Drilling CV Carb Slide Holes and Driveability

[JonnyC]

I have an 1983 Suzuki GS650. I have the DJ Stage 3 kit and drilled the slides. Never rode the bike stock, so nothing to compare to, but it seems that the driveability sucks. Very abrupt on/off throttle with almost no transition in between, so when navigating roundabouts I have to use the clutch to help feather the throttle so it's not hard accel or hard decel.

Do you think this might be the result of drilling out the slide?

And if so, does anyone know the stock size of the hole on a 650? I plan on using JB Weld and drilling to stock diameter.

Thanks!

[tom203]

".. so when navigating roundabouts I have to use the clutch to help feather the throttle so it's not hard accel or hard decel."

you must have some interesting roundabouts - I find myself mostly off throttle navigating them, so the slides are likely drooping! My spare carb set has about .090 drill hole thru slide.
I can't see the size of slide hole as causing this problem, but let's see what others think.

[JonnyC]

you must have some interesting roundabouts

Ha, they are normal roundabouts. The main issue is coming into the roundabout slightly on the throttle. Trying to roll off the throttle in the turn, the bike acts as if I just hit the brakes. Hard to explain, but maybe this is even normal. I guess I just haven't had this finicky of throttle on a bike before.

My spare carb set has about .090 drill hole thru slide.

Awesome, thank you!!

[JonnyC]

WOW. So it turns out I'm a bleeping moron (doesn't surprise me). I drilled out the slide holes with the 5/32 bit that was provided with the kit, but that was meant for removing the cap over the pilot screw. The drill bit meant for drilling out the slides is still in the wrapping! Off to buy some JB weld in shame...

[TxGSrider]

I've often wondered what would happen if those holes were too big.... now I know.

[JonnyC]

Well we'll see what happens when I get them back to the stock diameter. But I think with holes this large, it would limit the amount of vacuum pulling the slide up. But honestly I've given up on trying to figure out exactly how these CV carbs work.

[850GT_Rider]

PM sent.......

[TxGSrider]

Well we'll see what happens when I get them back to the stock diameter. But I think with holes this large, it would limit the amount of vacuum pulling the slide up. But honestly I've given up on trying to figure out exactly how these CV carbs work.

Im not sure how exactly they work either, but I understand that making the holes bigger actually speed up how fast they open. On the Dynojet GSXR kit I have they actually use inserts to make the slide holes smaller to slow down the opening speed.

[dorkburger]

WOW. So it turns out I'm a bleeping moron (doesn't surprise me). I drilled out the slide holes with the 5/32 bit that was provided with the kit, but that was meant for removing the cap over the pilot screw. The drill bit meant for drilling out the slides is still in the wrapping! Off to buy some JB weld in shame...

You're not the only one...... I did the same years ago.....

[JonnyC]

The JB Weld is curing right now. Should I redrill a new hole instead of drilling through the JB Weld? It seems the location of the hole shouldn't matter, as long as I don't drill into one of the holes where the tabs of the plastic cap thing go.

[Steve]

Well we'll see what happens when I get them back to the stock diameter. But I think with holes this large, it would limit the amount of vacuum pulling the slide up. But honestly I've given up on trying to figure out exactly how these CV carbs work.

It does not limit the amount of vacuum, it actually speeds the transfer of vacuum, letting the slides move faster, when they do move. It does not make them move any sooner, just the transition time is less.

Should I redrill a new hole instead of drilling through the JB Weld? It seems the location of the hole shouldn't matter, as long as I don't drill into one of the holes where the tabs of the plastic cap thing go.

I would not say the location of the hole is "critical", but you have to realize that it was in that place for a reason. Also keep in mind that the original hole was at an angle, not parallel to the needle. It was directly downstream from the needle, there might have been a reason for that, I don't know.

.

[JonnyC]

Ah, this is the mistake I made. Looking at this image, I assumed this is how our CV carbs functioned.

hqdefault.jpg

Source: https://www.youtube.com/watch?v=wyspAHrMbb8

But the only way air comes in and out above the diaphragm is from the hole in the slide. So yeah, it makes total sense that a larger hole will just mean a faster reacting (and possibly fluttering) slide.

I think I'm going to drill the OEM size hole just to the side of the hole plugged with JB Weld, at the correct angle. And then I'm going to return my jetting back to DJ160 and three notches down on the needle, since I've been chasing down a stumble on part throttle, which still might be there with my velocity stacks and all

[TxGSrider]

Ah, this is the mistake I made. Looking at this image, I assumed this is how our CV carbs functioned.

[ATTACH=CONFIG]47156[/ATTACH]

Source: https://www.youtube.com/watch?v=wyspAHrMbb8

But the only way air comes in and out above the diaphragm is from the hole in the slide. So yeah, it makes total sense that a larger hole will just mean a faster reacting (and possibly fluttering) slide.

I think I'm going to drill the OEM size hole just to the side of the hole plugged with JB Weld, at the correct angle. And then I'm going to return my jetting back to DJ160 and three notches down on the needle, since I've been chasing down a stumble on part throttle, which still might be there with my velocity stacks and all

I believe that when you fix the hole size, it'll make the part throttle mixture richer and may fix that stumble.

[dorkburger]

I may be wrong, but I think there is also a vent in the topside of the carb body to give it an atmospheric reference.

[JonnyC]

I called up Dynojet to get a replacement e-clip. I recall that I had lost one so I replaced it with one from the hardware store that is close in size, but it has a slight wave to it due to it being a little small. I swear I'm a better mechanic than I sound!

Anyways, the guy from Dynojet recommend just drilling through the JB Weld. I'll do that later tonight and post back the results when I get the carbs put back together. Thanks everyone for the help!