briann510 Posted October 2, 2004 Share Posted October 2, 2004 this engine has 20K on the stock rebuild. Has a 3.90 rear gear and 5 speed tranny with dual Z Therapy SU carbs and a 3/2 header. The car just isnt that quick. I mean its not suppose to be blazing fast but it just feels like there is no horespower and it really doesnt start to get up and go till about 4000 rpm's. this is a stock cam so i would think it would start the get up and go way sooner, especially with the 3.90 rear. it idles fine and starts up fine but just dont think its all there. Just the way it is? carbs out of adjustment bad? where to start just for some quick checks? Quote Link to comment Share on other sites More sharing options...
datsunlover Posted October 2, 2004 Share Posted October 2, 2004 Well if the carbs were off that much I'd think you'd have other issues; backfiring/poping out carbs and exhast, rough idle, sputter on acceleration, ect.. Have you done a compresion check lately? OR... check the timing? Could be it isn't advancing enough (bad points/vac advance?) or is set incorectly.. Quote Link to comment Share on other sites More sharing options...
briann510 Posted October 3, 2004 Author Share Posted October 3, 2004 my timing is advancing properly and i have it set at 17 degrees initial at idle. i need to make some marks and see where i am at total advance at higher rpm's. it idles smooth and the vacuum is about 15 inches at idle. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 3, 2004 Share Posted October 3, 2004 cam timing? Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 3, 2004 Share Posted October 3, 2004 Couple things... why is your timing so far advanced? Timing should be in the 10* BTDC range, I used to run 8* when I was running high compression and SU's. Unless you've recurved your distributor so that max advance is still around where it was stock, than 17* is too far advanced. Though it's rare, I've seen instances where TOO MUCH timing did hurt power noticeably. Secondly, I agree it would be a good idea to check cam timing. Third, I'm not sure but isn't 15 in. Hg. vacuum a bit low? I was under the impression that all Z engines tend to pull closer to 20 in Hg at idle... odd for a rebuild to pull so low Now the big question, surprised no one asked this... what oil are you running in the dashpots of the SU's? This factor can make a HUGE difference in the onset of power. Most people recommend running 20w oil in there, but this wont be the case for everyone. I actually had a much better power curve on the dyno when I completely removed all the oil from the dash pots and ran them w/o it. Some people have had better luck with thicker oils, some people have had great luck with auto trans fluid. Experiment... you'd be surprised how much difference in driveability this can make. you said you have new Ztherapy carbs. Did you install them yourself? did you check them for proper float adjustment and that the needles were seated into the pistons evenly? Mine were actually quite a ways off in couple aspects when I got them shipped new from ztherapy, luckily I checked them before isntalling them. The great thing with the SU's is that a pretty thorough teardown only takes a couple hours at most... if you're unsure, take them apart and check that the needles are seated evenly, that the floats are adjusted evenly and correctly, put them back together and retune them for fastest, smoothest idle. What needles are in the carbs? you said it was a stock motor rebuild, so a 2.4L motor yes? Most of the time Ztherapy will send out their carbs with the SM needles, which are good for running SU's on a 2.8L, but are probably a little too rich in mid and top end for the 2.4, which is why they are ideal for the 2.8 as it needs more fuel over what the stock 240 needles can provide. Quote Link to comment Share on other sites More sharing options...
briann510 Posted October 3, 2004 Author Share Posted October 3, 2004 i dont know much aboutthe carbs, only that they are fairly new from Ztherapy and have the 4 nuts rather than the 3 which i heards wasthe better older carbs. i will email the guy we bought the car from and see what if anything was checked on the carbs beforethey installed them. dashpot oil and have no idea what weight oil is in them. im not even sure how to check that as SU carbs i have never dealt with before. i think i can check the dashpot oil by unscrewing the top black nut? if the oil is wrong how do you drain it out of there and i assme you just pour the new oil in from the top? tearing them apart might only take a couple hours for someone thats dealt with them before but this is a daily driver and for someone like myself to tear them apart and back together again when car is needed everyday to drive might be a problem if i have issues putting them back together etc.. ill put the timing back down to spec and see if that makes a difference. thanks Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 3, 2004 Share Posted October 3, 2004 The two black plastic knobs on the top are dipsticks. You unscrew them and pull them out, and about 2" or so up the little stick (above the big thingy on the end) there is a high and a low line for oil. THEY ONLY TAKE A LITTLE TINY BIT OF OIL. I used to fill the cap for my Marvel Mystery oil can and put it in from there, and they would only take 2-3 little capfuls. If you overfill it is not the end of the world but it will smoke a bit until it burns off as it will just get shoved out the top of the tube when you floor it the first time, then it goes into the engine and burns away. As Bastaad said, most people get the best results with 20 weight oil. I've had friends who used 20-50 or 15-40. I used to use ATF, then Marvel Mystery oil, which is much thinner. You can play around with it and see what works best. I put ATF in my friend's SU's and it would cough and sputter pretty bad at low rpms. I think a lot of it has to do with how worn the pistons and domes are. The more worn the thicker the oil needed. I still think something else might be wrong though. I always thought the oil was done doing its bit at 2500 or 3000 rpm. Maybe I'm wrong. If you don't have a timing light that can read your total advance, I suggest you look at your distributor and get the number off of it. Then cross reference it here http://home.att.net/~jason510/dizzy_specs.htm and find out how much mechanical advance your dizzy has. The measurements on that webpage are in cam degrees, so DOUBLE them for crank degrees, and aim for about 35 total advance. Also, I would suggest you disconnect the vacuum advance if it isn't working or if you drive the car hard. Plugs a potential vacuum leak and those diaphragms in the dizzy are almost always dead anyway. Quote Link to comment Share on other sites More sharing options...
briann510 Posted October 3, 2004 Author Share Posted October 3, 2004 i just checkedthe dampner oil and it was below the lower line. i also moved the timing back to 10 initial with vacuum line disconnected, and as soon as i connected the vacuum line back up the timing jumped up to about 17. the idle is at 900 so i guess its already starting to pull advance. i have the idle as low as it will go using he fast idle screws up top, and im nervous about goint to the nut under the carb to get it down further cause if i dont get both carbs right on i might have more problems than before. any ideas? comments? thanks Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 3, 2004 Share Posted October 3, 2004 Apparently your vacuum advance is working. That's kind of surprising. Vacuum advance will give you better mileage and better throttle response at low rpm in a stock setup. If you run the car hard you should disconnect it, reason being at the end of a high speed run in 4th gear at 5000 rpm for example you'll actually start to get some vacuum again, and it can advance your timing at the worst possible time and cause pinging. If you disconnect it then you are going to use just the mech advance. On the 240 dizzys I want to say that they had something like 24 degrees of mechanical advace, so if you disconnected the vacuum you would need the timing set at about 10* total. I don't know this for sure, which is why I suggested you look it up for yourself. 10* advance at idle runs kinda crappy compared to 20* at idle (which you've already learned is what the vacuum advance does for you). So there are ways to limit the total mechanical advance so that you can run closer to 20* at idle and still get to the mid 30's total. You can close up the slots inside the dizzy so that the timing can't advance so far, or you can use a different dizzy that has a smaller amount of mechanical advance (280ZX has only 17* and is electronic, best of both worlds). If you don't want to get into all that right now, then you can either set the timing at the stock factory setting and listen for ping at high rpms, or you can disconnect the vacuum advance and set the idle so that you get into the mid 30's total advance. The latter is safer for racing or really high rpm driving, the former is going to give you better around town driving and gas mileage. Quote Link to comment Share on other sites More sharing options...
briann510 Posted October 3, 2004 Author Share Posted October 3, 2004 Thanks! its hard as heck to tell when the dashpot oil level is right cause i can hardly see the oil on the little stick. i added some oil to both carbs and it is between the 2 marks as close as i can tell. at my age my eyesight sucks! the idle is at 900 and i get 16 inches of vacuum and the timing with vacuum line attatched is about 17-18 degrees at that idle (if i unhook the vacuum line the timing drops back down to 10 degrees so you are right in sayingthe vacuum advance is already coming in at 900 rpm's. this distributor number (D612-54) wasnt listed in the above link so im not sure of the specs. i cant get the idle any lower unless i start adjusting those nuts under each carb, but i think if i do that i might make matters worse as i think those nuts are actually idle mixture also? im not sure if its striclty a idle rpm nut only or if those nuts actually changes the mixture also. i have been adjusting the timing using turning that phillips head screw and turning the cap section instead of loosening the actual distributor housing bolt and turning the whole distibutor as a unit. does this matter? took the car for a spin and it ran ok, but couldnt really tell of any big difference from these new timing settings if it is any different feeling. thanks for the comments and suggestions Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 4, 2004 Share Posted October 4, 2004 Well with the oil, in my oppinion it's better to just overfill it and let the carbs 'self balance' themselves. The first time the pistons go all the way up to the top they will expel any extra oil and set themselves to the full line. You can do this yourself by simply filling them and then pushing the pistons up to the top of their travel by hand (not with the motor running). Yeah so the motor will smoke for a minute as it burns off the oil, no big deal... at least you will know w/o a shadow of a doubt that they are filled to the right level. I used a little plastic syringe with a piece of rubber hose attached to drain them when I was trying different oils. Depending on what oil is in there the pistons will be fully up at WOT at around 4000rpm... with a thicker oil they will rise more slowly, causing the car to run more rich at first. I say drain them and put ATF or Marvel Mystery Oil in there. Or, one thing you can try is the "Norm Method" just to see how it affects driveability and if this may be your problem. Pull out the little dipsticks, leave whatever oil is in there, in there, just leave them open and go for a drive and see how it feels. This probably will NOT be the best way to run yours but it's an easy way to see how it will effect driveability... Norm runs his this way all the time and loves it. I strongly suggest you get the "Just SU's" video from Ztherapy if you don't have it. These carbs are SOOOOO easy to work on you'll really be surprised. When I said it only took a couple hours to tear them apart and check all that stuff... this was the first time I had ever even touched a set of SU's... I just watched that video a couple times and followed it's directions. You dont even have to remove them from the car. Tuning them is a snap as well, but you will need the little airflow measuring tool, which I believe you can get from Ztherapy as well for about $20. If you call and order just the tool you MAY be able to talk them into sending you the video for free. A regular tuning session with SU's averages about 30 minutes... it's reeeeally easy and the video covers this as well... watch it once or twice and you'll be able to tune them yourself no problem. I wouldn't disconnect the vacuum advance either, though I will say, it's odd that yours is having so much effect at idle. My car ran noticeably better during normal driving, mid throttle and throttle response was better with the vacuum advance CONNECTED. On top of that, for a long time I left it disconnected, and every so often I would get a bad backfire thru the carbs if I suddenly snapped the throttle open... I connected the vac advance and this NEVER happened again. If your vac advance canister is leaking or stuck it's definately worth replacing. My car did not "run kinda crappy" at 8* of initial advance... on the contrary it put down 170hp to the wheels and ran great in around town driving, and was my daily driver. I would never say it felt sluggish at low RPMs until I installed the hot cam, which caused power to not start to come on until about 3000rpm. I will say, if you do eventually want to mess with the advance curve it's not too hard to do, and a lot of guys do prefer running the higher initial advance, I myself never tried it. Quote Link to comment Share on other sites More sharing options...
briann510 Posted October 4, 2004 Author Share Posted October 4, 2004 i backed off the initial timing with vacuum hose unplugged from 18 down to 10 degrees. it didnt seem to affect much of anything, even the idle and that might be because when i hooked the vacuum line back up it jumped the timing back up to around 17 degrees while idling around 800 or so. i need to find this distributor number in a datsun book and see what the advance characterists are for it. i really need to find out if at 800-900 rpm's if the advance actually is already in 10 degrees or so, or if i have some weird distributor problem with my advance. all this really affected i guess was the total timing when fully advanced which i figured now im running 8 degrees LESS total timing when full advanced. i might have been way over advanced with it before. i need to get a timing light with a advance knob on it and check total advance and just work from there. i think i will get an SU book or the video just to help me out. the idle i think is too high stillat 800-850 ish and the actual idle screws on the top sides of the carbs are already out and the idle wont go any lower that way. i think the only otherway to get the idle lower now is with the nut under each carb? i thought that nut though was for actual idle mixtures but correct me if i am wrong on that please. i know when the car is cold if you apply the choke it runs very rich and wants to die or belch out black smoke. it warms up easier with no choke and just holding the throttle open around 1500 rpm's for a couple minutes. thanks for your help everyone Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 4, 2004 Share Posted October 4, 2004 Definately get the video, "Just SU's" from Ztherapy... really good, thorough and useful video. The adjustment nuts on the bottom of the carbs adjust the fuel mixture throughout the entire rev range, not just the idle. Same goes for the float level adjustments. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 4, 2004 Share Posted October 4, 2004 My car did not "run kinda crappy" at 8* of initial advance... Yes, it did. You just never knew what you were missing. It's not as though the lack of advance is going to make the car stumble or miss. It just won't accelerate quite as well off the line and won't have as strong throttle response as it would have if it could have run all the advance that the engine wanted. We're talking a noticeable improvement here, but it is only going to be slower and more boggy from idle until the mech advance comes in, so above 3000-3500 it's still going to run great. Quote Link to comment Share on other sites More sharing options...
briann510 Posted October 4, 2004 Author Share Posted October 4, 2004 so with that said, what is "usually the optimum total degrees advance? i have heard around 35-36. of course depending on what has been done to the engine i am thinking for a basically stock engine what would it be? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 4, 2004 Share Posted October 4, 2004 I've heard anywhere from 32-38. If you go 35 you should be really close and it only matters beyond that when you're trying to eek out that last hp. Quote Link to comment Share on other sites More sharing options...
briann510 Posted October 4, 2004 Author Share Posted October 4, 2004 thanks. im either gonna order a timing light with a advance knob or just run the crank balancer around a couple time and measure it that way with the tab marks and add up a a 20 and a 15 degree spacing and mark the balancer. im still trying to track down my distributor number information and see how much advance at what rpms it came with stock, but cant find that info anywhere. 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.