Help!!!! Low Vacuum, severe power loss

[Dan Cogan]

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.

Topic

[jay]

Dang,this is a tricky one…firing order? bad air/fuel mix on the front carb,valve timing but the last 3 cyls are in time.
I would try to check firing order and maybe swap carbs from fron to back if you can.

[Dan Cogan]

The firing order is 1, 5, 3, 6, 2, 4. I also should have mentioned that the problem started to occur with my original flat top carbs and I bought rebuilt ’72 round tops and it didn’t change a thing. I can’t understand why I’m getting such a low vacuum reading of 8-10″ with rapid occillation. I’ve researched the web on vacuum and they all list about 20 conditions and symptoms but none talk about mine (low with rapid fluctuations). The closest condition to mine points to the valves or weak valve springs, but I don’t know. the head that is on the car is a rebuilt one with 2K miles on it and I don’t know if the springs were replaced or my original ones were used. My original head had 100K miles on it. The problem started after about 1500miles on the rebuild and a severe overheating incident.

[jay]

Yeah, probably valve timing issue weak or broken springs or a lubrication/stuck valves issue,have you pulled the valve cover yet to see if anything looks wonky,maybe bar the engine over by hand to see if all is smooth.

[Deven McOmie]

It would almost sound to me like a ring issue. the rings need to have an off set when they are installed on the pistons and when the pistons are installed in to the engine. Other wise you can have mild blow by. how much bottom end work did you have done when the new pistons where installed? Of course you said this issue started prior to the pistons being installed. I would look at the rockers not being tightened correctly. Or faulty hydraulic lifters. In fact faulty or weak lifters could explain all the symptoms. I would look at the lifters.

[college man]

see if these links help.I believe you have a head/valve
problem. what kind of compression numbers did you get?

[Dan Cogan]

PROBLEM SOLVED!!!!!!!! Checked my front carb float chamber and it was bone dry, checked the rear chamber and it had fuel, so I removed the float and adjusted it like the rear one and that fixed the problem. Took it out for a test drive and it’s running great. I can’t believe I had this same issue with the front carburetors on both sets of carbs. Thats why I really didn’t think it was a carb issue. I guess it doesn’t pay to think sometimes. Thanks for all the input.

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