Ok... I'm stumped.. SU's...

[datsunlover]

Looking for advice here.. am I missing something? I must be..

1975 280z, 2.8l fresh rebuild, slight bump in C/R over stock, .510 / 280* cam, 2.25" exhast, 'cherry bomb' glas pack. Cam 'should' make power up to 6500, but about 5000 it's falling on it's face. 280zx dizzy, bpr5ey plugs, 72 SUs, good shape, worked well last summer. The needles (in them now) were from an old hot rod'd 240 with cam/header so I figured they'd be close to what I need..

 

Carbs are poping/backfiring like crazy. Idles lumpy, but ok.. and takes off good if I tip in slow. Quick 'bilp' into the throttle though, and "pa POW" out the carbs. Yes, the carbs are synced properly. I've adjusted the carbs lean, rich, advanced timing, retard it, and NOTHING does any good. (I've tried all difrent combos here, and get pretty much the same results. More advance has a bit more "umph" but it runs hotter and pops a little more.. and richer just stinks a bit more) Last year it would run like this for about 2 minits after startup and after that, (once it was warm) it was perfect...

 

ALSO, just about EVERY time I shut it off, the stinking thing diesels! My idle's set about 750-800, I've backed timing off, and STILL it wants to keep going! This is really pissing me off, as it had no problems like this last year.. and I can't understand what I'm doing wrong here... Could it be that the SU's just arn't up to the task of feeding this motor anymore? It's bee quite hot/humid here the past few weeks too.. I'm sure thats not helping things.. I'm running 190 to 200* acording to the aftermarket temp guage..

 

On top of this, it now wants to stall sometimes.. Idling ok around 800, and it will slowly drop as if you're letting the clutch out in gear while holding the brakes.. I think this may be an ignition issue though.. planing to install my MSD box this week.

 

Any ideas?

[Thumper]

Most of your problems point to being too rich. I would first check your float bowls and fuel pressure. If those are good try leaning half to a full turn at a time. If your float bowls all alittle off you might even have to fully tighten/lean the adjustment nut. If you want to be sure you have the air/fuel ratio I would recommend a colortune. Its basically a spark plug with a window so that you can see the combustion chamber flame. The upper rpm studder sounds more like ignition then fuel. First try a set colder in spark plugs. 6's vs 5's. 6's are what is recommended from factory now anyways. Then check wires, cap, and rotor. Good luck

[technicalninja]

It's possible that the needles are installed to deep in the pistons. The shoulder should be flush with the bottom of the piston not flush with the groove it sits in. Loosen needle, place metal ruler across bottom of piston (across the bore - parallel to the throttle shaft). Pull needle down until shoulder hits ruler; tighen and check how needle goes into jet. Is it centered all the way down?. Some times rotating needle in piston can help an out-of-align situation.

I have also seen the jets worn on one side (by a bent needle maybe?) that required replacement.

I have also found a lot of "HOT" needles (especially ones that have been modified) are not that hot at all for normal street driving.

If you still have the original needles put them back in and see how it does.

I have found minor tweaking of the stock needles position in the piston was a far better way to "tune" the carbs to the engine.

Put mixture adjustment nuts in the middle of their travel and move needles around until you find the center of the "sweet range" the use nuts for fine adjustment.

This is all dependant on and related to float level. Make sure it is right before messing with needles.

New needles and jets (just stock stuff) may make you job easier. Your set up shouldn't need much change from stock. These carbs were set up fairly rich from the factory (Much more so than a 280's F.I.) and all carbs meter fuel dependant on airflow and have a CFM range where they are most efficient. If you put a higher CFM engine behind them their sweet spot moves down in RPM. They still meter fuel just fine and their limitations are only noticed in high RPM/high load range. I think the stock S.U.s are a great size for a mild 280 (under 7000 rpm) and will give better low range drivability that ANY other carb currently available. F.I. will beat them but not by a large factor.

 

Rick

[datsunlover]

It's possible that the needles are installed to deep in the pistons. The shoulder should be flush with the bottom of the piston not flush with the groove it sits in....

Rick

 

 

You don't say... Well thats something to check for sure, cause I remember wondering about that, but not knowing for sure I guessed and put them back where they originaly were, flush in the groove. Funny thing is, from what I was aware of, I had them set prety close.. I'd push up on the piston lifter pin, and the rpm would go up slightly then fall back down. And I could try my old needles, but one is bent pretty bad and gave me tyrouble last year whenever I used the choke (would stick) BTW, I have no idea what the original needles were.. the ones in there now say "27" on them.. ??

 

As for the floats, I thought I had them set prety good too, but again, maybe I'm doing something wrong. I used to have a link to a good SU page, that gave a rundown on seting floats/tuning them but I can't find it. (girlfriends computer, she cleans house sometimes and some of my stuff gets thrown out.. ) Anyone have a link? Or a quick rundown on how THEY set their floats?

[Thumper]

Here are three good su tuning websites. http://www.zparts.com/zptech/articles/mal_land/ml_sucarb2/images4/SUcarb_111601d.htm

 

http://www.jetlink.net/~okayfine/sutech.html

 

http://forums.hybridz.org/showthread.php?t=82067&highlight=carb+tune

Good luck

[JMortensen]

I'd suggest leaning your mixture and trying a different oil. I used to run ATF or Marvel Mystery Oil in mine. The heavier the oil the more of an accelerator pump action you get. Kinda varies from set to set too. A friend of mine had a pretty similar setup as far as engine and exhaust, but her SU's liked 20 weight oil, where mine liked a much thinner oil. I put ATF in hers and the thing stuttered really bad when you took off from a stop, much like you're describing.

[datsunlover]

Thanks for the links Thumper, I've bookmarked them. After reading through the second link you posted, I'm almost tempted to modify my springs now.. I have two sets (actually, two complete sets of carbs, one 4 screw and one 3 screw) and from what I can tell, the springs are the same so if I mess a couple up, no big deal.

 

I'm a little fuzzy on using clear gas line to set the floats though.. I had set mine so the tops of the floats were just about 1" from the upper edge of the bowl (can't remember where I got that number) and it seamed to work, but now I'm not sure thats right..

 

Mark a line on the sides of your float bowls that is 23mm from the top lip of the bowls.

Attach a length of clear plastic tubing (ala fish tank stuff) to the outlet on the bottom of the float bowl. Be careful, you're dealing with gas!

If you have a mechanical fuel pump, disconnect the coil. Electric pump guys can never mind. To make it easier on your starter, you should also remove the spark plugs.

Make sure your car is out of gear and the parking brake is on.

Turn over your motor (key to "ON" for elec. pump guys) until the fuel in the plastic line stabilizes. Compare this to the mark you made on the float bowl.

Adjust your float lever until the fuel in the line matches up with the mark on the float bowl. Lather, rinse, repeat.

Now go and reset the throttle balance, sync the carbs, set the mixture, and you're off!! Simple...

 

In particular, (underlined text) how do you do this? Hold the line up beside the float bowl? I feel like a bit of a dumba$$ now but I'm not understanding this..

[datsunlover]

MAN am I pissed off now.. I spent the last few days going over evrything, changed to new bpr6ey plugs, cleaned the carbs up, set floats as best I can, got both needles set right now, and it does run better.. BUT (theres always a but you know..) it still pops out of the carbs ocasianaly, which seams to be more when it's really hot.. AND it has a new trick; On shut down, it doesn't really diesel anymore, instead it pu-pops out the exhast now.. Not a backfire really, sounds more like a loud wet fart. pardon the vulgarity..

[JMortensen]

Carbed Z's sputter and pop on decel. Not much you can do about it, other than put FI in. Just happens. Happens more with bigger exhaust IME. Did you mess with the oil in the carbs at all?

[PeterZ]

It has been my experience that thicker damper oil will help the sputter. The backfire through the carbs is lean-misfire indicator. If the carb pistons raise quickly air gets through but the fuel is heavier so there's a temporary lean condition. The damper rods help control that. I recommend leave the springs alone. I haven't checked the recommended links on tuning so I hope I'm not repeating somebody else or sounding way out of line. One trick, if you are careful, is to understand the relationship of the suction piston (the big one) to the fuel meetering needle and change your fuel curve from idle (too rich-dieseling) through the RPM range where it is too lean (popping through the carbs) by changing the taper of the needle. Datsun used to sell different tapers but I learned how to reshape the needles for engines to be driveable and still pass the smog test. I spun the needles in a drill press and machined the needle thinner in the places I needed more fuel. A few thousands in the right places works wonders. There's a lot to deal with and an exhaust analyzer helps tremendously. You can tell which carbs are lean by gently passing two or three fingers over a carb opening with the engine in the RPM where the popping occurs. You will essentially block off the air causeing the carb to pull more fuel. The engine will get "happy" and run faster. Good luck. If this doesn't sound like $#!+ to you I would like to ask a few questions and add a few more comments before you start sanding your needles.

[datsunlover]

Carbed Z's sputter and pop on decel. Not much you can do about it, other than put FI in. Just happens. Happens more with bigger exhaust IME. Did you mess with the oil in the carbs at all?

 

 

Well the sputter and pop on decel is minimal, and I can live with that.. (actually, I kind of like it ) but this pop out the exhast is just after I turn the key off. Engine stops, and seams to 'want' to diesel for a moment but doesn't. Then, about 1 second later it farts out the tail pipe. Oil in the carbs is 20w motor oil. My next thing is to go to ATF and see if it helps out.

 

Now.. how high should the pistons be lifting on light throttle 'blips'? I'm talking from idle up to about 2000rpm. Right now they're lifting about .5" or so.. If I momentarily block the carb mouth with my fingers, it does not get happy either.. it idles down, and wantes to stall.. And the lean pop theory sounds right, but if that's the issue, then why are my old plugs blackened, indicating too rich on fuel?!?!

 

I'm scratching my head on this one.. starting to think about going back to FI.. well, some sort of FI. Maybe rig a 300zx harnes/ecu to my original fi intake.

[PeterZ]

With light load the pistons won't lift very high. I think you have way too much fuel at idle and not enough off-idle. Maybe your cam needs lots of fuel to idle but we had run some pretty big stuff and it worked out well. We didn't recommend boring the carbs for larger butterflies unless they were going on 3000CC or larger motors... There are some air control valves and solenoids that can cure the backfire with key off. Your intake popping may go away with thicker oil but everything gets thin when the engine warms up. I would set up carbs to run the same damping oil that the engine used. I believe ATF is 15W so you might actually notice more popping. Use no oil and see what happens!... Are you lightly blocking the carb inlet at idle or at 2,000? try it after dialing the high speed adjuster screw until you get to 1500 RPM adn then 2200 RPM or so. Another test: if you are too rich at idle the engine will run better for a moment if you slowly pinch off the fuel line to the nozzle, right before it starves and dies... The mixture can change a lot as the needle moves up out of the jet. I have seen exhaust readings go from 6% CO at idle (Way too rich) to less than 1% (way too little) in just a few hundred RPM. Sometimes this is due to wear. If the needles and nozzles (jets) are not centered properly (or they were bent at one time) they can wear causing a rich mixture at idle. When you turn the mixture screw to lean out idle you also lean out the mid-range (and the top end if you have to go really far). Dave at Rebello Racing engines used to grind a neat shape in the needle for better top end. I found the drill press works great for fixing idle and part throttle problems. The tricky part is the first 1/4" of the needle at the thickest part, where the step is. If you are so rich that your adjustment has the nozzle resting very near the step then the needle must travel very far (while the piston is going up giving additinal air) before the nozzle sees any taper and gives additional fuel. Check the profile of your old needles.

P.S. the original FI intake runner is smaller than the porting in the head. I don't think it would offer better top end. I cut one open and it was something like 7/8".

P.S.P.S. Make sure your tack is right, we had a customer complain of the same thing and his stock tack was low by almost 2K RPM at the high end. Ever see what happens to valve retainers over 8K RPM?

[datsunlover]

P.S.P.S. Make sure your tack is right' date=' we had a customer complain of the same thing and his stock tack was low by almost 2K RPM at the high end. Ever see what happens to valve retainers over 8K RPM?[/quote']

 

Well, I can imagine.. I just put a new tach in (aftermarket) and it's reading about the same as the stock unit was. I know the stock one was malfunctioning smetimes (would 'stick' around 4500 and read really slow) but with the aftermarket one, I still only get to about 5500 and it's screaming, but feels like it's falling off on power.

 

As I said, it does run a little beter now and if I shift around 5200, it drops down to 3000 or so and feels good.. (3000 is right where the cam starts to come on) but the shaft is suposed to make power to 6500 and I'm nowhere near that. From the way it sounds at 5500, I'm aftraid to push it that next 1k for fear of breaking something..

[PeterZ]

Keep an eye on the tops of the valve stems in the retainers. At the higher RPM the valve starts to sink into the stock retainers eventually slipping into the cylinder or breaking off. My racer was required to use the stock retainers. Until I put a rev limiter a missed shift would tack out to 8100 (Tack memory) and I could see the damage before really bad things happened. If you like your fuel delivery except for top end you can narrow the taper of the needle at the pointed end. For a top-end test, you can pull the choke lever to drop the nozzles slightly at WOT. That works well to diagnose a lean mid-range too. Remember to turn the choke off when you shut her down. Or just forget about the idle mixture and enrichen the mixture screws on the bottom of the carbs several turns and see what happens to top end. You can map where you are rich and lean then work on "profiling" the needles.

Did you try running thinner oil or no damper oil yet?

[Clifton]

If I momentarily block the carb mouth with my fingers, it does not get happy either.. it idles down, and wantes to stall.. And the lean pop theory sounds[/i'] right, but if that's the issue, then why are my old plugs blackened, indicating too rich on fuel?!?!

 

If it was backfiring thorugh the carbs it was for sure too lean. Are you lifting the chamber in one carb to adjust the fuel on the other or blocking it off with your hand? You need to lift the chamber, blocking it off will just create a pressure drop and pull in more fuel. With one carb dissabled you should have the other one adjusted to where it will just about stall, then turn it down about 1/4 turn, adding fuel. If you are too lean/ verge of stalling it will increase emissions and most likely fail. I set a pair of carbs that a shop couldn't get to pass. Make sure your timing is good and your mechanical is advancing too.

[datsunlover]

Well, It seams to be better now.. I played with it some more today, adjusted evrything again and found that I had the needles poorly aligned for one thing... (front carb was too high in the piston, rear carb needle was low) also, as the day went on and I did test runs (low rpms and high rpm blasts..) I started to question my ignition... So once I had the carbs set as good as they'd get, I instaled my MSD box that's been collecting dust on a shelf for a year. It's got a 'soft touch' rev limiter with a 6000 chip.... Looks like I'll need to pick up a 6500-7000 chip now, as I can run the thing right up and 'hit' the 6k limiter. Still a bit week feeling after 5500, but I think some needle sanding will help that. As for ignition, my zx dizzy has a bum vac-advance, so I'm just running more initial advance. (worked well last year, still working great..) And at this point, I still have adjustment on the idle. IE, I can back it down to the point where it wants to stall, so I figure I can put some more initial advance in (bringing the idle up obviously) and get a bit more 'go' out of her.

 

I had a 'rolling start' race with a friend of mine tonite (here it comes... ) 98 civic; header/exhast, intake, decent suspension/tires. From 2nd gear (about 20 mph) we went, and it was pretty close at first.. but as he shifted to 4th, I was only at 4500 in 3rd. muhahaha.. so I finally put it fully to the rug.. I looked over at him and waved as I pulled away. I know, big deal, I killed a civic. but two weeks ago, I had to give her all she had JUST to keep up to this guy down a back road!

 

I feel pretty good about it now, and I think I'll let it be... see how it goes for the next few weeks, when the hot weather returns. I'm gona add an electric fan sometime soon.. (still runs too hot for my liking) Thank you guys for all your help.

 

Matt