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

[madkaw]

I have read a LOT of posts describing the SM needles are best for the bigger bore engines- 2.8 and higher

[H4LIGHTS]

You may be reaching a point where a dyno facility may be helpful in narrowing down the culprit of the 'hesitation'.

Gas analysis and the ignition pattern(s) revealed on a scope while running at the magic 4500 rpm.

[i'm betting rich mixture and ignition advance allowing for after burn. I've been behind Z's with a bluish flame traveling in the muffler/tailpipe]

But if it is truly missing the dyno will help in localizing the area needing repair or adjustment.

[Zedyone_kenobi]

I agree, I have called a few around me and they only work on Fuel injected cars.

There is nobody with a brain cell left that can work on carbs.

They all have been spoiled by OBD-II

I am going to invest in a wideband O2 and get my exhaust tapped so I can do some tuning on my own.

[Zedyone_kenobi]

UPdate, as I was starting to troll on Healy Z's thread.

I used a colortune to see where I was in my mixture, and low and behold I got a nice pretty blue flame at idle.

when I gave it gas I get a very orange color that never quite gets to blue again. I think I am still rich, but I am also starting to suspect other things. My dizzy is practically brand new from a professional rebuilt with new bushing/bearings, etc. All the advance weights actuate very well. I have the Vacuum advance blocked off, as I had the dizzy recurved to match the european specs.

I still need to find a wideband O2 sensor that can be read real time. Then I know for sure what is going on at higher rpm.

[Zedyone_kenobi]

Okay an update for any who care..

I was messing around with my colortune and came the realization that it does in fact give useful information, but really only on a gross setting at idle. I would not trust it beyond that. AN engine will react differently under load, so revving up to at idle to me is not an indication on mixture ratio whatsoever.

However, with all the mucking about I did, I decided to start from scratch. Again....

I checked my carb balance and verified it was good at both idle and 3000 rpm.

Next I set the A/F mixture knob to two turns form full lean.

Third, I started it up and found the highest rpm at 1.5 turns out form full lean, so I leaned it out a half turn.

Checked the static timing and max advance (folks get a digital timing light so you can do this, it is very critical)

Static timing was close, but my max advance is only around 32 degrees, where stock you could theoretically get to 47 degrees, but realistically only got to about 40.

I decided to advance my timing to close to what the european spec cars run ( 17 degrees) at idle. I could only turn my dizzy to get 15, but went out and gave it a try, and low and behold the popping was gone at 4000 rpm. Instead it was moved up to 5500 rpm and then only barely.

Methinks I need more total advance. I will try to hook up the vacuum advance again and see if I can eek out more timing out of it. I Car feels FAR better now by the way in all gears.

I should also note that I have a custom curved dizzy. Do not try to run your stock dizzy at 17 degrees BTDC as your mechanical advance and your Vacuum Advance will put you into a dangerous place.

[steve91tt]

A couple of data points for you...

As you are I am currently running Z therapy SU's with SM needles on my bone stock L24 without issues. I have also run them on a L30 stroker with no adjustment moving them between motors. They pulled stronger on the L30 than the L24 but other than that they seem to be equally at home on both motors. Incredible carbs.

I am running about 30° of total timing with no vacuum advance on my track car with no issues. I keep the timing retarded to keep the detonation at bay when I'm not using race gas. The car happily pulls hard to redline and beyond even with 30° timing.

Just my $0.02. Good luck tracking down your gremlin.

[240260280z]

Okay an update for any who care..

I was messing around with my colortune and came the realization that it does in fact give useful information, but really only on a gross setting at idle. I would not trust it beyond that. AN engine will react differently under load, so revving up to at idle to me is not an indication on mixture ratio whatsoever.

Exactly what Mr Obvious and I speculated. Colortune seems to be more of a novelty item or a crutch for someone who can't set jet depth to spec and tweak via plug reading.

As you mentioned above; reading the timing empirically with the correct tool made a difference. Likewise tuning with a O2 or similar gas analyzer will also take the magic out of tuning and give you real and useful data.

btw Someone should make a clip-on knock sensor and that would be fabulous to finish the tuning tool list...dang I wish I could patent my ideas.... can you imagine a knock-tune plug ™ from Blue Industries (also specializing in films from Europe)! It would be a plug with a built in knock sensor (or a clip on to a plug) and a lead to an led array where 4/6/8 plugs can be viewed while tuning.

[doradox]

I'd hesitate to blame timing or mixture for your woes. A few degrees one way or the other or a little rich or lean shouldn't be causing any popping. It doesn't make sense that you would need to be in "perfect" tune to run right. In my experience many a ridiculously out of tune Z has run just fine. How have you checked your ignition? H4LIGHTS mentioned hooking up to a tune up scope. Have you at least resistance checked your wires?

Steve

[Zedyone_kenobi]

yup, my wires are all reading very low .55 Ohms or therebouts. coil wire included...

I tend to agree with you doradox. It would seem that as close as my car is to 'normal' that something else may have been going on. I need to verify that my proposed solution is in fact the answer. I will also try to hook up a jumper from my battery to my coil to see if my tach is in fact causing any goofiness...

I will need to go for a drive today to verify my settings on my car and see if I can eliminate the popping with the addition of more timing. ( I am going to try to run 17 BTDC without a vacuum advance as the european dizzies run)

Steve, I read many of your other posts about timing. IF the stock OEM Datsun 240Z in 71 could theoretically achieve 47 degrees BTDC I would think that hoping for high 30's would be reasonable.

If there was a way to check to see if my pertronix was funcitoning properly I would do it, I have no other dizzy to swap out with it and I have not had points in my garage ever. I bought the car with the pertronix in it.

I have to admit, even though I read all the negative things said about the colortune, I wanted to try it anyway. Call it curiosity maybe. But having used it, I can tell you it would be far more useful for a car that cannot idle well in the first place and that is about it. For a car like mine that is faultless below 4000 rpm, it was just a toy that I plan to play with my lawn mower with now.

Everytime I put the colortune on my car I get a nice blue flame.... but I am an engineer, who really prefers quantifiable results. Not pretty colors.

Shame on me for not listening. Yes, I will look into a o2 sensor now, just for kicks.

also you all should note that I have a new fuel pump putting out almost 4 psia and a new fuel filter and a completely new set of all fuel lines.

[madkaw]

Glad to hear that the ignition timing made a change. If it were me and this was helping to fix the problem, then go with it. Put more advance in and don't worry about the vacuum advance. Listen for the knocking and keep advancing to see if it eliminates the your popping.

As far as the 47 degree figure, that would be with total timing INCLUDING vacuum advance and you would only see that at light cruise. I would shoot for the high 30's with your light, with the vacuum advance REMOVED, and high enough rpm for the all of your mechanical advance to be in.

I will also agree that a slightly out of tune shouldn't cause your problem, but what if your recurved dizzy has a problem that amplifies an out of tune issue.

Anyway, advance away and keep an hear out for knock.

[LeonV]

I will need to go for a drive today to verify my settings on my car and see if I can eliminate the popping with the addition of more timing. ( I am going to try to run 17 BTDC without a vacuum advance as the european dizzies run)

Steve, I read many of your other posts about timing. IF the stock OEM Datsun 240Z in 71 could theoretically achieve 47 degrees BTDC I would think that hoping for high 30's would be reasonable.

Vacuum advance is only active at part throttle. At WOT, vacuum advance should not be making a contribution to total timing. Empirically, MBT timing is typically 33-36 degrees on an L-series engine without serious combustion chamber work. If everything else is okay, and your timing is a few degrees off then the engine should still function fine, it will just be slightly down on power.

One thing that is apparent to me is that your mixture seems lean. A leaner mixture burns slower, which necessitates an increase in spark advance. You increased advance and the problem shifted to a higher rpm, telling me it's a mixture issue. Also, popping from the exhaust can indicate a lean mixture since incomplete combustion will then cause the unburned part of the mixture to ignite and "pop" within the exhaust.

The first thing I would do is check for big vacuum leaks and mal-adjusted valves. Just because they're quiet and were adjusted 2000 miles ago, does not mean that they're okay now. Make sure to include the PCV system during that check as well.

[Zedyone_kenobi]

WHat PVC system? I do not think my car has one?

All great advice Leon.

I am running SM needles and I adjusted my carbs per the SU video, finding the highest idle possible. 1.5 turns form full lean is not out of the ball park is it?

I get a nice pretty blue flame from my gimmicky colortune with that setting. Still, I can always check for vacuum leaks, easy enough, and look at my valves ( which I did actually adjust about 2000 miles ago! haha)

All my original tubes are hooked up to the air cleaner as the factory had them.

I will take her out for a drive and try to fatten her up a bit. I have exactly 2 turns clearly marked on my car so that will be a easy spot to reference.

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