DrBoost UberDork
4/18/12 6:08 a.m.

I've been spending what little spare time I have to resurrect my classic mini to sell it and my lack of knowledge of the SU carb has driven me here looking for help. The plugs would be wet with fuel after 10 seconds of cranking and the spark didn't look stellar so I replaced the coil. The spark is MUCH better now, but the plugs still get wet. So, I turned my attention back to the carb (I drained the tank, lines and float bowl already). I noticed the piston does indeed rise and fall, but there is about 1/4" of movement before the fluid dampens the movement. Is this as simple as needing to add some more ATF to the carb? The needle and seat look brand new BTW.

Mazdax605 Dork
4/18/12 6:30 a.m.

Hey DrBoost,

I have a dvd from when I had su carbs on my Datsun that you are welcome to. I imagine they can't be that different in a Japanese application compared to a British car. Pm me or email me at cdirado837 @ g mail dot com and I will mail it to you. The DVD is from a company called Just SU's so you can be sure they know their stuff.

Streetwiseguy SuperDork
4/18/12 8:02 a.m.

Side float bowl or bottom? Either way, if its flooding that quickly, you have got a leaking float valve, sunk float, or bad enrichment mechanism.

Graefin10 Dork
4/18/12 8:25 a.m.

What psi is the fuel? Sometimes after market fuel pumps provide too much pressure. Did you check to see if the floats have fuel in them? Did you look for any corrosion or deposits in the float bowls? Are the float needles stock? Sometimes it's really hard to keep them from leaking. There's an after market valve that uses a ball to seal the fuel flow that works quite well. Also "viton" rubber needles work better than stock.

The damper pistons are designed to allow a little drop on off throttle but I don't think 1/4" so it's probably a little low on damper oil.

Good luck!

DrBoost UberDork
4/18/12 11:25 a.m.

The float (singular) does not have fuel in it. This is a "ran when parked" situation so I would think the fuel pressure wouldn't be too high?
Streetwise, the float bowl is on the side of the carb. I found a video on youtube that explained how the carb works. IF I understand, the vacuum lifts the piston, uh, thing up allowing fuel in. The oil is supposed to slow that rise. My piston will rise about 1/4" or so (?) before I feel that fluid resistance. Is that bad? Am I understanding things correctly.
Masdax, you have a PM.

JohnInKansas Reader
4/18/12 12:05 p.m.

I'm at work and don't have my book in front of me, so I can't help much.

I know that on my Triumph, if you so much as LOOK at the gas pedal before the engine catches, it will flood.

The dampers (oil filled resistance doo-dads) are there to keep the piston from rising too far on snap throttle increase. I'm having to dig through the cobwebs to remember this, but with less than the recommended amount of oil, the pistons will rise too far too fast on abrupt throttle input and over-enrich the mixture. On Zenith Stromberg carbs (Triumph used them on some TRs), the damper oil fill level is something like .25" below the top of the damper tube. I seem to recall I used motorcycle engine oil in my Zeniths... don't remember what weight. Wasn't ATF though. Maybe it is for SU's, I dunno.

Fuel pressure is worth checking. If its got old air cleaners on it, get new ones or at least take the old ones off to try to start it.

Also, you might disconnect the fuel pump (or run the outlet back to the tank via rubber line) and try to get it to cough on ether... Since you've got spark and air, I would expect it would at least try to start with ether.

Or you could sell it to me and I can screw around chasing carb problems.

oldeskewltoy HalfDork
4/18/12 12:42 p.m.

Hmmmm, from my memory of using SUs on my early Volvo 1800Ss, the mixture is crucial to fueling. with the air cleaners off, and something propping the cylinders up, see how far up the brass jet is in its housing If it is more than a millemeter down from the surface... You might want to begin re-setting the mixture.

I also found (NJ) that she was a bit too dirty(emissions) when mixture was correct for power. When I would go through NJ emissions, I would have to lean it out ("tighten" the 17mm adjuster for mixture) about 1/2 to 3/4 of a revolution to comply with NJ emissions

DrBoost UberDork
4/18/12 4:28 p.m.

Ok, anybody in Michigan wanna make some extra money? I just don't have the time to keep screwing with this thing. I got it to stumble a few times on carb cleaner but with a job, 3 kids and a house, plus lots of other crap (where's the cheese to go with this wine?!?!) I'll never get it done with just time after work.
I'm looking to pay an honest sum if someone feels they can come reanimate this car.

Streetwiseguy SuperDork
4/18/12 10:39 p.m.

I'm only about 1000 miles northwest of you.

But I only work on SU carbs when they are screwed to something not British.

The dashpot on the top of the carb has a damper screwed in the top. It needs oil in the damper to run properly, but not to run. The dashpot just bounces around. If memory serves, the mini carb is facing the firewall, so you can't just take the airbox off and look down the bore. The next simplest thing is to take out the three screws holding the dashpot on and lift it straight up. It may come as an assembly, or the piston may stay behind. Don't drop the piston, because they always land on the needle and bend it, then you say bad words for a while. Now- look at the needle in the piston- there is a step on the needle right up close to the piston, which should be roughly level with the bottom of the piston. Sometimes they start to fall out of the sleeve, or the needle retainer falls out of the piston. Next, take a look down at the jet, which is the hole in the middle that would accept the needle- it should be maybe 1/8 " or so down from the body of the carb. The height of the jet is what sets the basic mixture, and on a side bowl carb, is also the cold start enrichment- their is a lever mechanism on the bottom of the carb that pulls the jet down when you pull the choke cable. If its more than 1/4" down, the jet, or the mechanical choke linkage, or the cable may be pooched. Now, go get a fire extinguisher, cause if you dribble gas, it will probably hit some component made by Lucas, and we all know about Lucas and sparks- the only thing they build that doesn't produce sparks is their ignition system. Have somebody crank the engine over. If the float valve is leaking, it will push fuel up through the jet. That is wrong, and needs a new float valve, or the chunk of dirt blown out of it, or some such thing. If there is no fuel pushing up while cranking, it either has no fuel or the right amount. Take the top off the float chamber and see if there is gas there. Get a container, stick the fuel line or the whole float bowl top and crank it again- should give you a reasonable amount of fuel. If not, look at filters and pumps, etc. Also, make sure the float valve hasn't screwed itself out of the float bowl lid. Clean stuff up, make sure the piston moves smoothly in the dashpot bore, reassemble it and it will be close enough for government work. There is really no simpler carburetor on the planet. There are no holes in an SU small enough to get blocked by anything smaller than a rock, or perhaps a complete mouse cadaver. They always work, but never work really well. They will not start cold without some enrichment.

If it all appears good, I'd pull start it. With the lousy Lucas ignition, and the lousy Lucas wiring, and the lousy Lucas starter drawing too much power and cranking too slowly, it may just need a bit of abuse to go.

Sorry about the Lucas digs, but I just can't be completely polite anymore with Brit cars. Too many memories, too much pain...

DrBoost UberDork
4/19/12 6:53 a.m.

Thanks streetwise. I'll take a look at this stuff tomorrow night.
Yesterday I made tripple-sure the float was not full of fuel, it's not. The float bowl is. I crimped the fuel line going into the float and put VERY little air pressure in the vent with the dashpot and piston out. Fuel did come out of the jet. The needle looks great but I didn't know how to check the depth.
I know these are supposed to be soo simple. But when you don't know how they work, they aren't simple enough.

Our Preferred Partners