Mechanical advance only?

[Sanchez]

So I finished a rebuild of a 1982 280zx Engine recently and put 4-screw SU carbs on it and a stock exhaust manifold with 2 inch pipe  and a turbo exhaust back so I can drive it daily with out waking up the neighbors. This is going to be pretty long so go ahead and ask questions and thanks for reading.

 

Now to my problem/question:

Driving now during break-in the engine makes a bothersome sound for daily driving that has not gone away in 200+ miles of driving. It has a very lopey and gurgle/burble exhaust and rev characteristic that sounds like a car with a cam that is way too hot for the application even though my cam is the stock grind. The idle is solid and does not jump around more than 100rpm on occasion and falls back to normal without dropping low after letting off the throttle. I fell like the carbs are well tuned and not the problem but I feel like the issue has something to do with timing and/or various tuning adjustments/gap settings of the car.

 

I cannot for the life of me find a 280zx distributor with a working Vacuum advance so I've given up on trying to find a working VA and come to grips with losing the mpg saving that come from the Vacuum Advance mechanism. After tons of research I found info on using mechanical advance only and ended up buying a distributor on a recently wrecked running car that I pulled off and put on with very little rebuilding.

 

1st. I put it on and centered the Stator/reluctor inside, checked for basic function, set plug gap to 0.040", checked compression (all cylinders ~160), rechecked my hot valve gap and did some small adjustments to specs. Wired up the E12-80 module (tach still works, lucky me). Also cleaned the ztherapy carbs out and set to baseline for initial start up. 

 

2nd. Then I set initial timing to 10 degrees BTDC and revved the engine to get mechanical advance all in (I messed up resetting the timing pretty badly at first and caused a few massive backfires, but corrected it quickly and nothing seemed damaged). I read 22 degrees of mechanical advance at 3500rpms which seemed higher than I would want for mechanical only advance.

 

3rd. I bumped initial timing to 15 degrees and got 37 degrees total at 3500rpms again and the car began to drive a better than when I had the pertronix and 240z distributor on the engine but it still seems to have a decent amount of noticeable engine/valvetrain noise and still doesn't have the free revving capability many people claim comes from the 280zx swap and from other Z cars I have driven in. Tuned in the carbs best I could with unisyn for the smoothest idle, the smoothest I got still seemed rougher than it should be, and got 22 inches of mecury at idle from a vacuum gauge at the balance tube.

 

4th. Checked the distributor advance reference spreadsheet for my model, D6K81-01 which should actually have 17 degrees centrifugal/mechanical advance at 2800rpms. If that is true than I should be able to go to at least 20 degrees initial which I think might be better for smoothing out the engine and driving feel. Should I trust my timing light here? I am new to timing and did tons of research before attempting it so do you think it is possible I may have used my dial on my timing light incorrectly causing the reading to be a bit off? 

 

5th. I have most of the parts from 2 280zx distributors I tore down as well as the 240z distributor so I'm going to try and rebuild a distributor from parts while I use the current one to drive my car daily and continue to finish engine break-in. The new one I build will have no Vacuum advance either cause none of mine are in usable, the 240z vacuum dashpot had a medium fast leak that keeps getting worse over time so it's not reusable either. I also heard that many rebuilt dashpots have the wrong advance setting and can damage the engine if used, plus I'm sure every breaker plate I have has the broken plastic bearing run. 

 

I'm asking here on hybridz cause we have no reliable distributor shops in the area so I can't have it recurved by a professional so I'll do it myself and learn from the experience. How do you go about re-curving the mechanical advance yourself? Like where do did you get different springs for the weights and how do you do this slotting thing to lower the total amount of mechanical advance? From what I understand about timing, advance, VA, idle/cruise mixture, and off-idle mixture it seems like I would want about 23 degrees initial and 12 mechanical to when all in. I would like to know how that sounds to you smart ignition guys and here suggestions or examples of your recurves.

 

Additionally I would like to here your thoughts on everything else I have mentioned that you think might be causing the odd driving and revving (e.g. plug gap, exhaust, etc...). Or if you think this is caused by a completely different issue than let me know, cause I have the original 4-speed that I did a mild overhaul on while out, but I have not done any work to differential or the hubs so something could be catching or clunking in there. I'm not very sure of the fuel delivery system (small leak from the Facet fuel pump and weird FPR, and no fuel rail or retunr line, but fuel pressure checked out) so I'm replacing that shortly anyway.

[madkaw]

Run 20 initial with no vacuum advance and you should be good. That will give you 37 all in on most ZX dizzys. Your issue might be more carb then dizzy

Edited August 1, 2014 by madkaw

[NewZed]

"Rebuild" implies that you tore things down pretty far.  Have you confirmed cam timing, reset valve lash, checked the cam for damaged lobes?  It's not uncommon to damage a new cam.

[z240]

I wonder if your "sound" problem is a small exhaust leak? They are often mistaken for or confused with a noisy valve train or contribute to that "too powerful" sound you are getting. Now that its all had a few heat cycles, go crank down all the manifold bolts again and check the downpipe flange. They like to be warped.

 

Since you can apparently can watch and measure your advance pretty easily, as long as it advances smoothly and its all in by 3200-3500 be happy with that. Get those fuel issues you mentioned settled, then report back.

Edited August 1, 2014 by z240

[Sanchez]

Thanks I'll work out the fuel in a few days hopefully when parts come in and as for exhaust leak that could be an issue. Just to get it driving I went with a stock manifold and one of the down pipe studs broke so I installed it with the other 2 and clamped down the side that had the leak with shaped sheet metal. When I have the cash I will get headers but with college loans to pay money is a little tight. What I have noticed is that the sound usually seems to disappear at certain RPMs as long as I am cruising at steady RPMs.

 

Edit: I'm just gonna try and get a new manifold from somebody around here since I have no need for a header on an unmodified engine. Just tried drilling out the bolt, everything was going great, and then the extractor bit broke in the damn stud, just my luck.

 

Edit: Another update on the condition of the engine is I forgot to mention that the engine shakes a bit even with the new motor mounts. It did this with the 240z distributor as well just not nearly as bad, which makes me believe a large chunk of the problem lies in the distributor/ignition/timing. It is almost as if a single cylinder isn't firing but all of the cylinders show good compression and all of the plugs are sparking alot better than with the 240z dizzy.

Edited August 1, 2014 by Sanchez

[NewZed]

If ignition timing is off then all cylinders will be equally bad.  Your timing numbers are in the right range for a smooth running engine, maybe not tuned for best performance, but still should run smoothly.  Ignition timing is probably not the source of the problem.  Injector imbalance is a possibility.  Pop each injector plug off while it's running, one at time and see how the engine responds.  You might find one or two that have no effect.  That would be a clue.

Edited August 2, 2014 by NewZed

[madkaw]

Zed- he has carbs on it now

[NewZed]

Oops.  Subtract all injector references.  Still, timing problem seems unlikely, assuming that the damper pulley mark is correct.

 

By the way, the stock distributors are usually done advancing at 2500 - 2700 RPM

[Sanchez]

Alright so I thought maybe I just wasn't being aggressive enough with the break in cause I was babying it just to be safe so I got the new fuel pump and took it to an empty stretch of highway and ran it through the gears up to 6500 in the 1st 2nd and 3rd... And Tasaday the engine noise went away, the idle went up and it took on a whole new rev happy life that sounded clean and pretty crisp. The shake went away and the idle increased from about 800 to ~1000. Unfortunately it became a tad too rich and tried dropping the mixture with out my unisyn or vacuum gauge and sorta threw it a bit out of tune but I'm sure once I get the tools from my garage it will all be sorted out. The only bad part about this is that my rebuilt starter now sounds like it's grinding a bit when I crank it or fire it up.