another su question

[510kevin]

alright, first off, i've searched and couldnt come up with any info on my problem.

 

i have a set of 1800 sss carbs on my l20b. for some reason, the rear carb (closest to the firewall) is sucking air like crazy while the front one barely pulls air at all. i dont have a sync tool, and im just trying to get it close enough to run it to ztherapy to get dialed in. any ideas what would make my carbs do this?

 

carbs have been recently rebuilt

emissions junk is removed

and im running atf in the pots

[SublimeDatsun610]

simply just adjust the carb idle screws. use a vacuum hose to your ear and place the other end in the carb opening and listen to the whistle. balance them out by adjusting the screws to even the whistle. its not rocket science. you can youtube how to sync or tune SU's... its helpful.

Peace

[Rustina 510]

The above advice is best if you don't have a sync tool

[Logical1]

1. Turn all idle screws/mixture knobs to full tight/lean/low idle

2. Turn Mixture knobs 1/2 turn both from full lean

3. Turn idles screws 1 full turn

4. Start car (if Possible)

5. Make SLOW (1/4) turn adjustments in idle screws until idle is decent

6. Make SLOW (1/8) turn adjustments to mixture knobs till it drives decent

7. Make sure you choke linkage (if hooked up) isn't fowling your mixture (it can keep the fuel jet down even if mixture knob is turned to lean and vice versa)

 

Also if your PCV hose is loose or in bad shape it causes a vacuum leak that is a pain to figure out. Re-built carbs? Are you sure the springs are new in the domes? Different rates can cause the asymmetrical problem your talking about. RARELY I have seen bent acceleration weight guides, but that is rare as hens teeth. Test piston lift with the lift pin on each carb when the car is off, if it doesn't lift easily and drop quick with a 'knock' when the piston hits the bottom of the carb, its a bent needle. Good luck!

[510kevin]

i think the rear must have a bent needle. it doesnt drop all the way. it stops about 1/4 inch from hitting the bottom

[DISLEXICDIME]

i think the rear must have a bent needle. it doesnt drop all the way. it stops about 1/4 inch from hitting the bottom

 

Check and make sure the needle is seated all the way

 

Chuck the needl in a drill then grab a piece of wood hold it against the wood and spin the drill

[exit64]

Go over to 311s.org and read my write up on how to straighten bent needles in the tech-wiki. Then read Keith O's write up on how to tune your carbs. If you still can't do it after that, yank em off and send to me and I will send you a beat up weber for trade. :cool:

[SublimeDatsun610]

i would also check for carbon build up in the bores causing piston to hang up.

[zerow]

Before you go taking them all apart...

Loosen all four of the dome top mounting nuts. See if the piston drops and clunks. Lightly tighten the screws back down and check it again. There is a very tiny amount of play that the dome tops will give to you for adjustment at the needle. I had this happen when I rebuilt my carbs and had the domes polished. Domes and pistons were matched, but the releation between the piston and the dome had to be re-established.

[510kevin]

Before you go taking them all apart...

Loosen all four of the dome top mounting nuts. See if the piston drops and clunks. Lightly tighten the screws back down and check it again. There is a very tiny amount of play that the dome tops will give to you for adjustment at the needle. I had this happen when I rebuilt my carbs and had the domes polished. Domes and pistons were matched, but the releation between the piston and the dome had to be re-established.

 

 

sweet, ill give it a try. i did have my domes off to get polished, maybe i got shit too tight when i put them back together...

[zerow]

sweet, ill give it a try. i did have my domes off to get polished, maybe i got shit too tight when i put them back together...

...or off center... :)