPENDENTLY from the power circuit (ever wonder why all the Flat Tops have the needles savagely removed---perhaps because they have a more agressive taper for the larger L26 that made more HP than the L24???)

 

Sure, the power valve isn't the greatest for the progression, but unless you are willing to taper your own needle you are experiencing EXACTLY what Nissan did when trying to get emission compliance, and came up to the same conclusion that spawned SU's invention of the HIF-6 Carburettor.

 

Separate the idle on it's own circuit, leaving WOT and High Load on the needle.

 

But remember, "Flat Tops Are Boat Anchors"...

Edited June 22, 2010 by Tony D

[WEBEZEEed]

I have the same sort of conditions you are describing with my 73, with 71 carbs on it. Idle is rich,cruise & part throttle acceleration is lean, then transitions to rich at heavy acceleration to full throttle. I then did some modifications to the carbs that increased air flow. I got the same circumstances only much more power & torque. I installed some SM needles, wound up at about the same with maybe a little improvement in mid range.i worked on the carbs a little more for air flow and have made new & improved air horns. Car pulls a little harder up top but air ratios are the same. I now have installed some RH needles and idle has improved a good amount along with acceleration not being as lean and for a shorter amount of time, however it still is quite rich(11-12 to 1 ) at full throttle. This is a stock L24 save for ignition & exhaust, wide band has been installed the whole time. I didn't have much luck in adjusting the needle jet. It would improve one area and hinder another. There is no telling how many profiles of needles it may take to optimize a particular engine, as Tony has mentioned a separate idle circuit go a long way in addressing this symptom.I have not as yet tried needles from some flat tops.

Edited June 22, 2010 by WEBEZEEed

[rayaapp2]

IMHO, those are kinda normal numbers for part throttle operation at least for how I tuned the SUs on my track car.

 

really? I would be pushing for close to stoic verging ever closer to 13:1 at WOT(at least that is my theory) On the 260Z with fuel injection I dyno tuned the car up to 11.5ish:1 at WOT pulls. I would think carbs would be at least 13:1 for max power in a track car? Id be curious about further details.

 

Check your damper oil.

 

Damper Oil only affects incoming acceleration of air not how far the slide will open or static flow rates... The spring tension on the top of the slider may affect a steady amount of air, but Im unsure as to what extent. Definitely not oil. I did have to change the oil from ATF to 20 weight to get the richer mixture on acceleration I wanted.

 

 

Try a set of FLAT TOPS.

 

They idle on a SEPARATE CIRCUIT from the needles, so the IDLE mixture can be adjusted SEPARATELY AND INDEPENDENTLY from the power circuit (ever wonder why all the Flat Tops have the needles savagely removed---perhaps because they have a more agressive taper for the larger L26 that made more HP than the L24???)

 

Sure, the power valve isn't the greatest for the progression, but unless you are willing to taper your own needle you are experiencing EXACTLY what Nissan did when trying to get emission compliance, and came up to the same conclusion that spawned SU's invention of the HIF-6 Carburettor.

 

Separate the idle on it's own circuit, leaving WOT and High Load on the needle.

 

But remember, "Flat Tops Are Boat Anchors"...

 

I have almost always LOVED the flat tops... I use to have a large collection of them at one point in time... Ive never tried to smog them though. I kick myself cause I let go of my last set 6 months ago for recycling while cleaning out.

 

I now vaguely remember(in part to the reference of separate idle circuit) a mod for SU carbs that leans out only idle... Some kind of Toyota vacuum switch hooked up to open a special metered leak only at idle... hmmm sounds like something I may have to try.

Thanks Tony! Hopefully that means you received my phone call? I tried to call you after the final smog I ran.

Edited June 22, 2010 by rayaapp2

[rayaapp2]

I have the same sort of conditions you are describing with my 73, with 71 carbs on it. Idle is rich,cruise & part throttle acceleration is lean, then transitions to rich at heavy acceleration to full throttle. I then did some modifications to the carbs that increased air flow. I got the same circumstances only much more power & torque. I installed some SM needles, wound up at about the same with maybe a little improvement in mid range.i worked on the carbs a little more for air flow and have made new & improved air horns. Car pulls a little harder up top but air ratios are the same. I now have installed some RH needles and idle has improved a good amount along with acceleration not being as lean and for a shorter amount of time, however it still is quite rich(11-12 to 1 ) at full throttle. This is a stock L24 save for ignition & exhaust, wide band has been installed the whole time. I didn't have much luck in adjusting the needle jet. It would improve one area and hinder another. There is no telling how many profiles of needles it may take to optimize a particular engine, as Tony has mentioned a separate idle circuit go a long way in addressing this symptom.I have not as yet tried needles from some flat tops.

 

Unfortunately, OR FORTUNATELY for California depending on how you look at it, my equipment does not sample max power pulls. So I cannot see what is going on beyond half throttle. It could richen up, but its pretty much a linear pull as far as my butt tells me after 4500 rpm that starts to drop off around 6500 rpm. Im running a pretty much original 1/71 L24 E31 combo with the exception of a "C" cam and an old Spin Tech 6-2-1 super long secondary header(by super long I mean the secondaries combine into the pre-muffler towards the end of the transmission and Im sure someones is longer so Im going to stop you there). The SUs are original. I have checked them for shaft leaks and found nothing. I went through the carbs and cleaned out all the typical deposits. I thought about modifying the bridge while I was in there, but decided I wanted to get a base line before I started compounding problems. I have smoothed that bridge out before on other SU's for engines in the past with good results.

 

When it comes to needles I have NOT done my homework and Im afraid I do not know the difference in needle designations or how to compare them. I have seen different needles and played with them a bit and that is the extent of my knowledge. I will have to sit down and figure that out soon I suppose.

[Tony D]

A solenoid or 'bump switch' to open a small orifice venting air around the throttle bodies is a scab fix. They were used on some cars (a late model Corvair, for instance) when emissions at idle needed enleanment but transition needed a lot of fuel...

 

Small spring-loaded arm set to be tweaked and crack a rubber stopper over a hole in the base of the carbuettor straight to the manifold. Soon as you cracked the throttle, the spring put the stopper over the hole and vacuum sealed it tightly.

[rayaapp2]

So my AB valve isnt working. It does not open when vacuum is applied. So I need a new one as I am running the air injection setup still and all the other "smog dog" stuff.

 

Im curious if a good one stays open at idle leaning the mixture out?

I know it is suppose to open under decel conditions but what about strong idle vacuum?

[Tony D]

A leaking anti-backfire valve... hmmmmmm that seems more like an open path from the air cleaner to the AIR system in the exhaust. I know if the check valve on the AIR manifold is leaking you EGR to your air cleaner....heavily

 

There is a reason I went digital...

 

Remember my longstanding commentary: "Anybody who says 'carbuerttors are easy' has never tried working with them in a precision environment!"

 

A couple of more months Ray, and you will start thinking of ways to convert those SU's into Megasquirt Powered TBI!

 

I know I did...