Which mikuni carb?

[JohnnyDatsun]

Engine specs

L28 n42 head and block .30 Over

Ross forged flat tops 87mm

10.2:1 compression

.460/.460 270/280 cam

Kameari 10 lb flywheel

 

I've got a brand new set of ztherapy carbs in the shop, and rebuilt triple solex 40s.

My question is should I move up to 44s or just run the 40s? Thanks, John

[JohnnyDatsun]

Any more information needed? It's going in a 1970 240z, I haven't ran the engine yet, I'm think of getting the engine dialed in with the su than putting on the miks and tuning them with a wideband

[budgy]

Since you have the 40's you should run them first. I don't see why it isn't enough carb unless you are running high RPM all the time. I know you can't always tell based on the specs but your cam doesn't sound like its made for a high rpm screamer more of an excellent midrange? I have 45mm carbs on a cam that's a bit larger and similar compression but its also almost a 3.2 litre engine. If you are planning on running an excellent crank dampener and revving to really high RPM's then you might need a bit larger carb to take advantage of that but you would likely also want more cam too. 

[JohnnyDatsun]

Thanks, I don't remember the brand off hand but It does have a high performance dampener from msa. Cam specs are supposed to make power from ~3k to 6.5k so no too high. Thanks for the response.

[Xnke]

The 40's are OK for an L24, for an L28 the 45's are always going to give better performance.

 

Run the 40's to break it in only if  you already have them, then replace them with 45's to actually tune the engine. Or break it in with the SU's, and you can even run with the SU's if you tune then correctly.

[JohnnyDatsun]

I have the 40s already they're jetted as follows

 

Venturi 32

Mj 150

Aj 220

Pj 60

PN 40

 

I think the pump nozzle should be bumped up to 45 or 50. Everything else is already above what mikuni recommends for a stock l28 with 40phh.

 

Any thoughts on my jetting?

[Samurai7one]

I have a similar CR and cam, running Mikuni PHH40s on a shaved P90.  I would not consider the 40s too small.  40's with that cam should be a good set up for you.  Great mid range power that is easily accessible..  I am happy with this set-up and it has exceeded my expectations.  Pump nozzle is probably good at 40.  I have a 200 air jet.  I don't know if you will be happy with the 40s, depends on your expectations.

 

Just some advice:

 I am still carb tuning but I think I am 90% there.  like a lot of other people, I struggled for a while with pilot to main jet transition, then I discovered if I take the top cover off the carbs and pull one of the jet blocks out, I could measure from the top of the jet block hole down to the fuel level.  To do this I used the rod that slides out of the end of a veneer caliper when opened up.  

 found not only were they all low, but also a 10mm difference between the high and low fuel levels (This was after I had set the floats as specified by the factory with the top of the carb turned upside down and floats hanging, set to 12mm). Using the external adjusting screw, I set all of the fuel levels at 23mm down from the top of the jet block hole.  Completely changed the running characteristics of the motor.  I even had to re-tune idle mixture (I use wide-band O2 & color tune tool).  Transition is now smoother than I was beginning to believe would be possible with these carbs.

[Xnke]

Put the car on a dyno with 40's tuned to the best you can, and then with the 45's tuned to the best you can...won't loose any drivability and will only pick up power and torque...that's what I mean when I say 45's will always outperform 40's

 

Just because you personally don't miss the extra power, doesn't mean it's not there.

[JohnnyDatsun]

Thanks for the responses, as long as the 40s pull harder than my su carbs did then I'll be happy. And certainly better than the 4bbl that was on the car when I got it.

[budgy]

I would strongly recommend getting a wideband gauge installed in the car to help tune the carbs the way you want them. Mikuni generally recommends hitting an AFR of 12:1 under WOT (wide open throttle) for maximum power but still with a safety net. Going a bit leaner than that will make a bit more power. Best to test based on being in a high gear like 4th or 5th for testing your AFR under the heaviest loads you can put the car under. If you put a wide band in and can witness what is going on in real time you can ultimately tune your car much better for power. Generally putting in more fuel will get you more power assuming it is actually being burned.