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Highway speed odd speed variance

  • Thread starter Thread starter tc80211
  • Start date Start date
T

tc80211

Guest
Hey all,

1981 GS750E
4 into 2 aftermarket exhaust
Pod Filters
Dynajet kit with larger jets installed


Recently switched out exhaust to a much freer flowing 4 to 2 then stock so I upped my main jets and now noticing that on highway when holding a constant 6500Rpm I am getting an odd speed variance. I hate to call it "surging" as the speed does not increase, rather with throttle held still it may drop by 5 mph which certainly is noticeable and then back up to the speed it was at...

I searched the forum and found other users having a positive surge in which speed would increase but not other way around like this.

Acceleration through all gears is smooth. Idle always returns to 1100-1200 range with no issue. No stalling either.

At first I thought maybe clutch was slipping? But then I should notice it in other gears to when accelerating. I checked for pinched vent hoses and all are good.

I have some popping when coming off throttle from exhaust so might need to lean out the mixture to fix that... Any thoughts on this highway issue would be much appreciated.
 
tc80211 said:
Hey all,

1981 GS750E
4 into 2 aftermarket exhaust
Pod Filters
Dynajet kit with larger jets installed


Recently switched out exhaust to a much freer flowing 4 to 2 then stock so I upped my main jets and now noticing that on highway when holding a constant 6500Rpm I am getting an odd speed variance. I hate to call it "surging" as the speed does not increase, rather with throttle held still it may drop by 5 mph which certainly is noticeable and then back up to the speed it was at...

I searched the forum and found other users having a positive surge in which speed would increase but not other way around like this.

Acceleration through all gears is smooth. Idle always returns to 1100-1200 range with no issue. No stalling either.

At first I thought maybe clutch was slipping? But then I should notice it in other gears to when accelerating. I checked for pinched vent hoses and all are good.

I have some popping when coming off throttle from exhaust so might need to lean out the mixture to fix that... Any thoughts on this highway issue would be much appreciated.
Click to expand...


sounds like your lean on the mixture and popping out the exhaust in my experience indicts lean not rich or a air leak which again makes it lean
a simple check you can do provided you have the VM carbs is pull the choke on see what difference that makes to the surging and popping
it wont harm anything....... dont try this with CV carbs that have the butterfly style chokes

ozman
 
Last edited:
I'd give a good look at pod filters for signs of tearing- at 6500 rpm, lots of air is moving thru them.
 
ozman said:
dont try this with CV carbs that have the butterfly style chokes
Click to expand...
Evidently you guys in Oz got different CV carbs than the rest of the world, because we got the same style enrichener (choke) that the VM carbs had.

.
 
I checked the pods, no tearing. Warmed up and did a few passes at that rpm range, pulled choke, no change... still inconsistent... I am only 2.5 turns out so I could theoretically richen the mixture still, should I try this?
 
Also, its been like 3 years since these carbs were synced... in know i know. never had any problems till now... Would this help?
 
tc80211 said:
I checked the pods, no tearing. Warmed up and did a few passes at that rpm range, pulled choke, no change... still inconsistent... I am only 2.5 turns out so I could theoretically richen the mixture still, should I try this?
Click to expand...


no change tends to make me say richen up alittle give it a try
 
tc80211 said:
Also, its been like 3 years since these carbs were synced... in know i know. never had any problems till now... Would this help?
Click to expand...
A sync will not do much at road speed, it mainly affects idle smoothness and very-low-throttle operation.

.
 
Richened the mixture. Fluxes the popping on deceleration. However the "surging" as I guess it can be called at highway speeds continue. Any other thoughts?
 
Running wide opened up a long hill, backing off the throttle slightly does it pick up power or blubber a little?
 
tc80211 said:
Richened the mixture. Fluxes the popping on deceleration. However the "surging" as I guess it can be called at highway speeds continue. Any other thoughts?
Click to expand...

maybe bigger mains it sounds like your still lean
have you done a plug chop that the speed where its surging ?
new plugs are easier to read, 6500-7000 rev for abit pull clutch in kill motor
check colour of the plugs on the ceramic centre section of plug
tan/light brown good white/light grey is lean

ozman
 
ozman said:
maybe bigger mains it sounds like your still lean
have you done a plug chop that the speed where its surging ?
new plugs are easier to read, 6500-7000 rev for abit pull clutch in kill motor
check colour of the plugs on the ceramic centre section of plug
tan/light brown good white/light grey is lean

ozman
Click to expand...

Ok. Problem solved. But no idea how. I did the plug chopping and plugs looked correct color. Then rode around at slower speeds. Hopped on highway and no surging! Had it up to 120 with no problem. So I'm completely clueless as to how the problem got fixed. Thanks to everyone for the input and troubleshooting help!
 
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