Idle speed in SU carbs

[Timothy Keith-Lucas]

Here's my "Getting to know my 7" question of the week.

 

I had problems with over heating; when I finally got her home I flushed the cooling system several times and am hoping that solved that.  But, possibly related (?) my idle speed is way up - about 2,000 RPM, and adjusting the idle screw (and the choke screw) all the way out doesn't change it.  The throttle cable is not the problem. It's a pair of SUs on aa Austin 1098 CC engine. Is something stuck inside? Am I going to find that one or both throttle cylinders is sticking part way up? I'm inclined to believe that "Carburetor" is a French word for "You're going to regret messing with this," so I'm asking  before I dig in.  Thanks.

[MV8]

I expect the throttle shafts individual linkage or combined linkage is hanging a little, keeping the butterfly cracked open. You can disconnect to isolate. Return spring tight/tensioned with the throttle closed? Check the fast idle screw tip is in the notch of the choke cable cam to allow the throttle to close to engage the idle speed screw.

 

Is the thermostat opening? During warm up, the upper hose will suddenly get much warmer when it does. Thermostats are often not very reliable. You could remove it and test, then reinstall if it's closed when cold. It can be tested on the stove in a pot of water.

 

Radiators collect debris and eventually clog. You can determine this by placing your hand around the core during warm up from overnight cold, a laser temp reader, or an IR camera to see the temp difference across the core. Warm coolant flows in from the upper hose.

 

I'd push water through the lower hose and out the upper port to attempt to flush. You could also try a safe for your radiator cleaner like CLR. Remove radiator, fittings up, mix cleaner in bucket, fill and let it sit a day or so, then flush from the lower again.

If your coolant was muddy, I'd use a cup of "metal prep" concentrate (phosphorous) added to water, run the engine to circulate, then drain and flush after a few days.

Edited April 8, 2023 by MV8

[wdb]

It's probably indication of some sort of affliction but I always enjoyed tuning SU carbs back when I lived by the wrench. A high idle that does not change with idle screw adjustments could be several things. Yes it could be the piston hanging but that would cause a bunch of other issues. Also it could be the throttle plate not being allowed to close fully due to mechanical misalignment or perhaps debris. It could also be air leaking past worn out linkage shafts and bushings. Finally you didn't mention the choke cable adjustement -- you want to ensure that the cable is not holding the choke open. A wee bit of free play in both cables is what you want.

[Timothy Keith-Lucas]

Thank you, MV8 and wdb. Those points give me lots of ways of chasing it down. Largely, you are telling me that something is keeping the throttle plate or the choke plate open a bit. I ca explore that.

 

Thanks also for the advanced course on coolant system clearing. The thermostat does appear to be working - at 160 F the coolant starts circulating. I think I'll block off the radiator and fill it with an acid cleaner for awhile, and wash the outside surfaces of the radiator fins as well.

[jbcollier]

Can you post photos of the carbs, specifically the linkage where the throttle cable attaches and the two carbs are linked.  There are a couple of different styles and it would be helpful to know what you have before commenting.

 

Do you have a sync tool?

 

Have you ANY experience with carbs?

 

General rule of thumb:  If there is a problem, don't mess with carb adjustments until you know what's up.

 

 

[sagamore]

In addition to the earlier posts, I would recommend that you try the MG Experience's Spridget website (link). 

 

You probably have, but make sure that both carbs have oil in their dashpots. I normally use 20W-50 in mine. 

[jbcollier]

Depending on your local climate, engine oil (20w50) may make the carb too rich on acceleration.  We ran Dexron 2/3 ATF.  It's lighter and better suits climates with less dense air (dry, high altitude, etc)

[Timothy Keith-Lucas]

Extensive experience with small engine (largely motorcycle) carbs, but no experience with anything resembling these SUs. I read "Tuning SU carburetor for Idiots" and found that the unfamiliar terminology meant that I could not understand what was being explained. It's pretty bad when you can't follow the "for idiots" instructions.

 

I'm in the middle of painting my 7; new BRG with a touch of flattener to keep it from looking repainted. Once that's done, I'll return to the carbs and the tach, which is a model that even "Clocks for Classics" has not seen before.  Thank you to everyone who has lent a hand to me above. I'll post pictures of the linkage upon a return.

 

Were 7 America's delivered painted or bare? The original paint job on SB1160 was very poorly done, right down to water drops under the paint and streaked sanding. I can't imagine the factory doing this bad a job.