SU Tune-up kit > Running too rich

[Jarvo2]

I'm hoping Scott or Bruce can chime in (or anyone else that has experience), as I'm having some difficulties fine tuning the SU's on my '72 240z. I've followed the instructions to the letter for installing the ZT-150 tune-up kit.

In order to have the piston-lift work as it supposed to (slightly higher idle and then falls back to normal when raised), I've lowered the mixture knobs down 8 full turns. The car drives and performs well, but I'm noticing that it smells very very very rich and the vacuum gauge is steady at ~17. When I lean out the mixture to about 3 full turns down (idle drops and engine almost stalls when raising the piston-lift), the immense smell goes away and my vacuum gauge is steady at ~20 and the car still performs well.

Here's what I've checked thus far: Float levels have been confirmed at 23mm, pistons drop equally, choke is 'off' once warmed up, ATF is in the dampeners, new SM needles are set properly, valves are adjusted, timing is spot on at 7 dbtdc, my vacuum advance does work as designed, and there is no un-metered air entering into the system. When I'm running 'rich' (or at least as I've described above) the spark plugs are black & sooty, whereas when I'm running 'lean' (or at least in my mind as described above) I'm seeing a nice whitish color on the plugs. Do I run any risks by not having the piston-lift test work properly?

I'm torn between the piston-lift test and achieving the highest amount of vacuum at idle. Any suggestions? I'm looking for the reliability, fuel efficiency, and performance that I know the stock '72 setup will provide me. Thanks a lot!

[Captain Obvious]

It all seems pretty normal to me...

Eight turns down for you = sooty plugs, and smells very rich, good performance? - That's because you are very rich.

Three turns down for you = clean plugs, smells normal, good performance? - That's because you have leaned out the mixture to closer to the correct mixture

What's the problem?

As for highest vacuum at idle, the knobs adjust mixture, not vacuum. I'm no expert, but I'm having a real hard time caring about what the vacuum gauge says when you're messing with the mixture knobs. Any change in vacuum when messing with the knobs is a second order effect caused by a shift in idle RPM. There's no primary link between manifold vacuum and nozzle drop.

[240260280z]

SM needles may be too narrow (rich) for a stock 240z.

[beermanpete]

It all seems pretty normal to me...

Eight turns down for you = sooty plugs, and smells very rich, good performance? - That's because you are very rich.

Three turns down for you = clean plugs, smells normal, good performance? - That's because you have leaned out the mixture to closer to the correct mixture

What's the problem?

As for highest vacuum at idle, the knobs adjust mixture, not vacuum. I'm no expert, but I'm having a real hard time caring about what the vacuum gauge says when you're messing with the mixture knobs. Any change in vacuum when messing with the knobs is a second order effect caused by a shift in idle RPM. There's no primary link between manifold vacuum and nozzle drop.

Vacuum at idle is an indicator of the fuel/air mixture. As the mixture is moved closer to ideal the engine will run faster. When the idle speed is readjusted to maintain a specific engine speed the vacuum will rise or fall because the throttle is opened either less or more. The idea is that "best vacuum" coincides with a smooth idle and the smallest throttle opening to maintain the specified idle speed. Adjusting the idle mixture to "best vacuum" is an old method from before strict emission control became the norm. Typically the mixture at best vacuum is rich by modern standards.

[Bruce Palmer]

I think you are nearing the "butt dyno" portion of the struggle. As was stated before 3 turns are getting you pretty close. It's now time to drive it and check for spit back through the carbs under acceleration or deceleration. When that goes away quit fiddling. You already know what fat looks,, runs and smells like.

As far as SMs are concerned, they were the first step up performance wise in the first several pages of the How to Modify Datsun engines and chassis copyrighted 1973 and have been in wide use ever since.....

[Jarvo2]

Thanks for the great info, just wanted to make sure I wasn't going too crazy with the piston-lift test failing! As for the ZT rebuild kit, it worked perfect, and is very high quality.

[Captain Obvious]

Vacuum at idle is an indicator of the fuel/air mixture. As the mixture is moved closer to ideal the engine will run faster. When the idle speed is readjusted to maintain a specific engine speed the vacuum will rise or fall because the throttle is opened either less or more. The idea is that "best vacuum" coincides with a smooth idle and the smallest throttle opening to maintain the specified idle speed.

This illustrates perfectly what I meant by it being a second order phenomenon. You aren't controlling vacuum with the mixture, you are controlling idle speed. It's the idle speed that's affecting the vacuum, not the mixture.

Vacuum at idle is not an indicator of fuel/air mixture. Peak idle RPM at a constant throttle position is an indicator of mixture.

I'm sure we're saying the same thing.

[Gary in NJ]

I really don't want to delve into a "this tool is better then that tool" debate, but I have found that tuning with a Colortune is considerably faster, more accurate and repeatable then using the lift-the-piston method. Yes there are other tools that are even better, but a Colortune is an inexpensive gateway tool to the other, more expensive options.

[pacemkr]

I really don't want to delve into a "this tool is better then that tool" debate, but I have found that tuning with a Colortune is considerably faster, more accurate and repeatable then using the lift-the-piston method. Yes there are other tools that are even better, but a Colortune is an inexpensive gateway tool to the other, more expensive options.

not looking to hijack this thread, but i would love to hear more input on this colortune item.

online info looks interesting.

[Gary in NJ]

Discussing the Colortune (it's benefits and limitations) would hijack this thread. If you do a search you'll find some recent (within the last year) conversations. Or start a new thread if you don't see the information you are looking for.