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Drilling Vacuum Slides - size??? '82 GS(X)400

  • Thread starter Thread starter TrikerJoe
  • Start date Start date
T

TrikerJoe

Guest
Hello,
I have a 1982 GS(X)400L/T (Canadian version). See sig for build log if you would like.
I am working on getting my carbs tuned in closer to where they should be and have come across the idea of drilling the vacuum slides to get better midrange response and allow full slide travel when mods have been done like i have with K&N filters and a MAC 2 into 1.

My question is, what size hole should I drill in my vacuum slides?

I have heard 3.5mm for a similiar year GS450, would this be a good place to start and then work on my needle position/float height/spring length once I can actually get out on the road (we still have snow)?
I currently have a 125 main, which seems to be too lean. I have been told in another thread that 140 to 150 mains are common with similiar mods. I am waiting for my larger size main jets to come in, but I need to make sure i'm getting proper slide travel to make sure i'm actually attaining WOT to tune in my main properly, if my understanding is correct.

Or am I just way out to lunch? That is always an option.

Thank you for your interest.

Joe
 
Hey Joe, you must've missed my jetting/tuning thread ;)

http://www.thegsresources.com/_forum/showthread.php?t=207055

I cut just under 20mm off the slide springs and have now drilled the vacuum hole from my measure 1.79mm to 2.38mm with a 3/32 drill bit.

The slide springs made the bigger difference so far, but I'll be going larger on the drilling yet, probably a 1/8 drill bit.

I'm also going to try dropping from the 150 mains back to 145.

I'll be keeping my tuning thread updated.
 
I recently purchaced a Dynojet kit for my GS1100 34mm carbs and the slide lift hole drill bit size was 2.90 mm this jet kit also covers the GS1150 carbs 36mm also.So looks like you are in the ballpark I just would't want to over size it and possibly mess a good thing up and have to tap it and install some kinda restrictor in the hole which I have seen come in some jet kits.
 
I am only guessing here (I tend to keep my carbs stock), but it is my impression that drilling the slides does not change when the slides open or how much.
What does changs is how quickly they respond to vacuum changes.

On a street bike, I am not sure if it's all that necessary for the slides to lift that quickly, and I don't think that they are going to be all that quick on a twin. :oops:

.
 
Standard on most CV's of that period is 2.80mm.
The first jump in size is usually the biggest - to say,2.95mm then I go up in .10mm jumps until response is sufficient.
It helps of course to have the drills - and over the years I've accumulated them.

It's too easy to get confused when switching between metric and inch drill sizes - and go for too big a jump. If you're in the US, find a supplier of metric drills.

Edit - answering Steve - The mods the OP is doing will change response time and probably leave a big mid range flat spot. Usually there's nothing wrong with the jetting or needle taper, but the vacuum signal the carbs see is different from OE so the rate of slide rise doesn't match the air requirements of the motor. To some extent it's similar to changing the slide cutaway on "normal" carbs.
I'd be surprised if it's going to need any jetting changes - for normal use anyway.
 
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Thanks very much for the responses folks!
I don't have drill bits in those tiny increments but I know people who do! ;)
Thanks for the numbers, now I just have to start small and experiment. And keep an eye on Pete's thread!! I'll update everyone as I get a chance to tune it up.
Right now i'm concentrating on the non-engine mods until I get my bigger mains and until the snow goes away!
Thanks again!
Joe
 
Steve and Greg... interesting input... I don't feel a noticeable flat spot on mine and I went to a 3mm hole last night. All my test riding is just my commute at the moment but I must say it feels quite awesome compared to how it was :)

Jury's out on a fun ride in the twisties until I get one in of course...

Steve M... very interesting! Thanks for that, certainly makes a difference!
 
pete said:
Steve and Greg... interesting input... I don't feel a noticeable flat spot on mine and I went to a 3mm hole last night. All my test riding is just my commute at the moment but I must say it feels quite awesome compared to how it was :)
Click to expand...

Pete, (or anyone of course!)
If I remember correctly you started at 1.73mm and are now at 3mm, do you think I would be safe to go from my stock hole of 2.82mm to 3.14mm (1/8") on my 400? Or will this not be enough of a jump to notice a difference and instead jump straight to 3.6mm (9/64")? This is still with the stock spring length (not yet measured).
Cheers!
 
Hey Joe, first up, do you know that 2.82mm is stock? Mine was measured to be 1.79mm which is quite a bit smaller...

The guy I've been talking to about this says he goes to between 3 and 4mm max before it's too large.

The safest way is to go up in smaller increments, but personally I'd probably stop at 1/8"... trial and error is the only real way to see and if you go too far you will have to fill the hole and re-drill.

One suggestion made to me if I went too far was to go further, tap it with a thread, then insert a nylon screw. You can then drill the nylon screw to get the vacuum hole back.
 
pete said:
Hey Joe, first up, do you know that 2.82mm is stock? Mine was measured to be 1.79mm which is quite a bit smaller...

The guy I've been talking to about this says he goes to between 3 and 4mm max before it's too large.

The safest way is to go up in smaller increments, but personally I'd probably stop at 1/8"... trial and error is the only real way to see and if you go too far you will have to fill the hole and re-drill.

One suggestion made to me if I went too far was to go further, tap it with a thread, then insert a nylon screw. You can then drill the nylon screw to get the vacuum hole back.
Click to expand...

Pete, no, I'm really not sure that is the stock size, just the size that I measured when I took the slides out.
I think I may leave the drilling for when I get the jetting/needle position sorted out.
I took it for a ride today (streets are pretty bad) but I was able to sort out that my midrange is popping and bogging out after 1/4 throttle which I take as way rich (I put in 140 mains the other day up from the 125s) so i'm thinking I will have to lean out my needle a notch and see if it improves.
 
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TrikerJoe said:
Pete, no, I'm really not sure that is the stock size, just the size that I measured when I took the slides out.
I think I may leave the drilling for when I get the jetting/needle position sorted out.
I took it for a ride today (streets are pretty bad) but I was able to sort out that my midrange is popping and bogging out after 1/4 throttle which I take as way rich (I put in 140 mains the other day up from the 125s) so i'm thinking I will have to lean out my needle a notch and see if it improves.
Click to expand...

Yep, it's normal...symptoms of richness in the midrange because the slides aren't lifting and letting in air !!!

Larger vacuum holes match the slide's rate of lift to the demands of the motor.
 
GregT said:
Yep, it's normal...symptoms of richness in the midrange because the slides aren't lifting and letting in air !!!

Larger vacuum holes match the slide's rate of lift to the demands of the motor.
Click to expand...

Thanks! That makes sense.
So today I will try to drill out the vacuum slide hole to 1/8" (3.14mm from 2.82mm) and see how it reacts!
It's nice to be out for even a short tuning ride, I missed the whole riding season last year. :(
 
Ok, so I've drilled out my vacuum slide hole to 1/8" (3.14mm).
I also have a spare set of carbs off a gs400TX and when I measured the springs, I found them to be about 4mm shorter than the ones in my carb (107/108mm compared to 111/112mm), so I swapped in the shorter springs.
I can now blip it through midrange to WOT and have done some drag style plug chops and the plugs are coming up white.
I am guessing that I need a larger main jet than the 140 I have in there, unless I am missing something???
I will also post an update on my carb tuning thread...
Thanks for your interest!
 
Not uncommon to need a larger main jet with a 2 : 1 pipe.
Just make sure you're using the right heat range plugs - too hot a plug will give you false readings.
 
Once you hit wot the slides are usually all the way up. Your getting the most air but not enough fuel.

Midrange is the oposit as greg says.

Start your tuning with the best main jet. Once its giving you nice plugs at wot. Then you can tune the rest.

Id say go to 150's and see if you can get the plugs darker. Then step down to 147.5, 145, 142.5 and see where its happyest.
 
GregT said:
Not uncommon to need a larger main jet with a 2 : 1 pipe.
Just make sure you're using the right heat range plugs - too hot a plug will give you false readings.
Click to expand...

Greg, I'm running what the manual calls for as stock, NGK DR8ES-L.
Do you think I will need to change my heat range for the mods I've done? I don't know anything about plug heat ranges...
 
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