Miles Posted September 17, 2005 Share Posted September 17, 2005 The SU carbs were rebuilt by ZTherapy two years ago. No problems until one week ago. 240Z L24 Engine idles very rough/ragged. It smoothes out at higher rpm, but there is a definite surge at times at high speed. Here is what I have done so far: • Replaced all ignition parts using Nissan parts. • Set points • Set timing per factory specs. • Set mixture and balanced the carbs. The new ignition parts and timing did not improve the rough idle. After balancing the carbs the engine still idles very ragged. If I lift the piston on the front carb the engine RPMs drop slightly and then the rpm recovers. If I lift the piston in the rear carb the rpm drops and stays low and the engine idles very ragged. When I drop the rear piston the RPMs increase, but the idle remains ragged. If I completely cover the throat on the front carb with my hand the engine continues to run smooth. Actually it runs smoother when I cover the throat of the front carb. Evidently the rear carb is capable of carrying the load of both carbs. If I cover the throat on the rear carb with my hand the RPMs drop and the engine will die. It doesn’t matter where the mixture/idle is set on the front carb it will still idle ragged and the engine will die when I cover the throat on the rear carb. Any help will be appreciated. Miles Quote Link to comment Share on other sites More sharing options...
datsunlover Posted September 17, 2005 Share Posted September 17, 2005 I'd check that the needles are in good shape (not bent/worn) and not sticking, and check the float setings on each carb as well. Also, it could be your pistons need a top up (don't know if you're running ATF or engine oil...) or maybe the front carb is low on/leaking damper fluid... I'm sure one of the 'experts' here can give you some more info, as I'm sure I don't know evrything. Quote Link to comment Share on other sites More sharing options...
SportZ2 Posted September 17, 2005 Share Posted September 17, 2005 I know this may sound way to simplistic but could it be bad gas. Everything you have noted as problems were exactly what I had happen the other day (rough idle, surges at higher rpms, etc). Realized it was bad gas because the car ran fine before I filled the tank and then ran like crap about 20 miles later. I drained the tank and put in some different gas and the car runs fine again. What a waste of $40.00. Quote Link to comment Share on other sites More sharing options...
quicker240 Posted September 17, 2005 Share Posted September 17, 2005 Sounds like you may have a vacuum leak somewhere to me.Check vac lines all over and also check float levels.Good luck... Quote Link to comment Share on other sites More sharing options...
BrandonsZ Posted September 21, 2005 Share Posted September 21, 2005 the first carb might either be too rich or too lean. Idle adjustment is just that, idle, won't fix this problem. I'd rebuild the first one (just take it apart, clean it, replace the gaskets and seals (if necessary) and you'll probably find a dead bee or something somwhere, it might be gummed up. Quote Link to comment Share on other sites More sharing options...
PeterZ Posted September 22, 2005 Share Posted September 22, 2005 I agree to check the front carb, To bee or not to bee, good call. Depends on what you are doing to cover off the carb inlet. Two fingers over the opening should be enough to make a change. If you put your hand over it and it makes a small change there's a big problem. To make one carb run leaner you can pinch off each fuel inlet hose at the nozzle. One way or the other (righ or lean) whould make the engine run happy if it truly is a fuel problem. Maybe a cootie got in and pluged the bowl vent on the top of the float chamber. Sticking a vacuum cap on them is a great way to trick a friend's car into not running right. You might lose a friend so be careful on how long you wait if he can't figure it out... Quote Link to comment Share on other sites More sharing options...
erb379 Posted October 8, 2007 Share Posted October 8, 2007 Sorry to revive an old thread, but i've been searchin around and this seems similar to whats going with my carbs. To elaborate, my front carb doesnt seem to respond to anything( lift pin method) whereas my rear carb does. And when I adjust both mixture nuts out equally, it seems the rear carb is running heavily richer than the front carb or atleast thats what my spark plugs are telling me(the front looks like its running lean and the rears are getting fouled) When I put my hand to cover the throat of the front, it doesnt do anything, where in the back, it just flat out dies like it should. Would this happen to be vaccuum related? My idle jumps up and down slightly anywhere from 750-1000 rpms, but the car still seems to drive fine. TIA! Quote Link to comment Share on other sites More sharing options...
Miles Posted October 9, 2007 Author Share Posted October 9, 2007 Get the video tape from Z Therapy and then balance and adjust your carbs. Be sure to properly balance the carbs per the video. As for mixture I found that my carbs work best at about 2 1/4 turns out. Miles Quote Link to comment Share on other sites More sharing options...
erb379 Posted October 9, 2007 Share Posted October 9, 2007 dang, didn't wanna have to resort to "the video" but guess its time haha. In the mean time, I've done a complete tear down on both carbs and put them back on balanced them, idles better but the richening problem in the rear is still there. I adjust out the same number of turns so Im a bit confused with that still. I can physically see more fuel flow from the rear carbs than I do from the front and the rear bowl covers are cooler to the touch than the front. Looking at the exploded diagrams I think I might be missing a shim on the mixture screw on the front carb which could be causing this... I'll have to experiment once again. Quote Link to comment Share on other sites More sharing options...
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