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

[doradox]

It points to a vacuum leak that occurs off idle and not to a persistent leak like an intake gasket, vacuum hose, brake booster. One thing that can act that way is the PCV valve. Since he's running no vacuum advance no cap on the ported vacuum port is another.

Steve

[Zedyone_kenobi]

Okay, lots of help here, let me address everything I can.

MY distributor was rebuilt by Advance Distributors, who put all new bushings, seals, and ensured that there was no slop in the timing up to 8000 rpm. The guys is very well respected from what I read from MG and other Datsun guys. I trust his work. With my light on the distributor, it was rock solid up to as high as I would want to rev it.

Now I have NO idea how solid my oil pump is, OR my crank gears, etc.

I am going to pull all the domes again, and try some straight 30 wt per recommendations, as one last ditch for the carbs. I will put the SM needles back in while I am doing that.

Then I am going to plug the brake booster (which I just replaced with a new one this year) and drive it and then plug the PCV and drive it. I finally got a plug for it.

After that, I will pull the valve cover and have a look at what is going on there.

Blue, I did not mean to try to confuse or 'cry wolf'. I am keeping track of a huge amount of data and I do at times forget to report in so to speak. When I first got the car it did in fact NOT rev at all past 4500 rpm, at part throttle, full throttle whatever. A new cap and rotor and actually timing it to 5 degrees BTDC like the factor recommends fixed that.

To the degree this problem may have always existed is tough, as for the first 2 years, I really did not push the car super hard, so it may have always been there.

What I am glad to see is even and high compression numbers on my car.

I do not know how many PSI it should have stock. I ran another compression test on it with the butterfly open and it came up to 182 +\- 2 psi on all the cylinders.

It is possible I have a bad brake booster, or a leak somewhere that only arrises under load. But the brakes work splendidly.

I have to admit, this has all the symptoms of a vacuum leak.

Again, I am sorry for any confusing info... The cap and rotor did let me rev beyond the 4500 rpm wall for the first time, that is true, but the Ztherapy carbs allowed me to run powerfully above 4500 rpm for the first time.

But blue, you may actually be 100% correct in the end.

Edited January 7, 2012 by Zedyone_kenobi

[madkaw]

Your cylinder compression levels are great-don't even give that another thought.

[Zedyone_kenobi]

You guys have been a huge help! I mean huge. I cannot thank you enough. I am going to stop posting results for a while and get my O2 sensor installed. I think my post has generated a little discontent from somebody whom I respect immensely in the Z community. I am going to get out my FSM and trouble shoot this as completely as I can before I bug the good gurus here. I thought I had done that before I posted the first time, but just to be sure, I am going to triple check.

Again,

steve911

madkaw

blue

doradox

Captain Obvious

5thhorseman

Diseazd

30 Ounce

and anybody else I have missed

And anybody else I may have missed. YOu guys are all 1st class. Always have been. I am going to stop hogging the resources of this board until I have something worth while to report.

[Zedyone_kenobi]

Cant help but report results...

plugged the PCV and the booster out of the manifold, same issues. If it is a vacuum leak it is from somewhere else. All bolts are torqued. I did put back in the SM needles and while I did that I cranked down the fuel mixture as far as I could...about .205 inches until it hit the stoppers. I had the domes and pistons out and the fuel was definitely below that... as it should be. But how far below, not sure until I can get the stoppers out to use 30 Ounces technique. While I was at it, I put 10w30 per the FSM recommendations in the carbs. Will drive it when the rain clears up.

If that does not pan out, then I will declare victory over the carbs and start measuring how much slop is in my mechanical drive that spins my dizzy. Oh after I get my O2 gage set up. I will probably pull the valve cover off Monday just to have a look around.

Sorry to keep this going,

[Diseazd]

Zedy.....One more thought. I know you replaced the distributor with Steve's with no change, but my guess is you still used your distributor cap. Cracks in a distributor cap or rotor sometimes can be almost invisible. Have you tried a new cap and rotor? A hairline crack in the rotor or distributor cap can leak voltage at high RPM causing symptoms similar to yours. Just a thought. Guy

[madkaw]

That's what I suggested and I got that idea from googling!!

[Zedyone_kenobi]

NO we actually switched the entire cap / rotor / dizzy as a whole unit.

I had already tried a new cap and rotor a while back. I did as much common sense trouble shooting as I possibly could in the past few weeks before I started this thread in frustration.

I am winding down to the last few ideas that I can possibly use on the carbs. I have checked every measurement possible.

The kicker is the part throttle nature of it all. What would cause this issue at part throttle but not at WIDE OPEN THROTTLE. The PCV and brake booster were not leaking.

I thought maybe worn throttle shafts that at part throttle that allowed a leak (unmetered air) into the system at half throttle, but not at WOT. But the bearings Z therapy puts in should have eliminated that as a possibility. I need to get on the ball with the O2 sensor and stop messing around chasing my tail.

[240dkw]

Zedyone: If you are going to start looking for a voltage leak do not forget to replace the two condensers on the ignition system. The one for the points and the noise suppression one on the coil if you have not done so. These can test good on the bench but leak or "spill over" at a higher voltage. This was the problem Arne had with his ignition and fixed it by removing the one at the coil.

Dan

[Zedyone_kenobi]

The one for noise suppression at the coil is in fact removed. I will check to see if the one for the points has been removed, since I am running a pertronix.

[Captain Obvious]

The kicker is the part throttle nature of it all. What would cause this issue at part throttle but not at WIDE OPEN THROTTLE.

Yeah, that's what's really got me stumped. I had the exact same thing but it was because I was lean at midrange due to the carb mods I made and the needles I was running. I switched to the N-27's and it went away. And you're running a richer needle than I am on my 260. Sounds like you're lean, but you shouldn't be.

Wish I had your silver bullet!

[Zedyone_kenobi]

I think the key to this is not to get stumped.

I feel like my car is a patient on HOUSE,

Some crazy ailment that I have to try half a dozen things to fix before some obscure thing triggers a eureka moment!

Still I cannot thank everybody enough for their time and energy replying to my dissertation of a post!