I am at my wit's end and need help on my 260Z race car. Ever since I got the car two years ago, I have had issues at high RPM and after trying lots of things, I'm pleading for help. Here's the engine background...

• L26 engine just rebuilt with new rings, valve job, and a mild cam
  
• Thermostat guts removed leaving only the ring and cage for a restriction
  
• Prior to rebuild a few weeks ago, it was 100% original
  
• 3 screw SU's with SM needles, floats set to spec and chokes working smoothly and carbs synchronized. N36 intake
  
• Previously used OE needles in carbs
  
• New mechanical fuel pump with 260Z electric pump removed
  
• Previously tried another new mechanical pump and an electric pump as well
  
• Stock exhaust manifold with a 2-1/4" straight pipe and a single glass pack up front
  
• Clean '75 fuel tank with new filters
  
• Previously ran a POR-15 lined '74 tank with new filters
  
• ZX distributor with E12-80 module. 10°BTDC at 800 RPM and 37° total advance at 2500 RPM
  
• Previously ran stock magnetic pickup distributor
  

So, the car starts right up and idles like a champ. It free revs cleanly and runs well at low RPM. On the track (car is not street legal, so all testing done on road courses), the exhaust note goes totally flat at about 5000 RPM and the car won't rev any higher. It makes no power at that point. Sometimes, the RPM limit before it goes flat is much lower than 5k. It's slightly better at part throttle, high RPM that at WOT, but the problem is still there. It doesn't always happen though. Some days, it runs fine for a short or even extended length of time and other days, it runs poorly right off the trailer. It happens at all temperatures and weather conditions.

In the past, we had some success pinching off the fuel return line, but yesterday that didn't help at all. I tried moving the needles up slightly to richen it up and the problem got worse. We ran out of time to try moving them the other way. In the past, we have tried running with no air filter, we also tried running it with the hood off. We have the fuel rail thermal wrapped and we have extra heat shielding between the intake and exhaust manifolds. Nothing helps at all.

The issue is exactly the same pre and post engine rebuild, pre and post fuel tank swap, pre and post distributor swap, pre and post fuel pump swap, and pre and post stock to SM needle swap.

We have a 25+ hour endurance race in 12 days and we are out of ideas. Does anybody have a clue what's going on?

EDIT: One thing I just thought of is that the temp gauge doesn't work (reads ~130°) even though I have swapped in another used sensor and tried a different gauge. The engine and radiator get hot to the touch like they should, but is there any chance it's running too cold?

Edited September 26, 201113 yr by Jeff G 78
Added info

OK, thanks all. I will add a (quality) thermostat back in and see if the temp gauge actually does work.

Bill, YES, please try to find the pump and let me know if you find it. I will be buying a pump tomorrow after work if you don't find it.

Randalla, no, the tank is good and we ran it with 4, 12 and 16 gallons of gas in that order with no change.

Steve, I agree that a wide band work be the best way, but we just don't have time to add one in time. We are thinking about going to a dyno this weekend (if we can get in) though to check and tune. I might also try the OE needles, but we did have the problem with and without the SM needles.

No other carbs available and no old hoses to try. I think the old hoses cracked when I removed them.

Gotta run and go pick up the Z from a race shop an hour away, but thanks all!

Well atleast look at the hoses and see if they have a tendacy to kink when moving the nozzle. It should be easily apparent if they tend to kink

We had similar symptoms on our 240Z. Ran great up to 5K RPM then went away, popping and not making power. It turned out to be floating points due to the wrong points cam. I know you do not have points, but the symptoms you report are to close to our experience to let it go by. Is your timing stable at high RPM?

Fuel pump - We are using a Holley "Red" fuel pump mounted in the rear where the original electric pump was. The mechanical pump has been removed. The Holley is rated for 71 GPH which should be more than enough for your mod level. You will need to use an in-line pressure regulator. We are using one of those little, round type (see image below) with good results but I suspect it might not flow enough fuel for higher power levels.

You mentioned you have 2 fuel filters before the pump. Perhaps there is too much resistance on the suction side of the pump as a result. Have you tried it with only 1 filter?

Are you sure the fuel pickup in the tank is not pinched or has POR-15 partially blocking it?

Just another wild idea.

madkaw, I will look closely at the hoses, thanks.

beermanpete, I thought about the dual filters, but have never tried to run just one. I haven't looked at the timing at high rpm, but it's dead stable at 2500 where I checked the total advance. I'll check at higher rpm too. I ran a holley red and a regulator just like that on a '72 with triple Webers. I called around and can't find anything local. I do like the idea of higher pressure and flow with a regulator adjusting it back down to the exact pressure needed.

pizzaman, this tank hasn't been treated. The first tank was and yes, the inlet was clogged with POR. That was one of the reasons for the tank swap. This tank is clean and free flowing.

Hi Jeff,

Of course, armchair troubleshooting is fraught with potential incorrect diagnoses, but from your description, I agree with the assessment that it's a problem getting fuel TO the carbs, not a problem inside the carbs themselves. Have to start with the simple stuff like filter screens though, right?

So next? A fuel pressure gauge between the pump and the carbs is a must at this point. And I wouldn't block off the return to the tank until you know what the pressure is. You don't want to overwhelm your float valves and blow fuel out the overflow tubes especially if you put an electrical pump in series with the mechanical.

You should be able to test the "sucking air" theories by running without a gas cap. I haven't completely digested the original tank venting system details, but I believe the original system is "supposed" to run at a slight vacuum. That vacuum is controlled at the stock filler cap. No filler cap - No possible vacuum - No pressure differential - No sucking air. (Of course, unless you're creating a pressure drop elsewhere with a clogged filter or something.) Just remember on left handers that you've got no filler cap.

The ID of the tube between the float bowl and carb bodies is 3/16 ID. If you want to do more than check the bowl levels and actually run the car with some clear stuff, I would recommend the gasoline grade F-4040-A Tygon. http://www.tygon.com/tygon-f4040a-tubing.aspx. How about some Tygon between the float bowl and the carb body and an engine bay cell phone video cam taped to the fender well?:bulb: If you see bubbles coming through that tube, you're sucking the bowls dry. At least you'd have the "what?". All you would need to do is the "why?".

I don't have any Tygon, but I do have a plenty of suitable Buna-N black opaque 3/16 ID tubing if you need some. If you're worried about kinking or collapsing at the bend, you could probably put on a length longer than usual and include a strain relief loop. PM me if you want to talk about it.

As an aside... 29 replies in 300 views. Now that's what I'm talkin bout! Glad I could break the ice.

Jeff - When I purchased my head from Dave Rebello (along with engine rebuild parts and pieces) he instructed me to set my timing with high RPM @ 3500 - you may want to do that just to get all you can get out of your timing advance - I'm sure if you do a search you would get a lot of info and benefits from the different settings - it's all going to depend on your engine - cam - and head for your final setting though.

Jeff,

One more long shot....(don't laugh), it could be the *Alternator*. Seriously.

On my 78 280z I found I could only rev to about 2000 rpm then it would die until the revs dropped a bit.

My memory of the trouble shooting is dim as it was 18-20 years ago, but I remember my first clue was when I happen to see the voltage on the dash gauge would drop steeply when the engine cut out. As I recall, disconnecting the alt belt made it much worse to my surprise. Disconnecting all the wires to the alternator made the problem go away.

I eventually opened the alternator up and found a wire on the rotor going to the commutator had a break in the insulation as it crossed very closely over something else. I surmised that at a certain RPM the voltage being generated would arc across the small gap. I unsoldered it and slipped on an insulating sleeve from a set of replacement brushes, then reassembled. It worked great for many, many years. I still have the alternator (and engine) stored away.

So yes, it is a long shot, but since you are desperate, and this is cheap and easy to try, I suggest you give this a shot. Disconnect (and insulate) *all* the alternator wires and see if it runs.

Just my two cents worth:

100 micron filter coming out of the tank (or stronger for racing - one the size of an oil filter would be best) to Electric fuel pump to Fuel Pressure regulator to 20 micron fuel filter to fuel line. Sounds like a pressure of fuel supply could be running short with the mech pump - I belive I have read that the mech pump can only supply 2.5 lbs of pressure. Sounds like you need a little more - keep us posted.

What pressure setting do you have on your electric fuel pump? - I would take it to at least 3.5 to 4 psi and give that a shot (monitor your carbs while you are reviewing - I would hate for your to force out fuel of your SU's on to your hot header - fire safety equipment is a must at this point!

I have to agree it makes since that you are running out of fuel under hard load. That would also explain the effect of raising the needles and increasing the instances and decreasing the RPM of occurrence. You have changed all other cause effect items (electrics, overhaul, etc...) Focus on fuel system-supply.

Thanks all for the continued ideas. I got the car back late last night and I'll start trying all of the great suggestions you have given me. My plan is to do all of the most likely fixes. From there, I'll prepare to do others at the track in case the not-so-likely becomes the likely once I hit the track. I have one week to get the parts needed and installed before I leave for my race.

Here's my game plan:

• Remove the mechanical pump and install an electric pump near the tank with a single quality filter.
  
• Install a fuel pressure gauge and position it to be in view from the driver's seat for testing.
  
• Check all fuel lines and hoses for possible leak paths.
  
• Check carb hoses for signs of kinking.
  
• Recheck carb inlet screens and float valves for debris
  
• Install a new quality thermostat.
  
• Recheck the valve clearances.
  
• Perform vacuum, compression, and leakdown tests on engine.
  
• Install an additional ground strap between the engine and body.
  
• Check fuel pressure w/return.
  
• Block fuel return system.
  
• Recheck fuel pressure w/o return.
  
• Set the SM needles back to the properl height.
  
• Retune/synchronize carbs.
  
• Adjust the fuel level in the bowls per Bruce's method. (set fuel level to 1/16" below top of nozzles)
  
• Check timing at high RPM.
  

If nothing jumps out as a smoking gun, I will *try* to get to a dyno this weekend. More than likely, I will not have the time, so we will leave a day early and pay for the Friday track day at Nelson Ledges to test the changes.

Assuming it runs well without hitting the 5,000 RPM wall, I will remove the excess fuel line and reposition the fuel pressure gauge to underhood.

If it still falls on its face and none of the other ideas help, I will swear and pout for a bit and then have a really crappy weekend playing rolling chicane for 25.5 hours.

Am I missing any of the ideas?

Hey BGM, did you locate the RX7 pump? If not, I'll buy the facet pump tonight.

Edited September 28, 201113 yr by Jeff G 78
Added info

Since you're looking for WAGs, how about the E12-80 module. From what I've read, and experienced, the electronic ignition modules seem to show different signs when they fail, but they all seem to have high RPM in common as a mode to bring on the failure signs.

I'll add it to the list Zed Head. Maybe Geezer will loan me his module so I can try it if needed.