Another SU question...

[Skatekov]

OK... So I finally got everything on the car and the round top SU carb'd 280zx is running. 
Idles little high (1000~1200), starts up fine. revs pretty fine too.  idles at 170~180F 

but when I try to tune the A/F mixture and push the piston on the front carbs up, it stumbles and almost dies (but ofcourse I rev it so it doesn't die) while the rear carbs on the other hand even if i push the piston up, nothing changes. no hearable increase or decrease in idle.  tried taking the fuel mixture pretty far down but still stumbles and dies on the front carb.

 

I've tried fiddling with the float level. No change so I put it back to the factory setting. 

 

all the gaskets are replaced. holes are cleaned. 

 

Tried different shoulder positions of the jet needle: one, the z therapy method where you have shoulders flush with the bottom of the piston: too lean, not enough for mixture nuts to compensate and the rear carb is still un-affected. coughs and dies.  

The straight edge method: flush with the needle mount, starts up without choke and idles but idle is a bit too high, even with the throttle all the way closed the engine still idles at around 1000. 

spark pluggs are fouled up. the front 3 are dry fouled while the rear 3 (cyl 4,5,6) are wet fouled 

My guess is: 
-Throttle shaft leak
-"modified" E12-9x ign module ******* up

-bad vacuum lines hook up.to dist etc I need help with this . 
-Me not using choke (but its like 90~100F out side...and starts up...) 
-needle position (where is the "real" bottom of the piston thats supposed to be flush with the shoulder? the little passage for idle or actual surface of the piston that touches the bridge. 
-and or bad fuel line and low fuel pressure. 

by the way this is what my fuel line looks like..  I havent got an chance to go get a OEM fuel rail

. 


As you know many will say "Use the search" but it  still havent gave me a clue or answer. 
little tight on budget and time for a DVD 

 

[Miles]

Make and model of car plus mods would help.

 

 

How many turns out for  each carb  on the mixture adjusters?

 

Not what you want to hear, but you may not find resolution in any forum/blog. Without understanding how the SU carbs work and how to tune them it is unlikley that you will solve the problem.

 

Go to Z  Therapy and buy the $25 DVD or tape "Just SUs"  at http://www.ztherapy.com/

Edited August 12, 2013 by Miles

[Tony D]

Balanced airflow before you started monkeying with them?

The FSM lays out balancing pretty clearly.

[Skatekov]

yup, airflow is in sync,  used unisyn.
did the standard 2.5 turn. with the idle mixture nut

Mods are SM needle 
I took apart everything, maybe I messed up the throttle butterfly clearance. 

ebay fuel pump... only pumping out like 1.5~2psi 
F54 P79 stock 280zx L28E engine. 

I understand how SU's work mostly. but I'm new to it so I lack experience with it. 

Edited August 12, 2013 by Skatekov

[Miles]

Check for leaks around the throttle shafts.

 

Check if  the needle valve in the fuel bowel is sticking.

Edited August 12, 2013 by Miles

[Skatekov]

Check for leaks around the throttle shafts.

 

Check if  the needle valve in the fuel bowel is sticking.

nedle valve is brand new and not sticking last time i checked it . 

 

will have to check throttle shaft leak. probably why.     But will it cause such an dramatic imbalance? 

[z240]

T in a clear chunk of 7/32 sight tubing into the bottom bowl to jet tube hose and really check the running fuel level on both carbs. If you have float bowl drains, you can rig up a sight tube from there. Should be right at the top of the rectangular raised boss on the side of the float bowl, which is 20 mm down from the lid gasket line (23mm down from roof of the inside of the lid as per FSM).  this will prove not only the correct float position, but also if your floats are free and if your float valves are sticking or not. 

 

Second important detail is fuel pressure. Just a bit over 4-5 psi might be held by one float valve, but not by the other. 2.5 - 3 is nice and safe.

Edited August 12, 2013 by z240

[Skatekov]

T in a clear chunk of 7/32 sight tubing into the bottom bowl to jet tube hose and really check the running fuel level on both carbs. If you have float bowl drains, you can rig up a sight tube from there. Should be right at the top of the rectangular raised boss on the side of the float bowl, which is 20 mm down from the lid gasket line (23mm down from roof of the inside of the lid as per FSM).  this will prove not only the correct float position, but also if your floats are free and if your float valves are sticking or not. 

 

Second important detail is fuel pressure. Just a bit over 4-5 psi might be held by one float valve, but not by the other. 2.5 - 3 is nice and safe.

can only check fuel level with it not running.  I have the simple style fuel bowl w/o drain plug. 

 

and I'm really low on fuel pressure or the Egay gauge isn't working. 

 

but I'm suspecting that and the fuel rail design the most. maybe the rear carb is getting fuel first. 

 

 

Edited August 13, 2013 by Skatekov

[Skatekov]

Changed fuel rail design, No change. 
Shaft isn't leaking. 


Could lack of fuel pressure cause this? 

(the regulator has been moved to the return side after this video but no change. 

Edited August 15, 2013 by Skatekov