popping through the exhaust at 4500 rpm or so.. HELP!

[madkaw]

I know it's tough, but hang in there. These are the situations that force me to study and research and relearn the basics. You will be the first to pop in when someone starts a thread on SU problems. !!!

Google SU carbs and their operation and critical float adjustment. The more I learn about carbs and fuel injection, the more I realize how good and easy FI is

[30 Ounce]

This is how I set my float levels and it worked great.

SETTING SU FLOAT BOWL LEVELS

We've all spent afternoons syncing the carb airflow, topping up the damper oil and making sure the jet needle/fuel nozzle levels are set properly (2 1/2 turns down, etc). But how many of us have checked to make sure the gas float levels are identical on each carb? This is actually a very big deal. If the fuel levels are different in each float bowl, even a small bit, one carb will always be lean or rich compared to the other.

The issue is that the SU uses a combination of incoming gas and the float closing the fuel-valve to create a steady fuel level at the tip of each fuel nozzle. If this fuel level varies between carbs then its also at a different level on each jet needle.

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.

HOWEVER: Even though adjusting the floatbowl levels to 23mm down is a factory setting, it doesn't guarantee that the fuel levels in the carbs are the same.

THE TRICK:

I did some measuring with a straightedge and found that on my 4-screw SUs a 23mm fuel level in the float bowl corresponds to exactly 1 centimeter (10 millimeters) below the fuel nozzle "ledge" in the carb. But how do you see to measure fuel 1 centimeter down inside the fuel ozzle? See below.

- Remove the dome and main piston from each carb so you can look down the tip of both fuel nozzles.

- Screw each mixture nut exactly 10 turns down from fully up. Each full turn drops the nozzle tip 1 mm, so 10 turns puts the tip of the fuel nozzle 10 mm (1cm) down....which happens to be the 23mm float bowl level.

- Then look down the tip of each fuel nozzle and adjust each float to set the gas level at the fuel nozzle tip.

I ended up taking mine to 9 turns down and set the fuel at the meniscus of the tip. Then when I went to 10 turns the fuel was exactly at the tip. A bit of a juggling act but after a few minutes you will get the hang of it. When you get them to match it's a great feeling.

NOTE #1: if your mixture nut won't go down more than a few turns, it's because the factory "stopper" next to the nut is still in place. Unless originality is a concern, you can permenantly remove both stoppers from the bottom of the carb.

NOTE#2: if the fuel level is too high and overflows the nozzle tip, you will need to drain each float bowl a bit before starting again.

The result was a lower, more solid idle and less choke needed at startup. I get no stumbling off idle and part-throttle acceleration is smoother, acceleration more powerful. Cruising on the interstate requires a lighter touch on the accelerator. Interesting.

[Diseazd]

I'm having a little trouble justifying a typical vacuum leak. You said your idle is perfect, and I would expect the impact of your typical "constant" vacuum leak to decrease as you give it pedal. That's why I was trying to think of more atypical vacuum leak sources that might get worse off idle.

Well whatever you find, I'm pullin' for ya!!

Could it be a leak on the other side of the throttle plate (maybe shot throttle shaft bushings)? Zedy....If I were you, I think I would accept Steve's (tt) generous offer to try his SU's. This would eliminate the carbs as a source of your problem. It's only a 30 minute swap. Why not?

[Zedyone_kenobi]

This is something I think I need to do. How many times can I reuse the stock cork SU fuel bowl gaskets?

I was going to try the clear fuel hose trick this evening if I could. I probably will not be able to get the O2 sensor installed for a week. Just too busy.

I have not verified my fuel level in my float bowls, but by God I will soon!

[Zedyone_kenobi]

Could it be a leak on the other side of the throttle plate (maybe shot throttle shaft bushings)? Zedy....If I were you, I think I would accept Steve's (tt) generous offer to try his SU's. This would eliminate the carbs as a source of your problem. It's only a 30 minute swap. Why not?

I just feel bad about asking him to take his carbs off his daily driver. I know it is just 8 bolts, but still. I will do that before I do anything drastic I promise.

first I need to get my fuel level in my carbs verified beyond a shadow of a doubt. As far as air leaking on the throttle shaft, I have to believe that Ztherapy carbs with their patented seals and bearings have eliminated this as a possibility... But a carb swap would be an easy way to verify this...

[Diseazd]

first I need to get my fuel level in my carbs verified beyond a shadow of a doubt. As far as air leaking on the throttle shaft, I have to believe that Ztherapy carbs with their patented seals and bearings have eliminated this as a possibility... But a carb swap would be an easy way to verify this...

I agree....and I would assume the floats were set correctly also.

[Zedyone_kenobi]

Maybe I should get data and not assume anything...

At NASA here in the cryogenic propulsion division, we have a saying...

'In God we trust, all others bring data....'

[madkaw]

reuse the gaskets until they fall apart--basically. If your careful you can take the covers off many times without issue.

[steve91tt]

Cork float bowl gaskets on the other hand are generally not reusable. I've made my own in the past out of gasket paper. The home made ones are good for several uses.

[Zedyone_kenobi]

Going to check out the float levels tomorrow when I hopefully get my O2 sensor bung welded in!

[Zedyone_kenobi]

I managed to get outside and check the float bowl levels using the clear hose trick mentioned by the wonderful folks of this board. I marked the outside of the fuel bowl .906" down from the fuel bowl rim (I know technically it is supposed to be measured from the inside lid, but it is close enough to see if I was off.

The front bowl was just about right up to that mark I made, not quite but close. The rear bowl was about a 1/4 to a half inch below that mark. I cranked the car for a good 8 seconds each time to give the bowls plenty of time to fill up.

So I removed the rear bowl and bent the little tang the slightest bit toward the float. I reinstalled it and presto, dead even with the line.

I have not had a chance to drive it yet, as tonight is date night with Mrs. Zedyone, but tomorrow morning, I will take that baby for a spin. I cannot find a shop to weld in my O2 sensor on a saturday, so I am stuck waiting a bit longer to get that working. But I will report back after my drive!

[Captain Obvious]

A quarter inch out of spec might be enough to be noticeable, but what could a low bowl level do to mid-RPM cruise that wouldn't be worse at high RPM WOT?

Hey, I forgot to ask you the other day... When you bought OEM stock needles, what needle number did you buy? Do the new needles even have numbers marked on them? My round tops have N-27's but I believe they ran a couple different numbers over the first few years.