SU Carb guidance

[Timothy Keith-Lucas]

The presenting problem is that my Sprite 998 engine with an SU AUC549 carb for the front two cylinders and AUC930R for the rear idles at 2500 RPM with the idle screw all the way out and a bit of slack in the throttle cable. The rear sparkplugs look just the right brown color while the front two are out-and-out wet after running to normal temperature. Overall power seems OK and I see some soot but not enough to be visible in the exhaust. Tightening what appears to be the jet adjusting nut (below the spring) on the front carb has no effect whatever. Not even Herb Adler's "The Idiot's Guide to Tuning SU Carburetters" is any help. 

 

What's going on here? Is there a guide that might help me?

[MV8]

Sounds like the throttle linkage cannot return the butterfly to the idle stop on that carb.

[ashyers]

I agree with MV8.

 

Maybe your carb sync is way out? Is one of the carbs returning to the stop while the other isn't? I'd take a real careful look at each carb's idle stops and see if things appear OK. In addition these carbs can hang if the carb's return springs are not adjusted properly or the shafts/bodys are worn heavily.

 

As far as the jet nut not doing anything, ensure that you're actually changing the jet height and not just rotating the entire assembly. That can happen on the old SU's if you bottom the jet jut on the jet holder assembly.

 

If your A series is running phenolic insulators make sure they aren't cracked and the gaskets are sound. They can lead to a big vacuum leak and all your tuning efforts will be a waste of time.

 

Andy

[Drakman]

50 minutes ago, ashyers said:

I agree with MV8.

 

Maybe your carb sync is way out? Is one of the carbs returning to the stop while the other isn't? I'd take a real careful look at each carb's idle stops and see if things appear OK. In addition these carbs can hang if the carb's return springs are not adjusted properly or the shafts/bodys are worn heavily.

 

As far as the jet nut not doing anything, ensure that you're actually changing the jet height and not just rotating the entire assembly. That can happen on the old SU's if you bottom the jet jut on the jet holder assembly.

 

If your A series is running phenolic insulators make sure they aren't cracked and the gaskets are sound. They can lead to a big vacuum leak and all your tuning efforts will be a waste of time.

 

Andy

 

[Drakman]

Wow, not sure what happened there.

 

Check the choke fast idle cam has returned and the choke in general, sounds like the choke is stuck on on the front carb.

[wdb]

Random thoughts:  You obviously have far too much fuel going in. High idle could be due to air leaking into the intake somewhere besides the butterflies; the extra gas is already in there so the idle is up. The carb shafts wear and can cause air leaks, plus a bunch of other intake related stuff. You could have a stuck float valve on the front carb. All that gas is washing the cylinder walls, causing wear, and wrecking the oil. 

 

Edited October 29, 2023 by wdb

[Timothy Keith-Lucas]

Thanks so much! Leads for me to chase down.  Drakman, I'm thinking about the choke (as well as the idle) because the front carb pulls very little suction. The back carb just about pulls my hand into the engine while I can put my hand over the front one with no effect. I'm also going to look for an air leak into the manifold.

[sagamore]

My suggestion would be to post your problem on the MG forum (LINK). Those folks deal with SUs all the time.

[S1Steve]

There is a Facebook page dedicated to just SU carbs. Wish I had such a great resource when I had 4 cars in a row that were running SU’s.🍻

[theDreamer]

I wished they'd had the internet back in the 70's when I was dealing with the Delorto's and other stuff on my '74 twin cam Europa!

[inchoate]

+1 for getting in touch with the MG people.  They were helpful to me in the past.

 

There's a Haynes book that's pretty good, but it's not always easy to find.

[MV8]

Tim, you have a PM.

[jbcollier]

Any SU carb work starts with fresh fuel, valve clearances and compression.  If the valves are tight, it will never run right.  Low compression, ditto.  Today's fuel go off much faster now.  So, check those three first.

 

Next, wiggle the throttle shafts up and down.  If there is a play, you need to repair that first as well.  False air at idle will make it very hard to set up.

 

All those things good?

 

Now check the carbs out further.  Is the recommended needle in place?  Same for both carbs?  Check that the shoulder of the needle is flush with the damper/piston.  Adjust your jets so they are even with the top of the brass guide.  Now lower each two complete turns (12 flats).  That is a good starting point.  Release the linkage between the two carbs.  Back out the idle speed screws.  Lightly seat them and then 1T further.  Lock the linkage.  Refit the piston and dash pots.  Does the piston drop freely?  Should fall quickly with a solid "thunk" at the bottom.  If not, you will have to recenter the jet.  Procedure is on the web.

 

That is your basic tuning starting point.

 

Do you know how to sync and set the mixture?