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jalexquijano

Setting Correct Float Level On 3 Screws Su Carbs For 1972 240Z

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i already set it to 9/16 (.55 inch gap) from the bottom of the float to the edge of the float lid and i  still have to turn the mixture knobs 4 turns clockwise when it was supposed to be 2.5 - 3 turns clockwise. Does the tang needs to be pushed a little bit more thus closing the gap less than .55 inches? Still dont understand the 1/16 inch below nozzle method at 2.5 turns.  

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You need.45 and .65 wooden sticks as go - no go guages.

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Well i tried using the sight glass method to set the gap between the float and float lid in the best possible way. I have posted pictures for your opinion on the levels. The first one is the rear carb and the last one is the front carb.

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Posted (edited)

That’s great. What you need to do now is get your My Little Pony spirit level and set it up across the flanges of the carbs and adjust it until the bubble is centralised. Then put on your Mr Magoo spy goggles and using a pink texta, transfer a line from the spirit level to the carb bowls front and rear. Now you need to carefully take a Bob the Builder drill with the same size plastic bit ( don’t want sparks near fuel) as your clear hose and drill a hole through each bowl on the pink line and shove the clear hose from the front bowl into the new hole of the rear bowl and vice verse the other hose. It’s a self compensating fuel level system that self regulates regardless of how many electric fuel pumps you are running. Make sure the car is on level ground before you start because your measurements will be out of whack but I’m guessing you know that.

Edited by TOO60Z

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Posted (edited)

To open another can of worms, to get it 100% perfect, you can ignore all that 9/16  float height setting stuff and set it by measuring the level of fuel in the jet, although I don't know what that setting is for 3 screw Hitachi SU's.  :-X

 

Well, there you go. According to Gav's link, the setting seems to be 7-10mm down from bridge. From memory, 1 3/4" SU's are 3/8" down, so pretty similar.

 

Setting float height is really just a way to get the fuel level in the jet correct, so why not go directly there to carry out measurements rather than the round about method of measuring fuel level in float chamber. Wind the jet down, or pull it down slowly with the choke, till fuel just starts to "spill" over the top of the jet. Measure down from bridge to jet using calipers, you will get an exact measurement that is independent of any other factors.

Edited by 1600dave

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Took the car for a ride yesterday and had to dial both fuel mixture knobs to 4 turns clockwise. 3 or 3.5 turns simply dont work. Could this be attributable to the fact that my engine is not stock and has been modified to 2.6Liters with a Schneider 274F camshaft and pertronix ignitor module and coil? I would really like to set it to 2.5 or 2.75 turns clockwise but it becomes to lean and lacks power.

I will test the car today without the airbox fastened to the carbs to ease the adjustments if necessary. Airtex 8012SX pump is only pushing 3.2 psi.

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Posted (edited)

If there was no need to adjust this setting, the factory wouldn't have made it adjustable.

 

If it runs fine @ 4 turns, but you simply MUST have it set to 2.5 turns, take the jet out from the bottom of the carb, place a 1.5mm thick washer on top of jet, put it back together and set it to 2.5 turns.

 

3.2psi is more than adequate, forget about this.

Edited by 1600dave

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Posted (edited)

I think you need different needles to suit your engine requirements.

By putting the vehicle on a dyno and monitoring the co2 levels this can be confirmed.

At the moment you have been discussing idle mixtures but, I think you will find that the mixtures will not be correct under load and this is a far more important time to have mixtures correct.

Lean mixtures under load can lead to engine damage!!!

Edited by Cozza

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Posted (edited)

Actually, serious answer - if you must have it set to 2.5 turns, get a new needle that suits the engine modifications.

 

The reason you need to drop the jet to 4 turns is because the modified engine needs more fuel. Dropping the jet allows more fuel. A different needle can also allow more fuel. What needle you need depends on where it is rich / lean when you set it to 2.5 turns down. Does it run lean all across the rev range ? Just at idle ? If you're changing the needle, you need to answer these questions to determine if you need a needle with more taper, a needle that is "thinner" everywhere, or e needle that is just thinner in various locations up and down the needle.

 

Best bet is to put it on a dyno with a dyno operator that understands SU's and go for it.

 

Or accept that it runs perfectly OK at 4 turns down, and setting it to 4 turns down is a perfectly acceptable solution.

 

EDIT: Yeah, do what Cozza typed while I was typing  ^^^^^^ ;D

Edited by 1600dave

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I had exactly the same issue on a modified 4 with SU's - you will need to find the right needles, or accept lots of flat spots and poor running... not sure who the go to person is in Panama though?? I know the right place in Melbourne...

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Posted (edited)

I used to get mine (38mm L4 carbs) done here in Newcastle by an older dyno tuner, absolutely brilliant, not just selecting the right needle but even went as far as "modifying" it by whacking the needles in a drill press and "pinching" sandpaper onto them at appropriate process to slighty re-profile the needles. It ran perfectly.

 

He also tuned the 1 3/4" British SU's currently in the car, again a perfect job.

 

Went to book the car in for a tune just before heading to Bathurst last November, only to discover he's retired and the guy who took over the shop no longer deals with carbs, only tunes efi :-[

Edited by 1600dave

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