How to: Adjust A/F screws

Post Reply
PrinceMo
Passed the Circle Test
Posts: 395
Joined: Mon Apr 04, 2016 11:13 am
My Bike: 1988 1400cc Intruder
Location: Los Angeles

Re: How to: Adjust A/F screws

Post by PrinceMo »

Thanks.
Good write up
However even with the screw all the way in my bike keeps running.
Still running rich.
Could the screw be bad?
Does screwing it in make it lean or rich?
Does the screw control air or gas?
(I'm thinking gas so in would be more lean?)
Does the mix screw have an O ring?

Thanks in advance.
PS: the bike is uncorked and ran great for years.
This popping and fouling plugs just started maybe last sept
I was in the hospital from dec 4 to march 7, 2016.

User avatar
YoDude
Joined a 1200cc Club
Posts: 11021
Joined: Sun Sep 07, 2014 5:30 am
My Bike: Suzi 1400
Location: San Somewhere. West Coast
Contact:

Re: How to: Adjust A/F screws

Post by YoDude »

The screw can't, "go bad".
Screwing it in makes the mixture leaner.
It controls fuel.
It has an o-ring.

You may have a leaking float valve.

Yo-
Intelligence is just the right thing to have, to render yourself extinct.

Spitfire
Joined a 850cc Club
Posts: 1378
Joined: Wed Dec 10, 2014 5:15 pm

Re: How to: Adjust A/F screws

Post by Spitfire »

Thought we finally ironed out that it was an air fuel mixture the screw controls and that it didn't really matter as it still controls the final air/fuel mix. [emoji106]

:soda:

User avatar
enforcer
Joined a 650cc Club
Posts: 1138
Joined: Wed Feb 03, 2016 7:29 pm
My Bike: 03 HD FLHT 01 Yamaha XVS650

Re: How to: Adjust A/F screws

Post by enforcer »

98VS1400 wrote:
YoDude wrote:The screw can't, "go bad".
Yo-
In a sense, it most certainly can. If a person tightens it down too hard, it will break off or deform the tip of the screw. That's why it is very important to only turn it until it seats. Once that mixture screw is messed up, you're screwed. (Ha, that was funny!)

While I do agree it meters fuel, there is a strong contingent of others who argue otherwise! See a previous thread on here where I was shellacked by Mike and a few others for saying it metered fuel.

:fu:
I don't know what everyone else's definition of meters is, but that's exactly what it does. But it doesn't really "control" that much fuel, really only one small passage on the idle circuit. That does add fuel to the others circuits, but not to the extent of actually flooding like the symptoms you described in your other thread.

A height adjustment could cure it, but seeing as how it's inconsistent, I doubt that's the case. A combination of both, coupled with a poor spark, could be causing the issue as well. It's all worth a look, but talking about it isn't getting it fixed. Time to get wrenchin', because without more info, no-one will be able to tell you definitively.
Current rides: 03 HD FLHT & 01 Yamaha XVS650(BIP)
Former: 87 Suzuki VS700, 94 BMW K1100LT & 91 Honda CB250

I don't want to own them all, just ride them all.
Image

User avatar
WintrSol
Joined a 450cc Club
Posts: 857
Joined: Fri Feb 26, 2016 2:02 pm
My Bike: CB450 GL1500CT
Location: Near St. Louis, MO

Re: How to: Adjust A/F screws

Post by WintrSol »

There are three paths from the mix chamber that creates the rich air/fuel mixture. One is controlled by the pilot screw, which sets the idle mixture, another is the 'choke', or starting enrichment circuit, and the third is the transient enrichment (coast) valve system. If adjusting the pilot screw has little effect, you may have the throttle valve open too wide (sync needed), or one of the other paths is open when it shouldn't be. Of the two, I seem to recall that the 'choke' valve tends to stick more often than the coast valve, but either could be a problem.
Florissant, MO
CB450K3, GL1500CTValkyrie

Spitfire
Joined a 850cc Club
Posts: 1378
Joined: Wed Dec 10, 2014 5:15 pm

Re: How to: Adjust A/F screws

Post by Spitfire »

:fu:
I don't know what everyone else's definition of meters is, but that's exactly what it does.
It was a silly debate on here a while ago that I let some other guys believe they won. Not saying any names, but one rhymed with Spitfire...

:fu:
:funnylast: Ah yup, silly but fun. Best I remember it became more an exercise of semantics than actual discussion of how to do it. :lmao:

User avatar
YoDude
Joined a 1200cc Club
Posts: 11021
Joined: Sun Sep 07, 2014 5:30 am
My Bike: Suzi 1400
Location: San Somewhere. West Coast
Contact:

Re: How to: Adjust A/F screws

Post by YoDude »

98VS1400 wrote:
YoDude wrote:The screw can't, "go bad".
Yo-
In a sense, it most certainly can. If a person tightens it down too hard, it will break off or deform the tip of the screw. That's why it is very important to only turn it until it seats. Once that mixture screw is messed up, you're screwed. (Ha, that was funny!)

While I do agree it meters fuel, there is a strong contingent of others who argue otherwise! See a previous thread on here where I was shellacked by Mike and a few others for saying it metered fuel.

:fu:
Alright smarty pants. The screw can be damaged by idiots that don't know the first thing about carbs and how to tinker with them. Generally however, it's not the screw that gets the most damage, it's the seating surface it comes in contact with. That surface can be cracked and split open when the screw becomes overly tightened. This causes permanent damage to the carb body itself and cannot be repaired. This is why you really get screwed when that happens. Otherwise, replacing the screw would fix the problem.

The screw does not, just, "go bad" on it's own.

Do ya dig what I'm sayin'?

Yo-
Intelligence is just the right thing to have, to render yourself extinct.

User avatar
WintrSol
Joined a 450cc Club
Posts: 857
Joined: Fri Feb 26, 2016 2:02 pm
My Bike: CB450 GL1500CT
Location: Near St. Louis, MO

Re: How to: Adjust A/F screws

Post by WintrSol »

Same goes for jets; they get plugged, and damaged when removed or installed, but don't go bad. Moving parts go bad, like float valves, diaphragms, throttle bearings, etc. Seals can go bad, too, if exposed to a solvent they weren't designed to tolerate.
Florissant, MO
CB450K3, GL1500CTValkyrie

User avatar
YoDude
Joined a 1200cc Club
Posts: 11021
Joined: Sun Sep 07, 2014 5:30 am
My Bike: Suzi 1400
Location: San Somewhere. West Coast
Contact:

Re: How to: Adjust A/F screws

Post by YoDude »

Yes, and especially the diaphragms are susceptible to solvents. At $80 a pop to replace them, best to use only soap and water to clean them!

Yo-
Intelligence is just the right thing to have, to render yourself extinct.

PrinceMo
Passed the Circle Test
Posts: 395
Joined: Mon Apr 04, 2016 11:13 am
My Bike: 1988 1400cc Intruder
Location: Los Angeles

Re: How to: Adjust A/F screws

Post by PrinceMo »

WintrSol wrote:There are three paths from the mix chamber that creates the rich air/fuel mixture. One is controlled by the pilot screw, which sets the idle mixture, another is the 'choke', or starting enrichment circuit, and the third is the transient enrichment (coast) valve system. If adjusting the pilot screw has little effect, you may have the throttle valve open too wide (sync needed), or one of the other paths is open when it shouldn't be. Of the two, I seem to recall that the 'choke' valve tends to stick more often than the coast valve, but either could be a problem.
Coast valves are shimmed and the choke is working.
However when the pilot/mix screw is all the way in the bike keeps running.
My thinking is the bike should stall with the screw all the way in.

Thoughts?

Thanks ,

Prince.

User avatar
enforcer
Joined a 650cc Club
Posts: 1138
Joined: Wed Feb 03, 2016 7:29 pm
My Bike: 03 HD FLHT 01 Yamaha XVS650

Re: How to: Adjust A/F screws

Post by enforcer »

PrinceMo wrote:
WintrSol wrote:There are three paths from the mix chamber that creates the rich air/fuel mixture. One is controlled by the pilot screw, which sets the idle mixture, another is the 'choke', or starting enrichment circuit, and the third is the transient enrichment (coast) valve system. If adjusting the pilot screw has little effect, you may have the throttle valve open too wide (sync needed), or one of the other paths is open when it shouldn't be. Of the two, I seem to recall that the 'choke' valve tends to stick more often than the coast valve, but either could be a problem.
Coast valves are shimmed and the choke is working.
However when the pilot/mix screw is all the way in the bike keeps running.
My thinking is the bike should stall with the screw all the way in.

Thoughts?

Thanks ,

Prince.
Only going to stall on that one cylinder, the other will keep running, pulling the other along. Could be that cylinder wasn't running in the first place.
Current rides: 03 HD FLHT & 01 Yamaha XVS650(BIP)
Former: 87 Suzuki VS700, 94 BMW K1100LT & 91 Honda CB250

I don't want to own them all, just ride them all.
Image

User avatar
WintrSol
Joined a 450cc Club
Posts: 857
Joined: Fri Feb 26, 2016 2:02 pm
My Bike: CB450 GL1500CT
Location: Near St. Louis, MO

Re: How to: Adjust A/F screws

Post by WintrSol »

PrinceMo wrote: Coast valves are shimmed and the choke is working.
However when the pilot/mix screw is all the way in the bike keeps running.
My thinking is the bike should stall with the screw all the way in.

Thoughts?

Thanks ,

Prince.
First, does it do the same with both carbs? If not, then the other guys suggestions are valid, but before you pull the plug wire, feel the exhaust, old school sync method. You should feel similar strength output pulses, and both should be warm. If one is weak, then it probably is not firing, maybe because the sync is far enough out that it can't. Or, you could have a problem with the spark plug boot, shorting out the spark; pulling the plug on the weaker or cooler side may reveal this, especially if you put a spare plug in that boot and hold it against the head.
Florissant, MO
CB450K3, GL1500CTValkyrie

Longhorn717
Practicing Bicycling
Posts: 42
Joined: Mon Oct 31, 2016 5:06 am
My Bike: 2000 vs1400

Re: How to: Adjust A/F screws

Post by Longhorn717 »

Hey guys. Need to resurrect this thread.

I looked for this "idle screw" y'all talked about but haven't found it. I did get the plugs drilled and adjusted the screws some. But only way I could get the idle down is to loosen the throttle adjuster on the handlebar.

By syncing the carbs you mean that the throttle cable begins to pull the on each carb at the same time correct? Or is it something else?

Thirdly. I've got some issues with riding it. It falls flat and I have to over rev it to take off. Then in first it pulls hard to wot. But in second, third, and fourth gear it doesn't want to cruise without surging or being flat at roughly 1/4 to 1/2 throttle. It will mostly cruise in fourth and fifth gear if I'm over 1/2 throttle. But it takes awhile to accept more throttle from a cruise to acceleration. Bout a 3-5 second lag. Then it pulls balls to the walls

Little about what I've done. I got the bike stock except for the exhaust it had been completely bored out. I put pod filters on to compensate and to allow room to relocate the battery further up. (My preference). I then added a stage three jet kit from factory pro. Put the biggest jets it came with in and put the new needles on the middle position. It also included one size larger pilot jets. Rear cylinder went from about a 145 to a 165 and front went to a 150 I think.

So what I'm think is I need to raise the needles one position to richen the 1/4-3/4 throttle range. But I'm not sure about the falling flat on take off. Thoughts?

Before I do too much I want to add some sort of muffler to it. Make the sound a little less loud and deeper with a smidge of back pressure.

User avatar
WintrSol
Joined a 450cc Club
Posts: 857
Joined: Fri Feb 26, 2016 2:02 pm
My Bike: CB450 GL1500CT
Location: Near St. Louis, MO

Re: How to: Adjust A/F screws

Post by WintrSol »

Longhorn717 wrote:Hey guys. Need to resurrect this thread.

I looked for this "idle screw" y'all talked about but haven't found it. I did get the plugs drilled and adjusted the screws some. But only way I could get the idle down is to loosen the throttle adjuster on the handlebar.
There are two things you could be referencing: the pilot (idle) mixture screws, which you apparently found, and the knob that adjusts the idle setting for both carbs at once, located on the rear carb on the right side.
By syncing the carbs you mean that the throttle cable begins to pull the on each carb at the same time correct? Or is it something else?
There are two sync settings: one that makes both throttles open at the same time, and one that matches the idle settings, so the knob can move them both the same. Both adjustments are made at the front carburetor, using vacuum gauges (or some other way to match vacuum). Both involve loosening a lock nut on a cable adjuster, and turning the adjuster so that the vacuum matches. The idle should be done first, and it requires loosing the throttle cable at the grip so there is adequate slack in the cable, allowing the idle sync cable full control of the front carb.
Thirdly. I've got some issues with riding it. It falls flat and I have to over rev it to take off. Then in first it pulls hard to wot. But in second, third, and fourth gear it doesn't want to cruise without surging or being flat at roughly 1/4 to 1/2 throttle. It will mostly cruise in fourth and fifth gear if I'm over 1/2 throttle. But it takes awhile to accept more throttle from a cruise to acceleration. Bout a 3-5 second lag. Then it pulls balls to the walls

Little about what I've done. I got the bike stock except for the exhaust it had been completely bored out. I put pod filters on to compensate and to allow room to relocate the battery further up. (My preference). I then added a stage three jet kit from factory pro. Put the biggest jets it came with in and put the new needles on the middle position. It also included one size larger pilot jets. Rear cylinder went from about a 145 to a 165 and front went to a 150 I think.

So what I'm think is I need to raise the needles one position to richen the 1/4-3/4 throttle range. But I'm not sure about the falling flat on take off. Thoughts?

Before I do too much I want to add some sort of muffler to it. Make the sound a little less loud and deeper with a smidge of back pressure.
You may not have the idle set high enough; it should be between 1000 and 1200 rpm. Set too low, it will try to stall when starting out. The flat spot could be the jetting is not yet correct for your filters, or the throttle sync is so far out that one cylinder is being pulled along by the other, instead of helping. Adjusting the jets well requires a dyno and an experienced carb man; you can, however, get close with throttle chops. With fresh plugs, run for as many miles as practical, under load, at a given throttle setting, then hit the kill switch and clutch at the same time. Inspect the plug color, then put in fresh plugs and repeat at another throttle setting. This way, you can find out if it is rich or lean at each throttle position; the needle jet/jet needle control the mixture between about 1/4 to 3/4 throttle, and the main jet above that. Below 1/4, the pilot jet and adjustment and the needle jet both contribute.
Florissant, MO
CB450K3, GL1500CTValkyrie

Longhorn717
Practicing Bicycling
Posts: 42
Joined: Mon Oct 31, 2016 5:06 am
My Bike: 2000 vs1400

Re: How to: Adjust A/F screws

Post by Longhorn717 »

All of the flat spots I've encountered are in the 1/4-3/4 range. Once I got past that it pulled super hard with what appeared to be no end in sight. I think the stumble off idle that I'm experiencing from what y'all are saying is the throttle sync. I may not have enough slack in the line though.

Ok this may be how tired I've been here lately. But I just thought of this. When I took the air boxes out the front air box had the throttle cable screwed to the lid near where it splits into two. Do I need to secure it some where some how to fix this varying idle I have?

User avatar
Herb
Joined a 1200cc Club
Posts: 19127
Joined: Sun Oct 26, 2014 9:28 pm
My Bike: 1999 1400 intruder

Re: How to: Adjust A/F screws

Post by Herb »

Longhorn717 wrote:All of the flat spots I've encountered are in the 1/4-3/4 range. Once I got past that it pulled super hard with what appeared to be no end in sight. I think the stumble off idle that I'm experiencing from what y'all are saying is the throttle sync. I may not have enough slack in the line though.

Ok this may be how tired I've been here lately. But I just thought of this. When I took the air boxes out the front air box had the throttle cable screwed to the lid near where it splits into two. Do I need to secure it some where some how to fix this varying idle I have?
Yes, the cable needs to be secured for the system to work correctly. If it isn't your throttle will not operate correctly.

On the old board there was someone that had used a plastic tie to strap it to something.

You should have about an 8th of an inch of free movement, in the twist grip, before it starts opening the throttles.
I can't seem to win the lottery. I think I have used up all of my good luck riding motorcycles.

Longhorn717
Practicing Bicycling
Posts: 42
Joined: Mon Oct 31, 2016 5:06 am
My Bike: 2000 vs1400

Re: How to: Adjust A/F screws

Post by Longhorn717 »

Ok that helps a lot herb. Unfortunately I've got some other issues that have sprung up that I'll have to take care of as well before I get to that.

Shift linkage broke. Someone before me welded or soldered both end and the rear joint broke this morning.

And I'm working on fitting some exhaust with baffle and packing.

Post Reply