Heat & mixture problem

[gmr714]

:disappoinI recently bought a 74 260z with Hitachi Carbs.I did a complete tuneup ++ and then had the carbs adjusted for $$$. It seemed oK at first. Now I have a problem with the way it runs if I start it while it is still hot. It runs pretty good for 50 miles but if I stop for gas or a soda then I start it back up it will stutter and run very weak. The engine temp stays low. I have to pull the hand choke all the way back to get power under load. It does not stall out. I have been through the vapor lock theory, and it is not the problem I think. I ran new fuel lines and insulated them , the pumps are new with good pressure. When I have this problem I can drive it for miles and it won’t smooth out, shouldn’t vapor be forced thru? I need to shut the car off for 15 minutes and it runs fine again. I found that the temp sensor on the air cleaner is missing and the air control valve on the intake does not function, should I close it so it only draws fresh air all the time? also the vacuum control valve is inoperative as the wire is torn off. The EGR has also been disabled since the diaphram was leaking and the VAC line is capped. When I pull back the choke am I cutting air or adding fuel or both? The vapor canister is also disabled.I have been attempting to figure out a way to adjust the idle speed and mixture also but all I have is an untrained ear and a screwdriver. I am a new Z fan and would appreciate any and all input on how to troubleshoot and get this car running smooth without replacing all of the above parts.I have antique plates ( no inspection or emissions) It is a driver not a show car. Thank you for your suggestions

Mark in PA

[beandip]

To solve your problem, email Ztherapy and order a set of SUs. Sorry but there is a reason that Nissan dumped the flat tops and what you describe is it.

[Weasel73240Z]

I'm with beandip on this one. I know you said you don't want to replace the parts, but I have a 73 240 that came with flat tops. I ordered the ZTherapy SU's, and now the car runs, idles accelerates like new at any temp, and no matter how long (or how hard) I drive it.

The ZTherapy carbs are worth every penny, and Bruce is great to deal with.

[z74z90]

In spite of what you have said, I would not rule out "vapor lock". I am the original owner of a late '74 260. It had the flattops and ran just fine for the first year. After that, those "vapor lock" type symptoms flared up. I took it to 3 different dealers and they just threw up their hands. Finally, a dealer actually suggested and did the swap to round tops. Unfortunately, that is not the end of the story. While the car did much better, it still to this day suffers those "vapor lock" problems. It still has the original heat wrap, and both the mechanical and electric fuel pumps. About 4 years ago I added a "fuel injector fan" from the S130 series. That made a huge difference in hot, humid weather. There are those that claim a properly adjusted flattop is a great set up. However, finding some one to deliver on that promise is not easy. Some suggest re-routing the fuel rails along with insulation, fans, whatever...but the most popular thing is to ditch the flattops. Strangely, some cars never seem to cause any problems and others can never lose the vapor lock issues. Good luck.

[gmr714]

Sorry I wasnt clear. I have the round carbs with the oil dipstick on top. Is there anyting I can do with these ?

[DeesZ]

Flat top carbs and round tops both have the plunger that you reference. The 'dome' that it screws into is the flat or round part.

Here is (I hope) a link to a good picture of flat tops that is on the eBay historical records: http://i11.ebayimg.com/06/i/001/1c/8f/65e7_12.JPG If yours look like this, they are flattop carbs.

On the other hand, if they look like the ones in my gallery (towards the beginning - maybe page 6?), they are the round tops.

[gmr714]

Thanks, I do have the flat top hitachis, I was confusing "flat top" with the webers. Can I leave all of the things I described disabled and still get it to run decent? How do I adjust the idle mixture? Thanks, Mark:sick:

[DeesZ]

The following link may help a bit.

http://www.jetlink.net/~okayfine/SU/tunacan.html

I have not run across anyone in the last several years that's running flat top Hitachis...

Where are you in PA?

[gmr714]

Thanks for the info. I live in Walnutport, PA 10 mi. NW of Allentown. I store the car at a friends and will try to use your info over the holiday to get a handle on what I am dealing with. I bought the car from a friend who redid the interior and exterior but not much to the engine after a fire. I will definitely look into the desmogging procedure. Thanks, Mark