Tuning this 3.0 on SUs...

[oranngetang]

I'm starting this thread to keep track of my findings, but please chime in if you have any thoughts, suggestions or critisisms on the matter.

 

The engine is a fresh 3.0L Rebello "Full-on" package, which have my zTherapy 4 screw round tops bolted to it, on a smoothed balance tube, and a 6-2-1 header. Compression is said to be 10.1:1, I noticed the head is slightly ported, and the valvetrain noise is overly aggressive even with a fresh lash adjustment. Cam is a '99DI', IIRC, which Dave himself said runs very loud. Rebello Racing bored these to 48 mm, and as it appears did some crazy grinds on the needles to put down 282 HP on my dyno sheet.

 

That said, my experience with the tune has a been a little frustrating.

 

For starters, this engine will not idle. With suspicions of the car running overly rich, I purchased an Innovate DLG-1 dual wideband gauge to monitor with banks of my header at the 3-2 collectors. The gauge has told me a lot, but most importantly that the engine IS running rich. How rich?

 

With the float levels at 20mm from top of bowl, at 3 turns down (2.96 mm by caliper), I'm putting down 9.1-10.1 AFR at WOT, through the rev range, and >20 AFR at an idle of 800 RPM.

 

With the float levels at 20mm from top of bowl, at 1.5 turns down (1.48 mm by caliper), I'm putting down 10.8 - 12.4 AFR at WOT, through the rev range, and the engine is much snappier above 2000 RPM, but below it has no power and will idle for only a few seconds with the idle screws generously opened up.

 

I suspect these fancy looking needles may have some, if not ALL to do with these problems. Thanks goes to Blue for walking me through uncovering them.

 

 

With the amount of material removed, it seems to be that I need to set the mixture to be excessively lean to compensate, leaving no fuel for the engine to idle on.

 

I'm going to try playing with these a bit, and am pondering the idea of spinning some SM or stock needles a little to see how it effects my specific engine.

 

In the mean time, I'm kind of wondering if setting these needles set higher into the piston, or filing off a tidbit of material at an idle/low end position may assist with the idle issue.

 

 

 

 

[Stanley]

Freaky needles, but 285 HP wow. Is this car driven on the street? Could you post the dyno sheets? Wondering about the AFR vs. power curves. Might be educational.

[Mark Maras]

 IMO, I would give Steve at Ztherapy a call. He will be able to sort out these issues & recommend the proper needles for your application. I suspect the horsepower will drop but you will have gained drive-ability. Frame the dyno-sheets & proudly hang them on the wall. P.S. Keep those old needles.

[madkaw]

I'm guessing that a 285 hp motor has a cam thar won't idle at 800rpm with carbs. Bet you will have to go with an idle above 1000 to get it to run right. It seems you are expecting street manners from a race engine.

Specs on your cam?

[oranngetang]

It is street driven. This engine wasnt really built as a race engine, though the internals would hold up well to some race abuse. The cam is more aggressive street than race, and seems to make good power and torque everywhere, but climbs significantly around 4500 rpm. I'll take some measurements of the cam next time the cover is off. An 850 rpm idle sounds like a stumbling V8 but is doable. The tradeoff to get an idle out of it is lowering the jets enough that the off idle mixture is always < 11:1. I can have the car idle, or I can have the car break loose in 2nd on a chilly day, but there isn't really a compromise where I can do both with these needles.

The dyno sheet is posted here somewhere. I'll try to paste the link. He sends the data as an Excel sheet screenshot, so I entered the data back into excel and plotted power vs time.

Been ignoring the car the last few days since my girlfriend is in town. Her flight home is this afternoon so I'll be back to car - mode by supper time.

Edited April 22, 2015 by oranngetang

[oranngetang]

[coop]

Those are what I'd call racing needles. They are filed flat down almost to the base. The purpose of this is to add richness, especially during quick application of the accelerator. Acts much like an accelerator pump. As far as idling goes, you need to look at the thickness of the needles down around the base(the thick end). That will determine the mixture at idle. Thick or large diameter = leaner mixture. I haven't preferred all of these Rebello style performance needles as Dave sometimes shaves them too far down the idle end for my taste leading to rough idling. Not a big deal for me since I run a race engine idling at over 1,500 rpm but still somewhat of a pain. I found that the needles that worked best for me all around on my ITS car were not shaved more than about half the length of the needle, leaving the thicker idle end stock. Your idea of trying some stock needles is good. Install them, warm the engine, then adjust the mixture screws until you get the smoothest/highest idle. Then, dial back the idle screws. Repeat this until you get the idle you are looking for. If you get the car to idle well doing this, you can then remove and shave only the thin half of these stock needles to get back your richness under load. WARNING! After you've tuned in the nice idle using the stock needles, do NOT go out and step on it hard as you will be lean. Make sure you shave those needles.

Also, this whole process still might not get you the idle you would like. After all, you're getting 285 hp on SU carbs! You may have to settle on an idle of 1000 rpm or greater.

[Stanley]

Why shaved flat instead of putting them in a drill bit and working them down round, as my modified N27's (for street only) were done? If it works, it's right, but I don't get it.

[73str86]

Those needles look crazy to me! I imagine if I attempted to grind some needles this is what they would look like. I agree with Stanley, I just always assumed custom-ground needles would be round. But if it comes from Rebello, it must work.

 

I'm running SM needles with an L29. With SUs and the cam, the idle is happiest at 1000 rpms. There could definitely be some fine tuning done, but these are 40+ year old round tops and I'm happy with the performance. 

 

When I called Z Therapy, Steve recommended trying the SM needles (before seeing if a custom set was necessary), but I also have stock SUs, not bored to 48mm. 

 

Congrats on 285hp!

Edited April 23, 2015 by 73str86

[oranngetang]

I'll get some needles ordered up from zTherapy tomorrow hopefully. In the meantime, is there any harm in cheating that needle up into the piston a little to get some more fuel flowing at the idle? I feel the filing needs to be done right to the base, rather than the other way around.

[240260280z]

You can do a few tweaks:

 

 

Part 1 [WOT]

Problem: Your WOT is 10.8-12.4 with the jet nozzle at 1.5mm down. You need this to be leaner at 12.5-13.5 so you need to come up a point in air fuel ratio. This means the orifice at the top of the nozzle needs to be plugged more by a thicker needle's cross-section.

 

Solution: Raise the jet to 0mm.  Use your fast idle screw on the throttle rod to get your idle at 1000-1500 rpm (or more... for test purposes)  so you are in a richer part of the needle and can  drive the beast at 0mm to do a test run to check WOT.  Hopefully your WOT AFR will come up a point.  If it has come up over 13.5 then lower the nozzle 0.5mm (1/2 a turn) and repeat until your WOT is ~ 13. Unfortunately you can not go to a higher AFR than with the nozzle at 0mm so that is your upper AFR limit at WOT.

 

 

 

Part 2 [idle]

Problem: Your idle is too lean.  The needle is too thick near the shoulder and blocks the flow of fuel out of the nozzle's orifice.

 

Solution: You need to thin the region of the needle just below the shoulder ~ the first 1/4".  Spinning the needle in a chuck and sanding then measuring the thickness with a caliper is the standard way. You can also try filing with a delicate small jewellers file but it is non-standard way.  This process will be try-and-test iterative AND you need to get both needles to the same shape to each pass the same amount of fuel.

 

 

It seems like a challenge but you should be able to improve what you already have.

 

You could also try buying some used needles and sanding the old fashioned way to make work.

 

Edited April 23, 2015 by Blue

[John Coffey]

The needle changes the jet orifice size.  Doesn't matter if the needle is tapered or has a flat cut in it.  The idea is to increase the orifice size some amount.  Purists prefer the taper.  Racers do what's needed between sessions with the tools they have at hand.  I've cut tapers and flats in needles, never noticed a difference between the style of mod on the dyno.