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38mm SU Carbs running rich at idle, but adjusted fully lean- wtf?


mhub91

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Got a set of SU carbs from Banahamuck and decided to throw them on. 

 

It seems like I can't get them to lean out. I've got the fuel mixer all the way in and they're still running WAYYYY too rich at idle. They blow black smoke like a diesel truck!

 

What should I look for?

 

I'm totally new to SU carbs btw.

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Pretty sure you can move the jet up and down on those models with a screw located under the carbs,,  not real sure without book in front of me though.   If they are super leaned out at the bottom ,, they had no power at the top.

 

 

Get a "How to SU" manual through Amazon or Ebay and quite a bit of information will be had.

 

 

 

PS ,, this is what i warned you over and over about before we even traded.

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Read up.....

 

http://www.zcarz.us/TechnicalInformationPageCarburation.htm

 

Turning the screw clockwise drops the jet which richens the mixture

Turning the screw counter clockwise raises the jet which leans out the mixture.

 

Even if one can get some decent (non-smoking...lol) A/F numbers at idle....doesn't mean the other stages like (WOT & cruise) are at all acceptable.

This is where a wide band comes into play.

 

..."HOW TO MEASURE FUEL LEVELS?

This has been covered in several places online. Be careful since you're dealing with gasoline(!) Mark the front of the float bowl 23mm down from the top edge of the bowl (not the top of the lid). These measurements are done with the carbs on the car.

Get some small diameter clear plastic hose from the hardware store, cut a piece about 18" long. Connect one end to a float bowl's outlet and run the other end up over the carb. Some people connect the hose to the drain plug on the float bowl but the fuel hose outlet is a lot easier.

Bring your fuel pressure to what it would be while idling. This means if you have a mechanical fuel pump turn the engine over a few times with the coil wire off. With an electric fuel pump just turn the ignition on to pump up the fuel pressure.The fuel level will equalize in the hose and should line up with the 23mm mark on the float bowl. By sighting the level in the hose and bending the tab on each float you can get the float fuel levels to match within a millimeter...."

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Pretty sure you can move the jet up and down on those models with a screw located under the carbs,,  not real sure without book in front of me though.   If they are super leaned out at the bottom ,, they had no power at the top.

 

 

Get a "How to SU" manual through Amazon or Ebay and quite a bit of information will be had.

 

 

 

PS ,, this is what i warned you over and over about before we even traded.

 

 

I know! I'm not knocking you, or them. I'm happy with them! Just getting some info!

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Do you have the air cleaner backing plates installed? If you do, make sure the vent holes are open. Some backing plates do not have the holes in the right place.

 

Also, if the needles were ever removed or adjusted, you may need to readjust them.

 

Really though, it sounds like the well jets are stuck in the choke-on position. You should be able to push them up from the bottom to seat them. If this is the case, then the carbs either need a good cleaning or new well jets.

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Do you have the air cleaner backing plates installed? If you do, make sure the vent holes are open. Some backing plates do not have the holes in the right place.

 

 

 

 

The aircleaners have to be put on correctly as there are drilled for that,,  but only for the correct sides.^^^

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Disconnect your choke before you make any adjustments and make sure the nozzle is actually coming up all of the way. Then reach inside the carb and make sure the piston is dropping. You can also do this by pulling the dipstick on the dome and reaching your pinky inside to make sure the piston is dropping. (Press down slightly)

 

Needle height is very important. You can do the 'drop test' to verify the needle is seating all the way with the nozzle at full up. ONLY do this after you've verified float height and all other adjustments.

 

 

Pull the dome and very carefully lift the piston. Be careful to not bend or damage the needle, these are sensitive and hard to straighten. On the side is a small set screw that holds the needle in place. Loosen it up just enough to allow the needle to slide loosely in the piston. Set the nozzle at the full up position and make sure there are no obstructions. Some SU's (I only know Z car SU's but they're not really different) had a piece installed to hold your adjustments or limit them. Once you loosen the set screw and have verified that the nozzle is at the full up position, CAREFULLY slide the piston/needle back in place and 'drop' them once it's centered. Carefully lift the piston back out, tighten the set screw and look at the needle position relative to the piston. The shoulder on the needle should be level with the piston. 

 

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If yours isn't level with the bottom of the piston, then you have an obstruction on the nozzle preventing it from reaching the full up position. A slight variance is normal, but it shouldn't be anything more then barely noticeable to the eye.

 

 

The reason I mention all of this is because my 240Z Su's had an identical problem I chased for a long time, first the rear carb piston wasn't dropping all of the way because the nozzle wasn't centered, then I found from the help of a Canadian guy from the ClassicZCar forums that was in town that the rear carb nozzle wasn't reaching the full up position because someone had mis-installed a small washer under the washer from a rebuild kit. I had the needles set flush with the piston as they should, but since the nozzle was .75mm off at full up, the rear carb ran super rich all of the time.

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Also, check the floats themselves for cracks if they are the old style brass series ( the later ones are plastic) They are notorious for getting cracks and filling up with fuel, then sinking and raising the bowl level. 

 

 

Both floats are brand new ,

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