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1972 Float Adjustment ...

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I have a 72 with 3 screw SU's and they have the long and short arms in the float bowl lids. Long in front carb, short in rear.

I intend to set the floats on my bench top by setting the lid with its float and gasket on top of a small clear drinking glass with a 23mm scribe mark at the correct spot.

Can anyone tell me whether the distance between the inside of the float bowl lid and the top of the gas is 23mm for both the front and the rear carbs on a 1972, or is the front 23mm and the rear something less ?

I'm looking for a definitive answer for myself and all my 72 brethren :)

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The 23mm mark is the same for both when you measure the bowls. If you use the upside down lid method, you use 16mm and 12mm. There was a huge thread on this in July.

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The 23mm mark is the same for both when you measure the bowls. If you use the upside down lid method, you use 16mm and 12mm. There was a huge thread on this in July.

Thanks for the quick on point response!

What's the title of the thread from July ?

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Yeah I had read that one, that's where I got the idea for the clear bottle, which I think is a genius idea BTW!

Thanks for ferreting out those numbers, until I hear otherwise I'm using them for my 72 carbs :D

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Because of the tilt of the engine, and the front float bowl being in front of the nozzle while the rear bowl is rear of the nozzle, the level in the front bowl needs to be lower than in the rear bowl by 2 mm. This puts the fuel at the same level in at the top of the nozzles. That's why the ears on the front are longer by 2 mm. The FSM for 72 SU's just says 23 mm down from the underside of the lid. So 23 mm rear carb, 25 mm front carb for equal level at the nozzles. I guess you could go 21 mm rear, 23 front - I had trouble with the rear carb overflowing out the vent when I tried that, but can't say for sure it's wrong.

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The engine does know know where the floats are... it only sucks in air. The fuel level in the jet should be the same for both carbs so that the suction at the top of the jet draws the fuel up the same height.

It is like drinking from a long straw vs. a short straw when you simply the process. The length of the straw is more important when the suction is weak due to gravity... as the suction force increases, gravity forces matters less.

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So I created this apparatus to help set the floats accurately and now I have even more questions.

To achieve a gas (water) height 23mm from the inside of the lid the float is nearly contacting the inside of the lid, that's doesn't seem correct. Its that way on both lids and floats.

Some assumptions :

1. The gasket and the inside depth of the lid adds up to about 4mm. So I marked the outside of the glass at 19mm.

2. Water behaves similar the gas for this test.

3. My valves are correct and in good shape.

Since I have them apart I thought I would use my calipers and calculate where the top of the gas is relative to the carb bridge assuming a fuel height of 23mm.

Here's what I got :

The bridge to the dome surface is 44mm

The top of the float bowl to the top of the dome surface is 33mm

So from the top of the float bowl to the bridge its 11mm

Add 4mm for the gasket and lid and you get 15mm from the inside of the lid to the bridge

Subtract 15mm from 23mm and you get 8mm, not 10mm as I've seen elsewhere ?

I'm thinking I should replace the values anyhow, where's the best place to get a proper set ?

post-4410-14150825436305_thumb.jpg

post-4410-14150825435179_thumb.jpg

post-4410-1415082543576_thumb.jpg

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A gallon of water weighs about 8.33 #'s a gallon of gasoline weighs 6.07 #'s so the float is going to ride higher in the water versus the gasoline because it has to displace more gasoline to equal its weight than water to equal its weight...

Charles

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Theorectically you could multiply the desired dimension by 1.3723 and that would give you the correct value for float setting in water. But I am not sure I did the conversion right... somebody want to check that?

C

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Well, I'm back at setting the floats and have fitted them with some new valves I ordered. I'm still using the glass jar setup, but I've eliminated the water pump arrangement and have a small funnel attached to the inlet port by means of some tubing. The glass is marked at 21mm as a reference and I can poor gas into the funnel and watch where the level stops. I was able to get the short legged cap set to stop at 21mm with the black plastic float about 1/8" from touching the inside of the cap when inverted, however, the long legged cap with the taller valve could not be adjusted enough to allow the level to come up to the 21mm mark. I stopped when I noticed that the tang was bent so far in that it was contacting the body of the valve when inverted and the plastic float was hitting the inside of the cap. I ended up using another short valve body and bending the tang up so that inverted the float was about 1/8" from touching the inside of the cap and that put the level at the 21mm mark. I think this is where I'm going to try them.

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Yeah perhaps, but the new valves behave just like the old ones I took out. I couldn't get the long legged cap dialed in with the old tall valve body either.

I just find it interesting that to get the correct gas level in the glass bowl required that the black float be nearly touching the inside of the cap. If I were to use the static 12mm, 16mm inverted lid method I'm sure that there would be insufficient gas in the bowl during acceleration (stumbling). Those numbers might work if the needles were solid and not internally spring loaded. IMHO, the spring loaded 3 part needles are just not designed for an inverted static setting. Does anyone know if the original needles were spring loaded, or were they solid ?

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Weighting the float could possibly be a solution to increase fuel height (and keep the float parallel to the roof at nominal)..... or cutting the high towers down and drilling new holes for the float pin.

Not sure how one would bond a weight to the plastic float in that environment, but its worth thinking about.

I like the redrilling idea, but I don't think the "towers" would need trimming down unless they interfere with the movement of the float.

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You could open the float at the top, insert weights, then seal so nothing external to hang up.

It's the sealant that worries me in a long time gas environment.

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Fuel level in the front bowl needs to be 2 mm lower than rear bowl. that's why the ears and valves are 2 mm longer (for the later 3-screw SU's) for the front carb. Then the fuel in both nozzles will be at the same level (the main thing).

Wonder if the 4-screws also ran longer nozzles in the front? The 72 FSM doesn't mention this change - wonder if Nissan engineers were embarrassed that they got it wrong with the 4-screws. Maybe the factory race team noticed the fuel level difference when they pulled the domes and checked fuel level in the nozzles. Easy enough to fix with a couple of washers, so maybe they passed it along to the designers who incorporated the change for the 3-screws. ???

Edited by Stanley

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It's the sealant that worries me in a long time gas environment.

The floats are solid foamed EPDM stuff . Nothing to seal. If you really really had to add mass, just screw in a tiny screw or embed a tiny needle or metal shaving.

I'm also wondering about all this lovely static fuel height setting and imagining how this relates to the case when fuel is spritzing in the top and flowing out the bottom as the car runs. Here's hoping its kinda similar.

Edited by zKars

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The floats are solid foamed EPDM tuff . Nothing to seal. If you really really had to add mass, just screw in a tiny screw or embed a tiny needle or metal shaving.

I'm also wondering about all this lovely static fuel height setting and imagining how this relates to the case when fuel is spritzing in the top and flowing out the bottom as the car runs. Here's hoping is kinda similar.

Ah I see, I didn't know they were solid, that makes a big difference!

Yeah I hear ya, with all of the fuel entering and leaving and the car going up hill/down hill over bumps, idling, accelerating, banking, etc. it's a wonder a float system works at all !

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Early Spitfires had floats, German Messerschmitts had fuel injection. In -neg G moves, the floats raised and caused the Spitfire's engine to stop. To circumvent this, the Spits had to roll first before pursuing...... giving the Me-109 time to get away. Think of this when you pull high G turns, etc.

I think putting the float on a gimbal would help (or the long buffer hose (anti slosh tank) between the bowl and jet solution I mentioned in another thread) however Weber and Hitachi (flat top) addressed it by putting the jet smack in the middle of the float bowl.

Edited by Blue

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