Doesn't pull thru entire rpm range

[59ghia]

Hello fellow Zer's

Here's the scoop, stock 72, my baby pulls really well up to 4500 rpm then starts feeling "unbalanced" and does'nt pull as well. Seems it should be smooth up till redline or close to it. The carbs have been rebuilt, and are sync'ed spot on. timing is on, valves adjusted etc. Compression is alittle low (150 psi) but even between all 6. Car sat for 5 years before I got her, so I'm think'in maybey the valve springs have weakened? Here's another possible problem- whats the total advance of this distributor? I'm getting about 10 degrees when i rap the throttle, is this enough?

Any ideas?

Thanks Dave

[2ManyZs]

Have you checked your points, cap, rotor, plugs, plug wires and the distributor internals? I'd check there before worrying about the valve spring condition. It sounds to me more like a ignition breakdown. How about the coil? If you go through a complete tune-up of the ignition and it still does the same thing, then it would be a logical step to look at the valve springs.

Another idea, what is the condition of the fuel filter? What about the air filter too?

10 degrees advance sounds right on to me.

[grantman240z]

mine does the same thing...and the other day the whole ignition system just shut down for no reason, had to take it into the mechanic...but that still didnt fix it.

[59ghia]

I new I left something out, Running a petronix with a new 40k coil. Fresh wires, plugs, cap and rotor. Was thumbing through one of my manuals and saw info on vacuum diagnosis, always saw this as fm, but has pics of guages under varing conditions. I think I have a vacuum guage in the garage somewhere. I'll give that a shot and see what I come up with. I'll let ya'll know what I find.

[2ManyZs]

Well, that blows that idea out of the water. Probably not ignition then, so that leaves air and fuel problems, of course the valve springs are still suspect. Another idea is cam problems either the cam lobes are worn, maybe the timing chain has so much slack the cam is out of time, or the rockers are shot. Guess this will keep you busy for a while. I'd try air and fuel next. If you are running the stock mechanical fuel pump you could try changing it or maybe using an electric as a back-up.

Another idea, probably an "off the wall" one but it will do the same things. How is the charging system and battery? If the alternator and/or battery is weak it might not be supplying the voltage necessary for the ignition. Just an idea, but one that is worth mentioning.

[grantman240z]

im slightly confused with the problem. Mine sounds like is red-lining at 4500 (load of smoke comes out the pipe too)...not like its losing a spark or starving for fuel, would the air/fuel system make a difference in that at all?

[2ManyZs]

It could depending on the color of the smoke, if it's black smoke it could be running too rich which means too much fuel (carb problem?) or not enough air is getting into the mix. Seems like there is always more than one having the same problems, wonder why that is?

[59ghia]

Ran the vacuum test, still fm to me but I think I get the idea. Not real far off considering its got 100k on it. No real funky readings but on hard accel the vacuum drops to around zero which indicates, according to Clymer, worn rings or diluted oil.--------- oils fine. But lets go to timeing chain, when I adjusted the valves I checked the chain, it seemed a little sloppy to me but when I checked the notches on the cam the slash was dead center to the groove and I think that means its not to bad. (should be to the left ?) Whatta think? new chain? I think I'll check the tensioner first, What should I look for?If so who has them cheap....errr inexpensive? Motorsport wants 85 for one.(chain) Ok enough ramblin' I need a beer!!!!

Dave

[Mike]

Sounds like a mechanical advance problem to me... but, I figured that would kick in at a lower rpm than 4500. Have you rebuilt your distributor?

Be careful when you do... there are springs inside that can shoot.

[Daniel]

I have seen incorrect/dirty air filter cause that problem.Remove your air cleaner for a test and give it arun.

[59ghia]

Mike,

When I installed the petronix I wanted to remove the plate but one screw wouldn't budge, the one thats kinda in a corner so i couldn't grab it with vice grips. I lubed the he!! out of everything, and went from there. The bushings are tight. I'll also give Daniels idea a try since its easy enough. 2many. the alternators new, old one had bad bearings and howled like crazy, not a sound you like to here when you first start a motor!

Thanks--

Dave

[grantman240z]

i fixed the problem with mine...and i think it was backpressure from the exhaust. I bought the turbo tip from MSA (the little dual exhaust vertical tip thingie) and now it pulls all the way up past 6000 rpm...plus it sounds awesome! So another thing to try is to check the backpressure in your exhaust sytem if you havent fixed the problem already