high idle when synchronizing SU's

[rossman]

I recently purchased my '72 240z. The PO tells me the carbs were rebuilt by Ztherapy. They look nice and clean so I have no doubts. My problem is as follows. When I synchronize the carbs the idle kicks up to ~1200 rpm. The front idle screw is all the way out and the rear is in about 1 - 1.5 turns. On the front carb, with the idle screw all the way out (i.e. not cracking the throttle valve) I get about 9 kg/h flow using my Synchrometer. On the rear carb with the idle screw all the way out, I get about 3.5 - 4 kg/h. If I leave both screws all the way out, it runs but idles rough and performance suffers. With them synchronized the engine idles at about 1200 rpm but it runs really well on the road. I have verified that the linkage and center idle screw is not holding the throttle valves open. The timing is set to 12 degrees which I think is low, Haynes specifies 17 degrees IIRC. I have checked for vacuum leaks everywhere but I don't think that is the problem anyway. Any ideas?

Thanks,

Ross

[biddljj]

What do the plugs look like ? Is there different colors from front to back ?

[Walter Moore]

Could your balance screw be so far off that one carb is never really returning to idle?

Balancing the SUs is a tricky thing. I strongly recommend buying the video from Z-Therapy and watching the balancing section about three times. I ended up balancing mine three or four times before I got the hang of it.

[beandip]

This is a long shot but , it could be a misalignment of a butterfly disk on the throttle shaft. It means you need to remove the carb to check it. Take the offending carb off and adjust the throttle adjustment, balance screw, so that the butterfly closes completely. Hold the carb up to a light so you can look through the barrel and see if you can see any light around the disk. I should seal off completely with zero light showing around the perimeter. If you do see any light, loosen the screws so the disk can move but not loosely. Close the throttle a couple of times firmly and see if the disk now seals. When it does tighten one of he screws and check again. If all is OK then remove the loose screw and use some lock tight and install the screw tight and do the same with the other one. this could vary well be the problem. Check for gaskets on the both sides of the carb spacers. I am thinking vacuum leak from somewhere. What ignition are you using? 12 degrees Before Top Dead Center is plenty of advance for the engine to run smoothly. L-24 with OEM points ignition calls for 10*. Gary

[rossman]

What do the plugs look like ? Is there different colors from front to back ?

Actually they all look quite good, all six roughly the same light gray.

[rossman]

Could your balance screw be so far off that one carb is never really returning to idle?

Balancing the SUs is a tricky thing. I strongly recommend buying the video from Z-Therapy and watching the balancing section about three times. I ended up balancing mine three or four times before I got the hang of it.

Which one is the balance screw? The one just to the side by the throttle valve?

[rossman]

This is a long shot but , it could be a misalignment of a butterfly disk on the throttle shaft. It means you need to remove the carb to check it. Take the offending carb off and adjust the throttle adjustment, balance screw, so that the butterfly closes completely. Hold the carb up to a light so you can look through the barrel and see if you can see any light around the disk. I should seal off completely with zero light showing around the perimeter. If you do see any light, loosen the screws so the disk can move but not loosely. Close the throttle a couple of times firmly and see if the disk now seals. When it does tighten one of he screws and check again. If all is OK then remove the loose screw and use some lock tight and install the screw tight and do the same with the other one. this could vary well be the problem. Check for gaskets on the both sides of the carb spacers. I am thinking vacuum leak from somewhere. What ignition are you using? 12 degrees Before Top Dead Center is plenty of advance for the engine to run smoothly. L-24 with OEM points ignition calls for 10*. Gary

Am I right in thinking that the car should die if the throttle valves are completely shut? I think you may be on to something here. The ignition is a Crane Fireball XR700. I sprayed starter fluid all over all the gaskets, joints and hoses and didn't notice any rpm changes so I don't think there are any vacuum leaks.

[rossman]

Oh and thanks for the replies

I think I'm going to take Walter's advice and get the Ztherapy video.

Cheers,

Ross

[Pop's Z]

FWIW, you might try calling Z-Therapy. 1-503-587-9800 (at least it was last time I called). They are really easy to deal with and they are THE source for S.U.(Hitachi) knowledge. Bruce Palmer is on here sometimes...actually I'm sort of surprised he hasn't chimed in here yet. Also they are usually at the MSA show on the last week in April. Very knowledgable people and very helpful. I know, I bought a pair of their carbs and they set them up specially for my altitude (5000 ft and up).

Cheers, Mike

[rossman]

FWIW' date=' you might try calling Z-Therapy. 1-503-587-9800 (at least it was last time I called). They are really easy to deal with and they are THE source for S.U.(Hitachi) knowledge. Bruce Palmer is on here sometimes...actually I'm sort of surprised he hasn't chimed in here yet. Also they are usually at the MSA show on the last week in April. Very knowledgable people and very helpful. I know, I bought a pair of their carbs and they set them up specially for my altitude (5000 ft and up).

Cheers, Mike[/quote']

I called them today and ordered the tune up video and carb-to-manifold gaskets. I'm gonna pull them off, inspect and clean. I talked to Steve and he said they will have "Ztherapy" and a date inscribed on the butterfly valves if they are in fact Ztherapy rebuilt carbs.

[rossman]

It turns out that the idle adjustment arm was bent such that it was contacting the aluminum stop before the butterfly valve was closing all the way. I bent it back straight, tuned and balanced the carbs now it runs great!

Also it turns out the carbs were rebuilt by Ztherapy on 1-6-06.

[Bruce Palmer]

Now this is the point where I jump in and take all the credit for single handedly solving this carb related issue... Nyuck nyuck