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TOO RICH! Fouling plugs at idle... L26 w/ 4 screw round tops

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Car supposedly ran "perfect" last time it drove 3 months ago according to the P.O... Got the car yesterday with it running super rich. It would only idle on 4-5 cylinders and puff out black smoke, and when throttle was applied, huge pops out of the carbs and the engine would cut down to maybe 2-3 cyl.


Today, I unscrewed the mixture adjustment knobs all the way, cleaned the needles and pistons in the carbs (Weren't gummed up much at all) set the timing to 10BTDC, checked fuel pressure (4 psi), ran a compression check (175-185psi on all 6) and verified that the chokes aren't sticking. The car runs much better, but still runs stupid rich, sputters and wants to die out below half throttle. Carb backfire is no longer present however. When I put my foot in it, the engine clears out and pulls good on all 6. Other than that, still having issues. Could this may be related to the carb floats being stuck or something? I'm kind of new to the carburetion game... 

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Buy the "Just SUs" video from Ztherapy.  They also sell SU parts.



On the way. In the mean time, I'd like to know how to at least get the car driveable/running on all 6. 


You say you unscrewed the mixture adjustment knobs all the way. Did you leave them all the way unscrewed? If so, then yes, you are running "stupid rich". Miles's answer will be the best thing you can do.

I left them all the way unscrewed, and for like 5 miles the car ran much better. Still idled very rich, but would run great above 3k rpm. Next day the car would barely start... Screwed the knobs in all the way this time, and car still runs like shit, but now backfires flames out of the exhaust when I try to gas it off of idle occasionally. 

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Time to stop turning knobs and read up on SU carbs while you are waiting for the video to arrive.  Watch the video five times and then go out to the car and identify every external part of the SU carb including every linkage and set screw.  You will need this information to tune, repair and ask meaningful questions.


Go to the download forum and download the Factory Service Manual (FSM) for the Z car  you have.  It will be a pdf file. Take the file on a thumb drive to Staples etc and have them print it out for you.


Google: "tuning su carbs 240z" 100s of how to tune/repair SU carbs on the web.  Book mark the sites.


Search HybridZ as there have been many threads dealing with your exact problem, some as recent as the last few weeks.


Note: worn throttle shafts leak air, worn/bent needles and/or improper float adjustment make it impossible to tune a SU carb by just turning the mixture adjustment nut.


You will need to purchase a "Unisyn tool" that is described in the video.  Ztherapy and MSA sell them. 

The Unisyn is used to synchronize the twin SU carbs. It is explained in the video. There is an adjustment in the center (round disk with bumps) that is used to adjust the sensitivity of the red puck in the clear plastic tube. I usually adjust it so the red puck moves between the bottom and the first line on the tube. The carbs are in sync when the red puck reaches the same height on the tube for both carbs at idle. Note: everything on the SU carb must be working and properly adjusted before you do this.


Do a Google image search for "SU carb float adjustment". There are some good graphics that explain why this is important.



Until you develop and understanding of the SU carb there isn't much we can do to help you.

Edited by Miles

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Ok, I've gone through the FSM/tuning pages and I have all the screws and knobs adjusted to factory settings. Can't narrow it down too much because my flow meter hasn't arrived yet. BUT! I discovered something interesting. Every time I give the engine around 2/3 throttle or more, the rear carb piston gets stuck up. I pulled down the piston with my finger and the engine started to idle much smoother. I believe I need to replace the needle and needle seat in the carb to fix this. Correct me if I'm wrong. 

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