GarageRatt13 Posted May 7, 2012 Share Posted May 7, 2012 Hey guys I just installed my new S.U. Roundtops and I cant figure out how to tune them. It starts and idles but its a really low idle and if you give it gas and let off it dies instead of dropping back to idle. I'm assuming its a fuel starvation issue but I cant seem to figure out how to tune them and none of the shops in the area know anything about these carbs. Any help would be greatly appreciated! Quote Link to comment Share on other sites More sharing options...
svMike Posted May 7, 2012 Share Posted May 7, 2012 (edited) I'm going to assume that you've already tuned the rest of the engine (point gap, timing, valves) since carb adjustment should be the very last thing you do. Here's a good video of how to do it: Edited May 7, 2012 by svMike Quote Link to comment Share on other sites More sharing options...
Miles Posted May 8, 2012 Share Posted May 8, 2012 Go to Z Therapy and buy the Video "Just SUs" it will teach you everything you need to know about tuning SU carbs. http://www.ztherapy.com/ They also sell quality rebuild kits and rebuilt SU carbs. Quote Link to comment Share on other sites More sharing options...
GarageRatt13 Posted May 8, 2012 Author Share Posted May 8, 2012 Well the car ran fine on my old flattops so would I still have to adjust everything on the motor again? I watched the vid and it seems pretty simple so i'll give it a shot this week. Also i'll look into buying that video from Z therapy. Quote Link to comment Share on other sites More sharing options...
Miles Posted May 8, 2012 Share Posted May 8, 2012 (edited) You will definately need to buy the Z Therapy video as it is Datsun specific and provides a complete understanding of how the SU carb works (important) and how to properly tune the carbs . It sounds like your carbs are initially too lean. The mixture screw on the Datsun SU carb is the knurled knob at the very bottom of the carb. Here is a quick and dirty starting point that may help you get the car running. The intial starting point for setting the mixture so the car will run is: 1. Looking top down at the carb turn the mixture knob counter clockwise all the way in (max lean). You can mark the knob with a marker so you can keep track of the turns. 2. Looking down at the carb turn the mixture knob out clockwise (richer) two and a half turns. This is your starting point to get the engine running. 3. Repeat setting the mixture knob on the other carb. Start the engine. Advance the throttle. If the the engine hesitates or back fires through the carb it is too lean. If it is too lean try turning the mixture out (richer) 1/4 turn on each carb. Experiment with the idle screws if you need to increase the idle speed to keep the engine running. You need to buy a 240Z Haynes manual as a minimum as it details the linkages and adjustments on the carbs which you must know in order to tune the carbs. This is also covered in the video. Check the linkages against the Haynes manual/video to make sure that they are installed correctly. There is much more to balancing the carbs and setting idel speeds etc., but this should get the engine running assuming that the floats and linkages are set close to being correct. You will need to buy a Unisyc tool to properly balance the carbs. You can get the Unisyn tool from Motor Sports Auto (MSA), BlackDragon and some auto parts stores. It is better that you set the mixture a little rich than too lean. A too lean condition can burn the valves. After viewing the video, and some practice, you should be able to do the entire carb tuning/balancing in about fiteen minutes. Edited May 8, 2012 by Miles Quote Link to comment Share on other sites More sharing options...
GarageRatt13 Posted May 8, 2012 Author Share Posted May 8, 2012 Awesome thanks Miles! I'll try that out tomorrow if it isnt raining. Quote Link to comment Share on other sites More sharing options...
Leon Posted May 8, 2012 Share Posted May 8, 2012 One tip, don't get the Unisyn tool. The Weber STE SK syncrometer is much more user friendly. Quote Link to comment Share on other sites More sharing options...
GarageRatt13 Posted May 9, 2012 Author Share Posted May 9, 2012 Cool thanks for the tip! can I get them at any part stores? or do I have to order them online? Quote Link to comment Share on other sites More sharing options...
Leon Posted May 9, 2012 Share Posted May 9, 2012 Online. I got it from here, it's the cheapest place I've found: http://www.texasaircooled.com/catalog/Weber-Carburetor-Syncrometer-SK-Type-1-30-kg-h.html Quote Link to comment Share on other sites More sharing options...
GarageRatt13 Posted May 9, 2012 Author Share Posted May 9, 2012 Damn its out of stock... I might check around the local shops to see if I can find one. My dad said the local motorcycle shop might have one. Quote Link to comment Share on other sites More sharing options...
GarageRatt13 Posted May 10, 2012 Author Share Posted May 10, 2012 Ok so I messed around with the carbs a little bit today and i adjusted the lean/rich as stated above and it ran but it was still really choppy and hesitant. If I enrichen the mix it starts to smoke pretty bad and if i lean it out at all it runs really bad. I tried messing with the Timing but it didnt seem to make any difference. Im going to look into it more this weekend once the weather clears up. Anything else I should look into? Quote Link to comment Share on other sites More sharing options...
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