GS750 1978 anemic on VM29 carbs?

[chuck hahn]

I had the 78 750 E as well. Heres what I found best for her ( she was stock with stock exhaust ).

Bottom pilot jets at 7/8 out and the air screws at 1 3/4 to start.

The bottom ones will be so close you wont have to fiddle with them, but the side mixture screws will be where you fine tune it.

Here is how I remember which way to turn them..If a cylinder is LEAN you want to ENrichen it right? Well, I just replace the "E" in Enrich with an 'I" and you get ......wiat for it.....INrichen.

So, to turn them IN will INrichen that cylinder. Stands to reason the turning them our will LEAN OUT a cylinder.

If you have anyone near you with a Colortune, they can adjust each cylinder perfectly in about 3/4 hr or less. The colortune works awesome on VM carbs but i have heard many that cant get it to effect the fuel/air mix on CV carbs.

[OldVet66]

Your pilot fuel screws should be just visible below the tiny hole in the throat of the carburetor when lightly seated. If they stick up in the throat of the carburetor when lightly seated, the hole has been enlarged and you are going to want to screw that one in further to compensate for the enlarged hole. The last set of carburetors I had before rebuilding the present set had all the pilot fuel screws sticking up in the throats of the carburetors and I could never get them tuned properly. Close but no cigar. Those 4 carb bodies are now in my scrap bin. The set I have now was never damaged and tune perfectly. Just to let you know that recommendations for pilot fuel screw settings may vary widely depending on what happened to the carburetors before you got them.

[chuck hahn]

Oldvet is right on that you dont know what these carbs have been thru. I posted what I have found to be the best BASELINE settings from my fiddling with them.

I also will tell you that the colortune will allow you to see flame color changes as you turn the pilot screws as well.

You will turn the pilots till the flame is the described "bunson burner blue" at normal idle. Then I turn it out and look for just the SLIGHTEST filcker of orange/yellow. At this point, the pilot is ever ever so slightly rich.

Then you will fine tune with the side mixture screws at the higher RPMS where they are transitioning to the needles...usually around the 2,000 RPM range. I run the RPMS up to the 4000 to 4500 range and observe the flame after a second or two to allow for the cylinder to "settle down" a bit. Do each carbs screws and move to the next.

I would suggest strongly that you borrow one...if possible.

[chuck hahn]

Please be aware that we can only offer up what baselines each of us have experienced with our own bikes. As with every bike, it is its own animal and your gonna have to do some fine tuning either way. We can only offer up general settings. Your bikes engine is gonna like what it likes.

[Steve]

Here is how I remember which way to turn them ...

I have always found it easier to just understand what the adjustment controls. In this case, the air screw is adjusting ... well, ... air. The convention for adjustments is that turning left, or out, will increase whatever is being controlled, turning right, or in, will decrease. Turn left, add more air, you will lean the mixture.

Same concept with the fuel screws, but you will be increasing the amount of gas when turning the screws left, that will richen the mixture.

.

[Agemax]

I have always found it easier to just understand what the adjustment controls. In this case, the air screw is adjusting ... well, ... air. The convention for adjustments is that turning left, or out, will increase whatever is being controlled, turning right, or in, will decrease. Turn left, add more air, you will lean the mixture.

Same concept with the fuel screws, but you will be increasing the amount of gas when turning the screws left, that will richen the mixture.

.

same as a washbasin tap. anticlockwise=increases flow (air or fuel) clockwise=reduces flow....

[bhm]

WikiScrew

Thanks guys!

To help me not mistake in and out (name of the best burger places in LA), and get a place to start.
I used Chuck's proposition (Bottom pilot jets at 7/8 out and the air screws at 1 3/4) tweaked regarding OldVet's sticking-out-parameter (screws sticking out in the throat when lightly seated would be a lesser 5 or 6/8 turns out to compensate)

The bike works really better. Still some weak zone around 2k Rpm, still slow take off but I didn't start fiddling. I had time to feel I could lean with more confidence already.

Anybody with a colortune in Los Angeles?

[chuck hahn]

Turn the side screws in another 1/4 turn and see if that makes it better..Dead spots can be caused by BOTH too lean and too rich.

So...Turning the side ones all in 1/4 turn will richen it up a bit.If this eases the symptom even better, then you were a tad lean.

If going in doesnt help, then return them back to the original 1 3/4 out and then back them out 1/8 turn at a time and recheck how she is running.

Yopu can kinda get the idea as to what some "ear tuning" is doing to get her close. If you can get a colortune..then youll be able to dial her in real nicely.

[bhm]

Yup,
So for this last step, I am riding with my screwdriver. I went 1/2 richer, since I had hanging idles and still a strange spot. Right now, it was working like a dream, I could just recognize were the off-idle yields to the main jet. but let's wait and try again... Tomorrow maybe another story, somehow. But we're definitively getting there.
Overall solution was fuel screws not tuned at all... I don't remember where the settings came from, but when I started this thread, I had a 11/2 turns out on fuel screw #1...

Again, solution was stock all GS750E:
"Bottom pilot jets at 7/8 out and the air screws at 1 3/4 to start."
chuck hahn

[bhm]

Almost there...
I've been told it might be a steep mixture difference between idle and main. Is there such a thing? Like, leaning the main and enriching the idle would allow to get rid of the off-idle bog/quick-full-throttle-killing-the engine?

[bhm]

Ok, now I have another surprise.
After a while, engine warm. Say at a stop, rpms go lower and lower and lower until it dies.
I adjust the idle screw, and at the next stop, guess what ? rpms go lower and lower and lower until it dies.
I looked up and couldn't find a thread about this... What the diagnosis' name? - apart from user inexperience ^^ -

EDIT: just found this http://www.thegsresources.com/_forum/showthread.php?t=199096&highlight=pilot+screw+adju stment

Characteristics of a rich condition are an inability to stay at the pre-set idle level, dropping off of idle level and dying at idle. If the bike starts to die at idle after a few miles, a too-rich idle mixture may be fouling the plugs.

Backing up on air screws

[koolaid_kid]

That is an indication that it is too rich at idle.

[bhm]

If I move the clip of my needles, do I have to redo a synch?
(recall: VM 29 ss 1978 carbs)

(ANSWER:
Yes you fin' Do)

[bhm]

Also found this, which today finally makes the more sense to me:

Just a note, the VM carbs are the reason I sold my 78 GS750E. A 2 stroke rider of over a decade, I was used to an immediate snap and star pointing position of the bike when I whacked open the throttle. The GS just bogged. It was soon replaced by another RD350.

Same thread:

ALL carburetors depend on having air moving across the jets at sufficient velocity to lower the pressure in the venturi to draw fuel up through the jets.
If you are running at a relatively low throttle opening (VM or BS carbs), your velocity will be high. If you suddenly whack the throttle open on the VM carbs, the engine, which has been creating a vacuum against the closed throttles, will suddenly have that vaccum filled with a rush through the carbs. Once that vacuum is gone, there is no longer sufficient air velocity through the venturis to draw the gas up through the jets.
.

Demonstration : https://www.youtube.com/watch?v=ZLnhKd8c1Kg

Will go on fiddling and post if there are any revolutionary improvements.

[koolaid_kid]

Just the nature of the beast, bhm. You have to roll into the throttle with VM carbs. This is why everyone eventually went to CV carbs; the design eliminates this flaw.