sputtering at 95 mph and higher

[zdude1967]

I have a 260z with 3 screw dome carbs. It begins to sputter and hestate at speeds approaching 100 mph. It seems fine up until the 100 mark then it is as if it becomes fuel starved. Any idea what this could be.

Thanks

Glenn

[d240zx2]

I presume you speaking of 4th gear. Does it do the same type of sputtering in 3rd at the same RPM?

[30 Ounce]

On mine I was running the float bowls dry. Check your fuel filters, change if needed. I had to run an electric fuel pump in place of the mechanical one. For a stock engine the mechanical one should be fine if its working properly. Once those have been checked move to the ignition.

[Bruce Palmer]

It's a built in governor to keep you out of the can.....

No, seriously it sounds like extended running at that speed is emptying the float bowls. Pains me to say this but you may want to go in there and raise them (the floats) a skosh at which time everything else will need to be re-set as the mixture will be off from raising the float levels.

Or slow down..... nyuck nyuck

[Marshall]

I have the same setup with exactly the same problem with my 260z. I just assumed it was God telling me to slow down.

[angus]

The best single thing for a 260Z would be replace your carburetors with early 1970-72 (4) hole dome top carburetors and manifold. I see some are available on the internet if you search hard and long, and they are not hard to rebuild.

Also, I experience a 'sputtering' problem with my '71 at higher RPM's (back in 1975) and installed a MSD system that solved the problem.

[Gary in NJ]

The best single thing for a 260Z would be replace your carburetors with early 1970-72 (4) hole dome top carburetors and manifold. I see some are available on the internet if you search hard and long, and they are not hard to rebuild.

The OP stated that he has 3-screw carbs, an indication that the '74 carbs are no longer installed.

[5thhorsemann]

I think you are not getting enough fuel, check your filter and blow out the lines. You aren't having the problem at lower speeds/ gears because you are smart enough not to run those RPM's in lower gears for an extended time.

[coop]

My old 260 had that problem. It had a boost boost pump back near the tank with a teeny pre-filter that would easily clog due to the rust in the gas tank. That pump and clogged filter could not pass enough fuel to keep the carb bowls full during extended high load situations such as climbing mountain roads or in your case, 95 mph runs. The filter in the engine bay looked OK, masking the problem. Check to see if you have that pump and filter back near the tank. If they are clogged, you'll need to clean them and possibly repair the tank to prevent further fouling. If they are clean and the engine bay filter is clean, the other advice you've read about adjusting float bowl levels is good. Another trick I used to do is to T a fuel pressure line between the mechanical(or electric) fuel pump and the carbs, and run the line to a pressure gage taped to the windshield. Take the car for a run and see if you get a dropoff south of the 1.5 psi range. If so, your fuel pump is weak or there is a clog.

[zdude1967]

Thanks everyone. I will try all the suggestions. Probably begining with the floats. I just got the SU cd from Z therpy so I am hoping this will help me with the float height. As an additional item I think I am experencing vapor lock as it is very hot in AZ now and after driving awhile the car begins to run the same was as it does at high speeds. I have a additional fuel filter by the mechnical feul pump and it is clear glass, you can see the fuel bubbling as if it is boiling when the car begins to crap out. I guess there isx no cure for this other then to move somewhere cooler.

Edited September 3, 2011 by zdude1967