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CGraves

L24 running extremely rich

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They are both filled to the correct amount of oil and they came off a 240z I'm not sure what year though.

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I figured it out. I'd like to start this out by saying either I'm retarded or zcardepot is retarded. The carb rebuild kids they sell don't fit any of my SU carbs needles and seats are either too big or too small. I'm about to rebuild 2 of them with the best parts from each other so hopefully I'll have 2 working carbs in a couple minutes.

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Okay new carbs went on and when you apply slow steady throttle it will rev up and work fine but if you give it a lot of throttle in one go it backfires through the carbs.

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Lean backfire. Not getting enough fuel for the air going in.

 

 

Look in the open SU and watch the slides. Do they both lift at about the same rate? If one jumps ahead of the other maybe the damper isn't working.

 

Needles too large or seats too small or both. Basically the jetting mixture is wrong. Now this could be on just on one of the carbs causing this.

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Okay mike first of all I'd like to say thank you for dealing with me and if there's anyway I can pay it back please let me know.

 

Anyway id like to update the post. IM RETARDED. I reversed the #3 and #4 plug when I changed the plugs. Everything runs like a top now.

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Ha ha, hey.... it was a cheap easy fix.

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Okay so another problem I'm experiencing a loss of power randomly. I've only had it happen in the middle of third and fourth gear but it will be fine then randomly a huge backfire will happen and then no matter how far I press the pedal I get nothing.

 

Is it just some water in the tank/bad gas or is there another problem I should look into?

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Happen under prolonged load, like climbing a hill? passing? that may be a clogged fuel filter. The engine is using fuel faster that it can be sucked through a part clogged filter. The filter may be new but the tank dirty so it's just doing it's job. They are cheap and easy to replace to find out.

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Nope I live at the beach so it's literally driving on perfectly straight roads cruising at like 40 MPH.

 

That's why I think it might be bad gas. Also I live in NC and there's currently a gas shortage so that leads me to believe there's something wrong here.

 

Also if it's a fuel filter is there one at the tank and one in the bay or is it a single filter?

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72 240z has a filter in the engine compartment.

 

74 260z likely has one between the tank and the electric fuel pump (if so equipped) and one in the engine compartment too.

 

Assume you are talking about the 240z.

 

Hey now, Thomasville is definitely not at the beach... you down a school at UNC-W or something?

 

dgi

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Okay so I've been cycling the bad gas out and I also replaced the fuel filter while I was at it. Prolly gonna change the plugs tomorrow too. It's running a lot better now!

 

@dgi yeah I'm going to UNCW for computer science right now. I just moved down at the beginning of the summer. It's been pretty fun so far. Found a good car scene down here too!

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Okay so I replaced the filter in the engine bay and I don't think it's the water in the tank because I can rev the engine in neutral just fine but as soon as I try it in gear it bogs and won't go anywhere. I did some research and is it possible it's a distributor/ignition problem?

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Revving the engine in neutral requires spinning about 50 pounds of crank and rods and very little fuel is needed. When in gear under load moving a 2,800 pound car things are vastly different. If the carb or carbs can't provide it the engine goes lean and falls on it's face, often accompanied with a backfire out the carb. 

 

The ignition either works or it doesn't and certainly can't tell when you are revving in neutral or driving.

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If it bogs when you open the throttle to take off under load and then picks RPM back up and pulls, perhaps even repeats that in 2nd gear or upon sudden acceleration under load in any gear and is often accompanied by backfilre, you have described a classic SU tuning issue.  

 

The suction piston is rising too rapidly which leans the mixture out.   Normally this is prevented by the oil damper.   If the oil is too light, this lean out condition occurs.  Also, if the suction spring is too light or missing, this lean condition can occur.

 

I suggest you read about the venturi control system for the SU carbs at this link

 

I personally find 20wt oil a little too light, so I mix 20wt and 30wt 50/50 trying to reach about 25wt.  That seems to be about right for me in warm weather.  I usually run 20wt in the winter time.. 

 

dgi

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It isn't lean backfiring so does that change anything? Is it possible that the carbs being unevenly filled would cause this? I just kinda poured some oil in them without making sure they were perfect.

 

 

But what exactly happens is first and second gear will pull fine. But as soon as you hit about 50 MPH in third or fourth it will bog and you can press the peddle down as far as you want but it won't accelerate but it will keep that speed. If you push the clutch in you can rev it as high as you like.

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Running well up to 50 under load and then falling on it's face, sounds like a fuel delivery problem. The carb float chambers are full of gas. If you take it out faster than it's put in it runs out and engine starves. This can be several things but if the float is set too low and the float chamber is almost empty it could easily run out. Didn't I post a link that shows how to set or check the fuel level???? I think you use a clear piece of vinyl tubing.

 

 

 

 

No doesn't always backfire.

 

Yes uneven float level changes everything.

 

 

 

Take for a drive (if possible) get it warmed up well and pull over, don't let it idle.

 

Pull plugs. They should be evenly colored at least. Darker or even black is too rich. Light or tan is about right. Very light or even spotlessly clean white porcelain is way too lean. You may notice the front 3 much different from the rear 3. 

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My plugs look great(I think). 

 

Okay so I have a mechanical and electric fuel pump but I normally don't run the electric but today I turned it on on the way home and then filled up at the gas station and it's running fine now. I guess whatever the problem was it fixed itself. I'm gonna drive it some more and see what it does.

 

Here's a pic of the plugs though. The one on the left is from the 1st cylinder and the one on the right is from the 6th cylinder.

http://i.imgur.com/ztiWJWw.jpg

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Plugs don't show rich mixture.  If you are running SU's be sure that the chokes are off and the fuel nozzles are UP

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ztiWJWw.jpg
 
Holy fuck that's LEAN!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Dangerous Lean running can melt a piston top.
 
Should be color of cardboard.


I don't know what the plug make is that you have.... but get NGK BP6ES

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Okay so I checked it and the plugs are completely black now. I literally changed nothing. Also both dash pots are low on oil but they are evenly filled. I have a set of NGKs for it but I didn't wanna ruin the good ones while I'm working.

 

Edit:Okay so I've come to terms with the fact that SU carbs aren't for me. If it's not FI or a holley 4bbl I'm clueless. I'm just gonna take the easy route and drop a L28 in. Gonna try and get it in soonish. I'm gonna run this motor till it blows I guess. I'll be sure to post a build thread too.

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So, it looks like you ran it down the road under load, cut it off and didn't let it idle, pulled a couple of plugs like Mike instructed.  You proved it is running on the lean side.

 

Then you drove it home, let it idle, pulled the plugs again and they were black.  So, you proved it is too rich at idle.

 

Read the reference I linked you to in reply #41 (again).

 

Are they the correct heat range plugs?   Mike told you the ones to make sure you are using.

 

Pretend you are debugging one of your computer science programs...  Really, that is what you need to do... Determine what the carbs are actually doing and then fix them...  After all, SUs are pretty close to the simplest carbs ever designed.  

 

You can fix them!

 

Otherwise bring the car home to Thomasville and make me a deal on it!

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Honestly I might take you up on that offer lol.

 

I was gonna change the door striker plate because it wasn't shutting very easy and someone cross threaded the bolt on the top or something and it FUCKING snapped while I was removing it. I'm just gonna get a trailer and take this thing home and steal my dads car until summer or something.

 

I did however get an L28 with the FI so expect a build thread /soon/

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