Help with SU's UPDATE and more problems...

[Guest bastaad525]

Hey all. Well, first here is the link to my first thread a few weeks ago:

 

http://forums.hybridz.org/showthread.php?t=103163&highlight=carbs

 

Basically, a friend and fellow hybrid Z member is having some problems with his L28 running SU carbs, so I offered to help him out. On my first shot, I pulled the carbs half apart, we cleaned them up really well, and then I reset the needles in the pistons (using the Z therapy method), and tried adjusting the mixture nuts, which were set very rich, at around 4 turns full open.. I put them back to 2.5 turns, but we ran out of time so had to stop there. It seemed to improve a bit, but didn't fix his main problem, which was bad bogging at freeway speeds.

 

I told him, and a couple of you guys also mentioned in that thread I posted above, that we also needed to check the adjustment of the floats, which I had wanted to do that night but we didn't have gaskets handy, so I asked him to bring it back another time.

 

So, he brought the car back tonite. First thing I did, just for the heck of it, was check his ignition timing. It was set pretty retarded, at 5 degrees BTDC. I know stock they are supposed to be either like 8* or 10*, and I used to always run my Z at 12*, so that's where I set his. Then he took it down the block and said it did improve a bit, no surprise there.

 

So then I took out the floats, first thing I notice, is they are adjusted WAY differently. One of them was set so high/rich that with the float lid held upside down, the actual rubber float was almost touching the lid! That would result in a way high fuel level and that car running very rich I would thing plus possibly fuel leaking from the nozzles? The other one was also adjust a bit high/rich, but not nearly as bad... this one had maybe 6-8mm between the top of the float and the lid itself when held upside down. Still a bit on the high/rich side, if I'm remembering the Just SU's video right.

 

So I opened them up a bit and set them even. I set them so that, when held upside-down, with the float resting lightly against the needle valve, there was 14mm between the top of the float and the lid itself. Can someone verify this number for me? I thought it was supposed to be either 14 or 17mm. But then, it's been 2 years since I tuned a set of SU's...

 

Anyways... so set them at 14mm, and put everything back together. He starts the car and it doesn't even want to idle... so I adjust the idle screws a bit, and then I start fiddling with the mixture nuts.

 

Well, adjusting the nut on the front carb, I richened it up one turn, and the idle smoothed out and it stopped trying to die, that's at 3.5 full turns. Richen it up any more and it would slowly start to degrade again, lean it out and any less than 2 turns full open the car would die. So I left it at 3.5 turns open.

 

now I go to adjust the rear carb, and NOTHING I do causes ANY change in the idle whatsoever... I closed the mixture nut all the way, full lean, nothing... remember, this is AFTER I set the needle into the piston properly using the Ztherapy method. Richen it up... 2..3...4...5 full turns... nothing.

 

Another thing I observed, is about those little buttons on the underside of the carb bodies... the ones you use to push up the piston slightly, for testing/tuning purposes. If I press up the one on the front carb, the car dies. If I press up the one on the rear carb, nothing happens at all.

 

Any ideas on any of this? I am completely baffled... I have now matched the carbs perfectly... needle setting, mixture nut, float level, and yeah also tuned the idle and high rpm air flow balances using the Unisyn tool... by all acounts the carbs should be running perfectly evenly. Yet adjusting one yields immediate, noticeable results, and adjusting the other does NOTHING.

 

 

Anyways... it gets worse. So after I mess with the nuts a bit and get it to idle smooth, he revs the motor, and now it just sounds horrible... a really weird sound that ... well the only way I know how to describe it is FLATULENT. I'm not trying to be funny here. The exhaust sound just totally changed and didn't sound right at all. And he's saying that it's revving way less responsively than it was before, then he drove it down the street and it's struggling just to go.... I'm stumped. The only thing I could think was I set the float levels TOO low/lean, and was going to readjust them but we ran out of time again. But otherwise... I mean... I dont want to put them back exactly the way they were, set so unevenly like that. Just doesn't make sense...

 

So please help me out guys... I thought I knew SU carbs really well, having watched the "Just SU's" Ztherapy video like 10x, and having rebuilt my old set with good results, then upgrading to a newly rebuilt set from Ztherapy and tuning those myself a few times... and never had any problems. But at this point I'm stumped and feeling really bad... I want to help this guy but somehow setting things the way they were supposed to be set made the car run like crap. Unless I was just totally off on that 14mm thing... that's the only thing I can think....

[wpnmkr]

"So I opened them up a bit and set them even. I set them so that, when held upside-down, with the float resting lightly against the needle valve, there was 14mm between the top of the float and the lid itself. Can someone verify this number for me? I thought it was supposed to be either 14 or 17mm. But then, it's been 2 years since I tuned a set of SU's..."

 

This information from Tuning S.U. Carburetters 2nd Edition - Speedsport 1972.

 

1-remove and invert float-chamber lid.

2-with the needle valve held in the shut-off position by the weight of the float only, there should be an 1/8 to 3/16 in. (3.2 to 4.8 mm.) gap between the float lever and the rim of the float-chamber lid.

3-the float may be set by bending at the crank unless a Nylon float is used, this being a non-adjustable assembly.

 

I don't know if this will help with your carbs, but it comes straight from the book I've got.

 

Chris

[Guest bastaad525]

wow.... thanks very much for that info. I coulda SWORE it was four-TEEN... I looked but I couldn't find my "Just SU's" video, so was just trying to go from memory... well damn now I feel really bad, especially that we didn't have enough time last night as I just had this feeling that was the problem. Well... wait a second though, what does it mean when it says "float lever"? I was measuring from the top flat surface of the black rubber float itself.

 

Okay so that explains some of the problem, but does not explain why one carb is behaving completely differently from the other one.

 

 

 

EDIT: Okay, just went and looked it up in the Haynes manual (almost forgot this covers tuning SU's), and it's agreeing with what I THOUGHT I remebered... turn the float lid upside down, and the measurement from the top of the float itself, to the bottom edge of the float lid is supposed to be between 14 and 15mm. Only thing different that it says is that I'm supposed to lower the float so that the arm of it JUST touches the needle, as opposed to just letting it rest on it's own weight... but that would only make 1-2mm difference, as that's only how far down the needle goes with the float resting on it with it's own weight.

[johnc]

Another thing I observed, is about those little buttons on the underside of the carb bodies... the ones you use to push up the piston slightly, for testing/tuning purposes. If I press up the one on the front carb, the car dies. If I press up the one on the rear carb, nothing happens at all.

 

The car is basically running only on the front carb.

 

1. Reset the floats to the proper numbers

2. Check to make sure there's proper oil in the dashpots and its at the correct level

3. Sync the carbs

[DCZ]

I have always set the floats so that there is 14 mm or 9/16 inch between the float and the lid so I don't think that is the problem. Definitely sounds like you are running on the front carb only. How do the plugs look in the rear 3 cylinders? Is it possible that they have been fouled and are not firing very well? Did you verify that the piston is not sticking on the rear carb and that the jet tube is centered on the needle? A worn jet and/or needle can let fuel flow past it no matter where the mixture nut is.

[Guest bastaad525]

yeah I was thinking this as well, especially the way the car was running, sounded like it wasn't firing on one or more cylinders... that was the 'flatulent' sound I described.

 

It's just odd that before I set the float levels to where they are SUPPOSED to be (thanks DCZ for confirming this along with what the Haynes said... I KNEW I was remembering it right!), the car ran fine, except it would bog at freeway speeds under hard acceleration, which is the original problem I"ve been trying to solve all along.

 

So yeah... the carbs have now been completely tuned and synced: float heights adjusted, needles set properly into the pistons, mixture nuts set to factory baseline (and tried thru almost their entire range). I checked both pistons for movement and they both move freely. I filled the dashpots with ATF, and also tried running them dry (my old N/A motor used to run MUCH better with the dashpots empty).

 

Though I haven't inspected them thoroughly, I did look at the holes in nozzles while I had the domes/pistons off, and they did not look abnormaly worn... they weren't oval or anything like that. The needles did not look abnormally worn either.

 

there is definately fuel going thru both carbs as far as I can tell, and before I set the floats, they were so high that some fuel was dribbling down out from the nozzles and the bottom of the carbs.

 

I will say, the carbs are somewhat worn overall as far as I can tell... I did the throttle shaft test, spraying carb cleaner around the throttle shafts while the motor idled, and it slowed down pretty bad on both carbs. As I said though the needles and nozzles LOOKED fine, but they may be pretty worn as well.

 

 

 

One more question guys, I forgot to mention... and actually, didn't even NOTICE this until the second time he brought the car to me (I feel very ashamed about this...). He's running some aftermarket air cleaners, dual seperate ones, and didn't have any air horns over the mouths of the carbs, as is present on the stock air filter housing. Is it at all possible that this could have been causing his total lack of power at freeway speeds? I know I remember reading/hearing that having the air horns IS important, but not to the point that it would just KILL power like that. Can someone confirm?

[JMortensen]

Air horns on SU's is a couple hp difference.

 

When I bought my first Z the PO had left it sitting for years. I had a similar problem where the car REFUSED to rev over about 4000 rpm. I tried everything, but it finally turned out that the spark plugs were bad. They looked fine, in fact they looked brand new (which is why we replaced the cap, rotor, wires, coil, rebuilt the distributor, etc but never changed the plugs), but new spark plugs were the cure.

[Guest bastaad525]

thanks for the tips guys.

 

 

Anyone else with any good advice for how to cure this 'only running on one carb' problem? Considering I've tried most of the possible fixes already..........

[Guest bastaad525]

eh... wouldn't you know it, it was something stupid.

 

He removed the float for the rear carb today, and didn't really do or change anything... just removed the needle-and-seat... checked everything out, couldn't see anything wrong. Put it all back together and now the car runs fine... DOH!

 

So I'm thinking maybe a piece of dirt got caught in the needle and seat or something.... I KNEW I had it all set right!!!

 

Now the last thing to see is if my adjusting the floats and timing finally got rid of his high end highway speed bogging... I guess he's out testing it as I type this, I've got my fingers crossed.

[JMortensen]

Good job dude. Sometimes its the little detail...

[Guest bastaad525]

Well he got back to me... no luck, the car still bogging at freeway speeds... if above 4000rpm at WOT, it just feels like it's got nothin.

 

So I'm still betting it's fuel starvation, either a clog, fuel pump issue, or still possibly the needles (meaning he might still need SM's).

 

However, the car IS running better now... it had a bit of a bog on take off as well, which is now completely gone, so the throttle response and low end have gotten better.

 

As for the fuel, we may have nailed that one too... turns out, when he first got the car, it had an electrical fuel pump out by the fuel tank, and then was also running the stock mechanical fuel pump off the side of the head. Thinking the electrical would be enough, he removed the stock mechanical. I would think the electrical would be enough as well... unless it's a REALLY low flowing pump... At any rate, he had also removed some smog equipment from the car at the same time, which I wouldn't think would cause the problem. He's not sure but he THINKS the car didn't use to have this problem before removing the fuel pump and smog stuff.

 

So... fingers crossed again, he's gonna put the mechanical pump back inline and see if that fixes it. If so then maybe I'll give his carbs one more attempt just to get them as perfectly tuned as I possibly can, as far as the mixture nuts and all.

[technicalninja]

Bad vac leak in rear manifold might cause problem but should also have other symptoms which would be obvious to you.

Might have something in fuel line to jet or jet itself.

Some (late 71/72 carbs) have different float mounting legs and as such have two different settings (and different length needle valves). I seem to remember that the long set goes on rear carb but I may be wrong.

 

An exhaust back pressure test might help decide if the exhaust is restricted which can poorly effect high speed/load drivability. Drill and tap exhaust pipe for 1/8 pipe- Brass fitting- 8' vac hose- standard low pressure F.P. gauge. For N/A anything above 1.2 psig is not real good, above 2 will seriously effect power, and good system should be .5 or lower.

 

Verify that mechanical and vac advance is working. Swap back three plugs with the front ones.

Verify fuel pressure while driving (tape gauge to window and check at speed)

Verify float vent is open and any line attached is open to atmospheric pressure (might have gotten plugged off when the emissions were removed)

Verify that airfilter housing does not restrict vent to vac chamber (pot).

Snap throttle while holding you finger lightly against piston to gauge piston load reaction. If the back carb is not opening as much (or opening faster) you will be able to "feel" the difference.

I would guess either the rear carb piston is reacting differently than the front or you have a reduction of fuel flow at rear jet.

Might be smart to run FP test line from rear carb fuel inlet.

 

Valves badly out of adjustment might cause these symtoms as well.

 

Hope this helps

Rick

[gogriz91]

I've had similar problems and ended up replacing all of my vacuum hoses.

[Guest bastaad525]

techninja - very good info and tips, and I have checked a couple of them already like the timing and vac advance. Unfortunately, a lot of that is a bit out of the scope of what I'm doing... I'm just trying to help the guy out but really am limited on time and also many of the resources I would need for the indepth stuff like some of what you mention.

 

I have recommended to him that he may want to replace all of whatever smog equipment he may have removed and see at least if that solves his problem.

[PeterZ]

OOOhh, this sounds like fun. Have you done all the basic checking already -fuel pressure and volume, secondary ignition, throttle adjustments, syncronization and even making sure the tack reads accurately at high RPM? If you are lean at high RPM you can easily test by pulling the choke levers to drop the fuel nozzles deeper to see if the engine gets happy. Properly set-up SUs can work very well. Many times the nozzles stick and cause all sorts adjustment problems beause they aren't repeatable on the mixture after every adjustment operation. If the fuel delivery hoses are made from bulk hose and not the formed hoses the nozzles will get side loads that cause sticking and too-rich mixtures. If the engine is all set the adjustments to tune the SUs throughout the RPM ranges can be pretty straight forward.