Idle mixture question

[pdqford]

I’m trying to get the idle mixture close on my 1980 GS750ET so I can finish the carb sink.
(I have set the valves and done the complete strip and dip with new o-rings. Bike is ALL stock!)
If I give the throttle a little blip the RPMs sloooowly come back to idle.
I have read where this is a sign of too lean of an idle mixture.

I’m confused by the idle mixture screw on the GS750ET.
When I open the mixture screw further, is it allowing more fuel in or more air in?

Thanks for your help.......Jim

 

[Steve]

Those screws control a MIXTURE, so turning them out (anti-clockwise) will richen the mixture. When I do a set of carbs, I turn the screws out THREE full 360? turns from lightly-seated. It is likely just a bit too much, but it acts a bit like a choke to richen the mixture to start a cold engine. And, ... a bike will run easier on a rich mixture than it will on a lean mixture.

You might still have to use a bit of "choke" to start the bike, but should be able to turn it off rather soon. Once the bike is running reasonably well off-"choke", you can do the carb sync.

Do the sync first, then tweak your mixtures. The reason for that is that you want to ensure that all cylinders are pulling through the carbs. If, for example, one carb is closed more than the others (will show a higher vacuum), adjusting the mixture won't matter much at all, because the carb is not open enough to contribute to overall power. When the carbs are synched, you will know that any changes you make to the mixture screw will show in engine speed.

By the way, if you leave the gauges connected while you are tweaking the mixture screws, you can see the vacuum level go up and down in all cylinders when you adjust the screws. While turning the mixture screws IN, you will want to find the point where the engine starts to slow down (vacuum will also drop), then turn the screw back out 1/8 to 1/4 turn, then move to the next screw.

.

 

[pdqford]

Steve, thanks for the reply. Your reply’s are so easy to read and understand. (And thanks for putting that word MIXTURE in caps. It reminded me that it not a fuel screw, but a MIXTURE screw, since it meters an air and fuel Mixture.)

So far, this bike is beating me! I have the mixture screw now at FOUR full 360 degree turns open and still can’t get a stable idle, or keep it from over reving on a little throttle blip.

I know that when I put the carbs back on I found a crack in one of the rubber intake pipes which I replaced. I’m gonna try and spray the intake pipes or maybe try my home made smoke machine to see if there is any vacuum leaks.

And some place in my computer I took a picture of the wet fuel levels to be sure they were all even. I’ll check that picture to verify they are at the correct height. The fuel level should be just below the parting line of the carb body and float bowl?
Does the fuel level in the bowl have a major effect on the idle mixture adjustment?

 

[KEITH KRAUSE]

"Rubber intake pipes" I assume, means the carb manifolds (between the engine and carb). If you found one was cracking, I'd question the condition of the other 3, assuming they're all the same age? Intake leaks will make carb tuning impossible.
You have to be positive of no intake leaks first. Often, spraying the manifolds while idling will reveal a leak, but sometimes you just don't hit the spot and your testing isn't positive. I always used water to check for leaks instead of something volatile. Water will drop the rpm's momentarily instead of raising it with a volatile fluid. The best checking is visual.
I'm no expert with the CV carbs but let me ask this. If you start the bike cold, then after a brief warm up you set the idle to 1,000 rpm's, then go ride, do the "idle" rpm's rise significantly as it becomes hot? If so, then you have an intake leak.
The fuel level in the bowls effect EVERY jetting circuit. To accurately test any circuit, the level must be in a certain range.

 

[pdqford]

So, I had a little time today to get back on this project.
I pulled one of the fuel bowels an all looks clean a shinny I there.
Even though I stripped, dipped and cleaned the carbs like three years ago,
they have only had fuel in them for 6 weeks or so.

When I lowered the float bowel, this little "weight" came down with it.
See attached (I hope).
To tell the truth, I don't remember these things being there.
And can't seem to find them in any rebuild instructions.

What are they in there for?
And does the tapered end go into the carb body first?
Or other way around?

Thanks.....Jim

 

[Nessism]

If the plug sits below the gasket surface you should use an O-ring to level it out.

 

[Steve]

That "weight" is just a plug. I have not figured out why it's there. There is nothing above it that needed to be machined, so its presence is a bit of a mystery. I have used some light taps with a small hammer to set them into place, then, as Nessism says, use an o-ring between the plug and the bowl gasket to make sure it doesn't drop. If it DOES drop, that will cause a rather major air leak, so that might be part of your idling problem.

.

 

[pdqford]

Thanks for the response Ed and Steve.

I *thought* the bowel gasket would seal that plug opening -see attached.

(But what do I know. That’s why I’m here :surprise

I looked at the parts explosion on bikebandit for the 1980 GS750E carburetor.
It doesn’t show that plug, but it does show a gasket (#34) at that approximate spot.
So I will see if I can pull all the float bowels and add o-rings to the plugs with the carbs still mounted on the bike.

Any thoughts on which way the plug should be inserted into the carb body?
Tapered end in first?
Or tapered end toward the float bowel?

 

[Steve]

It will only fit one way. Smaller end UP.

Good luck getting the bowls off with the carbs on the bike.

.

 

[pdqford]

Good luck getting the bowls off with the carbs on the bike.

.

Your right. Just couldn't get on the center two carbs float bowel screws.

I really didn't want to pull those carbs. They are a bear to get out and back in with it all stock!
But I got 'em off and pulled all the float bowels. See attachment.

From the left in the picture:
#4 had a thick gasket holding the plug up.
#3 had a rubber gasket holding the plug up.
#2 had an o-ring holding the plug up.
But #1 had nothing holding the plug up!

Embarrassing thing is I have no recall of these plugs or gaskets.

So I'm gonna put an o-ring in #1 and reassemble and see if it fixes, or at least, improves the idle.

 

[pdqford]

Okay. I have o-ringed the plugs to "level" them out

When putting the float bowels back on, I noticed an oddity.
Notice the orifice in the attached photo pointed to by the red straw.

The float bowel gasket doesn't completely cover that orifice.
Should it?

(Looks like I have to post next photo in next post.)

 

[pdqford]

Here is a photo of the gasket on the float bowel.
Doesn't look right.

 

[pdqford]

And a photo with the float bowel attached.
That orifice is not sealed.

I tried to spray some carb cleaner in that orifice and got a face full.
That orifice is definitely a dead end, so maybe it is okay?
Just doesn't look right.

Any comments?

 

[Steve]

No need to cover the hole.

.

 

[pdqford]

Thanks Steve.

Okay. So I put the carbs back together.
But what a time I had getting them remounted to the bike.
After a couple of hours of pushing, twisting, prying, got them all snugged down.
Hooked up the choke cable and then realized I couldn't connect the throttle cable with the carbs installed!
The throttle cable barrel won't fit in the hole because the #3 intake is too close to the hole!

Today I went through process again. Finally got them in there and look good.

I don't want to go through this again for a vacuum leak.
So I dug out my home made smoke machine and blew smoke in the air box

 

[pdqford]

After putting some smoke in the air box, I realized that with no fuel to shut the needle valves I would have to cap off the fuel line.
AND the air box drain tube.
AND the carb vent tubes.
AND the carb sync tool's vent.
AND the cyl head breather to air box line.
AND the vacuum port to the petcock

 

[pdqford]

Had a small wisk of smoke between #1 and #2 carbs connection to the air box.
The vent line between was not completely on the "T".

(Hard to see the smoke without a high intensity light.)

 

[pdqford]

Narry a wisk of smoke on the throttle blade side of the carbs.
I even held the throttle open so it got the full effect of the smoke.

Hopefully it will run tomorrow!

 

[pdqford]

It was too late tonight to start it tonight.
So just filled the fuel bowels with fuel,
checked for fuel leaks and verified the wet fuel level
at 0.20" below the fuel bowel parting line.
And set the "mixture" screws two turns open.

Wish me luck!