Guest bastaad525 Posted October 22, 2002 Share Posted October 22, 2002 Okay, I know some of you have been following along with me on this already. Specs are: L28 bored out to 2.9, not stroked, N42 head, rebuilt Ztherapy SU carbs with SM needles, and a mild cam. After some issues with uncertainty of tune of the SU's and some other little problems I decided to take the car to my friends smog only shop, and tune the SU's on his exhaust gas computer, with the car under load and checking it at different RPMs. After getting it about as close to 'perfect' as it would really get (which resulted in the car running a bit rich at idle, a bit lean under full throttle acceleration at about 5k and up revs, and just about perfect for everythign else), I've been driving it pretty much every day for two weeks. Well I pulled the plugs a few days ago and was kind of baffled by what I saw. The outer ring of metal housing (the part of the plug body with the screw threading), if you looked down onto the top of the plug, is covered in black soot, like it's running too rich. The white insulator is also is covered like this, BUT, each insulator also had a patch, sometimes more than 1/2 of the exposed visible part of the insulator, where there was no deposit at all, just clean white, like you would see if it was running too lean. The electrode tip itself, and the metal 'hook' that extends over it (where the spark jumps TO) were clean, no soot, maybe a little brownish. It's like two complete different readings on the same plug. Well anyways, as an experiment, I richened the carbs up by about 1/4 turn each. Well for one this made the car feel noticeably more powerful, smoother, and more responsive. So I've driven like that for a few days and pulled the plugs, and got nearly the same result, except now some of the insulators were almost completely covered in soot (the #3 and #4 cylinder plugs being the worst) and the rest still had soot with 'clean' patches, but the patches were now the right color as the whole plug SHOULD be when running right, i.e.: a nice tannish color. I then tried leaning them out a 1/2 turn, and pulled the plugs after a few days and got insulators that were almost completely white, but STILL black soot all over the edge of the metal housing!! Okay so... what's the deal here?? Is this usual for this type of setup? Does this make sense to anyone? Or does this indicate I'm having a problem? I'm beginning to think the SM needles aren't all they are cracked up to be, at least not for my car! The stock 240 needles sucked though as they would always run way to lean on the top end, unless I richened the car so much as to run like crap. I'm thinking I should stick with the richest setting I tried so far, as it definately 'feels' better, but I'm worried I'm shortening the life of my engine by causing it to build up carbon desposits rapidly. What do you guys think??? Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 22, 2002 Share Posted October 22, 2002 If you index the plugs you may get a more consistent looking plug. You can also play around with the needle, and perhaps drop it a little and see if that helps with your idle richness, or you can lower the float level a little. I think that is the main thing with SU's, you have to get them dialed in just right, and they will never be completely perfect since it really is just a controlled fuel leak. Just my opinion. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 23, 2002 Share Posted October 23, 2002 The needle is as low as it really should be, which is, it is exactly low enough that it will completely shut off the fuel if the mixture nuts are leaned out all the way. Lowering the needle otherwise should/would probably have the exact same effect as if I just tightened/raised the nozzle, which would lean out my overall mix. I don't really want to mess with the floats. They are set at factory spec, as recommended on the Ztherapy video. Anyways, what I more want to know... as I KNOW there's more than a few of you guys on there that run the SM needles, is if you guys get these types of results on your spark plugs also? Also I was wondering if running the car hard then shutting it off w/o letting it idle gave much different of a result? How long do you have to run it hard? I did a few 'passes' in low gear/high rpm on an industrial street (probably about a mile of driving) and pulled them and they looked the same. Quote Link to comment Share on other sites More sharing options...
gnosez Posted October 23, 2002 Share Posted October 23, 2002 I know exactly what you're going thru w/spotty plugs and A/F mixtures on those ZTherapy SUs. I have the 2" bored out version on my Rebello 3.0L and have been driven crazy by the amount of tuning time and mental abuse I've suffered thru. Yeah, I know, get a turbo or go EFI, but that's not why I got a 240Z. Here's what I did and where I'm at as of today...I installed two O2 sensors in the "Y" collector of my exhaust and have LED units on a velcro board located on the passenger kick panel. I have a small box w/velcro for when I want to see the readings either during driving or tune-up (the wire for the LEDs is that long. I tune the car to spec (w/BP7ES plugs - normal usage and 8s for the track)and check the LEDs at 45/55/65/75/85 and 105 mph. I have found that the readings are perfect for speeds over 70 but under the car runs too lean. It's all in the needles. A 1/8 turn of the adjustment screw too lean is enough to completely f$%&*k-up my car. I run mine 1/8 to 1/4 turn rich to cover all the bases. If you can document the AFM at different speeds on the same day, you should be able to get ZTherapy to grind a set of needles to your requirements. Or you can just give up a little power and run them a bit rich. Good Luck... Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 23, 2002 Share Posted October 23, 2002 I know.. that's what I really want to do is get a good sensor setup with the digital readout on my dash to see exactly what it's doing at all times. Right now I'm tapped for cash though, and will probably be for some time. This car has already been a very expensive hobby and has put me a bit deep into debt! So right now all I have to rely on is my own eyes, ears, and seat'o'the pants dyno. Oh and my trusty stopwatch To tell the truth I'm happy with the way the car is running now but all that black soot on the plugs is making me worry. My first ZX had a really bad fuel system problem and ran WAY too rich... I eventually had the engine from it rebuilt and when we took the head off it was nasty... a very thick, rock hard layer of carbon from a year of driving it like that (not to mention how long the previous owner drove it that way) had essentially turned the stock dished pistons into flat top pistons! No wonder the car felt so strong I'm just wanting to know if these spark plug 'symptoms' are something I should worry about. Oh, to 3L240, now that you've got it tuned good using your mixture indicator setup... what do your plugs look like now? Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 24, 2002 Share Posted October 24, 2002 I want fuel injection. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 25, 2002 Share Posted October 25, 2002 Hey LJ I'll trade you my EFI equipment from my '80 ZX for the turbo, manifold and efi from your car Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 25, 2002 Share Posted October 25, 2002 Don't think so. I want 300ZX fuel injection to replace my SU's. Quote Link to comment Share on other sites More sharing options...
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