Rev limiting problem at ~4000 rpm

[VaGuS510]

So, as some saw in my previous thread I am having a rev limiting problem. The symptoms are very similar to what is happening in some other posts on here, but I can't seem to identify my problem.

The car/engine: '73 240z with an e31 block, e88 head (recently rebuilt), petronix ignition, ztherapy carbs (3 screw), pacesetter header... everything in great shape

The symptom: At full or nearly full throttle when I hit ~4200rpm there is a pretty distinct hiccup... sometimes I can't rev past 4200rpm. Most the time it makes it through this hiccup and bogs down at ~4500rpm. Other than this the car runs fine, idles smooth and has decent power.

What I have tried:

• I first replaced my distributor cap (it wasn't put on correctly at some point and the rotor demolished the contacts. The only result was a smoother idle.
  
• Next, I removed the screens on the float bowl fuel banjo bolt. This resulted in my rear float overflowing and my problem was solved!!! I could rev right past 4500rpm:)... other than poor idle, the smell of fuel and redoubtably horrible gas mileage, all was good! Note that the fuel went through the vent into the air cleaner next to the carb... So apparently I was lean at high rpms
  
• Next, I sprayed carb cleaner around all of the gasket surfaces, I found a small leak coming from the rear carb's throttle shaft because the cover wasn't on completely... No other vacuum leaks found... No changes
  
• Next, I fixed the float and replaced my fuel filter (obviously things were getting past)... and a little crap was in the bottom of the bowl. The filter didn't change my problem
  
• Next, I checked the floats... they were significantly different, I set the levels to a fuel level of 22mm below the tops gasket surface by marking the side of the floats, removing the hose to the nozzle, checking the fluid height in the hose to the mark, and adjusting... I gave the carbs a fresh tuning and nothing changed.
  
• Next, I swapped out the RX-7 electric fuel pump I was running with the original fuel pump. This made no significant difference.
  

So now it looks like ignition could be my problem... I am almost out of ideas related to fuel. What worries me is how it responded to the rich condition of the rear float overflowing... That makes me think it is fuel related.

The way it pretty consistently hiccups at 4200rpm and bogs at 4500rpm makes me think ignition, but there doesn't seem to be much that could go wrong there.

Tomorrow, I plan to pull the plugs, take a good look at them and check my timing.

What do you all think? Any suggestions would be very helpful.

Thanks for reading all of my ramblings

Edited April 24, 2011 by VaGuS510

[cygnusx1]

You have been very methodical! Try new plug wires first. Then check your coil and wire connections at the coil.

Edited April 24, 2011 by cygnusx1

[Arne]

Sounds exactly like what I went through tracking mine down. Two questions -

1.) What happens if you change the timing? Say, 5 degrees either way. Does that change the rev ceiling?

2.) What coil are you running, and is it with or w/o ballast resistor?

[VaGuS510]

Thanks for the responses

What coil? I believe it is the original coil! It is a light blue/rust color:classic: It has acquired some sentimental value to me... so I really hope that it doesn't have to go!

Yes, there is a ballast resistor on the coil.

My connections are probably not the greatest either they are all a bit rusty. I am thinking the best thing to attack next would be to get a fresh coil, hook it up (probably without the ballast) and give it a whirl. This would eliminate any connection problems also.

My plug wires look brand new (came with the engine which I recently bought). I believe they are 8mm silicon but I have another high quality set sitting around, so I will swap them out if the coil isn't the trouble.

What do you all think?

[cygnusx1]

Plug wires would be my first test, even if they look newish, but since you say the wire connections are not great, starting there is a good idea.

[JLPurcell]

Check distributor for play, up down/side to side. You say that the rotor destroyed your cap, did you check the rotor for damage, inside and out, Next check your crankshaft / cam shaft timing marks to make sure your chain has not jumped time. I definitely believe your problem to be ignition/timing in origin. jlp

[d240zx2]

From all you've said and done, I'd lay $10 on you having a bad coil.

[tlorber]

Is your distributor advancing the way it should when the weights spool up?

[Zforce]

Sounds like you are running lean on top,I had the same problem. Try the sm jet needles from ZT,only 30.00 dollars. Also try running without the air filter cover and see if the bog goes away, I bet it's also choking at high rpm. I bought an air filter housing off ebay with holes drilled in it, since it was already swiss cheesed I made the holes even larger what a difference.

[Arne]

Quick and dirty first test - I'd temporarily bypass the resistor. If that is an improvement, connections and coil are the proper direction.

[d240zx2]

Since you're running Pertronix, the ballast resistor is not needed.

[Arne]

Since you're running Pertronix, the ballast resistor is not needed.

That's not always true. Pertronix recommends ~3.0 Ω total resistance. If you are using the stock coil (~1.6 Ω), you still need a resistor with the Pertronix. If you change to a 3.0 Ω coil, you should not run the resistor.

Of course, the above is good in theory. I have been unable to make my car run acceptably with either the Pertronix or with a ZX dizzy. I've tried both dizzies with several different coils and with and w/o resistors. My best guess is that there is too much voltage drop at the coil in my car to run electronic ignition. I fought this 4000-4500 RPM fade-out on and off for a couple of years. Eventually gave up and went back to points. Car runs great on points, pulls cleanly to red line. Sold the ZX dizzy, still have the Pertronix and matching 3.0 Ω Pertronix coil on a shelf in the garage.

Edited April 24, 2011 by Arne