Fuel bowl level and bending tab not working

[240260280z]

I normally set the needle valve closure point on carbs by blowing in the inlet and adjusting the tab so that the valve is shut at 90 degree point of swing (top of float is parallel to top of fuel bowl). The carbs I have set like this have always worked fine. However, I ran into some problems today with a set I did this fuel level technique too so I decided to check in more detail as I could not get the A/F ratio to work no matter what I did with the jet height.

I decided it was best to back track and recheck every thing. I first tried adjusting the fuel level at 23mm in the bowl and also the other technique of meniscus 10 turns of the jet down the well. The carbs initially were ~ 25mm and 23mm for the front/back bowls respectively (distance from bottom of gasket to top of fuel) with the same needle valves and tab bridge heights (not the 72 carbs). I readjusted the tabs and got the offending one to ~ 24mm and left it. After this I put new M49 needles in the pistons and set the correct needle depth using the "jet-push-piston-home" technique.

I ran the motor and it was super rich. I tried bringing the jets up but it was still 12:1 with zero turns on the jets! I then tried new N54 needles and it was only mildly leaner (14:1) at 0 turns of jet depth.

I then re-checked the fuel height and it was still 24mm. I then re-bent the tab many times (drain, bend, run starter for 15sec, wait for fuel to finish rising and I always got 24mm...I repeated ~ 5 times until the tab was nearly pushing the needle valve closed with only a few degrees of freedom. It was as if the new needle valve was not working. I then tried an older fuel bowl top with old float and old needle valve but the same problem...24mm of fuel level. I then thought that the fuel pump or return lines may be the problem. I was able to confirm the fuel rail return flow a few days ago, and today I blew on the return line to the tank and could pass air and hear it bubble so no problem there. I am thinking the lines are fine but the fuel pressure is too high.

I do not have a fuel pressure gauge nor an adjustable FPR for carbs so I am thinking of rigging a hose to check the column of fuel height the pump can head and then converting to PSI.... any input?

I am also lost as to why I still get 24mm of fuel height and not over flowing? It is like the needle valves are working independently of the tabs.

I had a quick inspection of the jet holes and they look fine. I put a 2.40mm brass rod in each and visually confirmed similar side clearance to the wall and round shape.

Also the floats are not cracked or sinking.

?????

Edited July 23, 2012 by Blue

[Captain Obvious]

I've had problems getting the bowl level correct as well. After an hour or so with what must have been fifty wet-set attempts...

All the things I can do, and that seemingly stupid easy task gave me the fits. Glad to hear it's not just me.

[240260280z]

It is nice to be in good company.

Actually it is the first time I actually measured the fuel height as the 90° technique has worked fine. This time I used my calipers and scribed two accurate marks on the outside of the bowl at 23mm down from the lip and at 25mm down from the lip. I used a brass "T" and three sections of clear hose to connect the bowl outlet to the jet inlet and to run vertical for measuring.

I'll swap fuel pumps today and see if that helps.

[Captain Obvious]

I'm from the opposite camp... Get in the ballpark with a dry measurement, but after that, I've always wet set everything.

It's been a while since I had the problems, but IIRC, it went like this:

Start with a dry bowl with a clear plastic tube on the outlet nipple bent upward and taped up to the side of the bowl.

Put fuel into the inlet until the level stops rising. Check the level against a Sharpie mark on the outside of the bowl.

It's a little low.

Drain the fuel, take off the bowl cover and bend the tang to allow more fuel.

Put it back together and repeat:

Still a little low.

Bend a little more and repeat:

Still a little low.

Bend a little more and repeat:

Level keeps rising and spurts out the vent nipple. :ogre:

Repeat for and hour and finally decide that a little low is "good enough".

[240260280z]

that is nearly the same... maybe we should take parts from each others carbs and the levels will be in the right spot.

How I managed to run rich with no turns on the jet knobs still confuses me. I think my carbs are haunted.

I have a second set of new needle valves and a used fuel pump that I disassembled today to ensure no surprises. I'll try theses and see if I can find the elusive parameter that is causing the problem.

[madkaw]

That's pretty weird. It seems you have covered the basics and more! It seems if you had excessive fuel pressure you would be dumping fuel . No holes in your floats is there?

[240260280z]

No holes, and I tried 3 different floats/tops.

[Zedyone_kenobi]

Are your AFR while driving or idling? If driving, is it possible that the fuel needle is not rising fast enough? A piston stuck to low in the jet would allow too much fuel to be added to the car at a given vacuum signal.

IF this is happening at idle, then I am not sure how you could run rich with the adjustment jets all the way up. Unless the hole in the jet is not round anymore, allowing more fuel. This is really weird. Be careful, SU problems can turn into LOOOOONG threads.

EDIT: just saw you checked the jet holes.

Edited July 24, 2012 by Zedyone_kenobi

[240260280z]

Yeah, everything check OK. I'll have another look at the fuel rail for an obstruction on the return. I'll also change the fuel pump today.

[240260280z]

Found the problem. Turned out that the new short N54 and M49 needles were the culprits. Grrrrrr. It was supposed to be a 240z kit they came from.

[Captain Obvious]

Found the problem. Turned out that the new short N54 and M49 needles were the culprits. Grrrrrr. It was supposed to be a 240z kit they came from.

I took a look at the numbers I got when I measured my N54 needles, and guess what... The N54's I have are way rich for the Z.

I had measured the N54's and recorded the numbers months ago, but I never actually took the time to compare those measurements against the other needles I've had success with. Well I just did, and I can see why they would run very rich. Sorry I didn't look at the numbers before... I could have saved you some aggravation.

I don't know if you and I both have manufacturing mistakes from the same aftermarket vendor or what, but at least I can give you a reason for your results.

It's not the length that's causing you a problem... It's the girth.

Five second check of the M49's indicate the same thing. (No surprise there, right?)

[madkaw]

That's why the SM needles are too rich for most stock motors. They are quite a bit shorter and richer at the top end.

Glad you figured it out