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SU Help 72 240Z


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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

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Was this a somewhat gradual or kinda overnight thing? From you starving the from carb of air and it running better means it running a bit lean , and sound like your rear might be a bit rich but I think it might be something else. Not trying to steer you in other directions but if this was kinda a overnight situation I would check the vacume lines to the dizzy and to the other areas of the motor < not sure if you have the emmissions at all> also if you are sure timing is correct and you have changed points, cap, rotor and plugs , just for laughs check the dizzy shaft for play. Also I highly recommend getting the z therapy tuning video which is great. Just as well check the oil in the carbs , also the advance for the dizzy. I belive if you crank down < shut down the carbs screw , on the bottom on the carbs and twist back out 2 and one half times that is the recommended starting point for them.

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Miles , take a look at the contacts inside the dist cap and scrape off any buildup. try reballancing the carbs , I hope you are using a uni-sun or equlivent. I don' t know what covering the carbs with your palm is supposed to do , but when you lift either piston the engine should run rough but not die . As what was asked before . did this come on all of a sudden or gradually. One thing , always check for vacuume leaks . Just for kicks check the point gap once more while you are checking the contacts on the cap. See that all contacts are clean at the coil and the plug wires are good.

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The distributer is one year old Nissan OEM with all new Nissan points, condensor, rotor and cap. Plug wires are one year old NGK from MSA. Problem started about one week ago. No emissions equipment left on the engine. Also, I thought it might be a leaking intake manifold gasket so I had the gasket replaced right after I noticed the rough idle. All vacuum lines are good and the vacuum advance works.

I went out and started the car a few minutes ago and set the choke full on. The engine ran even worse.

Could the float needle valve be stuck or possibly the jets are plugged??

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if you take the cover off the air cleaner and lift the pistons in the carbs with the engine NOT running . Do the pistons drop down easily with a clunk ? Do you think the engine is running rich or lean . I was wondering if maby a float might be stuck in the carb , either open of closed . You said that the engine ran worse with the choke , then It could be the choke sticking . The hoses that run from the float chamber to the bottom of the jet. are they stiff or brittle , they will harden with exposure to the heat from the header. If this is the case they can cause the choke to stick down slightly and cause a vary rich mixture and the engine will run like crap and the mileage will be bad. When the engine is running this way , try reaching down under the carb and pushing up on the jet and see if it moves up at all . Be shure the choke cable is adjusted so when you push the lever forward the chokes are off completely . These hoses are available from Nissan or Z therapy . the hose from the local parts store is too stiff . Did you check the ballance with a uni-sen ? Please check the contacts inside the dist cap . even if it is only a year old , and if there is any diposits on them scrape them clean .

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Miles,

If lifting the pistions one by one on the SUs by hand, they should act the same (like your front one)...since they don't, your carbs are NOT balanced. You should be able to do the piston test on both and not have too much different results for each (imo) and from my experience, I would say you need to let out your idle screw 1/4 to 1/2 turn on the front carb, or a few clicks 'down' on the bottom adjuster. SUs are fun to play with! esp. if yours are from Z therapy, becuase it's almost guaranteed that you don't have any shaft leaks.

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Make Sure You're not pulling air, as in a leak. If you cover the front carb, the engine should struggle at best. Make sure that you still have your brass/copper washers on either side of your fuel inlets where your screens are... (or are not).

Good luck-

72240z

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  • 2 years later...

Your carbs definitely aren't synced, but I don't think that's it, given the sudden nature. I'd wonder about the floats. If you don't have 'em, consider installing grose-jet float valves.

And see if the throttle shafts are leaking. If they were recently rebuilt, they shouldn't be, but leaky throttle shafts cause no end of SU problems.

[plug]If all else fails, ask on the Classic Motorsports forums. There's some SU whizzes there[/plug]

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Regardless of how long it has been....

"If you don't have 'em, consider installing grose-jet float valves."

This is something you should NOT do. I just ordered OEM float valves from Bruce (just received them in the mail Bruce, thanks)...these are just fine to replace worn out or gummed up float valves. I was simply missing one small clip that holds the needle from falling out....since I had originals, I just got some OEM ones, along with 2 new lid gaskets for the float containers, for $39 shipped. Better than any other price, I would recommend Ztherapy and Bruce for these parts. I checked around, and for japanese hitachis, this is where I would get them. Everyone else, you have to purchase either at the dealer or in a rebuild kit.

Grose Jets have been found to be overall defective--they have a high rate of 'sticking' in the open or closed position. Most owners with these will carry a ball peen hammer with them to 'hammer' the float bowl to release the ball bearings on the grose jets. NOT recommended!

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I had the carbs set too rich and the other problem was that the hose that goes from the float bowl to the jet/needle valve assembly on the bottom of the carb got trapped by the air box such that movement of the jet was resricted. As soon as I freed the hose the engine ran fine.

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Miles,

i know that you seem to have cured your problem but i was curious to your explanation. So was the hose kinked from the float bowl to the carb nozzle? The reason I ask because my carb did the same thing and it was a hose that was kinking.

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