popping through the exhaust at 4500 rpm or so.. HELP!

[motorman7]

Major frustration on this issue. I think I have tried all different combos now on timing and fuel mixture and pretty much nothing changes. Took timing from 10 degrees to 20 degrees, no change. Mixture screw from 2 turns down to .75 and no change. I am thinking this is the same issue that Arne was having with the different dizzy options he was trying. I do have the old one somewhere, but have yet to locate it. May try looking for it again tomorrow. May try a coil change out.

The only other thought is fuel starvation. Seems like it revs very well to 6.5K on the quick short bursts. Just doesn't like the prolonged high RPM's above 4.5K. Good thing is I rarely take it past 4K or even 3K for that matter in daily driving. Will post again when I get some improvement.

[LeonV]

Major frustration on this issue. I think I have tried all different combos now on timing and fuel mixture and pretty much nothing changes. Took timing from 10 degrees to 20 degrees, no change. Mixture screw from 2 turns down to .75 and no change. I am thinking this is the same issue that Arne was having with the different dizzy options he was trying. I do have the old one somewhere, but have yet to locate it. May try looking for it again tomorrow. May try a coil change out.

The only other thought is fuel starvation. Seems like it revs very well to 6.5K on the quick short bursts. Just doesn't like the prolonged high RPM's above 4.5K. Good thing is I rarely take it past 4K or even 3K for that matter in daily driving. Will post again when I get some improvement.

Your issue is fuel starvation, and it's very likely that the dirty tank clogged your filters. I would replace all fuel filters you have in the system (by the tank, in electric pump if you have one, in engine bay, and at the carb banjos).

Again, for those having problems, correct valve lash is absolutely critical. It does not take much effort to remove a valve cover and measure clearances, yet it has a huge effect on the way your engine behaves. Filters first, valves second, then worry about the rest. If your car will rev to redline occasionally, but bogs at constant speed then the problem is exactly what you would think it is, no magic to it.

[motorman7]

Your issue is fuel starvation, and it's very likely that the dirty tank clogged your filters. I would replace all fuel filters you have in the system (by the tank, in electric pump if you have one, in engine bay, and at the carb banjos).

.

Thanks Leon, I'll give that a shot tomorrow. Want to paint some black and yellow stripes on the valve cover anyway (that should add a couple HP ).

[Zedyone_kenobi]

MY car has 71k original miles on it. Since I am thinking that my issue lies not in my carbs but in my timing somehow,I was wondering if my timing chain may have some stretch in it, which is not allowing me to time it right. But I am not sure if that would make me run advanced or retarded. I will have to think about it. Nevertheless, I need to pull my valve cover and check to see where the heck I am when it comes to timing AND valve lash.

[doradox]

MY car has 71k original miles on it. Since I am thinking that my issue lies not in my carbs but in my timing somehow,I was wondering if my timing chain may have some stretch in it, which is not allowing me to time it right. But I am not sure if that would make me run advanced or retarded. I will have to think about it. Nevertheless, I need to pull my valve cover and check to see where the heck I am when it comes to timing AND valve lash.

The distributor is driven from the crank so timing chain stretch does not effect spark timing only valve timing. Stretched chain retards valve timing.

Steve

[madkaw]

Stephen,

forget about the timing chain-that's not your issue. Now valve adjustment is a possibility. Did you check the float bowl hoses-the ones that supply the fuel to your carbs from the bottom of the float bowls? Don't mean to harp on the hoses, but can't remember if you said that you checked them-because they have been a problem from z therapy. They collapse or kink and cut-off the fuel or part of the fuel to the carbs. It's hard to see -you almost have to go by feel.

And verify for me that have NOT changed your float settings yet during this thread. If you have NOT, you need to make that your next adjustment.

Oh and happy holidays!!!!!

[Zedyone_kenobi]

I checked my fuel lines and they are peachy. They are supple and are holding their shape well.

I have not touched my float levels. I have ordered a standard OEM set of SU needles AND a set of float bowl gaskets from Z therapy so when I tackle the float levels I will be ready.

copy that on the timing chain. I forgot my mechanics there. One more reason to check to see if the valves are adjusted correctly AND if the timing chain has any stretch. Can fix both at the same time.

By the way, while I am thinking of it. I would think you should check for timing chain stretch while the engine is hot. But I have not checked. Is that the case.

[madkaw]

I don't believe the FSM specifies hot or cold check on chain stretch. It would be worth to check while your in there, but it isn't your problem. If it's close, I would bump it up to the #2 notch. I don't see any ill effects, just benefits from advancing your valve timing slightly.

Glad to hear about the hoses. Mine were soft too---too soft and they kinked with the movement of the choke.

[doradox]

I don't believe the FSM specifies hot or cold check on chain stretch. It would be worth to check while your in there, but it isn't your problem. If it's close, I would bump it up to the #2 notch. I don't see any ill effects, just benefits from advancing your valve timing slightly.

Glad to hear about the hoses. Mine were soft too---too soft and they kinked with the movement of the choke.

What benefits would one expect from advancing the valve timing? More low end power?

Steve

[LeonV]

We've already discussed the chain topic, within this thread! The timing chain is not the problem, forget about it. If anything, a slightly worn chain will retard the timing, which would give you a mild improvement in high rpm performance. Advancing the chain to the next position will advance valve timing and move your powerband down, meaning more low end torque as doradox states.

[Zedyone_kenobi]

Just want to thank steve91tt for coming to the house today to give me a hand. Just to update the thread.

Steve took the dizzy with his pertronix out and we put it in my car and ran it with the same problem surfacing. Then we checked a different coil and it still happened.

However, steve then suggested that we take a look at the pistons and the SU domes for cleanliness. Well they had some varnishing on them but nothing too serious. I cleaned them off and reinstalled them (first time the domes have ever been removed since I bought them from Z therapy). The car ran better with no popping until about 5500 rpm. Very clean running

We also think we have the problem narrowed down to something in the domes, as the car will hold an rpm with little fuss, but the second I change the pedal position, it starts to sputter and pop out of the exhaust.

Since I am VERY confident now that I was wrong thinking it was a timing issue, I am convinced it is a carburetor issue. Which means the time has come to increase the amount of data we have to work with.

I will place an order for a real time wide band O2 sensor and we can further diagnose this issue. The O2 sensor will come to be more useful WHEN I get my triples one day, so it is money well spent.

[steve91tt]

I had a great time playing with the cars today. It's not very often that you get to see a couple of 901 silver Z's sitting with their hoods up next to each other in the driveway. Your's is the nicest looking and sounding 240Z I've ever seen in person. Now I've gotta go shopping for an Ansa exhaust!

I agree with your assessment, even when we switched out your distributor, Pertronix and coil the car still stumbled at 4500rpm.

It's interesting that the car pulls hard past 4500rpm and also works great at less than 4500rpm. It just has trouble in the middle of the rev band during medium acceleration. As we discussed, I don't see how it could be related to fuel supply to the carbs as there seems to be plenty of fuel once the engine gets past the 4500rpm bad spot. I also don't think it's a mechanical issue as the car runs great 98% of the time.

I'm not an SU expert but I agree with you that this seems like some sort of transition/acceleration issue in the carbs.