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

[doradox]

Remember that the WBO2 won't read accurately once there is a miss fire. It reads oxygen content of the exhaust and calculates fuel mixture and not actual fuel mixture. Is it actually leaning out or is it missing and making it look like it's leaning out?

Steve

[Zedyone_kenobi]

Doradox, I hear you, and to the best of my knowledge, I do not feel any hesitation or missing at all right up until it starts to pop and stutter. If I never revved the engine past 4500 rpm I would not ever even know I have an issue. To add more weight into the lean camp, my plugs look light brown when I pull them out, not the nice dark patina you want to see.

[240260280z]

Thinking out loud:

It seems fuel related but

- ZTherapy's trial ruled that out.

- high rpm at 13.9-14.2 under load can be lowered a bit from jut but pump seems fine.

- assume fuel is good.

Could also be spark but

- different distributors

- different caps

- coil tests ok

- no spark scatter at high rpms

Whats left?

- valve train problem? But why only at one point and under light load?

- other devices tied to manifold leaking air under light load (brake booster?)

- ????

[Zedyone_kenobi]

Blue, you thinking is very logical. I actually ran with my brake booster plugged off at the manifold and this still happened. Did the same thing with my PCV valve blocked off and the same thing happened. I also checked the check valve in line with the booster,and it pasted the FSM check.

I am just about ready to pull the head off the darn thing. I know there is lots of build up on top of all the intake valves. It is not unreasonable to think that I may have a burn valve. But I would think that would show up on a compression test. Still, I have replaced and swapped just about every single part I can think of. I am not so ham fistted at tuning that I think I have something that out of whack.

I am going to do my vacuum runs tonight if it is not pouring rain. But not sure what I will do with the data. If I show my vacuum dropping off as my rpm rises under part throttle, what do I do? I will do the spray can of carb cleaner around the sealing surfaces as well and listen for rpm changes.

If that does not show anything, well, then folks, I am going to admit defeat. The last test I will perform is the leak down test. Not sure how I am going to find TDC on cylinders 2-6 though.

My plan is to run the engine until it is hot.

Pull the plugs

disconnect the coil

remove the valve cover

rotate the engine until it is on #1 TDC

do leak down test on #1

Not sure how I am going to find TDC on #2-#6 though. I could stick something into the cylinder that is long and thin and when it stops moving down and starts going up I will know I am close. Guess that can work. But it will not be exact, it will only be close

One question I have is when you put 100 psi into the cylinder, do you have to keep the engine from turning over?

[LeonV]

At this point, I would do a cold valve adjustment. Get it warm. Take off the valve cover again and check hot lash. After you've checked/adjusted the valves hot, you can perform the leakdown test. Find TDC on other cylinders by looking at the cam lobes.

[Zedyone_kenobi]

Well just looking at the cam lobes will tell me when the valves are closed but it will not tell me when TDC is on each cylinder. Not really sure it matters, as all I am really measuring is make up flow, but everything I have read says put it on TDC.

Also, for those who have done a leak down test, did putting 100 psi in the cylinder spin the motor over.

[doradox]

One question I have is when you put 100 psi into the cylinder, do you have to keep the engine from turning over?

Yes you will.

[LeonV]

Well just looking at the cam lobes will tell me when the valves are closed but it will not tell me when TDC is on each cylinder. Not really sure it matters, as all I am really measuring is make up flow, but everything I have read says put it on TDC.

Also, for those who have done a leak down test, did putting 100 psi in the cylinder spin the motor over.

Make sure the valves are closed, chock the wheels, engage the hand brake and put the car in 1st.

[beermanpete]

The engine won't turn during the leak down test as long as you get it at true TDC for each cylinder. It is not as difficult as it might seem. Start on #1 since it is easy to find TDC. Then proceed in the firing order and turn the engine 120 degrees each time. If you need to, use a probe through the spark plug hole while turning the engine (by hand) to find TDC for each cylinder.

Regarding the mystery miss-fire, I know you have check every part 20 times, but have you tried to measure the high voltage at the spark plugs, or more importantly, the voltage capability of the ignition system? Your symptoms suggest inadequate spark that is not capable of reliably lighting the lean mixture at part throttle.

[Zedyone_kenobi]

Not sure how I would do that? I have checked that my coil is getting 13+ volts, and I have tried two different coils. I guess I could wire in a voltmeter to the coil while I am driving. Not sure how I would measure spark intensity though

As far as attachments to the coil, I do not have any capacitors attached to my coil or dizzy at all.

I am using a Pertronix flamethrower right now. And I am bypassing my ballast resistor.

[madkaw]

And we have crossed out that your running the correct impedance coil for the pertronix - this thread is so long to go thru!

And I thought the AFRs were much higher during the popping.

I'm leaning towards what Beermate at this point.

I'm not buying the valve scenario.

Have we tried a different ground for the ignition?

Edited March 9, 2012 by madkaw

[Zedyone_kenobi]

I will check the ground as best I can. I can honestly say I have not checked every ground on the entire motor. I will pull up a schematic to see where the ignition is grounded and see where I may be losing something.

I get into the high 16's and 17's when it pops madkaw.

I will measure my resistance through the coil again, but I am sure I hooked it up right. I think my coil is 3 Ohm so I bypassed my ballast resistor, but I will double check.