SU HD6 - Slow Idle Screw

[JBlubaugh]

Silly Question!

What is the purpose of the Slow Idle screw? Was making adjustments last night, following the manual, and it when it came to making the adjustments to the slow idle screw I really never noticed any type of change.

I've always made idle adjustments with small changes to the fast idle screw.

Am I missing something?

Thanks,
Jim

 

[John Turney]

The fast idle screws are used for adjusting the fast idle with the choke knob pulled out. The slow idle screw is for when the choke knob is pushed in. I leave a slight gap between the fast idle screw and its lever when the choke is pushed in, maybe 0.005".

 

[JBlubaugh]

Thanks John -

Can you explain the process for setting the idle with the slow idle screw? Per my manual, I close it all the way down then open 3 1/2 turns, making adjustments from there. When I do that, and make adjustments, I don't notice any gain or drop in rpm's.

Thanks,
Jim

 

[steveg]

Thanks John -

Can you explain the process for setting the idle with the slow idle screw? Per my manual, I close it all the way down then open 3 1/2 turns, making adjustments from there. When I do that, and make adjustments, I don't notice any gain or drop in rpm's.

Thanks,
Jim

Section D.11 describes this process. The manual calls the fast idle the 'throttle stop screw'. If you adjust them to a piece-of-paper gap per John above, the throttle butterflies will be completely closed when the engine is warm (choke off) the mixture will bypass the closed butterflies via passage 11 (see page D.12 - p110 in Bentley).

Only when you pull on the choke cable, will the throttle stop screws operate the fast-idle system.

On my HD8s, I closed the slow-run valves down to 3/4 of a turn open to get the idle below 1000.

If the slow-run valve has no effect, would guess you have the throttle butterflies open via the throttle stop screws.

 

[John Turney]

Jim,

If you don't see an effect with the slow idle screws, you either have the throttle plates open with the fast idle screws or both the throttle plates are not completely closed for other reasons. Did you make sure they were completely closed individually before locking them together?

 

[JBlubaugh]

Thanks John,

I will redo the process again this evening and see if I missed a step. Agree with you that it sounds as if the plates were not completely closed.

Thanks Steve -

Found a pretty good description, different from what I was using, in the BMC shop manual. Will give it a try tonight.

Jim

Jim,

If you don't see an effect with the slow idle screws, you either have the throttle plates open with the fast idle screws or both the throttle plates are not completely closed for other reasons. Did you make sure they were completely closed individually before locking them together?

 

[JBlubaugh]

Thanks Guys -

I think it is safe to say that for the 1st time in a very long time my car is on idle with the slow idle screws as opposed to the fast idle screws. Took a while to get everything dialed in but I just took it a couple of miles out and back and it ran great. Nice constant idle but the hiss of the slow idle bypass will take some miles to get used to.

Thanks for all the help.

Jim

 

[steveg]

Thanks Guys -

I think it is safe to say that for the 1st time in a very long time my car is on idle with the slow idle screws as opposed to the fast idle screws. Took a while to get everything dialed in but I just took it a couple of miles out and back and it ran great. Nice constant idle but the hiss of the slow idle bypass will take some miles to get used to.

Thanks for all the help.

Jim

Jim,
For closure, can you give us a little more detail on what exactly was wrong?

 

[JBlubaugh]

Hi Steve -

Sure, no problem. The short version is that when the engine was rebuilt the carbs were cleaned up but never overhauled, they ran fine prior to the rebuild. It appears that the previous owner had the slow running screw all the way closed and never used. The idle had been set by playing with the mixture and the fast idle screws, the choke was disconnected. It ran great, although has always been a tad rich. Wanting to lean it out and hook up the choke is what started this. Also changed some of the geometry of the throttle linkages to match the diagrams and specs. The PO had the vertical linkage coming off the pedal almost vertical, which was causing some hesitation. Moved it to more of a 2 o'clock setting.....which threw the OD cam off ....

It appears to be now set-up and running as it should. Will put some miles on it this weekend then pull the plugs to see how they are burning.

 

[steveg]

Jim,
Thanks for the closure -- adds value for future searches.

 

[John Turney]

Jim,
Thanks for the closure -- adds value for future searches.

 

[vette]

I don't think you should be hearing a hiss from your slow run screws. Do your slow run screws have the '0' rings installed on them. Can you tell us how many turns out your slow run screws are set at. Thanks, Dave.

 

[steveg]

I don't think you should be hearing a hiss from your slow run screws. Do your slow run screws have the '0' rings installed on them. Can you tell us how many turns out your slow run screws are set at. Thanks, Dave.

If the slow run screws are hissing you could test for the air leak by squirting them with windex.

 

[JBlubaugh]

Hi Steve / Dave - Let me correct that statement. It's not a hiss I am hearing from slow run screws themselves, it's the by pass of air making the sucking (hiss) sound when at idle. As the accelerator is applied, the noise goes away with the lifting of the piston. From totally closed down, I have the slow run screws opened to 4 complete turns. That appears to be point where the car will not drop in rpm's and stall out. RPM's on idle is sitting at 800 right now. I am going to spray around the carbs at temperature today with a can of carb cleaner to see if I can find any leaks anywhere. I'm not totally convinced that when the Carbs were cleaned up during the rebuild that everything was checked and changed, like bushings, throttle shafts, and jet diaphragms..... If I'm not happy with them will probably send them in to Joe Curto for a complete rebuild.

Thanks, Jim