So I finally got my own synchrometer (SKtype) and after having warmed up the engine and disconnected the throttle arms from the throttle shafts I took some baseline readings...

The gauge on the synchrometer goes from 0 to 30.

Both barrels on the first carb (the one closest to the front of the car) showed a value of 6 on the gauge, while the remaining four barrels on the other two carbs all registered at 3.25 to 3.5

The synchrometer manual/leaflet indicated that before any tuning could be done all carbs need to be within one gradation of one another... Obviously this is not the case here...

On the other hand though my stock NGK BP6ES plugs look fine, I've got an O2 sensor plumbed just rearward of the header collector which is showing barely a tick over stoichometric so that's good too... Am getting great power and torque and even better MPG on the freeway...

The Mikunis don't have that many adjustment features unlike Weber DCOEs for example and all three carbs have their idle mixture adjustment screws set at 2.5 turns out from seated as indicated in the Mikuni manual.

So what can I adjust?

Where would this discrepancy come from?

Any ideas or suggestions?

Thanks all & it's good to be back - yeah!

-e

hey jmortesen...did I spell that right? , well I've got an L28 with stock cams and N42 head. I ended up turning the pilot screws 2.5 turns out from seated position. I think the turns depend on what engine the carbs are on.

But before I did this, I bought the car with it idling relatively rough and had black smoke coming from the exhaust. When I was done adjusting the pilot screws I had no more black smoke, idle really quiet, but it tends to sputter more. Just a little during idle but more so when I take my foot off the throttle. I read in the Mikuni manual that black smoke is from the car being to rich; the pilot jet being to large and needs a smaller jet. This would also explain the sputtering right?

How am i suppose to know what size pilot jets I already have in the car. Are numbers stamped on them?

Edited May 22, 200915 yr by Tyrone

hey jmortesen...did I spell that right?

No. It's right there. You could copy and paste, or just write Jon if that's easier.

, well I've got an L28 with stock cams and N42 head. I ended up turning the pilot screws 2.5 turns out from seated position. I think the turns depend on what engine the carbs are on.

You CAN adjust them, but you SHOULDN'T.

But before I did this, I bought the car with it idling relatively rough and had black smoke coming from the exhaust. When I was done adjusting the pilot screws I had no more black smoke, idle really quiet, but it tends to sputter more. Just a little during idle but more so when I take my foot off the throttle. I read in the Mikuni manual that black smoke is from the car being to rich; the pilot jet being to large and needs a smaller jet. This would also explain the sputtering right?

How am i suppose to know what size pilot jets I already have in the car. Are numbers stamped on them?

The numbers are stamped on the pilots. Buy the How to Modify Your Nissan/Datsun OHC Engine by Honsowetz. It has a good although brief section on tuning Mikunis, has mathematical formulas to figure out where to start with jetting. You can also download the Mikuni tuning manual from Hybrid Z: http://forums.hybridz.org/showthread.php?t=122217

I would also suggest installing an O2 sensor and either a gauge or a voltmeter to read it. You can go crazy with a wideband if you want, I had pretty good luck with a $30 narrowband Bosch O2 and a $6 voltmeter. I've heard several people who bought the Autometer A/F mixture gauge say the sensor that comes with it is crap. The gauge works fine when you put a decent sensor in it though, apparently.

With an O2 installed you're no longer guessing, trying to read plugs (which just about everybody does wrong, including me), or smelling the exhaust. You will KNOW what is happening with the O2. It's really worth the hassle to install one.

My take on Mikuni tuning is that it is mostly about the pilot jet. If you have a stumble when you punch it, pilot jet (and venturis). If you're getting bad mileage, pilot jet, if you are running lean at cruise, pilot jet. For whatever reason it just doesn't seem to be that hard to get these things to run good at WOT. It's getting the right pilot for cruising around town and getting rid of the stumble going from cruise to WOT that seems to be the problem. You almost never want to mess with the pump nozzles, and main air and fuel are easy to dial in with the O2 sensor.

My take on Mikuni tuning is that it is mostly about the pilot jet. If you have a stumble when you punch it, pilot jet (and venturis). If you're getting bad mileage, pilot jet, if you are running lean at cruise, pilot jet. For whatever reason it just doesn't seem to be that hard to get these things to run good at WOT. It's getting the right pilot for cruising around town and getting rid of the stumble going from cruise to WOT that seems to be the problem. You almost never want to mess with the pump nozzles, and main air and fuel are easy to dial in with the O2 sensor.

gotta deadthread props for the best tuning tip ^ i could find on google