pilot air screws. strange but true!!!

[jerryz]

I recently did a full service on my M750 everything ,shims,oil, filters etc etc carb rebalance
she is running really sweet , set the pilot airscrews at 21/2 turns out but after riding a bit i thought she was a bit over rich at tickover and hesitant off the line , so i turned the pilot air screws out to 3 1/2-4 turns and wow she is much crisper off the line and tickover and starting are better as expected ..........but what I did not expect was the better midrange and top end response ?? those settings should not have an effect above 2000 rpm but the do and I noticed the same thing on my 750ss as well.

Today i set the carbs up on a friends M900 and same thing happened when we test rode it ....its not subjective its a noticable before and after effect and can be reversed .

anyone else had this effect or a scientific explanation
__________________

[ducpainter]

All of the carb settings affect all the ranges in the real world...just not as much as the setting that is typically thought to control that throttle range.

That's why they say to get the main jet right first as it actually affects the setting of the pilot air or fuel screw and the needle.

[motoxmann]

x2 to duc; all settings have an affect on the entire range.
a few thoughts though:
- they are actually fuel enrichment screws, they adjust fuel flow not airflow. turning the screws out ADDS fuel. which means originally it was running a little lean and you richened it up.
- the fuel that flows through the ports affected by those screws is a very tiny amount. the fact that it improved midrange and top end tells me that the bike was also too lean in midrange and top end, and may still be too lean in those areas. you may want to also try raising the needles one notch and/or going 1 size larger on the main jets and see if that improves midrange and top end even further.
- have the float levels been checked recently? from the factory the float levels tend to be noticeably lower than they are supposed to be as well as noticeably uneven. setting the float levels properly can have a huge effect.

[jerryz]

No that cant be correct !!!!!!!
turning the pilot air screw in cuts off air = richer ,  turning them out adds air makes the bike leaner .that what I was taught at automotive college  35 years ago  and on my S4 EFI its the same I can see it on the computer printout when i do the CO2 test.

any way the bike has always been a greaty runner the float levels and jets are spot on got perfect  4% CO2 on the gastester as well. Bike has open sils , stage 3 jets , open airbox ,k&N and zero closer shims ,I can catch M900s up to 120mph easy peasy.

[brad black]

the idle mixture screw regulates fuel flow on the mikuni carb.

on your s4 the air bleeds screws regulate air flow.

they're different things doing different jobs.

the slides don't fully open until around 5,500 rpm in my experience, so any fuelling under that and at lower throttle openings will be influenced by the idle mixture to some extent.  it's probably more variant on how much vacuum is over the transfer ports (little holes that are just in front of the throttle when it's closed).  if there's vacuum there to pull fuel out they will have an impact.

and it could just be too lean overall.

[motoxmann]

No that cant be correct !!!!!!!
turning the pilot air screw in cuts off air = richer ,  turning them out adds air makes the bike leaner .that what I was taught at automotive college  35 years ago  and on my S4 EFI its the same I can see it on the computer printout when i do the CO2 test.

you were taught properly; halfway at least:

-pilot air screws adjust air flow: turning them in decreases air making the mixture richer. turning out increases air making it leaner.
-fuel enrichment screws adjust fuel flow: turning them in decreases fuel flow making the mixture leaner. turning them out increases fuel flow making the mixture richer.

-carbed duc's have fuel enrichment screws. fuel injected duc's have pilot air screws.
as for other makes of bikes, some have pilot screws and some have fuel screws. it's different with every bike and every carb.

[jerryz]

and old Trumpets , Nortons, BSA's ,Harleys have Pilot air screws rather then fuel enricjhment screws and thats the sort of stuff we learned on back then.

[MonsterHPD]

If I remember correctly from my "carburettor days", if the screw is upstream of the throttle it's likely an air screw, and if it's downstream of the throttle it's likely a fuel screw. Don't know if it applies in the cases referred to, but it's generally been accurate in the carbs I've used.
 

[brad black]

tony tells me that amals have air screws, but all the dellortos and mikunis and keihins have fuel mixture screws ime.  luckily i've never had to touch shit that old.

[koko64]

If I remember correctly from my "carburettor days", if the screw is upstream of the throttle it's likely an air screw, and if it's downstream of the throttle it's likely a fuel screw. Don't know if it applies in the cases referred to, but it's generally been accurate in the carbs I've used.
 

Great rule of thumb that.^

Amal carbs,.. those were the days.. The "enrichner circuit", when you "tickled" the carbs until the fuel pissed out onto the cases. It was another time.

[ducpainter]

Great rule of thumb that.^

Amal carbs,.. those were the days.. The "enrichner circuit", when you "tickled" the carbs until the fuel pissed out onto the cases. It was another time.

Then someone had to go and invent injection...

[koko64]

Then someone had to go and invent injection...