SU's suddenly running rich

[madkaw]

Needle and seat assembly is tight and any appropriate washers are used on the valve assembly?

How did you check to see if the needle was sealing off the fuel flow?

[Captain Obvious]

The 73 carbs were the flat top variety and are very different than everything prior. There is little (if anything at all) that is transferrable from 73 to 70. In other words, So I'm not sure what it is you did...

 

Are you saying that you bought a rebuild kit for a 73 flat top and used the needle valves from that kit in your 70 round top carb?

And I'm also confused as to how you left the float bowl level situation. You said you couldn't get the levels correct and then what? You just left them wrong? 10mm high is way off and if you left them there, you'll surely run rich. What did you end up doing with the levels in the end?

[metalmonkey47]

Sorry I'm not familiar with the flat top-round top years. They are '72 rebuild kits. According to some basic research when I needed to buy the kits, they were supposed to all be the same from 70-72. In fact the carb kits list float specs for '70, and '71-72. 

 

I may be a little vague back there, let me try and back pedal and try and make sense of it. 

I used a clear hose to measure the fuel level in the bowls. Apparently 23mm down from the top of the bowl is the correct fuel level. I made some adjustments to the float, trying to get the level down to 23mm but regardless where I set the float, the level in the bowl always returned to 13mm from the top. I left it for the night and wnted to come back to it today with a clear mind. 

 

Today I pulled the domes and pistons and found that the front carb was flooding over the nozzle tip. The rear actually seemed to be settling a little low so I focused my attention on the front carb. Same issue fought the night before. Regardless of float height the fuel level in the bowl was always too high and flooded the nozzle tip. I swapped the float from front to rear and the problem did not follow, so I swapped the needle and seat and noticed the rear carb now reading high.  I ought to mention, when I say it's flooding, i don't mean that it's spilling out of the float bowl vents, the fuel level just rises to the top of the nozzle and occasionally dribbles over the nozzle. 

This has been a progressive issue from the beginning. It's been very inconsistent up until now.

 

Now it would appear the needle and seat would be the culprits. They do seem to seal which is strange. I hooked em up to a zero loss air pump to measure the point where pressure overwhelms the valve/float, and they don't seem to leak at all. I also flipped the cover over and let gravity hold the float to the needle tip, and at 2.5psi-3psi there is no leakage. 

 

I pulled both needle & seat assemblies and compared them to the old ones and found that the new ones have 1.7 marked on the side of the body, and the old ones are marked 2.0. Disassembled I don't see any major differences in the assemblies, although the tapered tip does seem to be slightly different.... Any idea what those marking refer to? 

[Captain Obvious]

70-72 rebuild kits makes sense.

 

About not being able to get the fuel level in the bowls to respond predictably to changes in the float tang, I've been there too. Been a while, but I remember having the same unpredictable issues where the fuel level didn't seem to follow the expected path after adjusting the tang. There are a couple threads here on the forum that discuss that issue. but you'll have to search for them. Blue and I were discussing it a few years ago. I unfortunately never got to the bottom of what was going on with my carbs, but on the good side (for me), I got the car running great without knowing and I stopped investigating.

 

I'm sorry, but I don't know what the numbers are on the sides of the needle seats are. I think there was at least one needle valve change in the first three years (70-72) but I don't remember if that was for a difference between front and rear carbs or a year-to-year change

 

So jumping ahead a little... What happens if you just put your old needle valves back in?
 

[Captain Obvious]

Oh, and by the way... I think the root issue to all of this is that your carbs are (using technical terms) "completely out of whack".

 

I think you've got mixture issues complicated by float levels, carb synchronization, and possibly vacuum leaks. I think you are juggling multiple different adjustment issues and at this point that I would start over with the basics.

Pull the carbs. Clean em up.

Adjust the bowl levels with a clear tube on the bench.

Don't progress any further until you are positive that the levels are correct.

Set the nozzles 2 1/2 turns down and put em back on using new gaskets.

Get it running well enough to synch the carbs.

And lastly fine tune the mixture.

 

 

 

 

[siteunseen]

That's about as Obvious as I've ever read, thank you Captain!

[Captain Obvious]

Haha!! It's my calling.   

[Stanley]

Don't know if this will help, but there's a little trick that will let you know if the float level is where you set it, when the car is running. With the sight glass tube attached (or with the lid/float assembly removed, however you set it),

get a little plastic straw from can of carb cleaner or something, pull off the vent hoses, and put the straw through the vent fitting. It will rest lightly on top of the float. Mark it (I measured 5mm above the vent) with a marker or scotch tape, etc.  When the car is running, measure it again. It should be easy to see where the floats are when it's running.

[metalmonkey47]

Thanks guys, glad I'm back on track again.   The car just started running EVEN richer now, so I'm a little annoyed. It was so bad I had to look for my exhaust leaks today and fix them  ::  Guess it wasn't hard since the pipe was literally cover in black soot around the leak. 

I've tinkered with the floats again and again but it seems that no matter what I do, I can't get them right. I've had the floats set literally so high that 2.5psi took forever to fill the bowls and it ran super lean. Just a hair lower and it floods. Which is strange because they seal, and the floats aren't leaking. I've played around for about 3 hours in 35 degree weather just trying to find the sweet spot. But part of me things the issue relies in cheap needle valves. The tangs on the floats probably won't survive many more adjustments. 

The old needle valves spray gas so putting them back in is useless. I had to replace them after getting the car because It was pouring gas out of the throat onto the exhaust. Dangerous. 

I think at this point I'm going to start by ordering a few gaskets, new floats and new needle valves from Z Therapy next week and take a few days off of work to pull the carbs and reset everything. The issue is getting progressively worse now and the car is running very inconsistent. 

But I guess you could say that 'out-of-whack' carbs is a good assessment. They're sync'd perfectly, but fuel wise........well you can see what I'm dealing with. 


Stanley, thanks for the suggestion! I didn't even consider that. Gonna give it a go and see exactly where my floats are shutting tomorrow. 


Although I don't have a choice, since the car is running poorly under load. Getting backfiring when accelerating. Probably due to excess fuel. 

[metalmonkey47]

For the record, this is the car, and it is WAY too clean to be running this bad  :: Seems like out of all the datsun's I've owned, the dirtiest looking ones ran the best.

 

[240260280z]

I am guessing you may have a set of needles for a different style carb. This happened to me once. I bought a "240z" SU kit but it was mislabelled. The needles were shorter and ran rich as stink.

Nice car btw!

[240260280z]

Where are you located?