Adjust Carbs? 71 240 l28

[71Nissan240Z]

Hey everyone,

I recently just joined here, I don't know too much about my car but I am trying to learn on my busy work schedule.

My car starts very fast when its warmed up and the normal few cranks when its cold with the choke on.

However, it used to diesel when I tried to turn it off and would not shut off until I put it in 1st gear to kill it.

I realized the idle was too high at over 1,000 rpms, so I turned the fast idle screw down, and now it idles around 800-900 rpms. Now it turns off good but sometimes after running for a while it will continue to clunk for a few turns before tuning off.

I realized that at low rpms, around 1,500-2000, it seems to have aslight misfire, I can feel it in N and while I drive. Once I bring it past 2,000 is has a lot of power and does not hesitate at all.

Once I reach 5,000 rpms the engine begins to shake a little bit and get worse as the rpms go higher. I can take it out of gear and put it into N and it gets smooth right away, so I know its something to do with the engine.

To me it sounds like its running slightly rich, but I am not sure. It has an L28 bottom and p79 head, double carbs with k and N filters. I ran some Seafoam through the break booster line, and it seems to make it better and run less rough at higher rpms.

Is there anything else I can do? is it just dirty carbs? I have carb cleaner, should I use more seafoam through the break booster line? or should I just adjust the carbs?

I have a repair manual but I just dont have time to read it right now. I'll have to wait until this weekend before, I can open up the book. But any advice from you guys would be greatly appreciated! Thanks!

Abdi

[Weasel73240Z]

Sounds like you may have issues with both the air/fuel mixture, and possibly timing.

By double carbs, do you means double SU's or double Webers (aftermarket)?

Get a good timing light, a Unisys (to balance carbs) and if you have SU carb's, get the ZTherapy video "Just SU's". I think that the run-on (dieseling) could be related to a float that is stuck too high in one of the carbs.

Getting the air/fuel mixture dialed-in is the toughest part in my experience. It took me at least 2 weeks of playing around to get it just right. Use the lifting pins on the underside of the carbs to lift the suction piston (SU's) about 1/8", until the exhaust note rises slightly, then falls back to about the original idling tone. If the motor sputters when you lift the pin and sounds like its going to stall, it running lean, if it races when you lift the pin, its running rich. Use the mixture nuts on the bottom of the carbs to adjust the a/f mixture. When it rises slightly, then falls back down to where it as at idle, the a/f mixture is about right.

Of course, if you have anything other than SU's I have no idea. :laugh:

[71Nissan240Z]

Sounds like you may have issues with both the air/fuel mixture, and possibly timing.

By double carbs, do you means double SU's or double Webers (aftermarket)?

Get a good timing light, a Unisys (to balance carbs) and if you have SU carb's, get the ZTherapy video "Just SU's". I think that the run-on (dieseling) could be related to a float that is stuck too high in one of the carbs.

Getting the air/fuel mixture dialed-in is the toughest part in my experience. It took me at least 2 weeks of playing around to get it just right. Use the lifting pins on the underside of the carbs to lift the suction piston (SU's) about 1/8", until the exhaust note rises slightly, then falls back to about the original idling tone. If the motor sputters when you lift the pin and sounds like its going to stall, it running lean, if it races when you lift the pin, its running rich. Use the mixture nuts on the bottom of the carbs to adjust the a/f mixture. When it rises slightly, then falls back down to where it as at idle, the a/f mixture is about right.

Of course, if you have anything other than SU's I have no idea. :laugh:

Thanks! I'll give that a try! Im not sure if I have the SU carbs or Weber, I think its the stock carbs with K and N filters. The filters are about 6 inches wide and sit verticle with crome in between them.

[Bruce Palmer]

If you need help identifying what you have go to www.ztherapy.com. There are plenty of pictures to help sort it out. We are all the way down in Salem if you'd like to take a run down this way and get aquainted with Steve who by the way kicked some major E Production butt last weekend at the Rose Cup Races.

Cheers

[71Nissan240Z]

If you need help identifying what you have go to www.ztherapy.com. There are plenty of pictures to help sort it out. We are all the way down in Salem if you'd like to take a run down this way and get aquainted with Steve who by the way kicked some major E Production butt last weekend at the Rose Cup Races.

Cheers

AWESOME! I am glad to have so many resources. It seems there are lot of 240z fans in the NW. I am probably the youngest, I am only 25. I see in there that you guys rebuild carburetors? I might need to go down there when I got some time. Are you open on the weekend? I would also like to go to one of your datsun meets as well.

[Bruce Palmer]

Unless it's a race weekeknd there is usually a crowd thing going on at the shop in the afternoon hours. Steve is very helpful helping new owners get their learning curve functioning.

[71Nissan240Z]

Okay so I have two carbs, SU carbs. The carb closest to the distributor does not stall when I push in the lift pin under the carb, the idle slightly goes down a little bit barely and then goes back up to normal. The other carb, when I push in the lift pin, it totally dies. Can someone tell me where the A/F ratio screw is? there is so many of them. I think it dies because its running lean. Thanks!

[Weasel73240Z]

Okay so I have two carbs, SU carbs. The carb closest to the distributor does not stall when I push in the lift pin under the carb, the idle slightly goes down a little bit barely and then goes back up to normal. The other carb, when I push in the lift pin, it totally dies. Can someone tell me where the A/F ratio screw is? there is so many of them. I think it dies because its running lean. Thanks!

The A/F mixture is adjusted at the bottom of the carbs. See #7 on the attached diagram.

SU.pdf

[71Nissan240Z]

The A/F mixture is adjusted at the bottom of the carbs. See #7 on the attached diagram.

Thanks, so do I have to adjust both carbs until they are identical when I lift the lift pins, or do I just need to adjust the A/F until it doesnt stall out when it push it up.?

What I mean is, The first carb idle goes down slightly when I push the pin in, the other completely dies. Do I need to keep turning the screw until I get the same result of the first carb, or do I need to keep adjusting it until it doesnt die anymore.

[beandip]

First off you need to use a air flow meter to balance the air flow of the carbs so that they are the same. Without doing this you are just chasing your tail. I have a couple of the things . Then screw in the mixture adjustment conter clock wise to the stop, and then back it our 2 1/2 turns , clockwise. Do the same on both SUs. The engine should run reasonably well at this point if the carbs are balanced first. At this point stick your finger in the throat of the front carb and lift the piston. This will take the front carb out of use, the engine will be running on the rear one. If the engine dies when you do this , the REAR carb is too LEAN. If the engine stumbles but keeps running , then leave it alone at this point. Now go to the rear SU and repeat what you just did on the front one. Again if the engine dies , too lean if stumbles leave it alone . Now to richen the mixture you turn the adjustment CLOCKWISE , to lean the mixture COUNTERCLOCKWISE. I repeat the air flow of the individual carbs must be balanced first.

Gary