GS750 1978 anemic on VM29 carbs?

[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.

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

 

[bhm]

WikiScrew

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+adjustment

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.

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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.

 

[Big T]

Falling idle is also a symptom of tight vales

Have you adjusted the valves yet?

 

[bhm]

Yes I did all the noob's homework around the propulsion (i.e. not brakewise nor exhaustwise, nor electrically (but electrical has been quite worked on yet) since I got it in January.

No way to put CV on my babe ?
Yeah there was a pre-BORN FREE 5 party And I could throttle that guy's GS450 like crazy. It was on CVs.
Oh well, I'll learn to handle it that way... It's my first bike also so I don't mind if she's not rushing... She was fine on the I-5 going 85 - and I felt she had a bunch more.

But still KoolAid kinda answers the whole thread on that quote.

 

[koolaid_kid]

Sorry about that. You'll get used to it. Once you are, you will appreciate that overall you have a very nice motorcycle.

 

[bhm]

HAHAHAHA ok, I just changed the jet needle clip richer while keeping a rather lean mixture, new sparks and HAHAHA handling it at higher revs like you said HAHAHA.
very nice motorcycle.

The bog gets "drowned" in the quick wrist. the full thgrottle killing I didn't try, I guess there is nothing to do about it but hey it's not a dragster anyway. I guess it was also my frenchy handling. I forgot we're in the States.

It's just about stopping whining and taking the time. Thank you guys. Not without you.

 

[bhm]

Removing the air filter cover, as I read somewhere here, was helpful, since I didn't want to mess up the rather balanced carbs configuration. It saved 2MPG without lessering power, for the price of a tiny bit of idle hanging, a good deal if you ask me.
It's running smoothly and powerfully, yet I am getting only 30MPG in town, which doesn't seem to be fabulous. Lotso off and back to work, 3 miles only with warming up 1 minute each time... Maybe I am good, what do you think?