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yet another carb question

  • Thread starter Thread starter sam78gs750
  • Start date Start date
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sam78gs750

Guest
a little background 1st... '78 GS750EC with airbox, jetting, and exhaust all stock. recent work: adjusted valves, sealed airbox with weather stripping/ new K&N air filter (not pods), replaced airbox carb boots, replaced intake boots & o-rings, cleaned and greased most of the electrical connections (still have to get inside the headlight bucket...), replaced various seals (tach cable, etc...), rebuilt carbs (completely disassembled and dipped each carb and parts overnight).

This bike has the VM carbs. bench synced & got them pretty close, I'm waiting on a set of guages to do the vacuum sync.

my question has to deal with the fuel screws on the bottom of the carbs, and the air screw on the sides.... I did a few searches and got a good starting point from some past threads. Started at 3/4 of a turn out on the fuel screws and 1&1/2 turns on the air screws. set the float height to 25mm. this put me pretty close to where I need to be, but it was running a bit rough. tweaked the air screws a little bit to get closer, then took the bike out for an "italian tune up"....after that it was running much better. the air/fuel mixture is still off a little bit. this is my first time dealing with carbs that have separate adjustment screws for air and fuel. so finally I get to the question....

What's the best order to go in for tweaking the mix? should I start by tweaking the fuel screws on the bottom, and then do the air screws on the side next? or should I just leave the fuel screws alone at 3/4 turn and just mess with the air screws? it's running a little rich at idle, but when i give it some throttle it bogs down on the transition off idle circuit (only with a quick snap of the throttle, a slow turn and it doesn't do it...). my thinking was to go out a little on the fuel screws and that would take care of it bogging down, and then tweak the air screws and that should fix the problem of running rich. should I even bother with the float height? like I said, all the floats are even at 25mm (spec range is 25-27mm) . I've got it all very close, and just want some advice on how to proceed....
 
sam78gs750 said:
a little background 1st... '78 GS750EC with airbox, jetting, and exhaust all stock. recent work: adjusted valves, sealed airbox with weather stripping/ new K&N air filter (not pods), replaced airbox carb boots, replaced intake boots & o-rings, cleaned and greased most of the electrical connections (still have to get inside the headlight bucket...), replaced various seals (tach cable, etc...), rebuilt carbs (completely disassembled and dipped each carb and parts overnight).

This bike has the VM carbs. bench synced & got them pretty close, I'm waiting on a set of guages to do the vacuum sync.

my question has to deal with the fuel screws on the bottom of the carbs, and the air screw on the sides.... I did a few searches and got a good starting point from some past threads. Started at 3/4 of a turn out on the fuel screws and 1&1/2 turns on the air screws. set the float height to 25mm. this put me pretty close to where I need to be, but it was running a bit rough. tweaked the air screws a little bit to get closer, then took the bike out for an "italian tune up"....after that it was running much better. the air/fuel mixture is still off a little bit. this is my first time dealing with carbs that have separate adjustment screws for air and fuel. so finally I get to the question....

What's the best order to go in for tweaking the mix? should I start by tweaking the fuel screws on the bottom, and then do the air screws on the side next? or should I just leave the fuel screws alone at 3/4 turn and just mess with the air screws? it's running a little rich at idle, but when i give it some throttle it bogs down on the transition off idle circuit (only with a quick snap of the throttle, a slow turn and it doesn't do it...). my thinking was to go out a little on the fuel screws and that would take care of it bogging down, and then tweak the air screws and that should fix the problem of running rich. should I even bother with the float height? like I said, all the floats are even at 25mm (spec range is 25-27mm) . I've got it all very close, and just want some advice on how to proceed....
Click to expand...
For the year bike, I don't think that the EPA will be knocking on your door.
Were any of the screws purposely covered/made inaccessible?
 
none of the screws were covered... the po had had messed with the carbs and had set them pretty out of whack....
 
Are these screws for IDLE Adjustments?
I would assume so; so, adjust for best IDLE?
Float height, in my experience, don't do dip other'n stopping carb. overflow/insufficient fuel.
Isn't that fuel screw on the bottom a sump drain?

For no manometer:
I personally would proceed at night with the exhaust manifold OFF.

The cylinder with an orange flame would indicate more fuel:air than the others (I would imagine).
The cylinder with a blue flame would indicate more "throttle open".

Adj. 1 cyl. @ a time, start with a higher "butterfly" than the rest so to "see"...
When finished setting to a nice BLUE flame, set butterfly to "flame out". Continue for next cyl.

But, Dey CRAZIER DAn ME!
 
Last edited:
teddux said:
Are these screws for IDLE Adjustments?
I would assume so; so, adjust for best IDLE?
Click to expand...

the screws are for idle adjustments. My question is which order do I adjust them in to get the best idle.... air screw 1st, or fuel screw 1st?

teddux said:
Isn't that fuel screw on the bottom a sump drain?
Click to expand...

on the CV carbs it is just a drain... My bike has VM carbs, which have a fuel ADJUSTMENT screw on the bottom

teddux said:
I personally would proceed at night with the exhaust manifold OFF.
Click to expand...

by exhaust manifold off, do you mean take the headers off?

teddux said:
Adj. 1 cyl. @ a time, start with a higher "butterfly" than the rest so to "see"...
When finished setting to a nice BLUE flame, set butterfly to "flame out". Continue for next cyl.
Click to expand...

again, VM carbs, so no butterflies... only slides.....
 
Leave the fuel screws where they are. Adjust each side air screw for highest idle. Do some plug chops on the pilot circuit 1/8th throttle. Adjust fuel screws accordingly. re-adjust side air screws for highest idle.
 
teddux said:
Are these screws for IDLE Adjustments?
I would assume so; so, adjust for best IDLE?
Float height, in my experience, don't do dip other'n stopping carb. overflow/insufficient fuel.
Isn't that fuel screw on the bottom a sump drain?

For no manometer:
I personally would proceed at night with the exhaust manifold OFF.

The cylinder with an orange flame would indicate more fuel:air than the others (I would imagine).
The cylinder with a blue flame would indicate more "throttle open".

Adj. 1 cyl. @ a time, start with a higher "butterfly" than the rest so to "see"...
When finished setting to a nice BLUE flame, set butterfly to "flame out". Continue for next cyl.

But, Dey CRAZIER DAn ME!
Click to expand...

WOW. Please don't try and adjust your carbs by taking the exhaust manifold off.

Proper float height is crucial for each circuit of the carb to work properly.

The carbs on a 78 750 are VM carbs. They don't have butterflies.

The fuel screw he refers to is a pilot fuel screw found on both the VM and CV carbs. They are also referred to as emmision screws.
 
renobruce said:
WOW. Please don't try and adjust your carbs by taking the exhaust manifold off.

Proper float height is crucial for each circuit of the carb to work properly.

The carbs on a 78 750 are VM carbs. They don't have butterflies.

The fuel screw he refers to is a pilot fuel screw found on both the VM and CV carbs. They are also referred to as emmision screws.
Click to expand...
Thank you.
 
teddux said:
No ManoMeter
Yes, you would be surprised at what you will SEE, no neighborhoods, but of course!
Click to expand...

You are confusing two proceedures. Looking at the color of the flame would be for adjusting air/fuel mixture. That can be accomplished by doing plug chops, or you can buy a Colortune.

A manometer is used to read how much VACUUM is being drawn through a carb, and for balancing muliple carbs to draw the same amount.
 
Last edited:
renobruce said:
You are confusing two proceedures. Looking at the color of the flame would be for adjusting air/fuel mixture. That can be accomplished by doing plug chops, or you can buy a Colortune.

A manometer is used to read how much VACUUM is being drawn through a carb, and for balancing muliple carbs to draw the same amount.
Click to expand...
You are jumping to conclusions...pls. read.
 
renobruce said:
WOW. Please don't try and adjust your carbs by taking the exhaust manifold off.
Click to expand...

And why is that? I do it all the time, in fact, I ain't got headers or mufflers.

"Proper float height is crucial for each circuit of the carb to work properly."

Really? Theroretically, sure.
Practically, have you tried it?
He said he was in specs.
That's enough to move on.
 
Last edited:
renobruce said:
Leave the fuel screws where they are. Adjust each side air screw for highest idle. Do some plug chops on the pilot circuit 1/8th throttle. Adjust fuel screws accordingly. re-adjust side air screws for highest idle.
Click to expand...

This sounds about right... that answers the question I was asking, thank you. (The pdf link that basscliff posted was full of useful info as well.)

As far as running with the headers off, that just sounds like bad business, sorry....

I'm not worried about the carb sync right now, like I said I got them pretty close with the bench sync, and have a manometer on the way. I will do a proper vacuum sync when it arrives. Right now I'm just trying to tweak the idle air/fuel settings.
 
teddux said:
And why is that? I do it all the time, in fact, I ain't got headers or mufflers.

"Proper float height is crucial for each circuit of the carb to work properly."

Really? Theroretically, sure.
Practically, have you tried it?
He said he was in specs.
That's enough to move on.
Click to expand...

Within specs is good. But saying "Float height, in my experience, don't do dip other'n stopping carb. overflow/insufficient fuel." is inaccurate. The level MUST be within spec for the carb to work properly.

And why would anyone need to take the exhaust off and wait until dark to do a carb adjustment? Do you know what plug chops are? Ever heard of a colortune? I suppose if you plan on not running any exhaust then your method should be fine. :eek:
 
sam78gs750 said:
This sounds about right... that answers the question I was asking, thank you. (The pdf link that basscliff posted was full of useful info as well.)

As far as running with the headers off, that just sounds like bad business, sorry....

I'm not worried about the carb sync right now, like I said I got them pretty close with the bench sync, and have a manometer on the way. I will do a proper vacuum sync when it arrives. Right now I'm just trying to tweak the idle air/fuel settings.
Click to expand...

If it's close now, I'd just wait until you can get them synched, then do your adjustments. Also make sure your valves are adjusted within spec..that can have an impact on your carb adjustments and your synch.
 
teddux said:
For no manometer:
I personally would proceed at night with the exhaust manifold OFF.
Click to expand...
sam78gs750 said:
by exhaust manifold off, do you mean take the headers off?
Click to expand...
teddux said:
Yes, you would be surprised at what you will SEE, no neighborhoods, but of course!
Click to expand...
renobruce said:
WOW. Please don't try and adjust your carbs by taking the exhaust manifold off.
Click to expand...
teddux said:
And why is that? I do it all the time, in fact, I ain't got headers or mufflers.
Click to expand...
teddux said:
Crikey, it's a SPACESHIP!!!
:dancing:
Click to expand...
I have been following this thread and a few others, and at this point, I am going to have to respectfully ask:

Teddux, stay the heck away from my garage and my bikes. :eek:

.
 
it sounds like you have both the carb sync and air/fuel screws good enough for the bike to run well enough to fine tune.

Now do a carb sync with the manometer. You maybe surprised how much of a difference there is between the 4 carbs with just the bench sync. As you know, you need all 4 slides moving in sync before you start tuning the air screws.

Not to guess your ability to perform an accurate bench sync but you will be off some and the manometer will show that.

Once the carb's are sync'd then start tuning the air screws. The 3/4 turns of the fuel are pretty good for the VM carbs plus it's a PITA to adjust these screws with the carbs on the engine.

If you can show us pictures of the manometer vacuums on the carbs before and after you adjusting them.
 
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