Hesitation after warm engine

[Palmettobug]

I have a '73 240z with automatic transmission and original L24 engine. I put it in the shop as soon as I bought it, as it needed all new brake line and master brake cylinder. I also had them put in a new flexplate, as mine was missing quite a few teeth. Add a new started. When I got the car back, I pulled the SU carbs off ( the PO said they were off a '72) and cleaned them. Placed 3-in-1 oil in the dampers, replaced spark plugs with NGK 6ES. Adjusted carbs with a flow meter and adjusted the mixture screws. Even put in a new in-line fuel filter under the hood. I turn the engine with the choke on and let it run at 1500rpms for a few minutes before taking off. It usually idles at 800-1000 while in park, 750 in drive. I for twenty minutes and the temp gauge shows halfway (200 degrees??). I can get it to 4000 rpm's at 90 mph. Then a few minutes later it starts jumping/studdering while going down the highway. If I go up a small graded hill, the rpm's drop and the speed drops to 50 mph. Once I get back on level ground, I can get the car back up to 75mph. 

I believe the PO put in an electronic fuel pump near the gas tank. I notice it "ticks" when I turn the accessory power on before fully starting. I've read quite a few other threads on this, but they don't seem to address quiet the same problem I am experiencing. This is my first Z, so I have all kinds of other stupid questions once we figure this one out.

[siteunseen]

Does it still have the mechanical fuel pump on the head above #1 spark plug or block off plate? Mine run great with just the mechanical pump. If it were mine I would put a Fram G-2 coming out of the fuel tank. Is your vacuum advance hose connected to the back of the front carburetor,  left side looking from the driver's fender. Are both sets of points in the distributor adjusted properly?  Just throwing some things out that could cause your problem.  Hard to say without knowing more.  Pull the spark plugs and see if they are whiter or darker. Keep them in order, the first 3 run off the front carb and 4 through 6 the rear. Post a picture of them if they don't all look the same off white color.

[siteunseen]

You may need to set the fuel level in the float chambers.

[Palmettobug]

Siteunseen: There is a mechanical fuel pump still mounted above the first carb, but the wires aren't connected and the hose is plugged off.

I just replaced the spark plugs yesterday. The first three appeared to be white. The second three appeared more blackish. I adjusted the fuel mixture after changing the plugs. Which was before I experienced the loss of power. I am running Seafoam through the system now. And its the second time I have experienced this. The first time was why I looked up the threads and started changing plugs. 

[siteunseen]

I think if it's inoperable,  the mechanical pump, I would pull it off and put a block off plate on.  Unless you have the Yorx A/C compressor in the way. That's a huge PITA to get it off, at least for me.

I would adjust the carbs to run independently on there on by lifting the piston or pushing down that pin which disables the carb. It'll barely run on 1 carb but it will run.  Like Blue says in his "quick and dirty" SU adjusting thread get them both to run by themselves equally as shitty. Google that, "Quick and dirty SU adjustment classiczcars.com"  that's my SU bible.

[Palmettobug]

Siteunseen: I did go buy the Fram G2 filter like you suggested. Must easier to see into than the plastic. There was plenty of gas in the one I removed. At least I know the electrical pump is working. The hesitation thing seems to be intermittent. I've been up and down that hill I described at least five times, only had a problem once. Not sure what that means.

[siteunseen]

Maybe it has something to do with the automatic transmission? 

@djwarner knows a lot about the automatics. He might see this and recognize a problem?  Sorry I can't be more helpful. 8^(

[djwarner]

Hi Palmetto,

Not too much I can add here other than to check the transmission fluid level with the engine idling. Usually low fluid or a clogged filter will cause a shifting problem. Band slippage would cause an increase engine rpm. not a drop.

The fact that the front sparkplugs are white indicates a lean mixture. Don't ignore this or think checking the mixture at idle will fix it. The fact that it only happens after a warm up suggests a vacuum leak. Only affecting the front three cylinders suggests the leak is at the balance tube or the heat isolator blocks.

Another cause is fuel starvation to the front carb. This can be caused by a sticking float valve or a partially clogged fuel line.

Just a few places to check next.

[SteveJ]

16 hours ago, Palmettobug said:

Siteunseen: There is a mechanical fuel pump still mounted above the first carb, but the wires aren't connected and the hose is plugged off.

I just replaced the spark plugs yesterday. The first three appeared to be white. The second three appeared more blackish. I adjusted the fuel mixture after changing the plugs. Which was before I experienced the loss of power. I am running Seafoam through the system now. And its the second time I have experienced this. The first time was why I looked up the threads and started changing plugs. 

Mechanical pump mounted above the first carburetor? Could you post a photo of that? Normally the mechanical fuel pump is mounted on the right side of the engine toward the front so it is actuated by the cam.

There could be an electric fuel pump mounted by the tank. As part of the effort to relieve the vapor lock issues with the 73, Nissan had a factory authorized modification to add an electric fuel pump. If that pump failed, a PO could have replaced it with another electric fuel pump. If it is there, it will be on the front of the fuel tank on the right side.

[siteunseen]

I borrowed a picture of Motorman's fine looking car for a minute.

The fuel pump should be on the passenger's side of the valve cover where the arrow is.  The vacuum hose off the front carb to the distributor is the other yellow arrow.  If that's not connected or plugged off, your front carb will be leaner than the rear.

 

[Palmettobug]

How do I post a photo? Having a memory problem here.

[siteunseen]

When you reply look down to the bottom left for a paperclip and CLICK TO CHOSE FILES. Hit that find your picture then click the "+" add sign in the picture once it uploads. When you click the + symbol it will put the picture in the body of your reply.

That's about as clear as mud, I know, but maybe you'll figure it out.