Need help ASAP - fuel starvation problem

[Jeff G 78]

I am out of spit and I only have a few days to figure out and fix the problem with my 260Z LeMons car or I will have a very long, frustrating race next weekend. I'm asking for thoughts you have. The biggest issue we have is that it's a track-only car and we cannot prove out anything we try until we are in the race next weekend.

Here's the story:

Car setup:

'74 260Z with 3 screw SUs on a 260 intake and stock exhaust manifold
Fuel tank was cleaned and POR15 treated and has two fuel filters that stay clean
Engine has good compression
SUs were cleaned/rebuilt and SM needles installed. Floats set to spec and they synched at idle and 1500 RPM
Pistons drop at same rate and needles move smoothly up and down in choke seats
New mechanical fuel pump - electric pump removed
New fuel hoses and vent lines
All hard lines flushed and checked for blockages
Stock magnetic pickup distributor with working vac and mechanical advance
Extra heat shielding between exhaust and intake and fuel rail well insulated
Performance coil running at full 12+V (no ballast resistor)
No EGR or water lines to intake manifolds
New MSA radiator with gutted thermostat

The sympoms:

Car starts immediately and idles perfect as low as 700 rpm
Free revs smoothly to 5k+ rpm
Runs great on roadcourses for 10-20 minutes with zero issues
After 10-20 minutes (depending on air temp), car begins to stumble at 4k rpm.
Within a few turns, the car hits a brick wall at 4k and will not rev higher
Backfires through exhaust 2-3 times on decel after a stumble
Runs worse and worse and within a few turns, car will not accelerate at all
Continues to free-rev to 5k+ without missing
Sometimes after car cools for 30 minutes or more, it will run well again for 10-20 minutes and other times, it continues to have issue
Car always free-revs fine and always restarts immediately and idles smoothly

What we've tried:

Checked fuel inlets at carbs, checked/replaced filters, floats, etc
Symptoms did not change with/without SM needles
Plugs are all the same and rather black after a stumble incident
Replaced Transistor Ignition Unit immediately following stumble
Removed gas cap to rule out tank vent issue
Ran car without hood to aid airflow to fuel lines in case of vapor lock
Replaced coil, spark plugs, plug wires

Our thoughts:

We are fairly certain the problem is heat related as is is worse when ambient temp is higher and only happens after car gets fully up to temp
We *think* the problem is fuel related since it's only under load. If ignition, would the car free rev OK?
We think the problem is in fuel delivery and not in the carbs since the plugs all look alike

Our next steps:

Install an electric fuel pump in addition to mechanical pump
Add fuel pressure gage so we can monitor pressure
Recheck mechanical advance in distributor

Like I said earlier, the car free-revs fine and it only happens under load, so we have no way of knowing if a fix works or not until we are on a track for 10-20 minutes. Our next opportunity will be DURING our race next weekend. We plan on trying anything and everything we can do in one week, so I'm open to suggestions!

[Chris280Z]

Are you running resistor plugs?

[Z train]

Plumb a temp fuel pressure guage to the hood and run the car.Is there a return line on the car?

[Jeff G 78]

Are you running resistor plugs?

Yes, we are running NGK BR-6ES-11, I think. I'm going from memory. They are the normal Z plugs everybody runs.

[Jeff G 78]

Plumb a temp fuel pressure guage to the hood and run the car.Is there a return line on the car?

We are running the stock fuel rail with return that has the small orifice in the outlet to regulate pressure. We plan to add a pressure plumbed to a gage mounted on the hood.

[Z train]

Process of elimination.FP needs to be verified before beating your brains out on other "possibles".

[Bruce Palmer]

Years ago when visiting Idaho Z, I was shown the inside of a Z tank that had been treated with the POR stuff and the upshot of that was that the outlet sock kinda filtery thingy (tech talk) was covered in the POR sealer which more than likely would have severly restricted flow. Of course the tank had to be cut open to show it....

[zKars]

Sounds like fuel for sure but don't discount plugs loading with use. If you're running a bit rich, those dang BP6..11 wide gap (1.1mm) plugs don't like it. Try good old BP6RES instead. Also the new mech fuel pump may be not so great. Got the right thickness of spacer between it and the head? Got the spacer at all? A pressure gauge will tell the story. Good luck on the race!

Edited July 30, 2010 by zKars

[Jeff G 78]

I've talked to a few more people and I might have gotten some good advice. My buddy who is a powertrain engineer at Ford listened to my tale and he's pretty sure the problem is ignition. Specifically, the distributor. He thinks that something is getting hot and I'm getting weak/no spark under load. Since the plugs look sooty after I come in from the track, he thinks that I have plenty of fuel, but a lack of spark causing the plugs to read rich. In reality, they just don't have the spark to fire. Also, since the bowls always look to be the proper level and the filters stay full, we shouldn't be experiencing the stumble when we first accelerate out of the pits. We should have gotten at least one straightaway of good running before the bowls got low. Instead, it coughed right from the very start of acceleration.

Now, assuming he is correct, what are my options? I have a '74 260 in Michigan and I need to be fully repaired in 5 days before leaving for the race. Getting a 260 distributor will not be easy or cheap. I have a '75 distributor, but no transistor ignition unit from a '75. I'm not sure if they are functionally different, but the P/Ns don't match. My other option is my '78 280Z. In it, I have a perfectly working stock ignition system. I can steal the distributor and transistor ignition unit from the '78 for the 260, but what else do I need? Will my '75 tach work? I assume I'll have to make a new harness between the distributor and the TIU, but what else am I missing? I REALLY don't want to have to cut any connectors off my '78, but I will if I have to. I have a lot of money and time tied up in this race and I cannot fail now. I'd rather rewire my '78 after the race than throw all this prep time and money away if the 260 doesn't run right.

Ideas?

[zKars]

If you have shaft play that would cause the motion to get out of hand or to get to a degree that you get reluctor gap issues, that might explain your problem. Or clearances get loose enough at temp to cause problems only when hot. Maybe. Don't buy the weak spark causes sooty plugs argument though.

The only thing in the dizzy that is likely to temperature sensitive is the coil in the pickup

winding. That shouldn't be RPM dependant.

If its the ignition module, anything stopping you from doing a GM HEI conversion as far as the race rules goes?

Tach will work fine with either 75-78 system. It comes right off the + side of the coil on both cars.

[Jeff G 78]

If you have shaft play that would cause the motion to get out of hand or to get to a degree that you get reluctor gap issues, that might explain your problem. Or clearances get loose enough at temp to cause problems only when hot. Maybe. Don't buy the weak spark causes sooty plugs argument though.

The only thing in the dizzy that is likely to temperature sensitive is the coil in the pickup

winding. That shouldn't be RPM dependant.

If its the ignition module, anything stopping you from doing a GM HEI conversion as far as the race rules goes?

Tach will work fine with either 75-78 system. It comes right off the + side of the coil on both cars.

I couldn't *feel* any shaft play, but it doesn't mean it doesn't have some. There is nothing in the rules stopping me from swapping to a GM HEI, but I just haven't done it before so I'm a bit ignorant. Could I use my spare '75 distributor for the GM swap? I always hear people talking about ZX distributors with GM HEI. I have no access to a ZX distributor unless I have one shipped from someone with a spare. I have a spare '75 dizzy and the dizzy in my '78 "good" Z. The '75 dizzy could be a permanent solution if I can make it work, but that engine last ran 17+ years ago, so ??? My '78 is in good working order with a newish pickup coil, but it can only be borrowed so I can drive that car after the race is over.

[Z train]

Borrow a MSD 6AL and hook it up to the 75 dizzy.