Help!!! Low vacuum, no power.

[AZ260Z]

I have a 1974 260Z. I'm getting low vacuum reading between 8-10" with very rapid fluctuations of the needle. Also, I can pull the # 1,2,& 3 plug wires off individually or all at once with no reduction in rpm's. I've checked and/or replaced the plugs, wires, cap, rotor, etc. I've also tested to see I was getting good spark to all cylinders, ensured the front carburetor was getting fuel. I've done a compression test and a leak down test all with good readings. Also checked for vacuum leaks using propane and a hose (very thoroughly). Also did a soapy water test around the head checking for a leaking head gasket even though I'm not losing any coolant nor are any exhaust gases getting into the radiator. I've also had it checked to ensure it wasn't a back pressure problem or that the timing chain hadn't jumped a tooth.

This problem started occuring intermittently sometime (but not right away) after a serious overheating incident where the fan clutch had failed and has gotten progressively worse over time. Needless to say the car is not drivable, running on 3 cylinders and I've spent countless hours and $$$ at mechanics trying to figure out the cause.

Is there anything else I can check before removing the intake/exhaust manifolds and head to inspect and/or send to a machine shop? I've run out of ideas.

[Captain Obvious]

What carbs do you have on the car? The original flat tops, or has it been converted to round tops?

Also, how did you ensure that the carb(s) were getting fuel?

[AZ260Z]

What carbs do you have on the car? The original flat tops, or has it been converted to round tops?

Also, how did you ensure that the carb(s) were getting fuel?

When the problem first occurred I had the original flat tops but replaced them with rebuilt 72 round tops thinking that may have been the problem. That costly bit of troubleshooting did nothing to resolve the problem and the symptoms remained the same.

As far as ensuring the carb getting fuel, I personally have removed the fuel line to the carb and turned the ignition to on and pumped gas into a container on multiple occasions. I also took the car to a carburetor specialist who checked both fuel pressure and volume, adjusted the carbs and said it wasn't a fuel issue. This was after going to 2 other mechanics who thought it was a fuel issue. I'd like to find out what is causing the low (rapid fluctuating) vacuum readings between 8-10".

[buysell]

I have a theory, I also have a early 260, and I had a rough idle, not as smooth as I would like. I have since taken everything apart and discovered why. Under the intake manifold there are a few plugs that are rusted out see pics. There are two between the head and the carbs; and two directly under the carbs. Check to make sure these are in place. -- Justin

and

[olzed]

A vacuum leak in the brake booster???

Block off the hose, and try it.

[FastWoman]

With the engine running, try shooting a bit of starter fluid into the front carb. If that makes the RPM pick up quite a bit, you probably have a fuel starvation issue of some sort in your front carb.

Somehow something was checked out as "good" that wasn't. In short, fuel + air + compression + spark = running engine. A working carb would give you fuel+air. You checked compression, and that's good. You say you have spark. Therefore your cylinders should fire. The fact that they don't fire means either your carb is bad, you're not really getting spark, or you have some sort of compression issue (not likely). It's not a timing issue if your other three cylinders fire. I suspect it's also not the brake booster, as that line would come off of 4/5/6, wouldn't it??

When you pull the 1/2/3 plug wires and don't see RPMs drop, do the wires spark to ground?

Are your 1/2/3 plugs fouled?

Are you able to determine how much fuel is in the float chambers of your carbs? If so, try this: Start your engine, and let the carbs fill. Shut off your engine and check that both float chambers are filled to the same level. Then start the engine again, and cut of the fuel supply to each carb at the same time. Run the engine until it dies. Then check the float chambers. I'm guessing the rear carb's float chamber will be empty, while the forward carb's float chamber will still be full.

Edited March 6, 2013 by FastWoman

[buysell]

A vacuum leak in the brake booster???

Block off the hose, and try it.

Sorry my bad, the first pic shows the plugs that are in the bottom of the intake manifold, directly under each carb, about the size of a dime, these look like small block freeze plugs. I also have a plug that is rusted out, center of second pic, not the brake booster pipe. My theory is the plug directly under the front carb is rusted, leaking or missing causing cylinders 1, 2 & 3 to not operate properly. --Hope I explained things better - Justin

[Captain Obvious]

I know you have said it's not a fuel issue, but I'm having trouble coming up with any situation that would be strictly confined to just the front three cylinders. Everything else would be common to all six cyls.

Humor us? Try what FastWoman suggested:

With the engine running, try shooting a bit of starter fluid into the front carb. If that makes the RPM pick up quite a bit, you probably have a fuel starvation issue of some sort in your front carb.

[AZ260Z]

With the engine running, try shooting a bit of starter fluid into the front carb. If that makes the RPM pick up quite a bit, you probably have a fuel starvation issue of some sort in your front carb.

Somehow something was checked out as "good" that wasn't. In short, fuel + air + compression + spark = running engine. A working carb would give you fuel+air. You checked compression, and that's good. You say you have spark. Therefore your cylinders should fire. The fact that they don't fire means either your carb is bad, you're not really getting spark, or you have some sort of compression issue (not likely). It's not a timing issue if your other three cylinders fire. I suspect it's also not the brake booster, as that line would come off of 4/5/6, wouldn't it??

When you pull the 1/2/3 plug wires and don't see RPMs drop, do the wires spark to ground?

Are your 1/2/3 plugs fouled?

Are you able to determine how much fuel is in the float chambers of your carbs? If so, try this: Start your engine, and let the carbs fill. Shut off your engine and check that both float chambers are filled to the same level. Then start the engine again, and cut of the fuel supply to each carb at the same time. Run the engine until it dies. Then check the float chambers. I'm guessing the rear carb's float chamber will be empty, while the forward carb's float chamber will still be full.

You're AWESOME FastWoman. You're tip about checking the float bowls jogged my memory of the last time I had checked to see if fuel was getting to the front carb by disconnecting the hose and having the fuel pump pump fuel into a container. Right afterwards I checked the front bowl and it was nearly empty with just a residue of fuel. I also noticed that the float seemed to hang up when you moved it up & down so I adjusted it. I thought that was odd but figured since I disconnected the hose that maybe it didn't get a chance to replenish itself. Also, since The problem surfaced while I had my original flat top carbs and wasn't cured by replacing with rebuilt round tops, I guess I didn't want to believe it was the front carbs on both sets.

Anyhow, once I checked the front float chamber today, it was bone dry and when I checked the rear one it had fuel in it. I then compared how both floats were adjusted and made the front like the rear and "bingo" that solved the problem. She started right up, cranking on all 6 cylinders, took it out for a test drive and it ran great. I had taken the car to 5 different mechanics but am especially disapointed in the "Carburetor Specialist" who had previously owned 2 different Z cars, but although it has been a long and expensive process, I've learned a lot and wish I had had your tip when it first occured. This forum is great, I owe you big time, if you ever get out to Arizona, I'll buy you a drink , hell I'll buy you dinner. Thanks again and thanks to all that responded.

[FastWoman]

Hey, cool! I'm glad you found the problem!

Often the best reputed mechanics don't have a clue what they're doing, and some of the shade tree mechanics are truly gifted. Often the very best mechanic is the owner, who knows the car better than anyone else. Anyway, I think you've found a carb specialist who you shouldn't let near your carbs again!

If I'm ever in Arizona...