Triple Mikuni thread

[Travel'n Man]

My AFR gauge (digital) moves around quite a bit as well - take your car up to 6000RPM's and get the average (it's not going to sit on one spot) and let us know. After talking to Wolf Creek Racing and Rebello Racing you can not predetermine the amount of turns on your pilot screw. They both said you really should not have to go beyond 3 turns (I know there is a lot of BS about one turn only but neither Dave or Todd agreed to that). My engine really like 2.25 turns so that is were I am currently. The only way to really read plugs is to get to wide open throttle (WOT) and immediately pull off the road and quickly take out your plugs. If you are pulling into your garage and casually pulling them they are already sutting up and changing colors. The AFR gauge is your best bet for determining the amount (or lack of) fuel into your combustion chamber.

My current jetting:

• Main Air - 190
  
• Main Fuel - 150
  
• Pilot Jet - 62.5
  
• Pilot Screw - 2.25 turns
  

1. 
   
2. Warm idle AFR @ 1000RPM - 13.1
   
3. 4th Gear @ 2000 RPM - 12.9
   
4. WOT - 2nd gear @ 6000RPM - 12.5
   

• From my understanding you should not be leaner at WOT than 13.2
  
• Ideal at WOT AFR setting should be 12.7-13.2
  

I have worked on jetting over the past two weeks and bought a lot of jets (of all kinds) - it's kinda crazy because I have came right back to where Todd originally had my set up. Here in a few weeks I will work on different pilot jets and trying to find some machined 36mm venturi's so I can see what my max horsepower is - my torque is excellent with the 32mm venturi's in but really want to see how much HP my engine will put out.

I am keeping a spreadsheet with all the different setting and the AFR based on each setting - that way I know all the combinations I have tried. If the AFR is too lean it is terrible for the engine and can risk all kinds of crazy heat and meltage of pistons and so forth. If you are too rich than the plugs will get all fueled up and have poor performance.

Naturally each engine will perform differently based on the engine build.

[madkaw]

I guess I never had much discussion, or Todd never mentioned about the BS associated with the pilot setting. I have been going by the Mikuni manual refering to the 1 turn rule. I am pretty sure that I am not running as rich as you, but I do believe that is where these carbs like to run. Todd never really gave me numbers to go by as far as jet sizes, infact he thought the 57.5's would be close on my engine, but they weren't even close. Like you said ,every engine is different, and my engine is especially unique running a CAI and a vacuum log. I have been trying to listen for detonation more than anything and using the butt dyno-which is usually not accurate.

Sounds like you and I are keeping Todd busy with all the jet orders!!

The biggest suprise for me is the size of pilot jets my engine likes to run, I haven't found many threads showing engines running this big of jets.

Do you have any bobbles at all during a hard tip-in of the throttle? I'm talking a quick tip-in?

[Travel'n Man]

I'm a little confused - if you are not as rich as me (and my setting is gauged on the ideal range for WOT with a AFR gauge, and I'm slightly on the fat side) then you are too lean and could cause piston and engine damage. But if you don't know what your AFR is at any setting (including WOT) then you are really lost on your entire tuning.............don't take it personally......IMO

I had discussed this with with Dave Rebello on increasing my venturi's to 36mm and you are running a larger Pilot Jet with a L24 than Dave recommended me to run with a modified L28 - I would really believe that you should log all your settings with the AFR from WOT so you can get a true idea of where your fuel / air is at inside your combustion chamber............I personally would restart with 57's and maybe go up to 60's

From my research:

• Pilot Jets: 20% throttle = idle = low end power
  
• Main Jets: Adds (or removes) fuel from the entire RPM range = higher number equals richer mix = top end power
  
• Air Jets: Smaller the Air Jet the higher the RPM and the more fuel comes in = higher number equals the leaner mix
  
• Power on Air Jets: Power on lower RPM band = 210/190 air jet
  
• Power on Air Jets: Power on higher RPM band = 140/150 air jet
  

Air Fuel Ratio Target based on Todd Walrich at Wolf Creek Racing:

1. 13.2 peak High AFR under load
   
2. 12.7-13.0 AFR ideal under load
   

[madkaw]

Nothing personal was taken, we are exchanging thoughts.

I DO have some idea of what my AFR's are doing, but in a limited range. At cruise-where the pilot circuit lives, my gauge seems to work okay. Only large swings in AFR throw it off. At cruise with the 65's I am running around 14(which I could dial in richer), which seems lean based on what you have learned and what seems typical with Mikuni's. Examining the plugs seem to verify this with the fact that they don't show any color, or soot, or anything but a slight tanning on the porcelin. Also my tail pipes are clean, actually cleaner than anytime I was running SU's with SM needles.

I can't explain the difference, but my engine runs better than it ever did with the SU's. I don't see going backwards on the pilot jets and Todd agreed with me to go bigger.

As far as the main air jets-not sure if I understand your statement. My understanding is the bigger the main AIR jet, the higher the rpms needed to draw in the main jet.

Also, you are saying that Todd suggests 12.7-13 under load-are you refering to cruise?

Edited July 4, 2012 by madkaw

[Travel'n Man]

12.7 - 13.0 is WOT at 6000rpms.........

[madkaw]

You never answered whether you experience any bobbles or any hesitation during hard/fast throttle tip-in??

[Travel'n Man]

Sorry - for the delay - been busy for the past couple days - Zero - none. Smooth acceleration up to 6000+ RPM.

[Seppi72]

I'm really liking this thread. I have a set of Mikuni 44s destined for my L28 stroker and am going to commit your info to memory when I get the time to put them on. I also have a set of Weber 40s that I bought as a back-up or to go on my "regular" Rebello L28 at some other point in time. Either way, it's a fascinating (to me) discussion among you guys.

[madkaw]

Well I finally got a new O2 sensor installed!!!!!!!!!

Now I know what happening with my AFR's at WOT-too rich! I'm dipping below 12 , maybe to 11.5 -so it's time to back off on the main jets.

Transition is still lean and I will be ordering bigger pilots.The AFRS bounce around 16+ in the dead spot.

So if you blow a headgasket, you will be replacing your sensor. I guess the coolant destroys the sensor.

[jmortensen]

If by dead spot you mean flooring it from 2000 rpm and the subsequent hesitation, I would suggest you don't tune that part out with the O2 sensor. That was what led me down the larger and larger pump nozzle road, and it got to the point where the O2 sensor would read lean while there was black soot blowing out the exhaust. It's been 10+ years, but I think the rationalization at that time was that when you floor it the airspeed in the intake goes way down and at the same time the pump nozzle sprays a big stream of gas into the intake. It's super rich, but because it bogs due to the lack of intake velocity the O2 sensor reads lean. Once the speed comes up in the intake then everything returns to normal. Solutions are not punching the throttle at low rpms and running smaller venturis (and losing top end).

Jetting is as much about the cam profile as it is about displacement, I wouldn't use any prescription for jet sizing as more than a starting point.

[madkaw]

Jon,

when I'm talking transition, I'm talking a slow transition, nothing to do with the accelerator pump. I can watch the AFR's jump around the 16 range when I get around the magic rpm of 3000 - while slowly increasing speed. It's not just the gauge, you can hear and feel the engine surging from lack of fuel.

I will use the spark plugs with the O2 to let me know whats going on as far as mixture.

[madkaw]

Well it didn't take too much to get the main fuel jet in check. I was suprised about how much change I saw dropping from 160 to 150 on the main. I went from running high 11's to 14's with the 150 jets. So I popped in some 155's I had and that seem to do the trick. Much closer to low 13's and high 12's.

Now I anxiously await my new pilots. Maybe I'll make a video today, the car is running awesome!!