datsunlover Posted July 10, 2004 Share Posted July 10, 2004 Ok, my car; 280z with 72 su round tops, 280zx dizzy with vac advance disabled. Fuel pump out back (generic aftermarket) riged to a switch inside and it pushes about 5 psi throught the stock feed line. At the carbs I split it 3 ways; one line to each carb, and one to the stock return line to the tank. What it's doing; At any rpm (ussualy while driving along though under light to heavy throttle) it suddenly bogs out and drops the revs. I can put in the clutch and keep it going, but barely. (it revs VERY slow and sound like it is really strugling) Sometimes it sorts out and goes away, sometime it stalls completly. It's like it starves for fuel... I'm thinking the filter gets cloged from sediment in the tank, and then the car runs off the float bowls for a minit before I notice anything. I have taken to killing the pump when the car bogs out (for a few seconds) and turning it back on. This ussually catches it within 20 seconds or so, and the car takes off again. (I figure the loss of presure lets whatever's cloging the filter to drop out, and then the fuel can flow again..) But I don't know if this is even reasonable... I thought maybe my zx Dizzy was craping out... but I'm leaning hevily towards a fuel issue...either the fuel itself, or possibly the carbs... it does run very rich, but if I lean it out any, it wants to stall when I get on the throttle and it backfires through the carbs. Can anyone help me on this? Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 10, 2004 Author Share Posted July 10, 2004 Just a litle update; it seams to do it EVRY time at WOT in 3d and 4th around 3500-4000rpm. Just hits a wall. I can rev out 1st and 2nd up to 5500 and get into 3rd, but then it dies out... please hep someone!!! Quote Link to comment Share on other sites More sharing options...
THUNDERZ Posted July 10, 2004 Share Posted July 10, 2004 I would double check to see that the fuel pump is keeping up at those revs. You can hook a cheap fuel pressure gauge in line and run an extension fuel line out of the hood so you can see the gauge when you drive. If the fuel pressure dives at those rpm's you know where the problem is. Absolutely check and or replace your fuel filter. If it is really dirty or clogged you most likely have some crud in the gas tank. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 10, 2004 Author Share Posted July 10, 2004 Yah, I was talikg about it with a friend (the guy who helped me prety much all along the way with this car) and we came up with a thought about that... I was thinkin of running the pump straight to the carbs (remove the spliter that has one line to each carb and one going back to the return line) and run the vent lines (off the float bowls) to a 'T' and then to the return line.. maybe it will keep presure better, and if it gets TOO much presure and blows past the floats, the gas will just go out the vent lines to the return... Or does this sound dangerous? I'd hate for my car to go up in flames the first week I have it out... Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 11, 2004 Share Posted July 11, 2004 The stock fuel rail goes to the carbs first, then has a smaller T at the end of the fuel rail with the 1/4" return line. You should definitely not have your return line plumbed in before the carbs. I seem to remember that SU's can deal with 5 lbs pressure just fine, but it's been a while since I've run them. How long does it take from WOT til the car starts to die? If your float level is set correctly I'd expect it to take a little while, because you'd be running off of that fuel and you'd start to stumble when that fuel got used up. I'd fix the fuel hoses right away and see if that helps, and if not then I'd check your float level. You can pull the fuel reservoir covers off without screwing up the gaskets if you're careful. Don't run the vents to the fuel return line. More than likely you'd end up pushing gas into the vents. If you have an electric pump all you need to do is switch it on with the engine off and wait. If fuel starts pouring out of the carbs, then you know that they won't hold the 5 lbs pressure. If it doesn't leak with the engine off it shouldn't leak with the engine running. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 11, 2004 Author Share Posted July 11, 2004 Well, as for time, not long.. Maybe 8-10 seconds? From an idle, I can get through 1st (shift at 5500) and through 2nd (go to third about 5000) and almost IMEDIATLY starts to sputter and choke up.... thing is, it's getting worse.. I just got home and as I opened my garage door, the car was ideling fine.. but suddenly sputtered and stalled. The whole way home, it would kick in and out in evrything from 1st to 4th, at any point through the rpm range. All I can think of is this; Posibly, the floats are sticking slightly.. so once the bowls are filled, they stick closed. At this point, the way I have my return line plumbed, the fuel goes the easiest route.. to the return. This causes less presure to the carb fuel lines, and with the floats stuck closed.. I run off the bowls till the car dies. I think going through 2 gears at WOT would probly suck back the whole bowl very quickly.. And here's the dumb part; I'm kinda ignorant about these carbs (hell it's the best way to learn... put them on and start tinkering! right? ) and I don't know where the floats are set. I don't even know HOW to set them, and WHERE to set them to! Also, ah... how many turns out should the needles be initialy? Cause mine are about 1.5 out, and car runs WAY rich.. just drinking the fuel! I will change those fuel lines tomorrow and see what happens... Thanks.. and any other info/hints that I might be missing would be great! Quote Link to comment Share on other sites More sharing options...
THUNDERZ Posted July 11, 2004 Share Posted July 11, 2004 I am thinking that you need to start eliminating things as far as what the real problem is. Check the floats, check the fuel pressure, maybe your relay for the fuel pump is going out. Double check to see that you dont have a loose wire somewhere that could be grounding out your spark. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 11, 2004 Author Share Posted July 11, 2004 Well, the pump is on a togle switch, so no relay... and it's new, with new wire and no breaks in it... Im going out there now to tinker with it.. I'll see what I can find.. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 11, 2004 Share Posted July 11, 2004 General rule of thumb is 3 turns out for the adjusters on the bottoms of the carbs. When you pull the choke on the SU's, the nozzles get moved down, which richens the mixture. It is very very common for them to get stuck down, causing a very rich mixture. I would suggest you just stick your hand down there and push up on the bottom of the adjuster and see if they move back up. If they were stuck down, that was the cause of your rich situation. You really ought to buy the video from http://www.ztherapy.com. Its cheap and it will answer all of your questions. I think you're right that you're probably running off of the float bowl fuel all the way to 3rd or 4th. Fix the return line thing and see if that helps. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 11, 2004 Author Share Posted July 11, 2004 Fixed! Well, sort of... I did check the choke assy. and they both work fine. No sticking or running rich because of that... I made a slight adjustment to the floats (trying to get as much fuel in as possible befor they close the valve... is that right?!?) As for the fuel lines, I swiched things around a bit, but it still did the bog/starving thing... so I did the redneck thing and clamped down on the problem. Literaly. I took a small pair of vice grips, and gently clamped them on the return line (which is now AFTER the carb feen lines) and went for a test run. On a back road I ran it up hard through the gears, and saw an honest 110 MPH in 4th and it was still pulling!! I lost my nerve though as I was running out of road in a hurry! (it's a side road thats quite straight but only 1/2 mile or so long) So now I guess I'll have to build/rig some sort of fitting to replace my 'backyard hick fix'... plus I like those vice grips.. I use them for other stuf sometimes so.. Anyway, I still have to replace my crudy air filter and set the carbs properly (I played with the wheels too, but they're only about 2 turns out now..) but it doesnt stall out anymore! I do plan on getting that book too.. heck, if I go with my present plan, I'm gona be ordering a set of overbored SUs to go with a hot cam and head work I want to do this comming winter... it's all time and $$ though! Anyway, Thanks a bunch guys! Quote Link to comment Share on other sites More sharing options...
THUNDERZ Posted July 11, 2004 Share Posted July 11, 2004 Sooo... Is it that since the return line is clamped and the system is essentially "dead headed" that it works? If so that means that your fuel pump is not adequate for the system. : Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 12, 2004 Author Share Posted July 12, 2004 Yah, I guess thats the whole problem then.. looks like I'll need to buy a better pump sometime.. but the one I got cost $60 and it works for now, so I'll leave it alone. It is 'suposed' to be 5-6 psi which I though would be the max I could jam into these SU's... but I'll have to get a presure guage and plumb it in there to see for sure what I'm getting. I still have to tune the carbs, change that dirty air filter and set the timing properly, as I am still running way too rich. Once all that is taken care of, it may be fine... but if not, then I will address the issue of 'what to get' for a decent fuel pump that won't starve my poor tired engine! Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 12, 2004 Share Posted July 12, 2004 The stock return line is 1/4", smaller than the feed line. I think the system needs a restriction there. You could run a smaller return, or you could just run it dead headed but the pump will die sooner with it dead headed. Maybe some others will chime in, but I'm not convinced that you need another fuel pump. Quote Link to comment Share on other sites More sharing options...
THUNDERZ Posted July 12, 2004 Share Posted July 12, 2004 I am not saying to run out and get another pump. I would absolutely get everything else set correctly first. I think jmortensen is on the right track. If you are using a stock return on a 280z it probably doesnt have a small enough inner diameter hole to hold pressure at the carbs with a low pressure low volume pump. Check the line that you are using for return. On my 260 the return line looks like it is the same diameter as the supply but upon closer inspection it come to a head and really only has a pin hole that returns fuel to the tank. Check it out and let us know. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 12, 2004 Author Share Posted July 12, 2004 Yah, thats what I was thinkikng anyway. It is the same size line, as my 280 had FI and I am just pushing fuel back throught the stock lines. I will try to rig up some sort of pipe w/smaller id. to serve as a restriction before the rueturn line.... after I do the other nessesary work to the car of course! I'll let you know how it goes.. it might be next week before I can throw any money at it again though.. darn bills and credit piling up... Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 17, 2004 Author Share Posted July 17, 2004 Well, I havn't fixed the fuel ine properly yet (vice grips are working well for now) but I'm having some more weird issues. I messed with the carbs a bit today, and tried to tune them a bit.. I think I've got them pretty close as far as syncronising (by ear) but I'm still WAY rich and stinky. I now have an awefull stumble/hesitation as I get on the throttle, and have to rev it a bit more than I like to get her going in 1st. I can hear the carbs suck, but it's like they starve for fuel for a second.. if I give it a little blip up to 1500 or so cleans it up, I take off, and it pulls hard right up to red line in evry gear. Trying to cruise slow though, it feels like it has a dead spot between 1500-2000 or so.. I push and push in the throttle (slowly, bit by bit) trying to maintain a steady speed, but it suddenly will kick in and ..wel.. GO! It's fine when I wana drive like an idiot, but not so good for rolling slow down the main drag.. ya know? I was thinking a new (read; CLEAN!) air filter would help a bit, but it seams to be more of a fuel/adjustment problem than just a 'lack of maintinence' issue IMO. Something else weird.. If I cover up the vent tube sticking out of the front carb, it will sputter, and stall. But not on the rear carb.. Any ideas? Quote Link to comment Share on other sites More sharing options...
A. G. Olphart Posted July 19, 2004 Share Posted July 19, 2004 Sounds like you are struggling a bit--- Perhaps (maybe?) these will help: http://www.team.net/sol/tech/su-tune.html http://www.vtr.org/maintain/carbs.html http://www.zcar.com/forums/read.php?f=1&i=152857&t=152847 It has been years since I worked on SU's personally; hope the links do some good.. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 19, 2004 Author Share Posted July 19, 2004 Thanks man, I actually was browsing the first two of those links last night and today! I am still fighting with the thing, but she runs better evry time I fiddle with it. I fixed the stumble (didn't have the carbs opening at the same time, and they wern't sync'd. Front carb was at about 1/4 open before the rear carb would start to open! ) but it still idles rough and runs WAY rich. I cant figure it out.. I assume a bit if it is caused by my dirty air filter (got a K&N on order) but it is really quite awfull.. big cloud of smoke when I gun it.. I roll up to stop light, and as soon as i stop, I get 'passed' by this white cloud of smoke! I dono.. I'll keep reading and messing with the thing and hopefully it'll get better! Quote Link to comment Share on other sites More sharing options...
Guest SeanyD300zxT Posted July 19, 2004 Share Posted July 19, 2004 along the same lines that you're thinking with the vice grips, but you might wanna consider clamping a welding tip inside the return line, depending on how much of a restriction you're looking for. I can't quite remeber, but I believe a stock FI return line is 1/4 inch? someone will catch that, but if upping the pressure helps, then you could use just a welding contact tip, and if its too small, just up the hole in the tip, using a drill bit or whatever. Just some thoughts...hate to see you lose a good pair of vice grips on the road!! Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 19, 2004 Author Share Posted July 19, 2004 Hmm.. I like that idea actually.. thanks man! And yes, it's bothersome to have a good pair of vice-grips stuck in the engine bay.. I really needed them the other day too.. I'll look at 'borrowing' something from my boss's shop.. or just turn something on the lathe.. as soon as its hooked up. (just moved shop, still sorting out stuff, unpacking, ect) I'll let you know how it works out. Quote Link to comment Share on other sites More sharing options...
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