Postby Hasbeen » 18 Dec 2014 01:16
How dare you tell us that SUs don't wear out Steve!
Strangely enough, that is my experience too. I've owned some pretty old, high mileage SU carbs, & never had spindle wear cause a tuning problem.
On that, I love Stags solution. Much less mucking around than spindle rebushing, & probably as effective.
On the sticking, not fully closing problem, --- dirt. Stop oiling the spindle area, clean it up to remove the dust stuck in the oil gumming the things up, & the problem is usually cured.
In my poorer days, I would solder up jeffremj's weird valves. Just leaving them there, solid, doesn't seem to effect performance at all.
The main problem I've had with SUs themselves is flooding. Fixing the fuel pressure, [2 PSI is best, but no more than 2.2 PSI], will usually tame a recalcitrant SU. Check the floats, & replace the float bowl needle & seats if it doesn't.
Getting them to idle is a different matter.
Balance is all important here, but remember it is mainly needed for that idle. A little variations in suction or butterfly opening are not really that important above about quarter throttle, but at idle, critical.
Most balance problems, with older engines like ours often have nothing to do with the carb, but the engine & it's attendant accessories.
First throw out all that anti pollution garbage, or make it inoperative with suitable ball bearings in all hoses & lines. Stick a solid plate under the EGR valve, throw out that fool air pump, & make the exhaust airtight. Oh, & stick another solid plate under the EGR, just to make sure.
If local law makes this impossible, you have my sympathy. Come back in a week to read the rest of this, after you have all that garbage working properly.
Now fix the vacuum of all the accessories you do need, distributor, brake booster, & engine breather. You can not tune a carb if it is sucking air from a split hose or diaphragm.
Check the plugs are clean.
Included in this is fixing any suction leaks in the inlet system, particularly those rubber mounts.
At last, now you can check the suction on each carb. I still use the hose in the ear method, but have switched from rubber to plastic hose, just to be more modern.
If it is a long way out, have a quick try at balance, but if it resists, don't waste too much time, go to the engine itself.
I presume you have done a compression test. If not do one now, it will revel any major valve or piston problem.
I have found it very hard to tune our engines to idle, if the valve clearances are more than a couple of thou out. If they are more than this, fix them before blaming that poor SU for the problem.
Now the SUs. Clean the dashpots, check the needles are straight & installed correctly, not touching the jet, & that the pistons rise & fall smoothly, easily & COMPLETELY. While the pistons are out, check the choke action, & that the jet slides fully up the jet tube, when the choke is off. Adjust the jets to level with the top of the bridge in the carb throats, then wind them down 8 flats of the jet adjusting nuts as a starting point.
Confirm there is no flooding.
You have now almost completed tuning your SUs by actually doing normal servicing. The rest is simple.
Disconnect the throttles of the carbs. Start the car, roughly adjust the idle, & run until warm. Blip the throttle about every minute & a half during the tuning & hold above 2000 RPM for a few seconds to clear the plugs.
With the plastic tube in your ear & the carb throat, adjust the throttle stops until the suction is equal on each, & the idle is about where you want it.
Now gently lift each piston a small amount. The revs rise a little, & then fall as you do.
If they stay higher, it is too rich, so adjust the jet up a flat of the adjusting nut.
If they fall immediately it is lean, so adjust the jet down one flat.
Continue with this until you get that slight rise, followed by a fall on each carb.
You may have to readjust the idle during this procedure, if so be sure to keep the suction equal.
Now your 7 should idle as well as the next car at the lights, even if it can't beat them away from them.
Hasbeen
