Triple Oer Carburetor Troubleshooting - Help

[adam.]

Hello all from Australia,

 

Long time browser, first time poster here because I finally can't find the information I'm looking for!

 

First off, my name's Adam and I have a 1973 240z which is currently running a standard L28, N42 head/block combo, with brand new triple 45mm OER carburetors, and Trust headers to a twin 2" exhaust system.

 

I have just installed a new set of OER carbs I purchased from Japan and am having difficulties sorting out the idle/low speed cruise setting.

 

Bit of details into the setup,

 

45mm carbs

34mm venturis

#190 air correctors (not yet tested to see if these are the best)

#130 mains

#60 idles 

#40 pump nozzle

#1.8 needle valve

3.5psi fuel pressure from the mechanical fuel pump into Holley regulator

31mm fuel level using the OER level gauge.

 

Idle screws are setup so that the first progression hole is just 100% covered looking through the brass cap/inspection hole. Carbs are balanced the best I can using a unisyn.

 

I have a wide-band O2 sensor probe in the tail pipe. I am running 98 octane fuel. Timing is at 15 BTDC with no vacuum advance connected. Valve clearances have recently been adjusted.

 

Now, onto the problem I am having..

 

Driving with the #60 idles installed, 1 full turn of the mixture screw turned out, slow acceleration and cruise is very rich, 10.5 and 11.5 AFRs respectively.

 

I also have #50 idles on hand which I have installed to try and improve the current rich scenario and I cannot even get them to idle. With mixture screws out 1.5 full turns (recommended range is only 3/4 to 1 full turn), they cannot idle on their own, only when I blimp the throttle which I believe is activating the pump jets.

 

With the #50s installed, AFRs are off the scale in the lean area. They can't idle let alone drive.

 

My question is, is going down from a #60 to a #50 idle jet that big of a jump that the car can no longer idle on its own? Or is there something wrong with my idle screw setup? OER offer a #55, but I would like to double check with the experts out there first before spending that money.

 

Any help would be appreciated. And please let me know if I am missing some information, I tried to be as detailed as possible.

 

 

Thanks,

Adam

 

[Chickenman]

Yes, going from a 50 to a 60 idle jet is a pretty big jump. A single step is 5 numbers in size.

 

Interestingly, on 44 mm Mikuni Solex carbs with 34 to 38 mm chokes, the #55 usually works out best from my findings. . The OER is based on the Mikuni Solex. I believe it uses the same Emulsion tubes and jets.

[adam.]

With the #60s installed and producing AFRs of ~11 at cruise, I just thought #50s would still run, albeit a tad lean.

 

Looks like I will have to get my hands on a set of #55s.

[grannyknot]

Adam, you could also drill the jets out, ultimately faster and cheaper.

http://www.classiczcars.com/topic/50563-silver-soldering-jets/

[munters]

As far I remember I have with my 2.75 stroked 2.4 and 45 OER

 

Ignition max advance 36

 

Venturis 37.3

Idle 60F9

Main 145

Air 155

Emulsion Tube F11

 

and the AFR is between 10.4 and 13.4 with those.

[Travel'n Man]

Adam

Tell us your knowlege of triple carbs so we know where you knowledge level is.  Personally I would not drill anything on your carbs at this point.

How did you come up with your start up jetting? 

 

You have a standard L28? 

• Any mods on any part of the engine? 
• Cam?
• Inigition? 
• Is everything stock? 
• Orginal engine or rebuilt and bored?

Edited February 24, 2015 by Travel'n Man

[adam.]

I have only really ever tuned the SUs in the past. These are the first set of triple carbs I have ever played with, so you can say my knowledge is fairly limited although I feel like I have done quite a bit of research into them, understanding how they worked, etc.

 

How I've come to my jets has been through research and following 'rough' jets of other similarly modified Zs/L28s.

 

The OERs originally came with 36mm venturis and I followed the venturi size multiplied by X plus X for air corrector formula. Idle jets were the only thing I guessed (following what others ran). From there, I tried it for myself, used my wide-band and ended up with the above jets which is currently running in the car.

 

And as for mods, I completely forgot to mention that I am running an electronic distributor from a 280zx. Other than that, the motor is standard. I have only replaced the head due to corrosion. Standard cams.

 

Cheers

 

Edit: I should probably also mention that I have performed a leak down test on the motor and all is well with leak figures measuring no more than about 18%. So the motor is still fairly 'fresh'..

Edited February 24, 2015 by adam.

[240260280z]

Adam. I re-checked a site that tunes OER's and the sweetshop for the idle enrichment screw is 2 turns out with +/- half a turn elbow room (so your should aim for an idle jet that works within 1.5 to 2.5 turns)

Check table 4 here: http://www.carry-back.com/CARRY%20BACK%20Carbu%20setting%204.htm

Edited February 27, 2015 by Blue

[240260280z]

FYI:

Here is an easy way to compare your #50 to #60:

Jet comparing at Idle

Turn out your idle screws to 2 turns and do not change.... then adjust the throttle valve stops to attain an idle of 900rpm or 950rpm or 1000rpm (forget the eyeballing through the progression holes). then measure your a/f ratios for each jet set (this way you are comparing apples to apples).

If your do this same measurements at 1.5 turns and 2.5 turns, you can plot your nice set of results to accurately see the differences between your jets.

Repeat at different fuel levels (29mm and 31mm) to get an idea of how much fuel level also impacts the data.

By doing the above exercise, you better understand the interactions and prepare yourself for road tuning:

ROAD TUNING

Grab a friend and have him record the a/f ratios at different RPM's as you drive:

For setting cruise, hold 35mph on a long flat and measure. Adjust the fuel level and/or change the idle jet to get your desired A/F cruise target (i.e. 15-16.5 typically for economy). Once you get this dialed in, double back and set your idle enrichment screw and throttle valve stops again to get your idle where you want it (i.e. 13.5 to 14.5 for smooth idle is common). Then recheck your cruise A/F again at 35mph on a flat as the idle enrichment screw will affect cruise it a little.

Next road tune up a long grade. Hold for 5sec at 2000rpm, 3000rpm, 4000rpm, 5000rpm and 6000rpm while your buddy records the a/f ratio.

Your main jet and emulsion tube will be the main players from 3500rpm to 6000rpm. Since you probably don't want to mess with E tubes, change your main to get the right A/F at 4000rpm then change the air to keep the same a/f as you get towards 6000. Repeat but at WOT runs to make sure the mains are not leaning. The air jet size will lean or enrich the high rpms depending on which you use.

If you are starting to run lean at WOT, stop testing and change your main jet. Running lean at WOT is not good for the engine. Aim for 13 to 13.5 for WOT from 3500 to 6500 (yes red line it)

Have fun!

Edited February 27, 2015 by Blue

[240260280z]

Some reference material:

http://www.classiczcars.com/topic/41332-understanding-a-weber-side-draft-carburetor-through-a-fictional-supposition/

http://www.classiczcars.com/topic/47089-sk-racing-carbs/?p=447754

https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCUQFjAB&url=http%3A%2F%2Fwww.classiczcars.com%2Findex.php%3Fapp%3Dcore%26module%3Dattach%26section%3Dattach%26attach_id%3D45109&ei=vrbwVP7jI8KdNqmag6AD&usg=AFQjCNHvDF_Pcf7lc4gwCFPgrpN9YfxubQ&bvm=bv.87269000,d.eXY

http://www.classiczcars.com/topic/44405-dcoe-documents/

http://www.classiczcars.com/topic/43089-tuning-40-dcoe-weber-151-on-an-l24/

[adam.]

Wow, thank you so much, Blue. I have read quite a few topics in the past and your posts have been very helpful in me understanding how.. Zs work! Not just carbs..

With the information you have just posted, together with what I already know, there shouldn't be anything else stopping me from getting these tuned. You've basically cleared up any doubt I had regarding mixture screw adjustments ( I thought 3/4-1 full turn was the aim) and tuning of the mains.

I will have a read of those links in your last post over time.

Cheers

[madkaw]

I would turn out your 50's as many turns as needed to get it running right. Don't be afraid to go out over 2 turns until you get some 55 to throw in.