Simple answer requested - tuning SU carbs

[Brandy]

Hi all...

Latest in my slew of apparently simple questions has to do with tuning the SU carbs. I had the carbs on my '70 240Z rebuilt, *supposedly* tuned by idiot mechanic, and now husband and I wanted to 'practice' using the synchrotester. Well, outside of the fact that the 'better' synchrotester by MSA is flimsy (read: broke), the carbs were vastly out of whack (front read 3 at idle, rear read 17). I ordered a uni-sys (sp) through the local O'Reilly auto store (a lot cheaper than MSA, even though its the same brand). Outside of the scary fact that the guys at O'Reilly had no idea what the thing was, I expected to be able to sync the carbs.

Digressing back to question: how does the unisys work? We tried setting it up according to diagrams and pictures in both the FSM and Haynes manuel, but the little red thingy sat at the bottom of the tube (tube was maintained at a perfectly vertical position). Yes, car was running, yes, the air filter was removed, no, fingers weren't sitting on anything that needed to move, and no, no directions were included.

Any simple ideas on what we are most likely missing?

[MikeW]

Digressing back to question: how does the unisys work? We tried setting it up according to diagrams and pictures in both the FSM and Haynes manuel, but the little red thingy sat at the bottom of the tube (tube was maintained at a perfectly vertical position). Yes, car was running, yes, the air filter was removed, no, fingers weren't sitting on anything that needed to move, and no, no directions were included.

Any simple ideas on what we are most likely missing?

You have to adjust the uni-syn by turning the center wheel thing. This controls how much air flows through the device versus how much air is forced to come through the tiny hole which gets its air through the tube with the red float. As I recall the more the main air hole is blocked the more air comes through the tube and therefore forces the red float up.

Keep in mind that the calibration marks really mean nothing. The entire purpose of this tool is to simply get the two carbs balanced. As long as the float is not resting at the bottom or floating all the way to the top you just want to adjust each carb so that the float is in the same relative place.

Another key to using it is to not hold it over the carb for very long. You just move the uni-syn into place long enough for the red float to move to a certain position but not long enough for the reduced airflow to have a noticeable effect on the engine.

[240ZX]

Mike is on the money with respect to his advice. In addition to what Mike said and before you even get to the point of syncing the SUs, be sure your valves are correctly adjusted, check your sparkplugs for correct heat range/gap/are they in good condition, check your timing, and go through the recommended procedure for insuring the SU throttle linkage it set correctly. I know I may have left a few items out! Basically, there are a number of items that require attention before you attempt syncing the SUs. It sound like alot and can certainly be confusing for first timers. Once you understand it all....well, of course it will all make sense at that point. In the meantime, you may concider contacting Z Theropy (see their website) for their information on going through this process...it may make it a bit easier for you. Hope this helps out to some degree!

[beandip]

Hi all...

Latest in my slew of apparently simple questions has to do with tuning the SU carbs. I had the carbs on my '70 240Z rebuilt, *supposedly* tuned by idiot mechanic, and now husband and I wanted to 'practice' using the synchrotester. Well, outside of the fact that the 'better' synchrotester by MSA is flimsy (read: broke), the carbs were vastly out of whack (front read 3 at idle, rear read 17). I ordered a uni-sys (sp) through the local O'Reilly auto store (a lot cheaper than MSA, even though its the same brand). Outside of the scary fact that the guys at O'Reilly had no idea what the thing was, I expected to be able to sync the carbs.

Digressing back to question: how does the unisys work? We tried setting it up according to diagrams and pictures in both the FSM and Haynes manuel, but the little red thingy sat at the bottom of the tube (tube was maintained at a perfectly vertical position). Yes, car was running, yes, the air filter was removed, no, fingers weren't sitting on anything that needed to move, and no, no directions were included.

Any simple ideas on what we are most likely missing?

Have you gone to the tec section and looked for ADJUSTING THE SUs If not do this first. The uni- sun is basicly just a device to be used to compair the volume of air traveling through a carb compaired to another. The position of the ball means nothing outher than the thing is working , so set the adjustment of the uni-sun so the ball is somewhere in the middle then put on the other carb and see how it compairs volume wise . Adjust the air setting on the carbs so that they are equal. Set the idle to about 800RPM and THEN adjust the gas mixture. Follow the step by step of the tec posting. Gary

[hls30.com]

While the tech article is excellent, I suggest you buy the ZTherapy Video on tuning your carbs!

Will

[Brandy]

You guys are great - if you lived nearby I'd make you all lasagna.

I went through the tech section, and literally downloaded everything from ZTherapy, it's just the basic simple how-does-this-thing-work that got me stumped. We replaced the spark plugs (the old ones were fouled), ignition wires (the old ones looked literally original), and I got a new ballast resistor since the original was cracked (not sure how important that was, but the new one was only a few dollars). I plan to *try* to use this car a few times a week to go to work (150 mile round trip every day), my commute is along a pretty deserted highway, and my husband didn't want to worry about my getting stuck when he goes back to work (he's a merchant marine). Plus, if my car works, I won't need to use his car (69 Camaro, but that car hates me anyway).

We adjusted the idle using the tach and idle adjustment thingy *a brandy-ism*, so the car idles at around 800 rpm (anything less, and she gets rough). It was at that point that we tried to sync the carbs. And I'm not kidding about the guys at O'Reilly's not knowing what the unisys was for....

Thanks guys - now to find another thing wrong with the car....

[Zs-ondabrain]

Here is a link that saved my life when it came to SU's.

http://www.teglerizer.com/sucarbs/index.html

The best one on that page is the 2nd one down (Blue square) with Tech Tip #6 in the square. It rocks when it comes to everything SU.

Good luck,

Dave.

I also just realized how old this thread is