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??

[Blue]

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/s...-xls-tool.html

[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.

[Captain Obvious]

It sounds to me that you're thinking the needles cause the fuel level to rise when they are inserted into the nozzle? If that's the case, you have to remember that unless you pulled the needles out first with the fuel pump still running, the fuel level was established with the needles submerged. That means the fuel level will go down a tiny bit when the needles are pulled out and then return to "normal" when they are reinserted.

And the amount of that change (either up or down) is only by the volume of fuel displaced by the portion of the needle that is submerged and that volume is very small. The needles are not splashing fuel up into the venturis when the pistons drop. If you've got fuel puddling in the carb throat, it's not because the needles splashed it there or caused the bowls to overflow because the volume of a submerged needle raised the bowl level.

That said, how are you regulating fuel pressure and what does that fuel pressure do when the engine is shut off? I'm wondering if the pressure goes up higher than intended when the engine is shut off and the fuel in the rail expands from heat soak. Are you sure that you're never exceeding 3 psi at any time? Ever?

Also, when you found fuel puddled in the venturis, how long had the car been shut off? I know you said you ran the pump with the pistons out and the level didn't change, but if you're looking for a slow leak and the car sat overnight? You could have a small seep around or through the needle valves that you don't see with the naked eye, but over a 12 hour sit maybe it adds up?

And about the poor fuel mileage and only getting 115 miles to a tank... Are you sure your needle and nozzles are in good shape? If they had been rubbing in the past or if material has left through corrosion, that will throw everything off. It doesn't take a lot of material removal to have a big impact. If you ovaled your nozzles and/or grooved your needles, then you could be sucking down way more fuel than intended even if the bowl level is correct.

Bottom line? My suggestions:
1) Check and set your bowl levels with a clear tube on the bowl outlet.
2) Put a fuel pressure gauge on the rail and make sure the pressures are what you expect at ALL times.
3) Inspect your needles and nozzles very closely for wear or corrosion.

[metalmonkey47]

Well to a degree I'm sure it displaces a little fuel in the nozzle, maybe not as much as I think. Probably not my issue now that I think about it.

My pump is installed at the fuel rail level with the pick up tube in the tank, and I have a regulator installed post filter set to 3 PSI. I have no gauge installed to verify at the moment, but I guess I'll have to put one in tomorrow. I happen to have one laying around if I can find it. I have no idea what the fuel pressure does when the engine is off, but I'm sure it's not dropping off. Now that I think about it, I've had a few times where I've pulled the float bowl cover and has fuel spurt from the needle and seat.

The only time I've looked was the other day, when I pulled the pistons/needles before starting the car to try and diagnose the hard start condition. That's when i notice raw fuel settled in the venturi. I'm curious if it's spilling over into the intake manifold as well.

The car is running terribly rich on both carbs, I've never had an issue with it rubbing or binding in the nozzle, but I can't speak well for the PO.... he was a bit of a tard and never touched the carbs so there's a chance that he never touched them in the 15 years he owned the car. When I disassembled and cleaned the carbs off of the car, the nozzles and needles were in awesome shape with no visible corrosion. They both drop fine with no binding.

I've gotta change the exhaust flange gasket tomorrow morning before doing any carb work (gotta get some of that rich exhaust away from the cab) and I'll reset the needles and get a fuel pressure gauge in place. This is annoying.

[Captain Obvious]

Yeah if you've got fuel pressure on the needle and seat even with the engine off it's possible that you're bleeding that pressure off into the bowls slowly overnight and starting flooded every morning. The original system with it's orifice based regulator allowed the fuel pressure in the rail to drop to zero as soon as the pump stopped moving. Sounds like your new setup doesn't allow that and you're asking your needle valves to hold that pressure back 100% forever and I'm not sure they're that good. Especially if you're getting a heat soak induced pressure spike after shut down. It might be part of the problem.

But that won't cause the long term high fuel consumption while driving though. That's got to be an adjustment or needle or nozzle issue. Keep in mind that the needles and nozzles can look fine to the naked eye and still be out of whack. My experience is that five thousandths (.005 inches) was enough to make the difference between purring like a kitten and stinky eye burning exhaust.

If you've got any doubt as to the condition and you've exhausted all other possibilities you might want to think about replacing them. 115 miles to a tank is certainly low.... BTW - How do the plugs look? Are they also indicating a rich condition?

[metalmonkey47]

Problem #1... I hooked up a fuel pressure gauge and found that with my regulator set to 2.5 PSI, I was getting double that at 5psi. Had to bottom it out to .5PSI on the regulator to get 2.5.

I reset the height on the floats, and the needle/seat are NOT leaking at all. In fact a detail I think I left out is that they are brand new.

Anyways, everything I did is hard to put into detail in one post so I made you guys a video, hoping maybe you'll catch something I don't. To be hones though, I'm about 90% sure at this point that the nozzles are going to be my issue. Bruce, I'll probably be giving you a call in the AM.

[Blue]

From the video:

- you are running on your rear carb only.

Q. What needles do you have?

Experiment: Try dropping the front jet down enough so that the car will run when you disable the rear carb... if it is at ~2.5 turns down and there is enough air to chug along then your front carb is fine. If this is the case then focus on the rear carb.

[Blue]

also disconnect the choke cables when trouble shooting.

[Captain Obvious]

What Blue said.

When you lift the piston with the little test rod, you disable that carb. So when you lift the front and nothing changes, it's because the front carb isn't doing anything to run the engine and those front three cylinders are just along for the ride. When you lift the rear carb and it dies, it's because the rear is the only carb that's adding fuel and you're running on just the rear three cylinders.

If the needles are installed properly (with the shoulder flush with the bottom of the piston),
and the nozzle adjustments are turned all the way up (zero turns down),
and the nozzles are fully seated all the way up (not stuck down or held down by the choke system),
and everything is in good shape (not worn or corroded away),

Then I would expect it to be so lean at idle that wouldn't run at all.

So again, what Blue said.

The fact that the front carb isn't running the front three cylinders with the nozzle all the way up is probably a GOOD thing. If the rear carb is still supplying enough fuel to run the rear three cylinders even with the nozzles all the way up (and even smoke at that!), then that rear carb is most likely a problem.

As for checking the needles and nozzles for wear or oval shape? If it's so bad that you can see it with the naked eye, then it's really bad. I can tell under magnification with measurement tools, but I don't have a quick and easy way for you to verify. How much do new nozzles cost? Can you borrow a pair from a known good set of carbs just to see what happens?

You're positive you've got the needles installed properly, right?

[Stanley]

2.5 psi is a little low and 5.0 is a little high, book says about 3.2 to 4.2, both carbs get about the same fuel pressure/flow unless the rail is blocked. If it's a Mallory low-pressure pump I think there's an adjusting screw on the pump, 3.5 to 4.5 psi or something. Maybe those regulators are crap.

Both carbs same fuel level at the nozzles. Hold sight glass next to nozzle to check.

The old emissions needle, the new needles, and the modified needles all look the same to me, even with my glasses.

[bartsscooterservice]

I looked up the FSM, the stock fuel pump should deliver a static fuel pressure between 0.24 and 0.30 kg/cm2