On my 1973 Z, I recently put on refurbished carbs, intake and air box with K & N air filter from a 1972 Z. Also, a 165 degree thermostat. When I start the car it will idle at 800 rpm. After driving the car for 20 minutes it will idle at 1700 rpm. Sometimes it will go as high as 2300 rpm.

Could anyone offer a suggestion on how to cure this? My mechanic is unable to figure it out.

Thank you!

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1973 240Z

I have read about a decelleration valve that was used to help with emmisions. It might be malfunctioning. It you look at the balance tube beween the carbs more toward the front carb. try disconnecting the vacuum line to the dashpot and plug the line then drive the car and see if that helps.

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Adam Perry
Datsun 260Z

Make sure the choke cables aren't too tight. If mis-set they may not be allowing the fuel nozzles to retract fully when you push the choke lever forwards. Symptoms of this are normal startup but higher idle when it warms up.

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Bryan Little
Datsunzgarage.com

1970 240Z - enhanced F54 L28 w/P90, Weber DCOEs, 4:11 R200, Nissan T5 5-speed

Also remove the air cleaner (air box) and ensure both your suction chamber pistons are at the same height while running. I had one which would stick after my carbs were rebuilt. it wasn't a lot, but as Bryan might remember, it couldn't be "adjusted away" and had to be sent back for rework under warranty.

You could feel both pistons with a finger in each throttle, or you could use a mirror to view them. Their only purpose is to raise/lower the tapered needle rods in the fuel jets, thereby letting more or less fuel into the engine. If one is higher then the other (or if either needle wasn't tightened well and slipped), one needle would be allowing more fuel to the engine than the other would.

I've heard of the deceleration valves but don't understand them well. The chokes on SUs do nothing more than lower the jets away from the needles, so a richer mixture enters the engine. If either choke is sticking, it will act exactly like a sticking vacuum chamber piston. Doesn't matter whether the needle sticks "up" or the jet sticks "down", the result is a rich mixture on that carb. Picture an upside down traffic cone in a big donut. Fuel tries to flow thru the donut hole and into the engine. If you lower the cone (the needle), it closes the donut hole (the jet) and no fuel can flow thru it. If you either raise the cone or lower the donut, fuel can flow thru the hole.

Also, with the engine off, lift and release each vacuum chamber piston (put your finger into the throttle and raise it). When you release it, the piston should return to the bottom quickly and smoothly, but not 'slam' down hard. Unscrew your black plastic thumbscrews on top of the SUs and pull the tiny piston out. Fill that little tubular reservoir up with 10wt oil (common household 3-in-1 "sewing machine" oil) then screw the pistons back in. Only screw the thumbscrews finger tight.

http://www.howacarworks.com/fuel-system ... burettor-1

Frank

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1970 240Z

Thank you for those suggestions! I forgot to mention, the engine was desmogged.

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1973 240Z

Please keep us on top of this, and tell us what you found wrong.

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1970 240Z

If it's not the choke cables, then maybe they need adjustment. There are only 2....the screws, (little wheel) on the bottom, and the screws on the top.

I took it back to the mechanic he adjusted the choke cables and checked the suction. Then he sprayed the moving parts on the carbs with oil. That did not work. I made a call to the place that refurbished the carbs and they said they never had anyone call with that problem. He said that it may take a little time for the plating to break in. Hopefully, it will correct itself. If not, I will get it fixed in the spring.

Thank you for your help!

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1973 240Z

Soooo, you've covered all the bases except one.

1. You've adjusted the chokes so that's not it;
2. You've checked the 'moving parts' and lubed them, so that's not it;
3. You've checked the intake vacuum at the mouth of both carbs and matched them, so that's not it;
4. You're smart enough to make sure it isn't something stupid, like your carpet getting pushed against the accelerator pedal, so that's not it;
5. Your mechanic probably also checked and lubricated the full throttle linkage from pedal to carbs and ensured nothing was binding, so that's not it.
6. The only thing left might be a vacuum leak somewhere after the carbs.

Any time you pull off the intake and reinstall it, there's a chance for leaving a leak. Leaks might not happen while the parts are cold, but as heat expands them, a tiny vacuum leak can lean out the mixture on one carb and make those 3 cylinders run hotter than the other 3. After the engine runs fast like this again for a few minutes, pull #2 and #5 spark plugs and compare them. If one looks whiter than the other, that carb is running leaner. Pull all the plugs and see if you can establish that that 3 look hotter than the other 3. That's the carb which has the leak.

Did you use new mono blocks, or reuse the old ones? If the old ones got put on differently than they came off, it can allow leaks. Aluminum manifolds expand a bit when they heat up; the mono blocks don't expand at all. The leak wouldn't show until the engine warmed up and the aluminum manifold expanded.

To check for leaks, spray a little starting fluid around the areas where leaks are likely (this could be a little dangerous when the exhaust manifold is hot, careful). If you squirt an area and the engine revs up, you've found a leak.

I assume the sorry response the rebuilder gave you ("never had anybody else complain about that ~ it's your problem. Ignore it awhile and maybe it'll go away"), means you didn't have them rebuilt by Z Therapy. ZT guarantees their work and will take your carbs back to chase away any type Gremlin you find once they rehab your SUs. Any reputable SU specialist should do the same, rather than shine you on and try to convince you to live with the problem.

One of the odd problems ZT warns against is a slightly bent fuel rod (metering needle), which can bind against the fuel jet during operation and prevent the vacuum chamber piston from closing completely, thereby causing a rise in rpm. Happily, that problem can actually be diagnosed and fixed right on your own workbench if you know how. All you need is an electric drill and a bit of clean lumber.

If you can get ahold of the 4hr Z Therapy video "Just SUs", you can just about cure all your own woes without leaving your own garage. Can i send you a copy?

Frank

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1970 240Z

Frank that was an awesome amount of information. Thank you very much!

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1973 240Z