Rich SU Carb <- Newbie

[Greengooey]

Hello, I bought a '72 240 a couple of weeks ago (I also have a '90 300zx) and I've been reading up on everything I can get my hands on. Anyways I noticed the other night that it stumbles a bit on acceleration under 2k rpm's (but not through the rest of the power band) and I could tell it was running very rich. I pulled the plugs today and saw that cyl. 1-2-3 plugs were very fouled with carbon while cyl. 4-5-6 plugs looked very clean. I've read a lot of threads about tuning the SU carbs and I was wondering if I needed to completely tune both carbs or can I just adjust the front carb to keep it from running too rich. Any help is appreciated.

[Doehring]

Greengooey, turn only the front carb leaner but clean it at first. There often is oil and dirt around the up and down going needle seat and maybe that by that even the choke doesn't work free and always is a little pulled. In that case you must not lean the fuel mixture 'cause the choke makes it too fat. Check that first.

Good luck

Rolf

[Si|v3r72]

Originally posted by Doehring

Greengooey, turn only the front carb leaner but clean it at first. There often is oil and dirt around the up and down going needle seat and maybe that by that even the choke doesn't work free and always is a little pulled. In that case you must not lean the fuel mixture 'cause the choke makes it too fat. Check that first.

Good luck

Rolf

Could you clarify this please? My front carb has always been oily on the bottom and I don't really understand what you said.

[Doehring]

Sorry, I'm not a mechanic and english is not my mother-language so I have some problems to find the correcct technical idioms and words. Let me try:

At a SU the air-fuel-mixture becomes richer when the little distance between the needle and the shaft becomes bigger. This happens either when the vacuum sucks the piston up (when accelerating) or when you screw the shaft down by turning the mixture screw or when the choke pulls the shaft down.

When there is sticky dirt around the shaft and the choke had been pulled once ago it can happen that the shaft goes not fully back upwards. By that the mixture becomes too rich. So make shure that the shaft works propery, does not stick and goes fully up to the basic position that is done by the mixture screw.In case you are then still too rich you have to lean the mixture. A great help to understand your SUs is the ZTherapy video.

Rolf

[Si|v3r72]

Originally posted by Doehring

Sorry, I'm not a mechanic and english is not my mother-language so I have some problems to find the correcct technical idioms and words. Let me try:

At a SU the air-fuel-mixture becomes richer when the little distance between the needle and the shaft becomes bigger. This happens either when the vacuum sucks the piston up (when accelerating) or when you screw the shaft down by turning the mixture screw or when the choke pulls the shaft down.

When there is sticky dirt around the shaft and the choke had been pulled once ago it can happen that the shaft goes not fully back upwards. By that the mixture becomes too rich. So make shure that the shaft works propery, does not stick and goes fully up to the basic position that is done by the mixture screw.In case you are then still too rich you have to lean the mixture. A great help to understand your SUs is the ZTherapy video.

Rolf

Much clearer thank you. Your english is fine, I'm just thick-headed. I think I may have the problem you've described, and although I do have the ZTherapy video, I've been too chicken to take my carb apart. Sounds like a good winter project for when the car is off the road.

[Doehring]

silv3r72, Good luck! I did that job without taking the carbs off. I unplugged the choke and when I tried to move the shaft up and down I felt that sticking. So I cleaned the underside and the whole mechanism and oiled it carefully. I also took off the piston with needle, turned the mixture screw at first fully in so that it was in level with the bridge. After that I slightly loosened the needle in the piston, pulled it a little bit out and put the piston with that loose needle back into the shaft. By that the needle's shoulder came in line with the shaft and closed the gap (means totally lean mixture). After that I fixed the needle in the piston and turnde the mixture screw until the shaft was 1,5 mm lower than the level of the bridge. That was my starting position for the fine tuning of the mixture. That took me a lot of time 'cause I didn't do it with the colortune that I have but by looking at the plug No. 2 about every 50 mls of normal drive.

[beandip]

:classic: Be sure when you shove the lever of the choke forward and turn off the choke that it acctually is off. Another thing , this happeded to me , the little fuel line that connects to the mixture adjustment had gotten brittle and was causing the choke to stick , just a little but enough that the front carb was running rich. I replaced the hoses and all is fine. One thing dont use just any hose, be sure it is for gasoline , and not vacume hose. Try nappa I have herd that they heve some, also from the dealer. Most after market hose is too stiff . another option is to try a Cycle shop . Some models at the hobby shop use gasoline in the large 1/2 scale also the large model boats. I bought mine from Z-Therpy. I think it was $8 .00 for the pair. Z Tharpy # 503-587-9800 If you dont own one get a uni-sun to ballance the carbs , you will need one , other wise you will have nothing but trouble. It will make it simple . Local speed shop or try a Harley shop , I paid $25.00 at the local parts store in the speed section. Victora British has them also , so does Motor Sports . All the best Gary:classic: