Hi guys, I've been away for a while. Decided to use the old GS as a track bike. I know, not the most power but it is really light and maneuverable.
I'm pushing it harder than I have before. At 65mph, I'm about 6500-7000rpm in 5th gear. If I go WOT quickly (seems I have about 1/4 twist more to go) the bike seems to bog down and won't accelerate. But if I open the throttle more slowly, the bike will slowly creep up to 75mph where it seems to top out. At 75mph, I can make the engine rev higher and make more noise, but it doesn't translate to more speed. These bikes are supposed to go up to 100mph no?
At lower speeds like 0-35mph it still pulls well in gears 1-3. Otherwise seems to run well, clutch isn't slipping. I've got it jetted as 20/60/147.5. Minus 1 gearing on the back sprocket.
Is this a matter of the vacuum diaphragm not opening up properly? Forgot where I got that needle shimmed. Or do I need a bigger main jet?
Couple of things, make more noise and RPM but not more speed might mean your clutch is slipping.
Regarding the power drop you have hot the dreaded 6 to 7 K RPM too rich dead spot.
This is the carb settign standard for 2007 bikes (see picture), you should have a 130 main. I understand you have gone up SEVEN steps in the main!
Go down on the main and all will be well.
Lots of folks say that the GS is jetted too lean, With all due respect I dont agree. Lots of folks who rejet talk about a 6K deadspot, I did too. After banging my head for way tooo long as you are (going richer must be the answer...) I went back to stock and all was well. You have highly modified bike so possible 1 or two up MAXIMUM.
Hey I think I got that jetting pattern from the wise old Buddha himself years ago! So you think I should go down to a 140?
Where's our other carb guru Josh /MR72?
Or could it be my crazy exposed K&N air filter? Not much I can do to fix that at this point, the tail is hanging wide open...
WOT at around 6 k RPM is where the main is fully in play (needle out of needle jet).
Your bike is going to be hard to tune over the whole range as its so customised, but the 6K flatspot at WOT is a sure sign of too rich at that point of the carb operation. Id start with 140 if I were you.
Bu then I have a sniffer similar to this ;-)
Quote from: Endopotential on August 11, 2019, 11:10:52 PM
Where's our other carb guru Josh /MR72?
Or could it be my crazy exposed K&N air filter? Not much I can do to fix that at this point, the tail is hanging wide open...
Lol I'm no guru! Especially on your mk2 carbs.
I agree your clutch has to be slipping. You only notice it at high speed because that's then the force is maximum. Mine did the same thing. I found the screw and locknut adjustment under the little cover to be very particular and not as easy as it looks to get right.
The open air filter is going to cause vacuum changes that will make it hard to tune. It may be causing the slide to move up less than it should. You could try the jet change and test herennow's theory, and if that doesn't work I'd shim the needle. If that makes it worse then move the needle the other way. You have stock needles right?
But really I'm useless with the 3 circuit carbs and open air filter. I'd be making uninformed guesses.
+1 to clutch slipping. More RPMS with no increase in speed without the clutch disengaged means the engine is turning faster, but the wheels aren't. Only way I'm aware of where that can happen is clutch slippage. :dunno_black:
As for the bogging on throttle, I think herennow is probably right in that your main jet is too large. I'm not a carb expert, but that was my first thought too.
I just went through this with a 150 and down to 142.5, still too big but I had to order 140. With 142.5 it's ridable but I have to throttle on slowly. Can't wait to get my 140 and see how it does,...
well I have the opposite problem... but I have mk1 carbs. I think my main jet is too small by one size. I have a set that are one size bigger. The bike runs just fine up to just under 9k rpm and then just stops pulling. I never noticed this with the stock 16t sprocket but with the 14t you really notice it running out of steam up top. Has to be main jet. But the 2-ckt carbs are much easier to figure out, especially with all stock airflow like I have.
Thanks guys! Glad to see the whole crew is still around!
I did that 3 step clutch adjustment a year ago, but I'll double check. What else should I look for, short of replacing the clutch? When I do my 0-30mph burnout, the clutch seems fine and doesn't miss a beat. Guess at higher speed acceleration there's more of a load?
I'll start looking around for some 140 mains.
Do some searching on the forum. I posted an Ali supplier where you get 6 jets for 10 bucks, but a month delivery time. Quality was fine .
Seeing as your stock is 130, I'd start with 132.5, 135 and 137.5. but hey, for 20 bucks you can get the whole range.
Quote from: Endopotential on August 12, 2019, 03:13:45 PM
I did that 3 step clutch adjustment a year ago, but I'll double check. What else should I look for, short of replacing the clutch? When I do my 0-30mph burnout, the clutch seems fine and doesn't miss a beat. Guess at higher speed acceleration there's more of a load?
Yes, there's more of a load at higher speeds. From wikipedia: A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome aerodynamic drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed, the drag (force) quadruples per the formula. Exerting 4 times the force over a fixed distance produces 4 times as much work. At twice the speed, the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, 4 times the work done in half the time requires 8 times the power.
Basically, as you go faster, the power that is required to overcome drag and maintain that speed increases exponentially. Which is why when the clutch first starts to go, it will only slip during high speed acceleration, as that is when the engine is working hardest. As it continues to wear, the speed at which it starts slipping will get lower and lower.