Flatspot RPM range 4000-6000 RPM 77-79 GS750

[Guest]

Hi!
I have been reading about others that
have had similar problems but not helped
me to fix it.

I have a 77-79 GS750 engine in a GS750
that have a flatspot in the RPM range 4k-6k.
It runs strong up to 4k and then just flatten out
until I get of the throttle,but if I then just let
the RPM rise gradually,then when hit 6k
it begins to pull like a train to 9k or redline.

I also have the slow to return idle/hanging idle that others have
written about and the strange thing is that if I brake
hard 2 times or so quikcly, the idle goes back to near normal.

Could this point at something?

I was thinking,The fuel in the bowls are forced forward
when braking and as the floats are hinged in their back
the fuel will try to hold it open and give a more rich fuelratio.
Am I wrong on this?

I have freeflowing 4-2 exhaust,(Marving slipons)
KN filter in orginal GS750 filterbox.
New intake Orings and cleaned the carbs
and assembled with new Orings on these also.

I use standard VM26 carbs with 17,5 pilotjets,115Main jets
Needles at the third grove(standard) floatlevel 23mmed
Carbs synced.

The plugs are a little sooty around the edges and the
electrode color are dark tan.

To me it feels like it has a lean spot but at the same time
maybe rich at other RPMS
The idle mixtures are set about 2turns out for highest idle.



Should i go up in number on the pilots,raisen the needles or
change the mains or whatever?


It is a little frustrating to not get this
sorted so all help is welcome.

In advance,
Thanks

gsman

[TxGSrider]

Usually hanging idle is the sign of leaness at idle. Tray turning the idle mixture screws out 1/4 turn and see if it helps.

When the engine has the flat spot from 4k-6k, does it only happen at full throttle, or does it happen at 3/4 or rolling on the throttle? Is it better or worse when the engine is cold or warm?

[gs11ezrydr]

I would possibly check the bowl vents to assure they are vented into a dead air space. Possibly they are exposed to the passing air and creating a negative pressure not allowing the carb bowls to fill fully. Just a thought .I have had this problem in the past on mainly full fairing bikes that had the bowl vent improperly routed. Hanging idle could be the slides out of sync. and of course a lean condition.

[Big T]

Where are the pilot fuel screws set?

Mid range issues usually need the needles raised

[gsman]

The float bowl tubes are routed together and ends in the air
between the frame and rear swingarm just in front of rear wheel.
I suppose this is close to standard routing of these vent tubes.

The flatspot is there,3/4 throttle,full throttle or rollon throttle.
If I am in low enough gears compared to the speed the rpm
climbs a little faster until it gets up to above 6k and then begins
to pull like it should.

Another side of the problem is that when it is in high gear
on the freeway,the airstream against the bike will overpower the enginepower
and it is just flattten out at 4k and above.If I then jumps down to third gear the
rpm will rise but it don`t help much when the flat spot are most apparent
when the engine are loaded in high gear and the speed will not increase further.

My sprocket combination are set for about 3000RPM/50mph (530 18/38 sprockets) in fift
which I see as a nice compromise of power and highway cruise RPM.

The fuel mixture screws are at the moment set at 1 turn out each.
I have tried with 1.5 turns out and that just made the plugs looks even
more sooty.

As some of you are indicating,there are a sign of
just to much air in the fuel ratio,But how to correct it?
That is the question.

I discoverred when I synced the carbs when carb topcovers off
that there was a small hole in top of the slides beside the needles and throttle link fasten bolts,
just like a third bolt hole for the throttle link.

When I last time was syncing the carbs I also did hear a surging sound from the slides at the locations
of these holes just like it got some unwanted air through these holes also.

What is this/these hole/s
intended for?

On some model years part drawings of GS750 carbs(VM26)
there are a plastic washer above the
needles (beetween the throttle link and the slide)which is not used on my carbs.

Is this a gasket?or what are the purpose of this plastic washer?


gsman

[hannibal]

Usually hanging idle is the sign of leaness at idle. Tray turning the idle mixture screws out 1/4 turn and see if it helps.

When the engine has the flat spot from 4k-6k, does it only happen at full throttle, or does it happen at 3/4 or rolling on the throttle? Is it better or worse when the engine is cold or warm?

Turning the mixture screws out will increase airflow and lean it out more. Turn the screws in to reduce airflow and richen the mixture.

[TxGSrider]

Turning the mixture screws out will increase airflow and lean it out more. Turn the screws in to reduce airflow and richen the mixture.

Uh, I don't think so.

If the mixture screws are on the engine side of the carb, they control fuel to the idle mixture (pilot system), and turning them out adds fuel, richening the mixture at idle and light throttle.

If the mixture screws are on the air box side, they control the addition of additional air to the idle mixture (pilot system) and light throttle and turning it out would increase "leaness".

[OldVet66]

I question the #17.5 pilot jets. Even with a stage-3 jet kit a GS1000 is recommended to use #15 pilot jets. Your pilot fuel screws should not be any more than one turn out and air screws starting at about 1 to 1.5. Generally a flat spot like yours that corrects when you back off the throttle and ease it back on is a lean condition. I suspect your plugs are coloring up darker because the pilot circuit is rich and the flat spot is because the main jet needles are set too lean. Hanging idle is also an indication of a vacuum leak on VM carburetors. Have you checked your Boots and Boot O-rings and properly sealed the lid of your air box?

[gsman]

Yes,I have also suspected that there is a vacumleak somewhere
as there is no change on the pressure value when doing a compression test.
With throttle open or closed,same value.
Then engine simply sucks the same amount of air whatever the throttle
is closed or open fully.This could be a indication of vacumleak
but it only if so happen when engine are fully warmed up.
As an example,engine idles normally shortly after going off the choke
until fully warmed up.
I think it should show a higher value with the throttle open than closed
if everything is as it should just as a test.
I have new intake orings.
Maybe I should get some other intake rubbers too as they are
old but have no visible faults I think.

gsman

[TxGSrider]

Generally, if the motor runs worse as it warms up, its a rich condition. If you had a vacuum leak, it would cause a lean condition.

[bobgroger]

Generally, if the motor runs worse as it warms up, its a rich condition. If you had a vacuum leak, it would cause a lean condition.

That was exactly the case for me with my recent issues. Hanging idle going to stall after fully warmed up. I kept richening it up, it got worse. Finally found the threads boogered on #1 pilot screw so what I thought was fully in, was actually several turns out.

[gsman]

OldVet66 are so far most clear on what it could be
as it is a combination of both rich and lean condition.

If I can get the midrange working ok then something
at least is solved.
As some say about the midrange,I think I will try
to raise the needles ONE step up.Or should I try
two steps when I am at it?

gsman

[Big T]

One step at a time