flat spot at 1/4 throttle opening

[dwfranke1]

working on a 80 gs750e, after installing new intake boots, raising the needles 2mm's, at setting the screws at 2 full turns from seated, i have a flat spot at 1/4 throttle, it does not however, happen all the time. the plugs are a lite brown shade, and all 4 look the same color, all 4 slides seem to move freely and the slide boots do not have anyy holes in them, the problem seem to happen as i accelarate from a stop through gears, then will go away for a few miles then return,

 

[Dave8338]

Sounds like the float levels are set a little too low or there is a restriction in the fuel flow. Do you have an additional fuel fliter on the bike?

Heavy acceleration consumes the available fuel in the bowls and it takes a moment or two for that fuel to be replaced.

Just a thought...

Also, a plugged fuel cap vent will cause a similar condition.

And one more thing, have you checked to see that the main fuel feed line from the petcock does not have a "high spot" in it?

 

[Guest]

Is the petcock new? Fuel flow is half the battle.

 

[dwfranke1]

yes it does have a little fuel filter inline, i will remove and see what happens

 

[tkent02]

1/4 throttle is hardly heavy acceleration. It runs fine with the throttle open farther? If so it is getting enough fuel.

Just curious, what made you think you needed to raise the needles 2mm? How did it run before this change?

 

[dwfranke1]

petcock is disabled, meaning nothing inside and the lines have been plugged at the petcock and carb location, and trust me flow out of the tank with the line pulled off is no problem

 

[Guest]

Did you use washers to shim the needle? Stick another on there if you did.

 

[Dave8338]

1/4 throttle is hardly heavy acceleration. It runs fine with the throttle open farther? If so it is getting enough fuel.

Just curious, what made you think you needed to raise the needles 2mm?

Now there is a good question. :!:

 

[dwfranke1]

one of the post i search before stated raising the needles 2mm's from stock was a cure all for old gs's so while i had the carbs off i raised them, when i bought the bike it had 4 1mm shims on each needle, the plastic shim was not there

 

[tkent02]

So you just removed two of the shims?

 

[dwfranke1]

when i pull away from a stop and this happens, if i open the throttle all the way it will eventually pickup and go

 

[dwfranke1]

yes i removed 2 shims

 

[Dave8338]

Stock box w/filter oiled and sealed up tight?
Have you replaced the intake orings?
Carb vent tubes open and flowing freely? (I have cleaned out more than one spiders' nest in those things...

 

[dwfranke1]

to answer the question of how it ran before was thae intakes boots were baddly worn and the airbox was not on properly so not well at all

 

[dwfranke1]

these are cv carbs, no orings, just rubber boots with clamps

 

[tkent02]

Stock box w/filter oiled and sealed up tight?
Have you replaced the intake orings?
Carb vent tubes open and flowing freely? (I have cleaned out more than one spiders' nest in those things...

1980 750E, stock is a paper filter, there are no O rings.

 

[dwfranke1]

yes it is a stock airbox and i dont think this filter requires oiling, everything is sealed and clamped tightly

 

[Dave8338]

1980 750E, stock is a paper filter, there are no O rings.

I just keep learning. OH...750...can read but don't always.



Check the length of the fuel line and see that the top of the loop is not above the center of the carb bodies. DITCH THE ADDITIONAL FUEL FILTER.

 

[dwfranke1]

CHEF right now i have 2 shims on each needle, you suggest 1 more for a total of 3?

 

[dwfranke1]

more info, the bike idles smoothly at 1100 rpms and has loads of topend, its just that middle part thats the problem