Checking correct fuel level at Round Top 3 screw SU CARBS Float bowls

[jalexquijano]

I guess its arriving today!!

[Mark Maras]

1 hour ago, jalexquijano said:

I guess its arriving today!!

Be sure to let us know what the float levels are currently set at when you get the new pieces installed. Useful information for us die-hard SU freaks.

[jalexquijano]

I read that Grose Jets are the solution to our problems?

[Mark Maras]

 I'm not sure if Grose Jets are still available. They were an improved (better mouse trap) needle and seat valve. I suspect they were more effective at shutting fuel flow off at higher than normal pressure which is 3 - 3 1/2 psi. I've never experienced a problem with the oem needles and seats. 

[jalexquijano]

Just received a couple of the su float level adjustment hoses that ztherapy is selling. They do look different than the ones Radalla sells. Anyway i am hereby attaching the sheet provided by Ztherapy which states the gap between the top of the float bowl lid and the float is .055". 2 questions:

1. How do i measure if the level is correct? Just plug these hoses  to both of the drains with the car engine on?

2. the correct gap is .55" or other measurement? My car is a 1972 240z with dual roundtop 3 screws su carbs and SM needles fully rebuilt by ztherapy on november 2014. 

[jalexquijano]

Is there any precaution i must take before opening the float bowls in the front and rear carb? i believe they must be filled up with fuel and i could easily damage the corks. I just want to verify why my mechanic couldnt tune up both carbs to 2.5 turns clockwise and had to drill some extra holes to the ears of the front lid. Even with this approach he left it at 4 turns clockwise on the front and 2.5 turns clockwise on the rear.

[Stanley]

I have no idea why your mechanic does what he does.

I take one precaution: drain some of the gas out of the bowls before removing the lids. They need to be turned to easily lift them out and its easier with some of the gas removed. That lets the floats hang down so it's easier to turn the lids. 

Also, someone recommended using longer hoses from the banjo bolt to fuel rail while setting the floats. Then you don't have to remove the hoses each time you remove the lids, so it speeds up the process.

Regarding 2.5 turns down: it's just a starting point. In fact it's only the starting point if the location altitude and temperature vs. mix setting graph in the FSM says so. Here are a few of the things that affect carb performance. If any of these are not stock then adjustments will probably need to be made.

Fuel pressure, air cleaner, exhaust system, engine displacement, cam, cylinder head flow/compression ratio, carb needles, carb springs.

 

[Mark Maras]

1 hour ago, Stanley said:

I have no idea why your mechanic does what he does.

I take one precaution: drain some of the gas out of the bowls before removing the lids. They need to be turned to easily lift them out and its easier with some of the gas removed. That lets the floats hang down so it's easier to turn the lids. 

Also, someone recommended using longer hoses from the banjo bolt to fuel rail while setting the floats. Then you don't have to remove the hoses each time you remove the lids, so it speeds up the process.

Regarding 2.5 turns down: it's just a starting point. In fact it's only the starting point if the location altitude and temperature vs. mix setting graph in the FSM says so. Here are a few of the things that affect carb performance. If any of these are not stock then adjustments will probably need to be made.

Fuel pressure, air cleaner, exhaust system, engine displacement, cam, cylinder head flow/compression ratio, carb needles, carb springs.

 

 Stanley, You spoiled the ending of the SU float drama. I was just trying to get Jalex to the 2 1/2 turn intermission before the fine tuning grand finale.

 Jalex, don't let this scare you. Setting SU floats is like eating an elephant. You can't look at the whole meal. Just take it one bite at a time. You can do it. Even with our help.

[jalexquijano]

Well at the end i decided to drain both float bowls removing the drain screw and open the box of surprises:

1. Mechanic drill a lower hole to Front float lid ears to compensate i dont know what maybe the size of the needle valve that ztherapy installed in that lid which should have been the larger one. Front and rear carb have the same size of needle. The .55 gap was also wrong it measure .61 inch.

2. There is a disadvantage on drilling those holes as the float gets to a point where it cannot move any further.

 

3. The rear float was also out of gap if you measure the gap from the top of the lid to the bottom of the float.

4. Setting the carbs at 2.5 shows the same depth as per the pictures.

Now what else should i do? Install the correct height needle valve, move back to the original holes in the front lid and get a gap of .55?

 

[Mark Maras]

@siteunseen Cliff, When you shimmed? the needle and seat in one of your carbs, did you have equal length needles and seats in unequal pivot ears? I'm kinda in uncharted territory. My experience is with 4 screw SU's where all things are equal. It seems to me that it would be best to get the different length needles and seats and go back to the stock pivot point. Opinions are needed.

[Stanley]

Yes, use the long valve in the front bowl and put the float back in the original holes like Nissan intended. The fuel level needs to be lower in the front bowl and that makes it easy. What happened with the ztherapy plastic sightglasses in the first post ? That's what I use. Or the ones by Randalla. Or use the "both nozzles at same height, same fuel level in the nozzle" method. If you can use either of those methods why bother measuring .55 inch or whatever ? That method is a little iffy, even if it's in the FSM.

Make sure the car's sitting level, not on a sloping  driveway while setting the floats.

[jalexquijano]

I want to be able to dial both carbs equally at 2.5 turns clockwise