SU Carbs different and not running good

[neoxman]

I bought a 73 240z with SU round top carbs. It hasn't been running very smoothly all the time. It's hard to rev past 4500 and when it gets into stop-and-go traffic, where it is a little hotter it starts to surge and won't go away until i floor or get out of traffic.

I've been reading the forums and I completely replaced the ignition except the coil. It's much smoother revving with the new ignition but i've tried adjusting the carbs with a unisyn first then setting the mixture and now i've developed a high idle and while driving the idle won't fall below 3000rpm. I've also noticed in this process that i've got 2 different round top carbs. The front is a 4 bolt float bowl, and the rear is a 3 bolt flow bowl with a fixed float. I'm wondering if this is the problem or if there is a vacuum leak. I have some pictures:

Front Carb

Rear Carb

Parts I'm not sure of... do i need this?

I also topped off the carbs with 20wt oil.

[beandip]

What do you mean fixed float ? there is no fixed float on a SU. As for a mixed set , it should be fine as long as the needles are the same and they are working properly . You said that you balanced the carbs with a air flow meter and adjusted the mixture now it has a 3K idle? Sounds like you need to go back and adjust things properly. This engine should idle at 700 RPM . What did you set the timing at ? Also the new Dist what is it , what kind of ignition are you using ? The parts you have circled can be removed as long as you don't have to stand a emissions inspection . One way to look for a vacuum leak is start the engine and take a spray can of carb cleaner and spray it around the intake manifold and suspected leak . If there is a leak the engine will change RPM when the spot of the leak is sprayed . DO NOT USE START FLUID FOR THIS. What is the casting # on the intake manifold ? Gary

[neoxman]

Thanks for the quick reply Gary.

>>>What do you mean fixed float ? there is no fixed float on a SU.

I'll take some pictures, but float pivot has a brass pin through it but no arm to bend to set the level.

>>>As for a mixed set , it should be fine as long as the needles are the same and they are working properly.

I hope the needles are the same. Are the labeled if you remove them?

>>>You said that you balanced the carbs with a air flow meter and adjusted the mixture now it has a 3K idle? Sounds like you need to go back and adjust things properly. This engine should idle at 700 RPM .

I've been reading up on the adjustment and I know now i've been adjusting this the wrong way. but even before i messed with the carbs it was exhibiting the surge problem.

>>>What did you set the timing at ?

10 deg at 700rpm. I'm not sure what the best timing should be with my setup running 91 octane.

>>>Also the new Dist what is it, what kind of ignition are you using ?

I purchased a new Euro Distributor from msa and using the stock coil. NGK BPR6ES plugs and custom cut Accel 8mm wires. All gapped including the breaker point in the dizzy.

>>> The parts you have circled can be removed as long as you don't have to stand a emissions inspection .

Great, i don't need it inspected so off they go.

>>> One way to look for a vacuum leak is start the engine and take a spray can of carb cleaner and spray it around the intake manifold and suspected leak . If there is a leak the engine will change RPM when the spot of the leak is sprayed . DO NOT USE START FLUID FOR THIS.

I did this with carb cleaner from gumout... i got some vapors around the carb inlet and idle went up slightly. but other than that my preliminary spray didn't do anything. I will do a more extensive test later.

>>> What is the casting # on the intake manifold ?

E88 head is also E88

[=Enigma=]

Either that is a completely different carb, or someone replaced the float bowl with another. They're detachable. I've never seen one with the fuel inlet like that 3 screw one you pictured. Perhaps from a british car? Are the round tops(dashpot) held down by 3 or 4 screws? I'd also measure the venturris and make sure they are the same width just to be sure. If necessary, obtaining a replacement or a set wouldn't be too hard.

[sakijo]

Either that is a completely different carb, or someone replaced the float bowl with another. They're detachable. I've never seen one with the fuel inlet like that 3 screw one you pictured. Perhaps from a british car? Are the round tops(dashpot) held down by 3 or 4 screws? I'd also measure the venturris and make sure they are the same width just to be sure. If necessary, obtaining a replacement or a set wouldn't be too hard.

My carbs have that fuel inlet fitting, but they're the smaller 38mm Hitachis made for the 2.0L L20. Since they're pretty rare carbs in America, I'd guess British car float bowl too. It shouldn't be a 38mm carb because the intake mount is square with 4 bolts; 38mms are diamond with only two.

[ezzzzzzz]

Somehow, I think you'll find that the two carbs are not using the same needle/nozzle. It is obvious that a mix-match of parts were used. The rear carb has either a JDM, british or volvo bowl top most likely. I'd look into getting a matched set (3 OR 4 screw) of carbs. They aren't terribly expensive or hard to find. Your ignition is probably fine if it runs and allows acceleration. I think timing should be around 16 degrees advance@700 rpms. Be sure the nozzles are not hanging and go up completely when the choke is off. Be sure you have oil in the damper wells. Be sure the pistons are not hanging in the domes and the needles are seating correctly in the nozzles by performing the drop test. You need to remove the dampers in the domes to perform this test. All of this is easier if you'll remove the air cleaner completely too. As for adjustment, loosen the top screw completely at the balance tube linkage. It is only used to balance the carbs at cruising rpm. Loosen the sync screw completely on the linkage between the two carbs. bottom out the mixture nuts (under carb body) and turn out 2.5 times. Turn idle screws in 1 turn. Start engine and allow to warm up. Using the unisyn or a length of heater hose (old school trick using your ear) adjust the carbs for equal flow by adjusting the idle screws. Readjust idle to 700-750 rpms by making equal adjustments of both idle screws. Resync for equal flow. Using the lifting pin (under body of carb on rear) raise the psiton about 1/16 inch to test for best air/fuel mixture. Rpm's should momentarily rise then drop back to normal. If rpm's stay up then the mixture is too rich. If the rpm's drop then the mixture is too lean. Turn the mixture nuts in or out in 1/4 to 1/2 turns to achieve the ideal effect (momentary rise of rpm them drop to normal rpm). Now, turn the top screw in until you achieve the cruising rpm (about 3000 rpm's). Using the unisyn adjust the sync screw between the carbs to balance the carbs for equal flow. Turn the top screw out until it does not contact the linkage at all and leave it there. Leave the sync screw alone at this point too. You should be close to the mark for the engine to run well assuming there are no other issues (vacuum leaks, valve adjustment, mechanical problems, etc.).

[neoxman]

Thanks for all the input guys. I will try figuring out if the dashpots are 3 bolt or not and I'll definitely try tuning it again.

So far the most confusing thing about the carbs has been the number of adjustments and confusing use of terminology refering to the different adj. screws on these carbs. Some people say "idle screw" and that is confusing because it can be the idle mixture or the fast idle. Now I have those straightened out. This site had some good diagrams:

http://www.teglerizer.com/sucarbs/techtip6.htm

But even their diagram had one less balance adj. screw than the Z's SU's.

I'm still wary of the rear carb and possible jet difference. Do you guys think the float bowls are different volumes? I don't know carbs so i'm not sure if that would be a problem.

I will let you guys know when i do the adjustments.

Thanks again!

[ezzzzzzz]

Float type or weight is only relative to setting fuel level. Once done it is not an issue. Ideally, the level should put fuel just below the nozzle tip when the mixture nut is backed off andthe nozzle is fully seated in the up position. There are several books on SU carbs that you could use for reference and learning purposes. Ztherapy's tape is very useful too. Needle and nozzle sizes would make a difference once you have the engine anywhere off idle (accelerating or cruising). It's easy to determine if your carbs are 3 or 4 screw. The number of screws holding the dome to the carb body is one or the other.

[neoxman]

So 3 screws hold down the dashpots, and I was successful at getting the carbs tuned up. The balance was way off, and now it's a lot better. I still think it could run better. I floored it to see how well it would rev and it gets up to 4500 nice but around 5000 it's struggling and feels rough and and it can't rev any higher than 5200 - 5400. But definitely is much smoother driving now that it's synced and the throttle was evened out.

So the part of the adjustment that threw me off was after I set the idle nuts to 2-1/2 turns and set the throttle adj. screws even... the instructions say:

"8. Turn in or out front and rear idle adjusting nuts simultaneously by each 1/8 turn until the fastest and most stable engine speed is obtained."

My engine speed did not change when i turned the nuts. Is it supposed to go up or down? i think i ended up at 3-1/2 turns out.

[Arne]

Hmm. Make certain that the butterflies are opening all the way when you floor the pedal. If the linkage is bent or mis-adjusted, you might not be getting full throttle opening.

[beandip]

If you have the correct carbs and no vacuum leak , 3 1/2 turn should be way rich . When you reassemble the carbs. Use 20w oil in the dampeners , if you don't have any 20, 10w30 will work. I like the others here wonder if the carbs are both for Zs. measure the opening of the inlet of the carbs to see if they are the same size . It would be best to measure the flange side of the carb , but for now just check the inlet side. So these miss matched parts is how you bought the car , right ? You didnt do this, right ? To answer your question on the markings on the needles. They were simply printed with ink stamp. Most likely the markings are no longer there. If you have a set of 1'' micrometers you can compare the two needles , but it isn't' easy because of the taper. Needles are still available from Nissan . If I were you I would start looking for a set of carbs. Just ask there should be some extra sets someone here on the web site would part with . Gary

[neoxman]

After a full weekend of removing and disassembling the carbs i cleaned everything and put it all back together. Had some problems because the rear float bowl has a bypass fuel nipple or extra nipple and i thought that was for vapor relief. I hooked up a hose to pipe it into the air cleaner and ended up with an air cleaner full of gas! Good thing nothing bad happened. I plugged that extra inlet as had been done when i bought the car.

Still not understanding the Idle nut adjustment. Is one turn 180 degrees or 360. I read somewhere the base adjustment 2 turns is .060" in depth from the top of the bridge for the jet nozzle height. Thats almost a 1/16" if i'm not mistaken. Mine at 2 360 degree turns looks like 1/8".

Thanks for your guys help so far. btw runs much better after the rebuild and 20wt oil.