Looking at a hard starting 1970 ish MGB. it seems like the float levels are low. it has the plastic floats, when I invert them they seem to be at correct height (not that you can tweak them anyway). I noticed there was a washer under the float needle/valve so I removed it (should raise the float level right?)
Anyway, back in my Spridget days you could pull the piston off the carb and peer down into the jet and see the fuel just bobbing below the bridge. On the B I dont see any fuel in the jets. I pull the choke, see the jets drop and still no fuel.
So i did the other float trick, add float gaskets. I got up to 6(!) On one carb and still didn't see any fueling the jet.
Details:
-carbs were taken apart and cleaned with new gaskets. I can spray carb cleaner through the jet and there is no blockage.
- car has spark
-compression is 160 psi cold
- fuel pressure is 3psi
Should I be able to see fuel in the jet? fuel may be suspect, it's been in there a few months. car will start on full choke but never really sets into an idle (lean pops and coughs). I've got a Weber I can put on but I used to be a SU fanboi back in the day, so I'd like to make those work.
Let me know what I'm missing. Thanks!
NOHOME
UltimaDork
11/14/18 6:07 a.m.
You seem to have most of the tricks figured out. And you are right, adjustng fuel level in an SU has become difficult and random with the POS parts that we can buy.
Assuming you have approved of the throttle shafts for wear? That being SU Carbs 101?
I use two versions of your observing the fuel in the jet. First is to pull the choke all the way down. ( with the jet adjusted 2.5 turns down) and look down the jet holder. I want to see fuel just over the top of the retracted jet. No more than 1/8" above.
Then I release the choke and use a piece of small tube to blow a puff of air straight down the jet. You are not trying to blow fuel back into the tank here, just a short puff is all you need. The idea is to just make the column of fuel in the jet bob down a bit. It should bounce back and juuuuusssstttt dribble over the bridge. I am looking for a film, not a flood. Subjective? Yeah, but once you do it you will get the gist pretty quick.
If you really cant get the fuel to where it behaves in the jet, then resort to adding fuel to the topless bowl until it gets to where it needs to be. This will eliminate any other issues in the jet itself. Is it possible that the plastic float is just to big?
Has the car been sitting too long? Possible that the passage along the jet to the float is constricted with shellacked fuel or smegma? This would allow the fuel to fill the jet to the correct level, but not supply enough fuel.
I am assuming this is a set-up that was known to be running. Are these new jets and needles? I have a high level of distrust with any new parts for these cars.
Recently, I have had a spate ( two instances) of the float needle not dropping and allowing fuel into the bowl. Drove me nuts and was only able to diagnose by doing the air down the jet test and fuel level observation. My thoughts are that the Chinese have substituted rubber for Viton on the needle tips.
The other thing to look at is a vacuum leak. I would put a vacuum gauge on the engine and have a look. Especially if the float levels check out. One place where vacuum leaks sneak in is from the flanges on the carbs and manifolds. Paint the flange and run it over some sandpaper on a flat surface to see if they are flat. The left hand side of this manifold looks a bit questionable. This may or may not have been handled by a gasket, but I did not take a chance.
A couple of weeks ago is the first time I had ever heard of the puff of air down the jet. I still don't understand it. But others say it works.