I had been reading up on the DR650 on the net - I think it'll make a nice street single - and came across a lot of talk of it being necessary to change or mod the carb. The carb mod seems simple enough and improves the throttle response in the low and midrange. So I thought I'd try it on the GS...

With the tank off I took the top off the carbs, pulled out the slides and drilled the holes either side of the needle 1mm larger taking care to level the burr on the hole from drilling. In my 2009's carbs the slide springs sit over the holes effectively halving the holes area...

I bolted it all back together and took it for a test run. As advertised the throttle response from 3-5000 rpm is better and now I can ease the throttle on at 2500rpm and not have the bike shudder badly - still shudders a little though. For a $0 mod I'm happy with the result so far.

I'll ride it like it is for a week or two and if I feel it'll be of benefit I'll do the next stage of the mod which is backing off the spring preload on the slide by trimming the spring.

Cheers

Interesting modification. Did anyone ever say if you go bigger than 1 mm if it negatively affects the response? Anything else that needs to be done as far as jetting etc.?

Interesting , thanks for the data point. is jetting all standard?

This is a common Harley mod. But there are as many sites for it as against it.   I cvonsidered it but fixed my juddering problem by taking bike back to stock (leaner)

I'd be interested to know the effect at WOT around 5k RPM.

Quote from: jar75 on January 20, 2021, 07:53:35 PM
Interesting modification. Did anyone ever say if you go bigger than 1 mm if it negatively affects the response? Anything else that needs to be done as far as jetting etc.?

I went 1mm bigger because that was the next size up drill bit I had. I would have gone 0.5mm bigger first if I had the drill bit. In the slides from my 2009 carbs there wasn't really the room for the extra 1mm diameter but I forced the issue.
The idea is to have the slide react a bit quicker to throttle openings but going too large with the holes would have it reacting too quick which could cause fuelling issues. The fuelling is still controlled by the needle and seems to be fine with the 1mm increase. The next stage of the mod is to back off the slide spring preload by shortening the spring and needs the needle to be lowered since the slide will sit higher for a given air flow.

Quote from: herennow on January 21, 2021, 12:55:46 AM
Interesting , thanks for the data point. is jetting all standard?

This is a common Harley mod. But there are as many sites for it as against it.   I cvonsidered it but fixed my juddering problem by taking bike back to stock (leaner)

I'd be interested to know the effect at WOT around 5k RPM.

My bike is totally stock, haven't even had the carbs off the bike.... The GS500 isn't really meant to be accelerated from 2500rpm which is the only time I get any shuddering. I'm lazy and try not to downshift when in city traffic. The biggest noticable difference is rolling back on the throttle at 3000rpm in 6th - pickup is smoother and quicker. Haven't found a drawback yet.

Here's a link to a vid showing the carb mod for the DR650. This is the accumulation of years of experimenting on the DR carb and won't translate exactly to the GS carbs.I can't find anywhere on the net where someone has done something similar for the GS but there are other bikes where the details have been sorted out. That's why i'm approaching it one step at a time and evaluating each step.
The DR slide spring is 112mm long stock and they cut it to 100mm - the springs in my GS carbs are 100mm stock so after a couple of weeks getting used to the first stage I plan to cut one loop of the spring off which will be about 5mm and test that out...

cutting that spring will actually make it stiffer. FYI. The "preload" affects how much force (vacuum) is required to begin moving the slide. Cutting it will result in less vacuum required to begin moving it but more vacuum to move it further. This will really affect mixture across rpm and throttle openings making it progressively leaner across the range as throttle and rpm increases. But it will be richer at initial opening and very low rpm and throttle.

Good thing internet users of dr650s know more about carburetor design for a bike they are not even working on than Suzuki engineers did when they designed the gs500.

Oh, this could be at work in the 02 carbs I could never get to run right especially past 1/2 throttle. Now in an 89-00 the slide comes up far too fast. That's our biggest problem, its solved by blocking 1 hole. But this may be at play in the 02 rack I have. I "solved" the problem by switching back to the older carb.
Let me check that video. Thanks guys.
Cool.
Buddha.

https://www.youtube.com/watch?v=aN1aU0G4YQE

I watched that video. That's a slightly different version of the 89-00 carb on that DR with 1 hole in the bottom of the slide.
I'll be careful applying its "lessons learned" to the 01 and later GS carb. The 89-00 carbs sure - except the GS has 2 holes in the slide and a well documented case of slides coming up too fast especially if we remove any restriction like pipe. So I'd say the DR setup is the opposite and they're trying to create a problem we already have. Maybe we could trade those guys our slides and get theirs in exchange.

BTW a carb trying to feed 650 cc's at 8K redline has a whole different situation than one trying to feed 250cc @ 10k. The CFM's are nearly 2X on the single, but they cant increase the venturi diameter to match. Then the bike will be unrideable at low revs. So they fit too small a carb on it anyway. That leads to a bunch of other problems like slide rising too fast etc.

Cool.
Buddha.

Quote from: herennow on January 21, 2021, 12:55:46 AM
...
I'd be interested to know the effect at WOT around 5k RPM.

I don't normally ride WOT but tested this out coming back from the shops when the bike was warmed up.
In first then second then third I held the bike to a steady 5000rpm then slammed the throttle wide open. Pickup was instant and it revved out fine.

Quote from: mr72 on January 22, 2021, 05:19:48 AM
cutting that spring will actually make it stiffer. FYI.

Should be a recommended budget fix for the soft fork springs then I would think.

Quote from: mr72 on January 22, 2021, 05:19:48 AM
...
The "preload" affects how much force (vacuum) is required to begin moving the slide. Cutting it will result in less vacuum required to begin moving it but more vacuum to move it further. ...

More vacuum is provided by the larger holes in the slides as mentioned in the first post.

Quote from: The Buddha on January 22, 2021, 09:50:56 AM
... a well documented case of slides coming up too fast ...

I'd like to read about that. Can you point me to any info on this?

Cheers

Quote from: moe_tunes on January 22, 2021, 05:15:36 PM

Quote from: mr72 on January 22, 2021, 05:19:48 AM
cutting that spring will actually make it stiffer. FYI.

Should be a recommended budget fix for the soft fork springs then I would think.

It is a cheap way to make a spring stiffer, except you don't know how much stiffer the spring will be. Its much more a guess and check thing. Better to spend the $100 on a known spring rate and be done with it. Maybe a physicist could tell you, does cutting a spring in half make it twice as stiff? Its probably a more complex relationship than that.

Now you want to introduce guess and check into your carburetor without an objective way to measure the results (dynometer)? Have at it if you like pulling the GS carbs off over and over but you are more likely to kill your fuel economy than get any measurable gains in HP.

Bluesmudge, if you had read the first post you'd know it isn't about "gains in HP".

Cheers

Quote from: moe_tunes on January 22, 2021, 05:15:36 PM

Quote from: mr72 on January 22, 2021, 05:19:48 AM
...
The "preload" affects how much force (vacuum) is required to begin moving the slide. Cutting it will result in less vacuum required to begin moving it but more vacuum to move it further. ...

More vacuum is provided by the larger holes in the slides as mentioned in the first post.

no it's not. Vacuum that pulls the slide up is above the diaphragm. That hole is a controlled leak. Air coming through it actually goes into the cylinder, making it leaner, mostly at idle. But there main purpose of that hole is damping, the larger the hole, the slower the slide comes up, and the faster it goes back down. A stiffer spring will also put the slide down quicker. This is all a delicate and precise balance where each change interacts with everything else. Very rarely can you change one thing and have no deleterious effects.

IMHO this is a misguided solution in search of a problem.

Quote from: Bluesmudge on January 22, 2021, 05:50:56 PM

It is a cheap way to make a spring stiffer, except you don't know how much stiffer the spring will be. ... does cutting a spring in half make it twice as stiff? Its probably a more complex relationship than that.

Actually it is exactly like that, pretty much. For active coils, it is precisely linear. It's easy to approximate inactive coils. With a calculator and a micrometer you can easily calculate spring rate as long as you know the spring material.

Problem with the GS spring is that it's so soft to begin with, you have to cut like 40% of it off to get it stiff enough and then it's too short to not impede travel. When you cut coils you also shorten the entire spring, reducing the space between coils available for movement. You make this up with a spacer but at some point you end up with coil bind bottoming the fork before you have used up all of the mechanical travel.

I did exactly this, cut about 1/3 off of my stock springs, and they were better but the .90 Sonic springs are way better. We'll probably put the cut down GS springs in my dad's TU250x. It doesn't have as much travel to lose and can handle softer springs.

Ask yourself a question..................

If the very easy (and cheap) placing of a tiny hole in the slide in the way you describe gives such an advantage.........why didn't Mikuni and Suzuki with all their combined years of in depth knowledge and experience do it themselves before the bike even left the factory?




I will leave that one with you 

Quote from: moe_tunes on January 22, 2021, 05:22:02 PM

Quote from: The Buddha on January 22, 2021, 09:50:56 AM
... a well documented case of slides coming up too fast ...

I'd like to read about that. Can you point me to any info on this?

Cheers

Dynojet offers plastic plugs to constrict and/or to close off those holes. I actually dynoed my bike with 1 hole closed off, it still showed signs of the slide coming up too fast. I trained myself to open the throttle slower.
When you whack the throttle open and the bike falls on its face when it was running along fine, and if in that same rpm/speed/gear it reacts fine when you open the throttle slowly, that's a sign the slide is coming up too fast.
Cool.
Buddha.

Quote from: mr72 on January 23, 2021, 05:49:00 AM

Quote from: moe_tunes on January 22, 2021, 05:15:36 PM

Quote from: mr72 on January 22, 2021, 05:19:48 AM
...
The "preload" affects how much force (vacuum) is required to begin moving the slide. Cutting it will result in less vacuum required to begin moving it but more vacuum to move it further. ...

More vacuum is provided by the larger holes in the slides as mentioned in the first post.

no it's not. Vacuum that pulls the slide up is above the diaphragm. That hole is a controlled leak. Air coming through it actually goes into the cylinder, making it leaner, mostly at idle. But there main purpose of that hole is damping, the larger the hole, the slower the slide comes up, and the faster it goes back down. A stiffer spring will also put the slide down quicker. This is all a delicate and precise balance where each change interacts with everything else. Very rarely can you change one thing and have no deleterious effects.

IMHO this is a misguided solution in search of a problem.

Where do you think the vacuum that raises the slide comes from?
I don't think you quite understand how CV carbs work. Have a look at Benoulli's Principle. It might help you get that bit sorted.

Cheers.

Quote from: sledge on January 23, 2021, 07:58:58 AM
Ask yourself a question..................

If the very easy (and cheap) placing of a tiny hole in the slide in the way you describe gives such an advantage.........why didn't Mikuni and Suzuki with all their combined years of in depth knowledge and experience do it themselves before the bike even left the factory?




I will leave that one with you 

I'm suprised you didn't tell me to get a different bike !

Cheers.