74 260z with round tops Car was 'restored' around 15 years ago and mostly sat, and had a 'tune up' around five years ago. its been in my possession and I am finally getting time to mess with it and drive it.

THE PROBLEM - After the car reaches operating temp, it will run and drive fine for another 20-30 minutes. Pulls great through 1st, 2nd, and 3rd, 4th feels crisp and responsive as well. Then, suddenly after enough time has elapsed, I accelerate hard in first or second and starting right around 3500rpm it will...stutter?  and it wont clear up. Third gear is very smooth from 2-5k, I haven't tried a pull that high in fourth because...well, I haven't, but it drives well/is crisp/responsive in 4th as well when the problem is occurring. 

EDIT: maybe I wasn't paying close enough attention, it does seem it happens a little in third when the problem is occurring, it just isn't nearly as noticeable and not nearly as consistent. 

Through multiple videos and just trying things, ive got it running rather well with the exception of the title. Cold start/hot start not a problem. Mixture is a tad on the rich side (going by the lift the piston a tad and the idle will rise, when I take around 1/2 a turn of fuel away from where they are currently set it will respond the 'correct' way. It feels better where it is set ) and the carbs are balanced. Car has a new coil, cap, rotor, (had spares with the car, so im not sure where they originated, some not out of box new) plugs, full exhaust, current carb oil is atf. Fuel coming out is clean, 4ish psi, plugs look good, I have played with timing and it seems to be happiest around 10* of base timing. (i've also seen conflicting info on timing) The motor was rebuilt at some point previously, so i'm not sure if the compression ratio has changed by enough to matter due to machine work?

I may need help identifying the distributor/ignitor combo. It's no longer points, looks like a 280dist. With an aftermarket ignitor possibility?

Plugs look good.

Happens with more than one ignition coil.

Plug wires all check well cold and hot, wiggle them around, no open condition.

Ballast resistor still in place. Is this an error? I've read conflicting info and it has a 12v coil on it as well as some type of non points (cap, rotor, ignitor) style distributor on it. Should the resistor be removed?

I can't identify exactly when it happens but it is rather sudden. It's not a condition that gets worse as it warms up. It will be fine, fine, fine, and then bam there it is. I'm wondering if it's electrical (ignition system specific) related. I'm leaning that way because it is a sudden change. Similar to if you had a bad coil or bad injector. The car will run fine until that component reaches the "critical temp" and then the error occurs and doesn't go away until it cools down. That all being said I'm not sure what/where to check for an error once the problem is occurring.

This is also my first post here, so hello! 

Edited February 12, 20232 yr by Kcusi
Updates

35 minutes ago, Captain Obvious said:

Haha!! Yeah, I got all excited when I saw someone working on a 260.  Then I saw the note about the round tops.      

He could have done something really weird like converted the carburetors into throttle bodies.

That's ridiculous. Nobody in their right mind does that.

It's good you qualified that statement!

So now that we've got all this distraction in this thread... I hope the OP has solved his issues!

Hahaha I have not. Actually just finished with the timing and new plug wires. Maybe something strange is happening under acceleration once the wires are hot and the way they sag or bounce around is causing an issue like previously stated. I did check the ends like stevej mentioned and couldn't find a culprit or fix my issue. 

 

I also have a can of compressed air and I'll be trying to cool down the ignition module during this next test drive if it persists. 

Have you tried to bypass the possibility of a dirty fuel tank and run the fuel hose that comes from the tank to a 2 gallon gas jug? I did this on my '72, put the can between the grill and the radiator and secured it with bungee cords. Drove around the neighborhood, it ran fine off the jug. That's when I found that I a tank full of crud. 

 

On 2/11/2023 at 2:44 PM, siteunseen said:

Have you tried to bypass the possibility of a dirty fuel tank and run the fuel hose that comes from the tank to a 2 gallon gas jug? I did this on my '72, put the can between the grill and the radiator and secured it with bungee cords. Drove around the neighborhood, it ran fine off the jug. That's when I found that I a tank full of crud. 

 

Yeah, I've pulled a line and ran about a qt into a clear container, it was clean. It has a relatively new tank (12ish years?) 

I have a Gm hei module, I was considering making a pigtail and temporarily converting to the hei ignition module. I found the writeup on this forum, i think it was a stevej  post. I'm not finding fault of anything in the ignition system that I can test. I've got a heatsink and hardware mocked up for it, so it could be worth a shot at this point. 

I dont want to throw parts at it and I've exhausted what I can test as well as swapped in spares of a few components and all have had no effect. If anything it'll give me something to do for a bit and would at least allow me to confirm/deny the ignition module. 

Even if it doesn't help, it's always good to have a spare ignition module ready to go.  That original one is going to fail eventually, probably sooner after you get the engine to rev right.  The typical parts failure cascade.

Just now, Zed Head said:

Even if it doesn't help, it's always good to have a spare ignition module ready to go.  That original one is going to fail eventually, probably sooner after you get the engine to rev right.  The typical parts failure cascade.

That was part of my thought process. Finding n.o.s. parts are crazy expensive and just going to a known good module that's easy and cheap to replace would be nice. I can just keep a spare in the bag I keep in the car.