GS750 1978 anemic on VM29 carbs?

[bhm]

Rhonda has been well taken care of under Nessism supervision.
I bought her three months ago in North Hollywood, at Johnson and Wood, where the engine had been rebuilt not long before by Frank.
Going to NoHo yesterday, I figured I was just near Johnson and Wood.
I met Frank there, and he took a look at the girl.

He said he'd never heard her run so well. Bravo GS resources!

He felt it's still a bit "anemic" though, and evoked several things around the carbs:
- the carburetor jet needle set to another notch could improve it
- the angle at which the slides are chopped is not the best i.e. maybe the carbs VM29 are not the original ones for this model: GS750E 1978
They are the right ones aren't they? Anybody heard of a happy swapping here?

He also said it's a bit lean, and while it's well tuned, it could be better: when throttling off, the RPM needle goes back down before setting to an idle and this would mean adding or removing 1/16 of a turn on the air screws.
Does this ring a bell?
I just adjusted the float height. Do I need to vacuum resync?
I know it's hard to fine-tune over the internet, but I witness so many miracles here...
And I wanted to acknowledge this one!

 

[Agemax]

if they are indeed VM29's then they are definately not the stock carbs for that model......

 

[bhm]

Maybe I am mistaken...
They are VM for sure, they take 32mm o-rings.
These are the one:

If these are not the right ones, what model should be on a '78 GS750E?

 

[Agemax]

measure the internal bore of the throat,where the carbs slide into the rubber boots on the engine side. that will tell you what size they are. 26's i think, but i may be wrong......

 

[Guest]

They are most likely the VM 26 carburetors. The 29's have a big float bowl drain plug on the bottom of the float bowl. The dinky little inline fuel filter might explain some of your problems though. The petcock screen is more than adequate. If the tank is not clean, that is another necessary maintenance job. That filter is probably starving you for fuel on demand.

 

[Road Rash]

Stock carbs on MY 78 GS750 are VM26SSes, which look just like what you have.

 

[Guest]

Those are the stock carbs, VM26's as mentioned, for your bike. You don't need to resynch after adjusting float height, only after doing valve shims and the like. If the bike is dropping below idle after 'blipping' the throttle. You can try adjust the carb settings a bit with the air screw, pretty easy to do, just adjust them and blip the throttle and see what happens. Have you done any plug chops or adjusted using the fastest idle method? the other thing could be a bit of an air leak, all kinds of places for those, but usually you will have a rough idle of some sort as well. I'm not exactly sure what was meant by the 'angle of the slides chopped', and no insults to OldVet66 but I've used those 1/4 inch inline filters on several of my bikes and have not had any issues, and they've been riden' like raped apes at times. Keep us updated on the progress.

 

[bhm]

Okay, they seem to be stock carbs indeed. So it's a fine tuning story.
As OldVet66 and Nessism both frowned at the dinky main fuel line filter, I removed it, but without noticeable improvements.
I don't think there could be any air leaks, I weatherstripped the air box's a** off, changed all o-rings. Maybe on the exhaust side... But it idles really well and low since the sync. It's about transitions.
Lemme find and try the fastest idle (high rpm) method.
EDIT: Found it here (basscliff's)

 

[Road Rash]

Eh, I don't believe for one second that the filter would give you any woes. I've run those in the past and have never had any sort of fuel supply issue, as a result. In fact, I was thinking about adding one to my current ride. I don't like to rely solely on the internal sock filter.

 

[Agemax]

Eh, I don't believe for one second that the filter would give you any woes. I've run those in the past and have never had any sort of fuel supply issue, as a result. In fact, I was thinking about adding one to my current ride. I don't like to rely solely on the internal sock filter.

it depends on where you buy them from. the real cheap ones dont flow properly and block up extremely quick as the filter element is way to fine.

 

[Road Rash]

Maybe I've just been buying the right ones. I'll be sure to keep buying them, then, just to be safe.

 

[Guest]

Maybe I am mistaken...
They are VM for sure, they take 32mm o-rings.
These are the one:

If these are not the right ones, what model should be on a '78 GS750E?

I meant to ask earlier, what are those 6 fat o-rings for in the bottom left of the picture??

 

[Testarossa]

I meant to ask earlier, what are those 6 fat o-rings for in the bottom left of the picture??

Those are the seals where the throttle shaft exits each carb body. If you look at the top of the picture you will see the two plugs that cap off the outside holes on carb one and four.

 

[Guest]

Bingo, of course.

 

[bhm]

follow-up

follow-up

So to follow-up, I received the new fuel needles from Z1. They seem to do the job as well as the former ones... No real improvement. Sometimes it overflows, sometimes it doesn't. I adjusted the floats and changed the needles, there is not much more I can do. I'll ride more and see if I want to touch the #3 float again.

I tried the highest rpm method, it drove me nuts, since screwing or unscrewing the mixture didn't impact the rpms, even by waiting after each adjustment to make sure. It just didn't work for me.

So I screwed in 1/2 each mixture, and test drove with my screwdriver handy. Going on and off from a stop. Leaner was a more powerful off idle. I enriched another last 1/16 to get the best result, but I couldn't get completely rid of that slight stumble. I'll ride more to see if I get another of these scary holes in the power - in the middle of a turn in 1st gear.

Listen, it's rolling pretty well. I guess the next step would be to take it to a garage, but it looks like bikes are too complex for tools to be cheap and too simple for mechanics to be good. Nessism told me I should be happy with it. As far as anemic, I'd have to try another bike to compare... I think I saw a gs750(or 800)G in the neighborhood...

 

[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.

 

[Guest]

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.

.