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

[Zedyone_kenobi]

Thanks Adam, I am not even running my vacuum advance at this point. I am relying totally on mechanical advance.

Well I got my carbs back last night, and I plan to put them to the test.

After I get the car back together and I do all my other trouble shooting, if it still does this I am going back to timing.

I picked up a newly rebuilt D611-54 single point distributor. I ordered a new pertronix, more new wires (I wanted red ones anyway), new cap, new rotor, new vacuum advance and will try timing from scratch again.

I know that distributor is not OEM for my car, but it should work well enough.

[Zedyone_kenobi]

UpDATE:

Had a talk with Dizeasedz and he suggested I do a leak down test. I have acquired a leakdown tester today and will try to get that hooked up tonight to see what I find.

PS-It was great talking with you Guy. Thanks for all the VERY helpful advice on the engine build. I will be in touch soon to discuss that further.

[LeonV]

I think you said that popping occurs at 4500rpm and only partial throttle, correct? If this is the case, your distributor was probably not providing enough timing advance to light off the more dispersed, leaner mixture. I recommend swapping in a distributor with a functioning vacuum advance. Have you still not checked valve lash?

[Zedyone_kenobi]

Yes I checked valve lash. Nothing is crazy out of whack or lose. Hard to say for sure, as it was not hot and it was not cold. More warm. But the feeler gages felt within the realm of acceptability.

I have a dizzy in there with a working vacuum advance. It is just plugged up. When I get her running tonight hopefully (I installed the carbs last night) I will be able to play with timing again with AND without vacuum advance.

On top of that I have another OEM distributor that has not been recurved with a NEW pertronix, new cap, and new rotor, and new wires, I may swap that in, set it 5 BTDC and give that a try. I will flush out the ignition part of this thoroughly by the end of this weekend. Although, keep in mind, I already swapped dizzies with somebody else and this problem still existed. So this is one last ditch effort to run this to ground on the ignition side.

While I know no two engines are the same, but many other people have run without a vacuum advance and high static initial timing without any issues. So part of me thinks this is something more clever.

A leak down test will give another data point, as will the vacuum readings

[LeonV]

Yes I checked valve lash. Nothing is crazy out of whack or lose. Hard to say for sure, as it was not hot and it was not cold. More warm. But the feeler gages felt within the realm of acceptability.

I have a dizzy in there with a working vacuum advance. It is just plugged up. When I get her running tonight hopefully (I installed the carbs last night) I will be able to play with timing again with AND without vacuum advance.

On top of that I have another OEM distributor that has not been recurved with a NEW pertronix, new cap, and new rotor, and new wires, I may swap that in, set it 5 BTDC and give that a try. I will flush out the ignition part of this thoroughly by the end of this weekend. Although, keep in mind, I already swapped dizzies with somebody else and this problem still existed. So this is one last ditch effort to run this to ground on the ignition side.

While I know no two engines are the same, but many other people have run without a vacuum advance and high static initial timing without any issues. So part of me thinks this is something more clever.

A leak down test will give another data point, as will the vacuum readings

Well, most people that run without vac advance do so when there are no provisions for a vac advance, e.g. when switching to triple Webers. In any case, load sensing spark advance (vac advance in this case) is a good thing. What you have going on sounds like a bad combination of fueling and spark timing. Sounds like you were leaning out at partial throttle and high rpm, while your spark timing did not advance enough to burn the leaner mixture on time.

I don't run a vac advance on my triples because there isn't a provision for one, but my triples also run rich. A richer mixture will burn quicker than a lean one, thus the spark doesn't need to be as advanced in order to light-off the mix at the right time. However, clearly this is not good for efficiency.

Sounds like your new carbs are set richer and you can test with vac advance connected. I think a properly working vacuum advance will cure your problem.

[Zedyone_kenobi]

I wish you were right my friend.

I just tried the ultimate overkill ignition test tonight. I left my old dizzy on and installed the newly reset carbs from Ztherapy. Started right up and idled great as always. Drove well below 4000 rpm as always, but again, I was leaning out and popping at part throttle as the rpm reached north of 4000 rpm.

So I got out of the car and hooked up the vacuum advance. Drove the car again, with exactly the same results as before. My dizzy is free inside to move, I tested the VA diaphragm and it help vacuum just fine.

So Now I was ready for the swap.

I installed a brand new pertronix into my D611-54 dizzy, put on a new cap/rotor, and installed it sans VA hooked up. Drove it again, and whala. Same thing. NO change at all.

My first dizzy was timed to 17 deg BTDC with and without VA .. .leaned out at part throttle

Second dizzy was timed 11 deg BTDC w/o VA ... leaned out at part throttle

I am ready to write off the ignition has anything to do with this.

I have tried Three pertronix, Three distributors, 2 coils, and anywhere from 7 deg BTDC to 17 deg BTDC, with and without a VA.

It was getting late so I did not want to annoy the neighbors, but all the VACUUM gage stuff is next. I will try to see the following:

I will see if the VA nipple is pulling adequate vacuum while driving to operate the VA

I will see what my manifold vacuum is doing when this leaning is going on vs WOT

I will get back you guys on that. Then I will perform the leakdown test on all 6 cylinders

[240260280z]

Wow this is frustrating. Maybe it is carbon/compression problem or valve train problem.... does not seem like spark or fuel.

btw I assume different distributor caps and rotors were used?

[Zedyone_kenobi]

yessir.

I have three sets of brand new rotors and caps! haha

I still need to perform the carburator cleaner spray check. But I am saving that until after my vacuum round. I am getting desperate.

How desperate you ask?

I have already called Rebello and Datsun Spirit about an entire rebuild, from oil pan to valve cover. I can leave nothing to chance.

That 2.7 purist motor Rebello has is looking VERY tempting right about now! Then again, I would really love to tear it down myself and rebuild it with my own two hands.

[240260280z]

Does the popping change when you rev by hand with car stationary vs. slow acceleration vs. aggressive acceleration?

Middle top (weak coil at acceleration).

I am still wondering about slop in the spindle to the distibutor causing scatter but that would get worse with rpm.

Could be some weird harmonic in the valve train.

[madkaw]

You keep saying-"leaning out", what does that mean?

Are you saying that by looking at your wideband guage the engine is leaning out?

[Captain Obvious]

You keep saying-"leaning out", what does that mean?

Are you saying that by looking at your wideband guage the engine is leaning out?

Yes, when he says "leaning out" he means that at certain RPMs and throttle positions (4500-5000 RPM and light to medium throttle position) his wideband indicates that his mixture is lean.

Details are in post #229 (page 10), but here's the Reader's Digest version:

2nd gear pull - part throttle

2000 rpm----13.9

3000 rpm----14.5

4000 rpm----15.8 Popping started

5000 rpm----16-18 Popping too strong to continue at part throttle...

2nd gear pull - WOT throttle

2000 rpm----13.9

3000 rpm----13.8

4000 rpm----13.9

5000 rpm----14.0

6000 rpm----14.2

You can see from the results that at the same RPM, he's running leaner at part throttle, than he does at WOT.

The recent interest in ignition timing details isn't because it would do anything to add fuel to the mixture, but it might reduce the effect. In other words, the root problem is that he's lean, but it might be OK to be that lean, if it weren't for the fact that it's causing performance issues.

[Zedyone_kenobi]

Blue, the car can rev to 7000 rpm without any issues stationary, and revs pretty freely... This phenomena only happens under load and only on part throttle. With the throttle opened 30% in 2nd gear I get to darn near 5000 rpm before it happens, in 3rd gear with the throttle opened 30% I get to about 4500, in 4th gear with the throttle opened 30% I get to 4000 rpm. THe harder the car has to work against a mechanical load, the sooner the AFR hits 16+:1 and starts popping and the car stops accelerating. But if I am currently popping and stuttering, if I floor it, I go strait to 12.8:1 and all is right with the world.