Guest bastaad525 Posted October 22, 2002 Share Posted October 22, 2002 Another SM needle question for ya. I've read three different 'sub' methods for tuning SU's that are SM equipped. Some people say just tune them for the fastest smoothest idle and this should give you the perfect mixture (or use another tuning method like Norm's 'lift the piston' method to get your mixture dead on at idle). Some people say that this will give you the perfect mixture at idle only and to do the 'lean-drop' method which is to tune for the fastest idle, then lean the carbs out just enough to drop your idle 50rpm or so. While some other people (currently, myself included) seem to think that tuning for the perfect mixture at idle will actually result in running LEAN under load, and that the only proper way to tune them will result in you running pretty rich at idle. I have tuned my car on a smog places computer, under load, and it SEEMS that this third example would be the case, at least in my car. My main question here is what experiences other guys have had with SM needles on their SU's and what is the usual thing with them or what to expect. This also relates to my earlier post one or two threads down about strange spark plug readings. Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 22, 2002 Share Posted October 22, 2002 I have a set of sm needles, and they are a pain to set. If I set them for the fastest idle, I have problems keeping the car running until it gets warmed up good. Stock one's are better in that regard, but I have to really move the adjustment knob to affect the idle. Sm's affect the idle dramatically with little adjustment. I need fuel injection. Quote Link to comment Share on other sites More sharing options...
katman Posted October 22, 2002 Share Posted October 22, 2002 The only real way to go is to get the idle smooth, which is usually kinda rich because of the nature of L series combustion chambers and intake tract, and use a wide band O2 sensor for everything else. After hours and hours of needle shaving and experimentation you'll discover that a) carbs suck, and over 4000 rpm the only way to affect the mixture on an SU is by changing the exaust system since the needle is already out of the jet. No substitute for a dyno day with the right instrumentation. Best of luck. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 22, 2002 Share Posted October 22, 2002 Have your vacuum advance hooked up helps with the idle also Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 22, 2002 Share Posted October 22, 2002 I want fuel injection. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 23, 2002 Share Posted October 23, 2002 Heheh LJ well I held onto my EFI manifold from the ZX along with all the injectors... I may still go that route one day. But for now the SU's FEEL great, and give me no real problems. Start up fine in the morning, with no choke, runs good cold hot or otherwise. Feel plenty powerful and even better since I richened them up. It's just these weird reading on the plugs. Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted October 23, 2002 Share Posted October 23, 2002 Bastaad my way of tuning is not done to get the best smoothest idle. That's easy to do, just lean the sucker out. No, my way is to get a balance between a liveable idle quality and a NO SPIT or Skip from the carbs when the throttle is floored with the car in neutral. A little extra fuel mix does give ya more power, but without an MSD unit the plugs will tend to foul if the car is left idling very long when using performance cams. Later,norm Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted October 23, 2002 Share Posted October 23, 2002 BTW, the weird richer mix on 3 and 4 is due to the intake manifold design whereby 3 and 4 tend to run a little rich. When I switched to the N36 intake manifold oddly enough my 3 and 4 plugs started looking just like the rest of the plugs. Later,norm Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 23, 2002 Share Posted October 23, 2002 Hey Norm Yep I had read that way back in one of your posts about the N36, when I was trying to figure out if they really made more power. I'm definately planning on installing some when I can, whether they make more power or not I like the idea of even running cylinders I've tried the tuning method you posted on Zcar but my car doesn't react the same as what you posted. For instance, if I lean one carb all the way out then raise the other piston the car won't die. Maybe your floats are set at a leaner position I don't know. Also, whenever I adjust the tuning on my carbs I do the neutral rev thing like you say and it never coughs or stutters. And my tuning has covered a pretty big range from too lean to too rich. Only at the very leanest settings would it cough occasionaly or sometimes backfire thru the carbs when suddenly rev'd. So Norm what do YOUR spark plugs look like when you pull them??? Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 23, 2002 Share Posted October 23, 2002 Oh btw I do run an MSD-6A and Blaster 2 coil and still getting fouling on the plugs but I think the fouling is occuring at idle, since I drive in city traffic every day I spend a lot of time just idling. Really need to hit a nice stretch of highway and cut off the motor clean and then have a look... Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted October 23, 2002 Share Posted October 23, 2002 My plugs look kinda light brown. Later,norm Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 23, 2002 Share Posted October 23, 2002 you should have stuck with EFI Have you try to drop the fuel pressure down alittle? Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 23, 2002 Share Posted October 23, 2002 Yeah well... I WANTED to stick with my stock ZX EFI but it would have cost me like another grand to have it swapped over and wired and all that plus a 280 fuel tank and pump. I was already out of money... and anyways the FI system itself wasn't running quite right either when in the ZX... had lots of little problems that several Z mechanics and even the Nissan dealership couldn't figure out, so I figured better to start with a clean slate and get the rebuilt SU's. And anyways people keep saying 'shoulda went EFI' or 'why didn't you stick with your EFI' well to you I say DUDE the damn SU's run GREAT, get great gas mileage, and start right up every day with no choke. I don't think odd spark plug readings should equate to 'dude you need EFI'!!! Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 24, 2002 Share Posted October 24, 2002 Well I don't have an MSD yet, although I was looking for one on ebay the other day. If I could get around the freakin resistor, I would be way ahead of the game. Norm you also have a bigger engine then I do. Maybe I should run some sort of heavier fluid to dampen the pistons. I am using ATF now, again per Z therapy. I have the N36 intakes too, and the K&N filters, but no velocity stacks yet. I think I even have a set of the more valuable 3 bolt SU's. They are not like the other one's. They have a big adjustment knob but it is thin, and there is a tab that keeps it locked in place. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 24, 2002 Share Posted October 24, 2002 ATF is thinner than 20w. I'm just not well educated on the SU's or any other carbs. LJ, check the post in the ignition Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 24, 2002 Share Posted October 24, 2002 LJ I thought I'd need heavier weight oil also since my engine is technically and L29. I started using straight 30w but then tried Norms trick of just taking the dampers out and it made a huge difference. Indicating that the oil was too thick. Anyways I went back to putting oil but this time ATF and the car runs better now. I thought the four bolt SU's were the more valuable ones, as they were the ones on the original 69-70 Z's? By the way LJ I've heard from several people that running without air horns/velocity stacks/whatever they're called can be really hurtful to performance... Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 24, 2002 Share Posted October 24, 2002 I am pretty sure the 3 bolt ones are the valuable ones. Mine have the drain on the bowl which according to what I was reading on the ZT website makes them the rare ones. I need to get a set of air horns. Victoria British has them and they are cheaper than anywhere else, but I haven't tried them. I like the ones MSA has on the website to. I need to order a set. Quote Link to comment Share on other sites More sharing options...
Dragonfly Posted October 24, 2002 Share Posted October 24, 2002 The value of the 3 screw carbs vs the 4 screw carbs is negligable, the differance between the two which is why ZTherapy will not let you exchange 3 screw for 4 screw and vice versa is that on the 4 screw the brass sleave that holds the jet in place is threaded and therefore adjustable while the 3 screw is pressed into place and can not be adjusted. I talked to Scott Brunning in person at length about this and he explained to me that the pressed in sleave will sometimes be pressed in at an angle (not perpendicular to the bridge) which causes the jet and the needle to come into contact with each other creating abnormal wear and poor behavior. My personal recomendation is that if you have the 3 screw carbs pull the piston and check the needle to see if it has been rubbing on the jet, if it has and they are ZTherapy carbs you can exchange them. Dragonfly Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 24, 2002 Share Posted October 24, 2002 Well then that sucks. I have a set of 3 bolt and 4 bolt, and the 4 bolt ones don't run as good as the 3 bolt ones. I have never seen 3 bolt one's that I can recall until this set I got. I did drop test them before I put them on, and they were fine, so I don't think I have any contact. I got to see about some velocity stacks now. Anyone tried the ones from Victoria British? They are alot cheaper then the one's from MSA. Wonder if they will fit under my aircleaners? Quote Link to comment Share on other sites More sharing options...
Dragonfly Posted October 24, 2002 Share Posted October 24, 2002 The 3 screw carbs are the newer style and the reason for the sleave being pressed in is to remove an adjustment that should never be made by a novice. Every thing about the 3 screw carbs that are different from the 4 screw are there to make the carb more user friendly, so as long as yours are not hitting the needle against the jet they are going to be easier to set up and tune. The flat thumb wheel with the stops are so the mechanic who has never seen those carbs before can not screw up the settings as bad, with the thick thumb screw you can turn it down until the jet falls out the bottom. It usualy takes me about 20 to 30 minutes to undo the 3 hours of adjusting some one else has done to a set of SU's then less than 10 minutes to properly adjust them using a color tune and a flow gage (manometer). Since you have two sets the set with the best (condition) throttle shafts will be the set that runs the best. Dragonfly Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.