car misses at high rpms (4500)

[e_racer1999]

my car pulls fine through 1st and most of second, but if i'm under load or in third gear my car starts missing at around 4500-5000..... i'm guessing it could be the timing, but does anyone have any other ideas?

[2-4-T-Z-MAN]

Jason,

I see that you have a 72 240 listed. Is that's the car you are referring to?

If so, your first guess could be correct. What you also need to check is the plug gap, the point gap or dwell, make sure the vacume advance is working smoothly. How old are the plug wires? Are they a good quality brand? What about the coil?

On the other end would be weather or now you are getting enough gas.

Check the filter. Are you running SUs?

Did the problem just start? What is different if the problem just started? Did you replace or repair anything? If so you might just need to back track.

Good luck.

Barrie

[biggels_7]

I had a similar problem and i discovered that one of my carbies float levels were down alot compared to the other .

[e_racer1999]

yes i'm referring to my 72 and i do have SUs... i just set my timing yesterday (you won't believe me if i told you it took 1.5 hours and $36 to change my timing!) and i haven't had a chance to "open it up".... i'm going to start a thread chronicling my events from yesterday so i'll keep you posted... anyway, thanks for the ideas!

[beandip]

if the engine is cuting out at higher RPMs , look at the ignition first. Something is braking down. Do the easy things first , remove the dist cap and check the contacts for buildup on the ''contacts'' and clean them off. If you are still using a dist . with points , check them for pitting or deposits , either condition change them. They are plated and if burned or pitted it is a waist of time to try and save them. Always change the condenser when you change points. If the plug wires are old change them and get some 8mm silicone filled wires . Take a good look at the plugs and if there is any question on these change them . Use NGK plugs. I use BPR6ES nothing fancy and they work the best for me. The only thing left is the coil , they dont fail vary often but some times they will . Usually in hot weather. Some times if you look at the engine while it is running in total darkness you will see sparks from bad plug wires. They have to be really bad though. If your timing is set at 8 to 10 degrees BTDC , and there is no pinging heard under load you should be good there. my 3cts

[AxtellZ]

I'd double check the float levels, make sure they are adjusted properly. One carb may be running out of fuel under load, causing the starvation..

[Chino 240Z]

how funny just posted about this simular problem I had. Starvation above 5000rpm. I walked that path for 6 months! :tapemouth

"3 to 4 psi is all you'll want for carbs." I found I need a solid 5 psi when above 5000rpms to keep up with the ZTherapy (stepped up) SUs. Spent alot of time trying to get rid of my high rpm problem I thought was electrical, miss-fire, or carb adjustment. Hard to believe 3 to 3.5psi wouldn't supply enough fuel, like everyone kept telling me. After trying 4, then 4.5psi problem almost was gone. Now at 5psi, problem solved. SUs handle it just fine. 5psi is not for all SUs set up for daily street driving, but I believe you need to step out of the norm sometimes to see if the norm isn't the problem.

"just fuel for thought"

My current setup that works for me is:

Holley Red Pump (somewhat loud for daily driver).

Holley regulator set @ 5psi

Mechanical guage.

ZTherapy SUs with SM needles & .101 nozzles.

Full radius air horns & K&Ns

No fuel return line.

[ksls94]

check fuel filter for trash of stopped up

[biker]

Another one to look for is the Dizzy.

Several months ago I had the problem of once hitting 4500rpm it seemed like I was hitting a rev limiter.

Turned out that the bearing/bush on the shaft of the dizzy was worn allowing movement against the points as revs built therefore alterin the gap at the points.

To check it, remove the rotor cap and see if there is any large rocking of the shaft/cam that the points runs against.

biker

:classic:

[beandip]

another thought , there are fuel filters at the enlet to the floats , check this . They are unneeded toss them.

[dspillman]

Jason,

I had similar miss at 5500-5800, turned out to be leaking/failing brake booster, short term fix, plugged line at manifold, went away...wasted 2 race weekends, and 45 min on chassis dyno to track down. Shoot a little carb cleaner around carbs, linkage and such listen for rpm fluctuations.....helps me find big vac. leaks..

FWIW,

David Spillman

[Chino 240Z]

Looks as if we all have had a simular "miss" at RPMs and there are several things related that can cause this result. I guess it's now your job e-racer, to take these hints and one by one check, re-check and aliminate each one. Investigative research! Possibly a little of this and alittle of that problem combined to make the symptom show up.

I hear you David, we farted with that "miss or skip" at the track for 3 events, then acouple of hours on a chassis dyno too!

The good side of it all is: I now have tune-up knowledge of these SUs, ZTherapy SU Video Tapes, 3 SU carb books, rewired electrical & ignition system, clean grounds, new ground straps, calibrated & tested fuel gauge & fuel pump, a distibutor tested to 8K rpms, a tested Pertronix ign. system, 1 spare MSD 6AL box, 2 spare caps & rotors, a new friend with a chassis dyno, Mrs. Chino put down the shotgun- from me complaining so much. . . but man does our #44 baby scream now. Finding & fixing a crazy problem is almost as good as getting laid in a Zed!

:eek: (this is my Ooh face)