How to tune Su's without flowmeter

[Reverend]

Hi

I recently made a post about my 240z carb behaviour in Engine section http://www.classiczcars.com/forums/showthread.php?t=38755

Now i'd like to know how to tune this carbs without flowmeter. After reading all the posts made by this community i think it's running too lean. What's the most safest way to make it run richer without messing up carbs more?

Also i'd like to know the reason why it occasionally puffs out white smoke from carbs after car is shut down.

[Arne]

If you are really good, you can gauge the amount of air being pulled through a carb by comparing the sound using a length of fuel line held to the carb mouth. But at best, this should be considered a very rough approximation. To do it right, you need a flowmeter of some sort.

[Reverend]

So adjusting the mixture is also impossible without flowmeter?

[Arne]

The carbs must be pulling the same amount of air before accurate mixture adjustments are possible.

[JimmyZ]

Ditto what Arne said... Gotta have a flowmeter. You'll be surprised how fine an adjustment it takes to sync the carbs. (fractions of a turn)

You can adjust mixture without synchronizing but it's not the proper thing to do. You'll have three cylinders using a different volume of air than the other three. Without the carbs sync'd the usual "raise a carb piston" while running method won't work well.

I suppose in a pinch you could make a flowmeter with the proper tools. It's much cheaper to buy one. I have often thought how nice it would be if our Z's had vaccum nipples like motorcycles. Then synchronizing would be as easy as hooking up a set of mercury gauges and would be much more accurate. A mercury gauge set is much more expensive than a flowmeter.

As far as making the mixture richer there are several things affecting mixture when it comes to SU's. Assuming the fuel system is delivering properly to the bowls,the carb parts are within tolerances and the float level is right there are two things determining overall mixture... The nozzle adjustment and the needle.

As you lower the nozzle by turning the adjusting wheel more fuel is administered. This is because the needle is tapered towards the base and is now permitting more fuel to pass. If you have completely bottomed the nozzle adjustment and the car is still running lean then you have other problems.

Keep trying! You'll find it's a easy/fun car to maintain once you're familiar with it and have a few essential tools. There's a certain pride/feeling of accomplishment in owning a Z. The Z will teach you a LOT as well. It's changed my life for the better and I'm not the only one.

Edited June 8, 2010 by JimmyZ

[Reverend]

Alrighty, i'll order the flowmeter then. Eventually i need it someday so why not order it now.

[ChrisZ]

Just make sure to get the right kind http://www.cbperformance.com/catalog.asp?ProductID=572

[Gary in NJ]

The carbs must be pulling the same amount of air before accurate mixture adjustments are possible.

Right. It's an iterative process. Once the gross mixture is adjusted (float height) you can adjust for balanced air flow. Once balanced air flow is achieved, fine mixture (nozzle adjustment) is conducted. Once the mixture is determined and set, final airflow/balance at idle is performed and then high-speed balance is set.

That's three times that you need the air flow meter.

[Carl Beck]

A different tool for "Flow Measurement"

http://www.zhome.com/ZCMnL/tech/Tools/STETools.htm

They don't seem to be available any longer. This does allow you to run the engine and observe the behavior of the two SU's...

FWIW,

Carl B.