SU Float Setting Questions- Need yous help again z gurus

spawpoet

spawpoet

New member
FINALLY we are back out on track following our rebuild, and mostly the car ran very well. Tim got his first race in, and by tomorrow should be one step closer towards pulling off those x's. He had very good drives today, and by the end of his race today was turning laps within 6.5 secs of the ITS leaders. That said we did have some fuel delivery issues, and I had a couple of questions.
I'm not sure we have the floats set properly on the SU's. As I understand it when the floats are set properly with the suction chamber/piston removed you should be able to see the fuel just below the bridge/nozzle interface down in the carb. I can only see the fuel in our nozzles if I open the chokes up. Additionally, I have to have the mixture nozzles turned as far rich as they will go to get enough gas for the car to run, and I still think it's a bit lean with them there. I CAN'T make it too rich with the mixture knobs. As it's set though we seemed to make good power until Tim was winding it out where it would lay down a bit at higher revs.
One last quirk was that a couple of times we had fuel overflow the bowls out the float lid vent. Is it possible to overflow the bowls, and have the floats set too lean at the same time? This would happen coming off of power where the fuel was backing up in the bowl. My guess on this was that fuel pressure was too high (4.5-5lbs) so we turned it down to 3.5 lbs, but haven't run at that setting.
The carbs are z-therapy units, and are in very good shape. Timing is at 34 deg total advance, and the ignition is working well. Plugs come out with a good burn on them, and there are no vaccum leaks. What really has me miffed is that I can't get meaningful adjustment in the mixture knobs. At full rich setting it runs, and anything else is too lean to even hold an idle. Any tips with this is greatly appreciated.
 
Just thinking out loud Chris, take it with a grain of salt..
4psi max. IIRC float level is ~12mm.
Yes, too much fuel pressure will over run the bowls.
Have you checked needle height?
2 1/2 "flats" should get the car to run and then you can balance and tune the carbs.
I have had the float level change when trailering the car and it caused issues.
Check for any vacuum leaks around the carbs.

Good luck, glad to hear it is back on track!
 
OK then I'm fairly confident that the overflow was from fuel pressure. We HAD a vac leak, but cured that before heading to the track (backfire out the carb blew out a throttle shaft seal). We set float level from the ceiling of the bowl cover to the top side of the plastic float at 14mm (setting per Clymers/Chiltons), so we will dial that down to 12 mm. Am I correct that the measurement is between the ceiling and top of the float?

If I set mixture at 2.5 turns it won't even keep running. It's just not getting enough fuel at the nozzle unless it's wide open. I'm gonna check the needle height tomorrow. And also make sure there are no clogs in the nozzle lines. Thank you very much for your advice Paul!!
 
Chris, it may be 14mm float level and you are measuring correctly, from lid to top of float. I was going from memory and would have to check the manual so you are probably right.

Check the nozzles and make sure they move up and down and make sure the choke lever isn't binding. I have one that is a little sticky. 20wt oil for the pistons, make sure you haven't overfilled.

Good luck, just some thoughts as I watch Auburn blow it at Georgia. Dang!
 
Still had issues today but I think were figuring it out. I'll let Chris chime in with the details but we shaved another second off today. Even though it never really ran great for an entire session the car is coming along real well. Went 4 off for the first time today... that was entertaining. As far as PBIR... I love the track but they still REALLY lack in SCCA accommodations...
 
pballance said:
Chris, it may be 14mm float level and you are measuring correctly, from lid to top of float. I was going from memory and would have to check the manual so you are probably right.

Check the nozzles and make sure they move up and down and make sure the choke lever isn't binding. I have one that is a little sticky. 20wt oil for the pistons, make sure you haven't overfilled.

Good luck, just some thoughts as I watch Auburn blow it at Georgia. Dang!
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Thank you again Paul. After reading this and going through some of the old related threads I'm pretty sure I have too much oil (it is 20wt) in the dampers. Ron said in another thread he used something like a medicine dropper to fill the dampers, and I just poured in. My guess is way too much. We adjusted the installed needle height (they were out too far) and I was able to get closer to normal mixture adjustment on the mixture knob. The biggest problem we are having is similar to Ron's from a couple yrs ago where in hard rt hand corners we have one carb's cylinders going cold (rich) on the egts, and the car misses badly til it burns off the extra gas. We have the bowl overflows vented back into the air cleaner assembly, and it's going rich when the bowls overflow I think. We are going to vent into a catchcan at our next event, and I still think our floats may be a bit high. Also we have N-27 needles in there which I believe run a bit lean, and I'm going to try the N-54's in there to see if it helps a bit more.
 
Wish I could help you out but my carbs are a totally different design than your carbs. You've got external bowls and so on. Mine is basically like a big Mikuni carb - float on the bottom, piston, dampener rod, jet, needle. You've got to get your float height to a happy place though and that may not be the factory setting. My factory setting is fairly close, but not quite perfect.

And you've got to get the right amount of oil in the dampener section. I'm sure the 240Z guys could help you there, on the 260Z it is about 2ccs.
 
Ron Earp said:
Wish I could help you out but my carbs are a totally different design than your carbs. You've got external bowls and so on. Mine is basically like a big Mikuni carb - float on the bottom, piston, dampener rod, jet, needle. You've got to get your float height to a happy place though and that may not be the factory setting. My factory setting is fairly close, but not quite perfect.

And you've got to get the right amount of oil in the dampener section. I'm sure the 240Z guys could help you there, on the 260Z it is about 2ccs.
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Ron, was it the float height that was causing you to have one carb's egts going cold Ron?? I have the bowl sight tool on order from z-therapy, which should give us a little more accurate look at the float settings. Actually what you've said is a help. I've been trying to religiously follow factory settings, but it's helpful to know we might have to drift a bit from stock specs to find what's going to work for racing. Moreover I just never realized that the amount of oil added to the dampeners was all that important. BTW have you run your car with the new motor in it yet?
 
We ran 3in1 oil vs 20wt also look into diff spings in the dampners some of the english stuff is lighter than nissan. Also look into custom ground needles the two you mentioned will leave you fighting lean a lot(they are designed for mid range response). Remember most of the time you are at full throt. and slowing the piston will only help at low rpms and makes response slugish in the higher rpm range.
The MG guys can tell you where to get the light wt springs and several people have custom ground needle sets cheap or if you are talented try your hand at grinding your own (we modifed a set of english ones for our autocross car back in the day).
I think you will find at top rpm is when you are leaning out and either the needle profile is wrong or you are not getting full piston travel. On the EP car we dont run any oil and just use the spings for idle, pistons go wide open at 1500 and have the needle profle set for the cam. We run 7400-8000 on 3.5 psi pressue and have no problems with lean out. :eclipsee_steering:
 
spawpoet said:
Ron, was it the float height that was causing you to have one carb's egts going cold Ron?? I have the bowl sight tool on order from z-therapy, which should give us a little more accurate look at the float settings. Actually what you've said is a help. I've been trying to religiously follow factory settings, but it's helpful to know we might have to drift a bit from stock specs to find what's going to work for racing. Moreover I just never realized that the amount of oil added to the dampeners was all that important. BTW have you run your car with the new motor in it yet?
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With respect to one of the problems I fought for a few races I had a carb that was a bit off on the float setting and the fuel level was too high. I have sight glasses but the carb that had the setting off had a poor glass that didn't allow you to see the level. After resetting the floats many times I finally got my hands on proper glass and fixed the problem. I then further fixed the problem to make it easy to set the floats by using a "go no go" wire gauge inbetween the float tab and a fixture cast in the bowl so I don't have to worry about it - I set it with the wire gauge and I know it is good. I arrived at the proper setting by using the carb that was working well as a master.

Damper oil - Steve P and I have compared notes here and both of us agree that no damper oil doesn't work well. It worked well on Sam's dyno, but on track having no damper oil seemed to allow the pistons to slam open quickly and we both experienced throttle transition stutters. Bear in mind Steve has a 240, I have a 260, but we both had the same experience.

Too much oil, like filling the tube 2/3 full or higher, seems to cause a bit of a sluggish response. I know that description is hard to quantify but that is about the best I can do.

Mine seems to like about 2ccs (2 mL), and I use 20 weight motorcycle fork oil (I don't think this makes such a bid difference, you just don't want a multiviscosity oil in there). Why 20W motorcycle fork oil? It was what I had on the shelf that was single weight. I'm sure you could get similar results with 30W motor oil.

We have run the new motor on the track but not the dyno. On track I think I think what we have is a bit more hp throughout the range than the old motor. Beyond that they seem somewhat similar. I'm looking forward to putting it on the dyno and actually getting out and doing some racing with it!
 
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oil...weight

You do need springs, you do need oil. Withoutem piston slams open at slightst throttle opening.......throws mixture outta whack...IMHO.
Try a little lighter weight to 5-10 weight....YMMV.... Yeah, and hope Katman will throw his 2 cents worth in.....

david
 
How do you guys vent your overflows? Our biggest issue with stumbling on track right now is a temporary rich condition we are getting on one carb in hard right hand turns, but only after the car has been on track for 4-5 laps. We have vented the bowl overflows back into the air cleaner, and I think when the bowls heat up we are overflowing back into one of the carbs (rear) in the hard righthanders causing the problem. Is it a mistake to vent the overflows back into the filter housing? As for the needles/oils/springs we intend on spending some dyno time experimenting with different combinations. We still have yet to get O2 sensors into the exhaust to get better mixture info, so that will help tune it too. Really it mostly ran very well, but the main problem was it "richening" out in those hard corners. We smooth that out, and our laptimes should be just 3-4 secs off the pace with an undeveloped car and driver. Thank you again everybody. I really appreciate everything the community on the forum has to offer.
 
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spawpoet said:
How do you guys vent your overflows?
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I don't think I have a choice. My carb has an air horn that comes up right into the airflow stream into the carb. I'd love to move it because it certainly is probably 10% of the area of the opening. So, my bowls are vented into the opening of the carb that is in the air filter housing.

I'm not sure why this is done. Other carbs I've worked on that are somewhat similar have vents to atmosphere that are not in the airflow to the venturi.
 
One of our thoughts was that the fuel was getting sucked out the over flow into the air filter housing and running straight into the carb... flooding it out... Do you think this might be the case or is it most likely over flowing into the carb body itself due to float settings?

We have a stock air filter housing w. stock vstacks modified to fit a single ITG air filter. The vent lines go to the stock location in the backing plate.
 
I am far from an expert in this area but here are a couple of observations.

My vent is a simple loop tucked in alongside of the carb and I have not seen/smelled fuel venting from the float chamber. I have read elsewhere that a 1/2" drill bit used as a gauge for setting float levels should result in proper float level with a slightly "loose" fit.

Needle height is important and float level gives you the gross fuel adjustment for the carbs. Just a thought here. remove the float valves from both carbs and see if they are different. I remember something about rear valve is different from the front and it is the length. I don't remember which is which, but it is to compensate for differing gross mixtures under g-loads in the float chamber. It has something to do with the way the floats hinge and the location of the bowls since they are on opposite sides of the carbs. I will have to check my notes when I get home.

Hope that helps. Work has been getting in the way of fun these days with travel.

Paul
 
Sorry to confuse the issue, on the EP car we are running a big cam in a higher rpm range. I agree you def need oil just go light wt was all. I have seen problems with heaver oils.
I talked to one of my english guys and he thought there was a either green or blue spring set they used on the race cars (I think they have colored paint spots on them?)
Also he said that there may be a float jet/seat issue and to replace them if in doubt, this was a common problem in the race cars. He has seen them stick when they heat up, also if you have bad vibration (I dont think the L series have much) will cause sticking and overflow (this was on the euro SUs so he was not sure if it applied to the jap stuff). But you def will want a set or two of custom needles.Sam makes good ones!
Good luck!:)
 
Step 1: Buy Factory Service Manual.
Step 2: Do what it says for a stock car.

You can ignore those two steps if you have tons of money for dyno testing, in which case you will discover that in the end steps 1 and 2 are pretty dang close to best performance even for a fully developed ITS engine. I was constantly amazed over the years I was racing Z's in IT by how many guys I made faster by undoing the crap (ie speed secrets) they had done to thier engines in favor of getting back to stock. 20wt or 10w30 for damper fluid (ie per FSM), stock piston springs, stock float levels, stock needles (no SM's or "performance regrinds", we ran N54's), stock idle mixture setting. I always chuckled to myself on the grid looking BEHIND me at all the Z's that had to keep reving the motor to keep it cleared out from too much gas while we sat quietly purring along at a pleasantly stock idle.

Go back to basics and assume nothing is correct. Remove the float chamber lids and first confirm whether or not you have the same size float pin locator tangs on the lids or two different lengths. Then make sure you have the right needle housing for the tang setup (short tang housing aft, long tangs in front). By tang I mean the protrusions that the pivot pin is captured in. I'm just going from memory here, so somebody check me. I believe the '72 carbs (4 screw) were equal tang lengths, and float level should be 14mm. Turn the lid upside down, and when you lower the float lever seat (the curved metal piece that touches the needle stem) until it touches the needle stem the distance from the inside surface of the lid to the upper (when installed, lower since we're currently upside down) flat surface of the float lever seat should be 14-15mm. That gives you 23mm of distance from the inside surface of the lid to the actual gas when all installed and running. Of course that is practically impossible to measure. In the field I just turn the float lid upside down and see of the float is about horizontal, then I unhook the float and set it in the gas and the gas level should be about 3/4 inch below the gasket lower surface (ie top of fuel bowl). Bottom line- no tricks, make it stock. If these were Z Therapy carbs, unmolested by a previous owner, I'd be surprised if there's anything wrong there but assume nothing. Stock fuel pressure was 3.4 psi, so 3.5 should be okay assuming that's what you are actually getting.

Yiou should never have gas coming out the vents. Um, you don't have Grose jets in there do you?
 
kthomas said:
Step 1: Buy Factory Service Manual.
Step 2: Do what it says for a stock car.

You can ignore those two steps if you have tons of money for dyno testing, in which case you will discover that in the end steps 1 and 2 are pretty dang close to best performance even for a fully developed ITS engine. I was constantly amazed over the years I was racing Z's in IT by how many guys I made faster by undoing the crap (ie speed secrets) they had done to thier engines in favor of getting back to stock. 20wt or 10w30 for damper fluid (ie per FSM), stock piston springs, stock float levels, stock needles (no SM's or "performance regrinds", we ran N54's), stock idle mixture setting. I always chuckled to myself on the grid looking BEHIND me at all the Z's that had to keep reving the motor to keep it cleared out from too much gas while we sat quietly purring along at a pleasantly stock idle.

Go back to basics and assume nothing is correct. Remove the float chamber lids and first confirm whether or not you have the same size float pin locator tangs on the lids or two different lengths. Then make sure you have the right needle housing for the tang setup (short tang housing aft, long tangs in front). By tang I mean the protrusions that the pivot pin is captured in. I'm just going from memory here, so somebody check me. I believe the '72 carbs (4 screw) were equal tang lengths, and float level should be 14mm. Turn the lid upside down, and when you lower the float lever seat (the curved metal piece that touches the needle stem) until it touches the needle stem the distance from the inside surface of the lid to the upper (when installed, lower since we're currently upside down) flat surface of the float lever seat should be 14-15mm. That gives you 23mm of distance from the inside surface of the lid to the actual gas when all installed and running. Of course that is practically impossible to measure. In the field I just turn the float lid upside down and see of the float is about horizontal, then I unhook the float and set it in the gas and the gas level should be about 3/4 inch below the gasket lower surface (ie top of fuel bowl). Bottom line- no tricks, make it stock. If these were Z Therapy carbs, unmolested by a previous owner, I'd be surprised if there's anything wrong there but assume nothing. Stock fuel pressure was 3.4 psi, so 3.5 should be okay assuming that's what you are actually getting.

Yiou should never have gas coming out the vents. Um, you don't have Grose jets in there do you?
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Thanks katman... No, they dont have grose jets and they are 4 screws... Its quite possible the fuel was building up in the carb and not coming out of the overflow. The more I think about it the more it seems to just be set too rich (float level) ((unless the needle and seat WAS stuck thus dumping excess fuel out of the overflow))

We're set to run the Turkey Trot next weekend and we'll have additional tools that'll help us with float setup. We also just finished modifying our airbox adding heat shielding between the carbs and header. Lastly, the wideband will be installed to get logged overall mixture readings.
 
tim240z said:
Thanks katman... No, they dont have grose jets and they are 4 screws... Its quite possible the fuel was building up in the carb and not coming out of the overflow. The more I think about it the more it seems to just be set too rich (float level) ((unless the needle and seat WAS stuck thus dumping excess fuel out of the overflow))

We're set to run the Turkey Trot next weekend and we'll have additional tools that'll help us with float setup. We also just finished modifying our airbox adding heat shielding between the carbs and header. Lastly, the wideband will be installed to get logged overall mixture readings.
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No Tim, we definitely had fuel coming out the vents. I saw it in the paddock on both carbs. And those floats are right @ 14mm. If anything, I think they are lean from the view I get looking down the carb body at the fuel nozzle. If the bowls were set too high we would be rich all the time. We are just getting a temporary rich condition in one or two corners, and only when everything's hot. I think the heat shield will help, and I think we need to dial the fuel pressure down to around 3lbs given the extra volume we get to the bowls with the aeroquip feed lines. Another thought. I wonder if we are getting some pulsing from the fuel pump, and are occasionally exceeding the fuel pressure we set for the car overpowering the needle valves.
 
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