8/71 SU Carb Help, Float Bowl Adjustment

[Pinbill]

Hello,

I had my carb adjusted pretty well but wanted to get it running better. Here is the backstory.

So far I replaced all the intake gaskets, cleaned the carbs thoroughly, adjusted the timing and set the valves. I checked the float level by measuring from the top to the float. I set the mixture using a gunson color tune. The car would run good but would pop a bit on deceleration and the plugs always looked a bit rich. When lifting the slide I could never get the idle to go up 100 rpm and then settle.  No matter how I set the mixture the idle always would drop off when I lifted the slide slightly. I have read that if the idle drops off that means the mixture is too lean. It seems conflicting to have rich plugs and lean condition when I lift the slide.

I played with it this afternoon and made it worse. I want to start from the beginning. I would like to thread something into the bowl at the drain and set the float using the clear sight line float adjustment method. The procedure looks pretty straight forward but I don't know the thread size for the drain plug. Does anyone know the thread size for the bowl drain plug so I can order a nipple for the clear line?

Thanks for the help,

Bill

 

Edited July 24, 2020 by Pinbill

[Mark Maras]

 Was it running the same before the refurbish? 9/16" float measurement? Nozzles 2 1/2 turns down? Any possibility of a vacuum leak?

[Pinbill]

Hey Mark,

Thanks for getting back to me.

Before the refurbish the car hadn't been run in six years, the carbs were real dirty, the intake manifold gaskets were original ect.. It ran ok but it wasn't the greatest. After the refurb it was running much better. Then today I changed the float level from 14mm to 10.31mm. The car ran much better at 14mm. To set it I blew through the fuel inlet line and made sure the valve was fully closed and not letting air through at 10.31mm. Is that the best way to do it?

I found this spec in the service manual and tried 10.31 because my car is 8/71 build date. Should I use the 14.28 setting?

I will rig up a vacuum test tomorrow.

 

 

[heyitsrama]

Is there a difference in the float needles when compared front to rear?

[Mark Maras]

 Do the carbs have 4 screws or 3 screws that fasten the suction chamber to the carb body? It's the 3 screw carbs that use 2 different float settings due to the difference in the float pivot heights.

[Pinbill]

It's a four screw suction chamber. Headed out the the garage in a few hours to test the vacuum and set back to 14mm.

Do the springs in the needle valve wear out over time?

[Pinbill]

Got out to the garage. I changed the float setting to shut off at 14mm. There is a bit of studder under acceleration. One of the plugs looked fouled. I am headed out to get a new set and then mess with the mixture settings.

I also tested the vacuum. I am getting 15 at idle with altitude of 5200'.

Bill

[Mark Maras]

 You may be able to get rid of the stutter by increasing the wt. of the oil. Thicker oil temporarily richens the mixture during acceleration. I always used ATF but Nissan recommended 20 wt. Do have the yellow springs under the suction chambers?

[Pinbill]

Alrighty,

It looks like I am moving in the wrong direction. I took the car out on the street and the hesitation under load got worse. It would buck and now I could hardly get up the driveway. The car starts but runs erratic, idles terrible, and shuts off.

I have been through the carb system a number of times. Double checked the float valves. Took the nozzles off and checked for clogs. Cleaned the spark plugs I bought yesterday. I checked the suction chamber spring and doesn't have paint on it. It was running well with them at one point so I think they are OK.

I am starting to think the problem is electrical. My points, condenser, rotor, and wires are new. I know the wires are hooked up right and the rotor is clocked correctly. I am currently searching the web to find out how to test the resistor and coil.

If anyone who knows how to test the resistor and coil I would be forever grateful.

Bill

[Pinbill]

Update,

The points gap is set correctly.

I got 1.6 ohms across the resistor.

On the coil I got 1.7 ohms from positive to negative. Then for the secondary I got 8.95 ohms from positive to the main coil wire and 8.95 ohms from negative to the main coil wire.

I checked the plugs and I am getting a yellowish spark on all six cylinders.

My battery is at 12.1v

With the car running measurements on the coil were 7.6v on the positive tab and 3.8v on the negative.

With the car not running and the key in the on position the coil is getting 5.1v and the ballast resistor is getting 10.9v and giving out 5.1v.

Does anyone know how much voltage I should see at the coil and resistor?

Thanks for the help,

Bill

 

 

 

 

Edited July 27, 2020 by Pinbill

[Mark Maras]

 While it's running and bucking erratically, ease the choke on and see if the symptom changes. If it improves, lower the nozzles in 1/2 turn increments until the choke no longer improves the performance. Hesitation, under a load is caused by lack of fuel.

[Pinbill]

Thanks for the reply mark. No matter where I set the nozzles the car will only run for 5 seconds at a time. The idle is rough and terrible and then it dies. When I open the throttle it bogs. Not sure where to go from here. I will check the floats again tomorrow.