anyone cut a slide in carburetor

General forum on engines, transmissions, gearing and modifications to each

Moderator: rztom

Post Reply
Message
Author
graham heise
Posts: 291
Joined: Sun Sep 16, 2012 7:10 am
Location: gold coast Australia

anyone cut a slide in carburetor

#1 Post by graham heise » Mon Oct 22, 2018 6:41 am

Gday. I have recently changed my carbs to vm 30's. I have spent quite some time dialing in the jetting.
the only area i have an issue is the, of idle to quarter throttle responce. It is rich in that area (splutters).
I am leaning towards cutting the slides. The vm 30's come with 2.5 slides, i am considering raising the cutout to lean the responce on the slide. Has anyone done this? The next size slide is a 3 which is .794mm or 1/32thou higher in the top of the cut out then the 2.5 slide. I'm thinking of using my dremell and raising it somewhere in between 2.5 and 3. From my research gary from spec 2 and others have cut the slide on vm30's to improve responce in this area.
I can live with the current jetting, but in the pursuit of perfect jetting i am hoping to improve this area of the curcuit and get some tips from those who have done this mod. Any guidance would be appreciated.

current jetting. vm30 carbs. 30 pilot, air screw 1.75 turns, 6dh3 needle clip 2nd from top. 159-p5 needle jet, 2.0 air jet, slide 2.5, main jet 310-320. uni pod indvidual foam filters. banshee reeds, cylinders ported, chambers, 1mm squish, 31k ignition.
yamaha fanatic.

brrrappp
Posts: 603
Joined: Tue Sep 18, 2018 10:21 pm
Location: USA

Re: anyone cut a slide in carburetor

#2 Post by brrrappp » Mon Oct 22, 2018 2:48 pm

I have cut slides on my old dirt bikes before flat-slides were popular. You will want to make sure you can buy a replacement slide before you start because, like porting, it's hard to go back to baseline if the mod fails to produce a desired result.

Does the bike idle? What RPM is your idle set at? Sometimes you can get a feel for slide cutaway by simply raising the idle (I am not saying the cutting of a slide will raise idle by the way).

I would be curious to know if you have already tried dropping the pilot jet size significantly? Like going all the way down to a #20 from your current #30.
Also, I find float height can cause blubbering at low RPM/Low throttle opening. Some types of carbs are way more sensitive to float height than others.

Palolo
-----
-----
Posts: 129
Joined: Sun Mar 11, 2012 12:53 am
Location: Honolulu
Contact:

Re: anyone cut a slide in carburetor

#3 Post by Palolo » Mon Oct 22, 2018 5:03 pm

Yes you can cut the slide. I've done it for my H2 (VM30 carbs) with an angle grinder. Of course you want to take it gradually and make sure both slides match.

You might also think about changing the needle jets as they affect the throttle range where you're too rich. Take a look at this chart: http://kawtriple.com/mraxl/carb/mikunineedle.html" onclick="window.open(this.href);return false;

graham heise
Posts: 291
Joined: Sun Sep 16, 2012 7:10 am
Location: gold coast Australia

Re: anyone cut a slide in carburetor

#4 Post by graham heise » Tue Oct 23, 2018 12:25 am

Thanks for the replys and advice. A few things you mentioned i have been through. The 6dh3 needle is allot leaner then the stock vm30 needle and has been established as the needle to use for this application on other forums. Although some have used a leaner needle jet 159-p4 opposed to 159-p5. I started at a 40 pilot jet and went all the way to a 27.5 but needed to close the are screw to far which upset other area's of the jetting. I am bang on in the middle of float hight 23mm (22mm-24mm recomended) . The jetting i have is Very close to what others are using from info on other forums. Except most others are running with the 40 pilot. At the moment the engine purrs like a kitten at 1500rpm on idle. It will pick up the first few hundred revs before stumbling by the time it reaches 4k the stumble is gone and pulls smooth through mid throttle to red line. Mid, and wot plug chops are good just need to address the low throttle (1/8th-/4 I'll try a leaner pilot again and change float high, play around before i cut the slides.

Sent from my SM-A520F using Tapatalk
yamaha fanatic.

RZtuner
Posts: 152
Joined: Sun Sep 17, 2006 10:28 pm

Re: anyone cut a slide in carburetor

#5 Post by RZtuner » Tue Oct 23, 2018 1:18 pm

It sounds like an off-idle lean bog to me. I had the identical problem on my new RG500 build this summer. Starts first kick, idles nicely, engine bogs off idle to 4 grand then picks up. I had the stock #25 pilot jet in it at 1.5 turns out. Raising this to 27.5 completely fixed it immediately.
Hope this helps.
350 Prod. SCTA Bonneville World Landspeed record holder
500 Prod. SCTA Bonneville World record holder
350 Prod. AMA Bonneville National record holder
350 P-MG AMA Bonneville National record holder
AMA No.1plate holder x2
Pioneer- Motorsport Hall of Fame

Bare
Posts: 1196
Joined: Sun Jan 29, 2006 2:18 am
Location: vancouver

Re: anyone cut a slide in carburetor

#6 Post by Bare » Sat Nov 03, 2018 11:26 pm

Perhaps Retrofit a pair of New Genuine Keihin 28 PWKS.
These are of more sophisticated, advanced design, consequently Very easy to jet.... as in close enough is usually Far better.. than Spot on with Mikunis.

75RD350rider
Posts: 601
Joined: Thu Jun 19, 2014 9:52 pm

Re: anyone cut a slide in carburetor

#7 Post by 75RD350rider » Sun Nov 04, 2018 10:47 am

Gary at Spec II also cuts slides when he preps his VM 30s for sale. I am running a 40 pilot in my Spec II prepped VM30s and the same needles with airscrews at 3/4. Main jet is a 360 with Uni pod filters. Mine is linear from idle to rev limiter at 10.5K.
I love the smell of Blue Marble in the morning!
Tim Steele in Front Royal, VA

graham heise
Posts: 291
Joined: Sun Sep 16, 2012 7:10 am
Location: gold coast Australia

Re: anyone cut a slide in carburetor

#8 Post by graham heise » Tue Nov 06, 2018 4:16 am

thanks for all the replys guys. i have been changing jets and shaving slides. jetting is now
320 main
40 pilot
159-p2 needle jet
6dh3 needle 2nd clip
I shaved the slide in increments. there is a .791mm difference in cutaway between a 2.5 and 3.0 slide. i first shaved .2mm and noticed a nice difference between 1/3rd and half throttle but the rich stutter at 1/4 throttle is still there. i then shaved another .2mm of slide, still no difference to the 1/4 throttle stutter.
Since then i have adjusted float hight 1mm as i thought maybe i'm getting some pilot jet extra fuel by having to high a fuel level. No difference. i have tried smaller pilots all the way down to a 22.5 and with that the bike won't pull of idle without choke. pilot feels best at 40 with 1.5 turns air screw.
I have taken notes with every change and the best the bike has been is with the settings outlined at the top, fueling and power is fantastice from 1/3rd throttle to wot.
All good except for the rich stutter at 1/4 throttle which is unfortunatly cruising around position.
Just to confirm it is a rich condition i have ridden the bike at 1/8 to1/4 throttle where it stutters the most and then turned the fuel tap of. after 10 or so seconds the stutter goes and the engine picks up revs, confirming it's rich.
I am now considering going to a 159-P0 needle jet to address the rich 1/8th to 1/4 throttle rich condition. i also wish i had a suduco mikuni tuning manual to look at other options.
From what i have read there are others who are running the same or similar jetting to me without any report of this rich stutter.
I am also considering the condition of my carbon reeds and wonder if they are influencing in any way.

thank palolo, for the jetting chart it has been usefull. 75rd350 does your set up use the p2 nnedle jet or a p0?
yamaha fanatic.

Palolo
-----
-----
Posts: 129
Joined: Sun Mar 11, 2012 12:53 am
Location: Honolulu
Contact:

Re: anyone cut a slide in carburetor

#9 Post by Palolo » Tue Nov 06, 2018 11:33 pm

I think the P-0 will help however the changes in the slide cutaway might complicate tuning since the slide cutaways have been changed.

My experience with changing needle jets is with my Kawasaki H2's. I have an RD350 that I bought a couple of years ago, haven't put it on the road yet.

brrrappp
Posts: 603
Joined: Tue Sep 18, 2018 10:21 pm
Location: USA

Re: anyone cut a slide in carburetor

#10 Post by brrrappp » Wed Nov 07, 2018 3:05 am

Definitely check the condition of the reeds. Gaps and fraying are bad and have the most impact at low RPM.

Something else you can experiment with is extending the length of the intake tract between your k&n filters and the carb bell. Some right-sized PVC and some rubber hose or old air-box boots might allow you to add an inch or two. It could create some added velocity or a slight difference in the pressure wave. Maybe someone you know has one of those two into one air filters That Toomey sells and could loan it to you for a day of testing.

graham heise
Posts: 291
Joined: Sun Sep 16, 2012 7:10 am
Location: gold coast Australia

Re: anyone cut a slide in carburetor

#11 Post by graham heise » Wed Dec 19, 2018 6:56 am

G'day. I wanted to add some info for those who might be reading this thread to find some answers.
In the end i found that because i was using some aftermarket manifolds without provision for the crossover tube, that was the reason for the of idle 1/4 throttle rich stumble. i ended up turning down the carb mounting spigots on a lathe so i could use the stock manifolds and cross over tube. this cleared the stumble and the bike performs very well without flat spots. my final jetting is. mains 320
pilots 42.5 1 and half turns air screw.
6dh3 needles 2nd clip from top
159 p2 needle jet.
on a hot day when temps are above 30 degrees celcious the bike won't pull full throttle with the 320's. i drop to 310 mains and open air screw to 2 turn.
yamaha fanatic.

Post Reply