SU's suddenly running rich

[metalmonkey47]

So just a little back story on the car:

It's a bone stock L24 with L28 N47 head (E31 is getting some special work done) and because of the N47, a low pressure electric pump and regulator set at 3PSI.

Over the last few weeks I've noticed when starting my car cold it seems to start very hard without opening the throttle a tad bit or revving the snot out of it. For a few moments after it starts. The car seems to be running WAY rich all of the sudden, at about 2 turns down. Today it didn't want to run very well leaving work on my lunch break, and was quick to stutter and choke. You could smell the raw fuel. I've been having issues with my SU's running rich for a while and I haven't been able to rectify this, it seems to be getting worse, and now I'm just getting frustrated.

I've already:

-Checked needle height - they are flush with the piston about 1mm down or so, followed all of the interwebz rebuild guides

-Pistons move freely and are dropping fully

-Chokes are shutting off fully and the nozzles are up all of the way

-Float is set properly, using the clear hose trick to measure the fuel level in the bowl

The carbs were recently rebuilt by yours truly, which was a learning experience all in itself. I LOVE the simplicity of these carbs but now this problem is starting to drive me crazy. It's getting worse and worse and I can't stand it anymore. Plus, 14mpg sucks!!!

It almost makes me wonder now that i think about it, if one of the new needles is stuck or dirty, and one of the carbs is flooding or spilling over when the engine is off, or possibly when I turn on the ignition and the electric pump starts. Seems crazy to think that a piece of dirt that large could make it through the pump, pre-filter, and post filter all the way to the carb.

Any ideas of what else I might be facing??

[240260280z]

My guess would be the fuel pressure regulator failing or the fuel pump malfunctioning.

Cold engines need more fuel to start (what a choke does) but once the engine is up to temp, the fuel metered has to match the oxygen density in the air. To match the air temperature, the jets should be lowered 1.25 turns down from their summer hot day setting to match a cold morning just above freezing.

Here is a more accurate tool for setting standard jet heights.

http://www.classiczcars.com/forums/su-carb-technical-articles/43477-su-jet-depth-calculator-temperature-altitude-compensation-xls-tool.html

Edited October 15, 2014 by Blue

[Captain Obvious]

Not sure what's going on, but tossing out some stuff to think about.

How do you get a continual rich condition?

- Nozzles not adjusted properly - Shouldn't be rich at 2 turns down (assuming nothing is worn out).

- Float level too high - You checked this and it's OK

- Float needle being overpowered by the fuel pump - You got a 3 psi regulator

- Nozzles getting stuck down instead of returning fully up - You checked this and it's OK

- Needles not installed correctly in the pistons - You checked this and it's OK

- Pistons getting stuck and not rising properly - You checked this and it's OK

How do you get a transient rich condition upon startup?

- On a hot shutdown, you boil the fuel in the bowl and as it percolates, it bubbles up past the needle and into the carb throat and then runs into the intake manifold.

- After you shut down, you slowly overpower a slightly leaking needle valve(s) with residual fuel pressure in the rail. The pressure in the stock rail and pump system drops to zero when the engine stops, but with your electric pump and a regulator, I don't know how you have it plumbed. If you're applying three PSI to the needle valves even when the engine is off, you might have a small leak in one or more of the needles that isn't a big problem when the engine is running and consuming gas, but might be an issue when the engine is off.

[bartsscooterservice]

The whole float setting thing is a myth. You either have the proper factory level or it's off, meaning it's worn ( or blocked with gunk ) and it overflows. These carbs where not build to have an electrical fuel pump feed them. ( like captain obvious says above here )

If you need to open the throttle to get it started, and the worse mpg means only 1 thing: it's getting to much fuel.

[JR Ohio]

I just refreshed my round top SUs. I set the float levels prior to installation according to the directions included and elsewhere. The result was that the car would not start. I checked the float levels with the clear hose method also and all was correct. The car would not start. It would kick a couple times but that was it. Starting fluid - just a few seconds of running.

I pulled the domes off and removed the pistons. Put the mixture adjustors to full top, then cranked them down 10 full turns equaling 10mm to check to see where the fuel level was. It was WAY below the top of it. I removed the bowls and kept adjusting them until I got the fuel just at the top. Then reset them to the typical 2 1/2 turns down.

Reassembled everything and on the 1st turn of the ignition, it fired up and ran pretty decent! After adjusting the balance and then the mixtures, and again the balance, I took it for a test drive and it ran marvelously!

Your problem could be different than mine, but this is what made mine work.

John

[metalmonkey47]

Sorry for the delays guys, and thanks for the suggestions. I got my copy of Just SU's so I've been trying to find time to watch that before trying to fry my brain.

The car still runs rich, even AFTER going through needles, centering nozzles, etc etc. I'm only getting about 110 miles to a tank. Yikes.

I figure this HAS to be related to float height, i have a verified 3PSI @ the carbs and there is no trash in the needle and seat. I'm about 3 turns down and if I'm anywhere above that, the car refuses to run properly at operating temp below 50 degrees.

I want to make sure I'm understanding how float height 'should' be set. You would want the fuel level in the nozzle to settle right at the tip of the nozzle with the adjustment set for driving correct? So if 2 1/2 turns down, the fuel level in the nozzle would be around the tip of the nozzle? Or would you want it slightly below the tip of the nozzle since the needle displaces a small amount of fuel when fully down?

[Bruce Palmer]

I say about a sixteenth of an inch below. Gas puddling on the nozzle things will be fat...... Gas way down the nozzle, things will be lean......

[metalmonkey47]

Sounds like that's where my issue is then. My float levels leave the fuel in the nozzle right at the tip, so I bet it's flooding over with the piston down. I'll lower the float a tad and see what it does.

[metalmonkey47]

So I took the time to pull the needles and pistons today to look at my float levels, and I found that I'm having fuel puddle in the carb venturi's with the engine not running. Looks like when the pistons drop, they're splashing fuel into the venturi's causing the hard starts/uber richness. I got 115 miles to the last tank. That's terrible.

I lowered the front carb a tad, now I just need to lower the float in the rear carb tomorrow and hopefully I'll be able to get my mixture dialed in better.

[madkaw]

So when you opened up lid on that float was it high?

Make sure your needle and seat are sealing. Make sure the needle assembly is tight in the housing and fuel is not bypassing around the threads

[metalmonkey47]

So when you opened up lid on that float was it high?

Make sure your needle and seat are sealing. Make sure the needle assembly is tight in the housing and fuel is not bypassing around the threads

I wouldn't say it was 'high' I didn't even measure it to be honest. I measured it by the level of fuel in the nozzle. I suppose I need to pull the bowl cover off and hold the needle and seat closed together and see if they seep with the fuel pump on. I ran the fuel pump with the pistons out and didn't see the fuel level in the nozzle change even the slightest so I guess that's a decent indication that they aren't leaking.

[madkaw]

You can do the old blow thru test to see if the needle is closing off the flow.