Ideal pilot screw turns out?

How many turns out did Amal intend a 939 pilot screw to be set at?

I want to know because float level has direct influence on this.

I set my floats so 1.5 turn out give best idle. It seems close
to the .080" standard, but each carb likes a bit different
level to hit 1.5 turn best idle.
What would Amal do?

Float level sets base for all the rest, so would be nice
to have idle screw indicate which way to bump float seat.

hobot

What's a 939?

Concentrics ideally dial in at 1-1/2 turns. Pretty sure the MKII's are the same.

Each carb is slightly different. Both carbs should not be set the same on pure assumption that 1-1/2 turns out is the "proper" setting. The PROPER setting is the one where the bike runs properly.

My issue is that, with the pilot screw set at the point where the bike idles well, the plugs foul.

There are unpteen threads on the topic, but your bike has several fuel-related parts & settings that ALL have to be in good condition and tuned properly.

-main jet
-needle jet
-needle
-slide (surface)
-slide (cutaway)
-low speed mixture screw
-internal fuel passages
-float needle
-float height
-low speed jet (when so equipped)

...and don't forget air filter condition and flow rate through petcocks...

...and I'm sure i'm missing one or two other things that are strictly fuel related.

1 1/2 turns out is the "base" setting that will give a reasonable idle but, as noted it will vary a bit but it shouldn't vary much if everything else is set correctly. On my 850, less than 1/8 turn from that setting results in the smoothest idle. As you noted, fuel level (float setting) affects the idle mixture - higher level=richer mixture so if you change your float level, the settings on your idle screws will have to be adjusted appropriately.

hobot said:

How many turns out did Amal intend a 939 pilot screw to be set at?

hobot

Click to expand...

There is no "ideal" set number of turns for your settings screw. Although 1 1/2 turns is the place to start, the ideal setting is the one that works for you. There are too many variables. Altitude, the gas you use, the wear on the carb, the age and miles on your machine, your riding style....it goes on and on. Also, when your ideal setting is attained, don't consider this as a perminant setting. As things wear and environments change, so will the settings. It may be only a 1/8 to a 1/4 turn, but some time that's all it takes to make an ill mannered Norton behave. I always carry a mini screw driver.

I am sure Amal will concur with all said here.

Ok fella's If 1.5 turns out is the default to start from, then I'll
assume if everything else tip top, that should give best idle.
I'll continue to bump float so its so.

Yes each carb and bike are a bit unique but almost all
of them can have float fudged to give 1.5 turns out as
best Amal idle. Other brands seem based on same default.

Thanks I'm happy camper now.
hobot

hobot:

Adjusting the float level to correlate to 1-1/2 turns out for best idle may totally screw you up in several other places in the rev range!

In other words, DON'T WORRY ABOUT THE NUMBER OF TURNS ON THE LOW SPEED MIX SCREW! just so long as it has a definite effect on the idle (if it doesn't, you have plugged internal passages).

Properly setting the float level is a whole other situation.

Check Amal's tech guides and do each adjustment in PROPER ORDER, as they can affect each other negatively if not done in order (sorry, I don't recall the proper order from off the top of my head).

Totally concur with GP; set the float level to spec, forget about corellating it to the idle mixture screw. Adjust the idle mixture screw to get the best idle after the carbs are back together and everything cleaned up. Note that changing the idle speed will normally require another adjustment of the idle mixture screw. For example, lets say you have set the carb slide stops to give you an idle of 1000RPM and you have adjusted the idle mix screw for best running. If you decide to change your idle speed to, say 800 RPM and lower the slides appropriately to do so, the mixture screws will likely need some tweaking. These settings work together - moving the slides changes the idle speed which usually requires a mixture adjustment.

Properly set up/operating Amals will idle smoothly at 700 RPM but many people like the idle somewhat higher to reduce the chance of accidentally stalling the bike at a stoplight and having to go through the kickstart routine with horns honking.

It's a tedious process, to be sure.

Don't forget properly adjusting the cables BEFORE attempting get the carbs tuned properly, so that both slides rise together. Easy way is to work in about 1/8" of cable slack in both cables, have a helper crank the throttle all the way open, then make sure both slides are just butted all the way to the top of thier bores (adjust cables to equalize).

Of course, you'll have to re-adjust AFTER carb tuning, to ensure that they rise together equally.

Finally, a proper syncronization with a set of mercury sticks or vacuum guages is in order to get them nailed down.

It was on a years sat up 850 that we could not get to stay running
on idle that made me find float level off and diddled it
w/o measuring to get fine idle 1.5 turns out. Been my habit since.
One can then fudge float up or down from that base line
to further tune the response of the other Amal circuits and
reset pilot as needed.

I'd been told that Amal pilot was set 1.75 turn out so just
wanted a broader reality check. Happy with feedback.

BTW 939 dual Amals on a clapped out Combat with worn
points each miss set can still idle happy at 400 rpm.
My P!! dragster would settle to 600 if both jugs firing.
Main issue of slow idle is such low oil pressure and
not charging above ignition draw so may die of lack of
spark before running out of full gas tank.

hobot said:

BTW 939 dual Amals on a clapped out Combat with worn
points each miss set can still idle happy at 400 rpm.

Click to expand...

932 Amals? (900 series, 32mm)

Yes 32 mm factory issue standard with manufacture, age and handling deviations.

My bench mark with the professional built P!!, low low idle
yet just off idle response enough to spank Kiwi 750 triples
yet chug and lug off throttle though deep sugar sand ruts in
Fla. back woods.

Two factory Combats, a '71 and a '74 850 confirm float set for 1.5 turns
are a delight. Maybe a hot roded Cdo may like higher
float level, but then again its float fudge tends to make 1.5
center of idle adjustment diddle. Try it and see.

hobot