Jetting advice needed

[Tobi/DFL]

Hi everybody,
I recently finished updating my engine (new heads, cam and raised CR) and already made some test runs on the street and quarter mile. I´m using a LM-1 with rpm converter. At first some engine specs:

- 2176cc (78.4 x 94mm)
- 11.8:1 CR
- CB CNC Comp Eliminators, reworked by Udo Becker, 46x38mm valves
- GB 316 cam (FK-89)
- 50.5mm IDAs with 43mm vents, 60 idle, 170 main and 165 air correction jets
- 3rd bypass hole
- F11 emulsion tube
- idle holders are 120s

The lambda readings are quite okay when driving on the street (between 0,83 to 0,9 from idling to max. 8000-8200rpm) with just some minor ranges when lambda is a little higher or lower. BUT if I want to accelerate the car as hard as possible the lambda reading is getting much too rich (with reading of up to 0,68) and the car doesn´t accelerate as it should (it hesitates to accelerate - I don´t know the correct english expression). I guess that changing the main jet to a smaller 165 wouldn´t really solve the problem as the engine could run a little lean when driving on the street (I´ll try that this anyway this evening). I guess that maybe my pump jet should be a little smaller. What do you think? The (italian) carbs are fitted with the stock pump jets. Are smaller pump jets (the old screw type) available anywhere?
Or could another emulsion tube solve the problem? Anything else?
Thanks in advance for your help!
Bye,

Tobi

[Bruce]

.... if I want to accelerate the car as hard as possible the lambda reading is getting much too rich (with reading of up to 0,68) ....... I guess that changing the main jet to a smaller 165 wouldn´t really solve the problem as the engine could run a little lean when driving on the street.....

This is your mistake.  Jet it to where you want it for full throttle, then let the part throttle A/F ratio fall wherever it wants.  The part throttle A/F ratio doesn't matter.  It can be SUPER lean and nothing will happen.  Mine is quite happy at 18:1 at part throttle on the freeway.

[Tobi/DFL]

Thanks for your advice, Bruce! I will try that.

Tobi

[JS]

This might be a stupid question, but why doesn´t the engine run hot if you cruise on the freeway with a 18:1 AFR?

[Rasser]

When Cruising:  Idle jets will take you to 100km/h or above (depending on weight of car, gearing etc. etc).  My car will run 110-120km/h on idle jets alone 

Try to do a run without the mains, and you will clearly see when the AFR readings start to raise and go lean - this is where you want your mains to kick in! Not sooner or later. Float height, venturi size and offcourse main air and fuel jet all make this happen (among other small things). Try cruising at constant speed, and increase speed a little step at a time - you will probably see your readings start to go lean at around 100-120km/h and then  a little later they start to come down to normal again. Or you will see that the readings suddenly will richen a bit.... This is your mains kicking in!
You will have to drive with a steady foot on the freeway, and watch the readings carefully.Then you will see exactly when the mains kick in... and you will then know what to do.

But the very first thing to do, is to make the idle circuit run perfect. So make sure you have finetuned the idle circuit before concentrating on getting the transistion from idle tro main work. You will probably notice that when driving slowly in town at low revs (below 2000rpm), and then trying to increase speed just a little, will reveal a flat spot (running lean for a few seconds). This is the signal to your idle circuit that isn´t strong enough. To make this signal stronger you will have to make the hole in the idle air jet holder smaller, and after doing this you will discover that the idle circuit driving is a bit richer and you might find the need for a smaller idle jet.
when you have worked out what idle air jet and idle jet that works perfect on your engine - then is the time to start jetting for the perfect transistion between idle and main circuit, and at the same time jetting for perfect WOT readings.   ITS NOT EASY  

And you jet/tune your engine so it gets the right mixture all up through the range, and getting the idle-main transistion can be really really hard. If you have the transistion to rich, then you might want to step up on the main air (and perhaps the main fuel too - WOT tests will tell later on).

You don´t jet for WOT, and then just settle for whatever idle circuit jetting you can find useable afterwards.You tune/jet for the perfect compromise/transistion between idle and main circuit and at the same time makes sure that WOT readings are good. But first thingn is idle circuit tuning.


Good luck

[Tobi/DFL]

Hi Rasser,

thanks for your long answer! I started the tests of the engine with the jetting I had before (same engine, but welded 040 heads with 44/37.5mm valves, 11.0:1 CR, engle W140 cam with 1:1.25 rockers): 55 idle, 185 main, 140 air correction, 43mm venturis and F11 emulsion tubes. The first problem was that I couldn´t really get the engine to idle smoothly (as far as that´s possible with that cam) below 1700rpm. Cruising was no problem at all but whenever I wanted to accelerate hard the engine hesitated and got slower before finally accelerating. Next step was changing the jets: 175 main and 165 air. It now was better when trying to accelerate from a standing start but I tried a 170 main jet afterwards and it got better. Idling was still horrible so I tried bigger idle jets (60s at first then 65s) and decided to use the 60s as it allowed me set idling rpm to a "modest" 1250-1300 rpm at ca. 0,88 lambda. It´s now running reliable at idle as soon as the engine is running for some seconds so I´m happy.
Yesterday I tried a smaller main jet and went for a test drive once again. Transition from idle to mains could be smoother but it´s okay because the car is mainly used on the strip. When the engine was warm I accelerated it hard at full throttle. It seems to be a little better now but the lambda readings are still below 0.80 between 3200 to ca. 6000rpm and under 0.85 above 6000rpm. Next time I will try a 160 main jet to get lambda readings in the 0.85 range. Maybe a little smaller air correction jet will be possible but that´s no problem.
I would like to try to run without mainjets as well as you suggested to see where the main jet really begins to work on my engine. So I just screw everything else (emulsion tubes, airjet etc. in but leave the main jet away, right? And another stupid question: wht´s the meaning of "WOT"?
Thanks in advance!
Bye,

Tobi

[Rasser]

WOT is WideOpenThrottle 

Just unscrew all the etubes with air and fuel jets, and leave them out . Only thing that is gonna happen is that your car will only suck air when entering the main circuit.