SU Carb question - B20B 1971 145

Car is a 1971 145S, dual SU HIFs, stock mixture manifold with butterflies removed & shaft holes plugged. New manifold gasket, rebuilt carbs. Mixture seems OK, maybe a tad rich based on smell, but passes the lift pin test. Carbs are in sync as verified with airmeter.

Wondering if the following behavior is normal or indicative of something that needs to be adjusted or corrected:

Whenever I am driving the 140, after several minutes of driving at speed, say 5-10 minutes, when I come to a stop, my idle will be high. Maybe in the 1200-1500 RPM range. Typically, it will take about 2-5 minutes for the idle to slowly creep back down to its typical equilibrium of about 800-850 RPM.

I would like to know if this behavior is normal. The only 'problem' with this is that when I arrive at my destination, I have to wait a few minutes for the idle to go down otherwise I might get an "afterfire" BANG from shutting the engine off at too high an idle speed. It happened once when I turned the engine off at high idle and I'd really like to avoid having that happen again...

Temperature gauge consistently reads within normal range, radiator was acid flushed at a radiator shop.

Is it potentially an issue with the bimetallic jet adjusters in the carb? Or is this just the way things are with SU carb'd B20 cars?

On the plus side, you don't have a choke mechanism to futz with the idle speed via the throttle stops on the HIF.

I'd be looking at the timing? I've had weird things happen with idle speeds in the past due to wonky timing. I.e. it's spinning fast enough to get into some degree of advance in the dist, this helps support the higher idle speed, only slowly does it claw back some RPM's and removes that advance.

Also possible in conjunction with a vacuum retard (emissions thing that also affects idle speed on that era of car).

JohnMc said:

On the plus side, you don't have a choke mechanism to futz with the idle speed via the throttle stops on the HIF.

I'd be looking at the timing? I've had weird things happen with idle speeds in the past due to wonky timing. I.e. it's spinning fast enough to get into some degree of advance in the dist, this helps support the higher idle speed, only slowly does it claw back some RPM's and removes that advance.

Also possible in conjunction with a vacuum retard (emissions thing that also affects idle speed on that era of car).

Click to expand...

It's got a vacuum advance, but the vacuum solenoid is of a different number than stock for some reason.

Previous distributor that was on the car had centrifugal advance but still acted the same.

Static timing seems to check out OK, I keep it around 12-15 degrees, car seems to act better at that setting. (rings are a bit worn, compression test reveals 150, 150, 140, 140)

That's a good idea though, take it for a spin, pull into the driveway when idle is running high and measure the timing. Will do that today

I haven't really ever messed with HIF's as much as the 60's HS carbs. Do they suffer from throttle shaft wear and subsequent idle air leaks?

JohnMc said:

I haven't really ever messed with HIF's as much as the 60's HS carbs. Do they suffer from throttle shaft wear and subsequent idle air leaks?

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Not really

I've sprayed aerosolized hydrocarbon at the throttle shafts and got no response.
I think I did it with an unlit propane torch too.

I'm pretty sure they aren't leaking

Do the throttle blades have the decel valves in them? If so and the springs are weak it can hold the idle up. Common practice is to solder them closed.

Also check the hot start valves. They can stick and hold the idle up as well.

hiperfauto said:

Do the throttle blades have the decel valves in them? If so and the springs are weak it can hold the idle up. Common practice is to solder them closed.

Also check the hot start valves. They can stick and hold the idle up as well.

Click to expand...

Speaking of hot-start valves, what is the proper adjustment of them? When the car is at idle/no throttle input, should the plunger be depressed?

The service manual says to depress the plunger all the way and there should be a 1mm gap between the adjusting screw and the end of the plunger.

hiperfauto said:

Do the throttle blades have the decel valves in them? If so and the springs are weak it can hold the idle up. Common practice is to solder them closed.

Also check the hot start valves. They can stick and hold the idle up as well.

Click to expand...

Huh. Yeah mine have little bypass valves on the throttle plates. Is there no ill-effect from soldering them closed?

My hot start valves are adjusted within greenbook spec, but I don't really have a way to verify that they are opening and closing properly. I could test and see if they're sticking open by applying hose pinchers on the hoses coming from the hot start valves while the car is idling, right? Because if the carb was sucking in air from the hot start valve, the hose pincher would block that off and lower the idle immediately, or is my plan flawed?

The valves on the throttle plates reduce emissions by lowering the amount of vacuum at the jets so it doesn't draw in as much fuel on decel (which isn't needed). It probably also helps with fuel mileage minimally.

The issue with the hot start valves is mechanical. The plungers stick which in turn hold the throttle plates open. Try pushing in the plungers on the hot start valves when the idle is high and see if the idle comes down.

I'll bet the intake manifold is warped.
Works fine when cold, but as things heat up, there is an intake leak at the gasket.

Don't be surprised if you have to (shudder the thought) use high-temp gasket sealer on the intake gasket to eliminate the problem. I generally would never recommend that on a B20, but it is what fixed mine when I had the same issue.

Hi
put a stronger return spring to close butterfly's
and oil the linkage
try the simple things first

alschnertz said:

I'll bet the intake manifold is warped.
Works fine when cold, but as things heat up, there is an intake leak at the gasket.

Don't be surprised if you have to (shudder the thought) use high-temp gasket sealer on the intake gasket to eliminate the problem. I generally would never recommend that on a B20, but it is what fixed mine when I had the same issue.

Click to expand...

Ya know, this did cross my mind. Wonder if it would be too pricey to have the manifold resurfaced...

I’ve been chasing a high and erratic idle with my hif6’s. Rebuilt them, new 123ignition distributor, timing is correct, valves adjusted. I have to back the throttle screws all the way out to get the idle below 1k, and it lands somewhere between 700-1100 after blipping the throttle or stopping at a light.

Have you measured vacuum? Mine is low at idle and I haven’t figured out why.

svinkle said:

I?ve been chasing a high and erratic idle with my hif6?s. Rebuilt them, new 123ignition distributor, timing is correct, valves adjusted. I have to back the throttle screws all the way out to get the idle below 1k, and it lands somewhere between 700-1100 after blipping the throttle or stopping at a light.

Have you measured vacuum? Mine is low at idle and I haven?t figured out why.

Click to expand...

Sounds similar to my problem. I'll measure vacuum and report back

My idle screws are also backed almost completely out.

Blipping the throttle used to work, but now it seems like it just requires time.

I think my pistons used to be sticky, catching on the vacuum chambers, or the damper was catching on the piston tube. That might be what you're experiencing

I think the most important thing here is that I've discovered that I indeed have an issue, and this is not how it's supposed to be

alschnertz said:

I'll bet the intake manifold is warped.
Works fine when cold, but as things heat up, there is an intake leak at the gasket.

Don't be surprised if you have to (shudder the thought) use high-temp gasket sealer on the intake gasket to eliminate the problem. I generally would never recommend that on a B20, but it is what fixed mine when I had the same issue.

Click to expand...

First thing I thought of when I read your question. Try snugging up the manifold nuts.
High idle is air leaking in somewhere.

Ian, you may want to try fattening up your idle mixture 1/4 turn richer.
Slow to return to idle is a sign of being lean.
How do your spark plugs look?

I also second the "tightening the **** out of the manifolds and carburetors" motion.