Wow....What a Difference 1/4 Turn on the Pilot Air Screw Makes

[knotscott]

I've only had my '79 GS 750E for a couple of months, and it's been showing plenty of signs of running lean, including lots of backfiring on startup, slow to warm up, and sputtering in the 3100-3400 rpm range. I made a 1/4 turn clockwise adjustment to pilot air screw, and the vast majority of those issues are cured! Running like a champ now. It's so much nicer to ride when the bike's running well....

[tkent02]

I've only had my '79 GS 750E for a couple of months, and it's been showing plenty of signs of running lean, including lots of backfiring on startup, slow to warm up, and sputtering in the 3100-3400 rpm range. I made a 1/4 turn clockwise adjustment to pilot air screw, and the vast majority of those issues are cured! Running like a champ now. It's so much nicer to ride when the bike's running well....

You should try riding it when it's running perfectly. Keep going, it's in there someplace.

[gbrown]

I've only had my '79 GS 750E for a couple of months, and it's been showing plenty of signs of running lean, including lots of backfiring on startup, slow to warm up, and sputtering in the 3100-3400 rpm range. I made a 1/4 turn clockwise adjustment to pilot air screw, and the vast majority of those issues are cured! Running like a champ now. It's so much nicer to ride when the bike's running well....

I presume you made a 1/4 turn to each of the four carbs? I also have a new (to me) 78 GS750E and, until I can afford to adjust the valves and rebuild the carbs I might give this a try. Please advise, thanks.

[chuck hahn]

Cost just the price of a gasket to check the valves..and a day or two for Ray at the shim club to see if he has what you need for the tighter ones. Cost many hundreds if you burn up a valve or seat...plus the down time. I would advice you dont procrastinate on at least this bit of maintenance.

[rippledub@gmail.com]

trying to figure these things out . . . beginner I am
I was under the impression that the adjustment screw is actually a fuel screw, not an air screw, so 1/4 turn CLOCKWISE would be reducing the fuel, which seems to be opposite of what you would want if you were runnin lean?

[tkent02]

There is a fuel screw and an air screw. Which one did you turn?

[allojohn]

There is a fuel screw and an air screw. Which one did you turn?

And what bike are you running....

[Steve]

trying to figure these things out . . . beginner I am
I was under the impression that the adjustment screw is actually a fuel screw, not an air screw, so 1/4 turn CLOCKWISE would be reducing the fuel, which seems to be opposite of what you would want if you were runnin lean?

That depends on what carbs you have.

On the VM series carbs ('79 and older fours), you have a fuel screw and an air screw. Counter-clockwise on either one will add more of what it controls. Clockwise on either one will reduce what it controls.

On the BS series carbs (all twins and '80 and newer fours), there is only a mixture adjustment screw. Counter-clockwise on that also adds more of what it controls, which happens to richen the overall pilot mixture.

.

[tkent02]

He said '79 750.

[Steve]

He said '79 750.

Knotscott said he had a '79 750.

It was rippledub that asked about the function of the screw.

.

[rippledub@gmail.com]

Ok, sorry I may have created some confusion due to my inexperience.

I have a 1980 gs1000g which most likely has a different carb, so my comment may be off base. I was referring to a note in John Bloemers carb rebuild turorial in which he writes:

Note #2: Bill Kingston (wkings41@pop.infi -net.mindspring.com) has brought it to our attention that the references to a Pilot Air Screw may be incorrect. He states that on these carbs use a Fuel Flow Screw instead. This means that you are controlling fuel running through the passages with this adjustment, not air. So when turning the screw counterclockwise you are increasing fuel flow and when you turn it clockwise you decrease fuel flow.

[tkent02]

O k

[Steve]

Yes, you have the BS series carbs (also called "CV-type" carbs). The only adjustment on them is a mixture screw. It controls how much of a pre-set mixture is admitted to the air stream at low throttle openings, so counter-clockwise will richen the overall mixture, clockwise will lean it.

Note that the factory settings were made with EPA emissions compliance in mind, not overall ridability, so they were set lean. This also made for long warm-up times. It it a simple matter to richen the mix a little to make the bike (and you) much happier. The stock settings were probably in the 1 1/2 turn range, most of us find that 2 to 2 1/2 (sometimes a bit more) works much better.

Of course, this assumes that your carbs are CLEAN, synchronized and working well.

.

[knotscott]

I presume you made a 1/4 turn to each of the four carbs? I also have a new (to me) 78 GS750E and, until I can afford to adjust the valves and rebuild the carbs I might give this a try. Please advise, thanks.

Sorry I didn't notice this sooner, but yes, each carb got the same 1/4 turn clockwise adjustment to the air screw.