Vacuum slide operation (Mikuni) - Triumph Forum: Triumph Rat Motorcycle Forums
Hinckley Classic Triples 885cc Classic Styled T3's: Legend, Thunderbird, Thunderbird Sport & Adventurer.

Reply
 
LinkBack Thread Tools Display Modes
post #1 of 13 (permalink) Old 06-02-2013, 02:19 PM Thread Starter
Pole Position
Main Motorcycle: KTM 1290 Super Adventure
Senior Member
 
ArferBrick's Avatar
 
Join Date: Jun 2011
Location: Köln
Posts: 3,282
Other Motorcycle: '96 Adventurer
Garage
Vacuum slide operation (Mikuni)

I've been doing a lot of research lately on how CV carbs work, and the more I learn it seems the less I know!

That lead me to do some experiments with 2 different sets of vacuum slides that I have, one set is from a Trophy 1200, the other set are the standard ones that came in my Adventurer.

The slides have an air hole in them, some call it a "transfer port", which allows the air to be sucked out of the diaphragm top by the air rushing into the engine, raising the slide and opening the main jet needle to supply more fuel.

I wasn't quite sure if a bigger hole would make the slides rise faster, so I did a little experiment with our vacuum cleaner.

Here's how the Trophy slides react, they have one very tiny hole -

http://christiane.homeip.net/pub/adv...602_140308.mp4

And here's how the Adventurer slides react, they have 2 big holes in them -

http://christiane.homeip.net/pub/adv...602_140408.mp4

As you can see (if you watch the videos!) is that a small hole makes the slides react very slowly, and bigger holes make them react instantly.

I later tried both sets in my carbs - with the original slides (big holes), the engine would bog down severely when blipping the throttle, with the Trophy slides in the exact same carb, the response was instant and no bogging.

Exactly the opposite of what I would have thought would happen .

Trophy carbs (Mikuni BST36) have 4 differences to the standard Adventurer (and also Legend & Thunderbird Mikunis) -

1) They run a 117.5 Main jet
2) the air intake for the emulsion tube is 1mm instead if 1.2mm for ours
3) Slightly different needles
4) The throttle slides have the one tiny air hole (transfer port).

I went for a blast around the block with the Trophy slides in and what a difference it made!! I could actually feel the front wheel start to get extremely light, not saying I was close to pulling a wheelie, but definitely more get up and go, especially at higher throttle positions.

She would pull like a devil once I got over 4000rpm.

But I still have a major flat spot at take off. Idle is perfect, and top end is perfect, just a really annoying flat spot between the two .

That's not caused by the slides by the way, I've been having a lot of problems for the last 2 weeks, and with the standard slides in it would nearly die on throttle application - the Trophy slides actually make it rideable, and once I solve that hole I'm convinced she's going to be running very well indeed .
ArferBrick is offline  
Sponsored Links
Advertisement
 
post #2 of 13 (permalink) Old 06-02-2013, 06:40 PM
Site Supporter
SOTP Vintage Series
Main Motorcycle: 99 Legend Sprint GT
Lifetime Premium
 
IrlMike's Avatar
 
Join Date: Jan 2008
Location: Cork Ireland
Posts: 6,462
Are you sure those Adventurer slides are stock with two ports (holes) in them?

The size of the holes (as you've figured) affects the transient, or damping, response of the slides to pressure variations in the venturi. Presuming we're talking about similar sized holes, doubling up is a huge change. The Keihins fitted to all the T3 variants AFAIK only use 1 x 3mm hole. Since the flow dynamics of the engines are much the same, it would be surprising to see such difference in carbs of the same venturi diameter.


98 Tbird tank/seat, Sprint fairing, ZX7R forks+Racetech Gold Valves, 08 ZX6R shock, short sw/arm, Alcon 6-pots, Ignitech+TPS, T'bike 3-1+Beowulf can, airbox/jet mods, tubeless wheel mod, lots :) per mile, 55mpg ave(imp)
IrlMike is offline  
post #3 of 13 (permalink) Old 06-02-2013, 07:27 PM
Formula Extreme
Main Motorcycle: tiger 800
Senior Member
 
hawk#1's Avatar
 
Join Date: Apr 2010
Location: BLACKBURN LANCS
Posts: 792
Other Motorcycle: none
Extra Motorcycle: hmm maybe a t140
Right daft question here arfer but if your saying its air being sucked into the carbs from the engine would you not need the hover hose on the other side of carbs cause from what i can see from vids your sucking the air out engine but like i say maybe am asking a daft question unless your blowing air in opposite direction that is good luck arfer hope you get her sorted soon
hawk#1 is offline  
 
post #4 of 13 (permalink) Old 06-02-2013, 11:10 PM
SuperBike
Main Motorcycle: 1996 Thunderbird
Senior Member
 
BRG-BIRD's Avatar
 
Join Date: Nov 2007
Location: Somewhere in cheese land
Posts: 1,742
Other Motorcycle: 2009 Moto Guzzi Norge
Extra Motorcycle: 1975 Honda CB550F
There are two holes in the slides for Mikuni carbs in the T3 classic line. I tried the Trophy slide to reduce the bounce the slides are rumored to have at cruise. With stock jetting it would idle fine and go down the road fine but quick acceleration would creat a major flat spot. I suppose rejetting would have worked but I got lazy and installed Kehins instead for durability...

"You are either onto something or on something"........The Common Man
“Sometimes you are best off dealing with the Devil you know” anonymous
BRG-BIRD is offline  
post #5 of 13 (permalink) Old 06-03-2013, 05:12 AM Thread Starter
Pole Position
Main Motorcycle: KTM 1290 Super Adventure
Senior Member
 
ArferBrick's Avatar
 
Join Date: Jun 2011
Location: Köln
Posts: 3,282
Other Motorcycle: '96 Adventurer
Garage
Quote:
Originally Posted by IrlMike View Post
Are you sure those Adventurer slides are stock with two ports (holes) in them?
I'm not sure they're original, but they are the ones that I've always had in there and I haven't drilled them. Strange because she always ran well with those.

Quote:
Originally Posted by hawk#1 View Post
Right daft question here arfer but if your saying its air being sucked into the carbs from the engine would you not need the hover hose on the other side of carbs cause from what i can see from vids your sucking the air out engine but like i say maybe am asking a daft question unless your blowing air in opposite direction that is good luck arfer hope you get her sorted soon
The vacuum cleaner was pretending to be the engine, sucking fuel and air in. What you see in the video is the back of the carb, the part that attaches to the airbox side. Carbs and their operation is very confusing!

Quote:
Originally Posted by BRG-BIRD View Post
There are two holes in the slides for Mikuni carbs in the T3 classic line. I tried the Trophy slide to reduce the bounce the slides are rumored to have at cruise. With stock jetting it would idle fine and go down the road fine but quick acceleration would create a major flat spot. I suppose rejetting would have worked but I got lazy and installed Kehins instead for durability...
Oh good, so you can verify that they have 2 holes standard?

I've just ordered a brand new airbox and intake rubbers, hopefully that will sort my problems out cos it's driving me nuts right now. I've taken the carbs out 5 times over the weekend and completely stripped/cleaned/reassermbled them 5 times. Each time I get a slightly different result!

My last result was great idle, flat spot from idle to takeoff, amazing midrange to top end, and a sooty black #1 plug .

I've also ordered a ton of new parts from Allens and I'm going to replace all the O-rings with brand new ones.

I'll try my Trophy carbs once more with their original (117.5) jets in them to see what result I get before I install 90s in them and use them as a backup set.

The slides do flutter quite badly, and from what I've read that's "normal" behaviour as the slides are constantly re-adjusting to the airflow, and the airflow isn't constant, it chuggs on each intake breath. This is what causes the incredible wear on slides, needles, emulsion tubes, and slide guides that we see in Mikuni CVs.

I wanted to eliminate this flutter by using the Trophy slides with the smaller ports, they don't respond as fast and should do the trick.

It's all very interesting stuff though .

Last edited by ArferBrick; 06-03-2013 at 06:05 AM.
ArferBrick is offline  
post #6 of 13 (permalink) Old 06-03-2013, 07:41 AM
SuperBike
Main Motorcycle: 1996 Thunderbird
Senior Member
 
BRG-BIRD's Avatar
 
Join Date: Nov 2007
Location: Somewhere in cheese land
Posts: 1,742
Other Motorcycle: 2009 Moto Guzzi Norge
Extra Motorcycle: 1975 Honda CB550F
Arfer I am positive the two large holes are standard...

"You are either onto something or on something"........The Common Man
“Sometimes you are best off dealing with the Devil you know” anonymous
BRG-BIRD is offline  
post #7 of 13 (permalink) Old 06-03-2013, 08:05 AM
Team Owner
Main Motorcycle: 1977 Triumph Bonneville
Senior Member
 
Join Date: Sep 2011
Location: Grass Lake , Michigan,USA
Posts: 5,281
Other Motorcycle: 1996 Ducati 900ss/cr
Extra Motorcycle: 2007 Ducati S4RS
Ducati ss uses Mikuni cv carbs. They suffer from premature wear of the emulsion tube, sometimes just a few thousand miles. FActory Pro makes a tuner kit that contains a nickel plated emulsion tube and titanium needle for much improved wear rate, plus jets and instructions for tuning. This kit smooths out engine operation from idle to 3000 rpm on the Ducati motor. They probably have one for your application too.
duc96cr is offline  
post #8 of 13 (permalink) Old 06-03-2013, 08:31 AM Thread Starter
Pole Position
Main Motorcycle: KTM 1290 Super Adventure
Senior Member
 
ArferBrick's Avatar
 
Join Date: Jun 2011
Location: Köln
Posts: 3,282
Other Motorcycle: '96 Adventurer
Garage
Quote:
Originally Posted by BRG-BIRD View Post
Arfer I am positive the two large holes are standard...
So we can safely say that we both have two big holes! lol

Quote:
Originally Posted by duc96cr View Post
Ducati ss uses Mikuni cv carbs. They suffer from premature wear of the emulsion tube, sometimes just a few thousand miles. FActory Pro makes a tuner kit that contains a nickel plated emulsion tube and titanium needle for much improved wear rate, plus jets and instructions for tuning. This kit smooths out engine operation from idle to 3000 rpm on the Ducati motor. They probably have one for your application too.
Yes they do, but they cost a fortune .
ArferBrick is offline  
post #9 of 13 (permalink) Old 06-03-2013, 09:08 AM
Site Supporter
SOTP Vintage Series
Main Motorcycle: 99 Legend Sprint GT
Lifetime Premium
 
IrlMike's Avatar
 
Join Date: Jan 2008
Location: Cork Ireland
Posts: 6,462
Ok, so two holes was stock on the classic Mikuni T3s. That then makes me wonder if it was the restricted rubbers affecting transient response (or really frequency response) which necessitated the reduced slide damping?

But there is an issue remaining which definitely rings bells imo. Those 117.5 main jets. There is no way they should be appropriate without a very leaky airbox (or one drilled for at least double the intake area).

But the main jet mainly matters at the top end only, with some lower interaction with the needle. Maybe what's been going on with those oversize mains is some serious suppression of needle travel by the Trophy (1 hole) slides to compensate. Go back to original slides & the great gob of fuel as the slide opens too fast completely bogs the motor.

I once raised the DJ needles in mine by one notch (out of five) to see what happened. Didn't get more than 10 metres or 1st gear. Opening the throttle by even an 1/8 or so resulted in a momentary shove (good!) & then it bogged completely & would go no further. Not really possible to even get 2nd gear & hold it. (Fortunately needles are accessed without carb removal)

In theory, at such low throttle (& revs) it should have made little difference - supposedly still on the straight part of the needle, not the taper. In practice however, it seems obvious from this that the transient response of slide (& needle) must be pretty important.

So that would make me very reluctant to mess with slide holes much - butt dyno tuning is hard enough as it is!

ps have you read thru' the 'Hinkley faqs' collated from numerous T3 triple tuners? (they are around the forum somewhere I think)


98 Tbird tank/seat, Sprint fairing, ZX7R forks+Racetech Gold Valves, 08 ZX6R shock, short sw/arm, Alcon 6-pots, Ignitech+TPS, T'bike 3-1+Beowulf can, airbox/jet mods, tubeless wheel mod, lots :) per mile, 55mpg ave(imp)

Last edited by IrlMike; 06-03-2013 at 09:10 AM.
IrlMike is offline  
post #10 of 13 (permalink) Old 06-03-2013, 12:06 PM Thread Starter
Pole Position
Main Motorcycle: KTM 1290 Super Adventure
Senior Member
 
ArferBrick's Avatar
 
Join Date: Jun 2011
Location: Köln
Posts: 3,282
Other Motorcycle: '96 Adventurer
Garage
Quote:
Ok, so two holes was stock on the classic Mikuni T3s. That then makes me wonder if it was the restricted rubbers affecting transient response (or really frequency response) which necessitated the reduced slide damping?
But surely that would mean that they would run worse when "unrestricted" rubbers were installed, and that's (from my experience) not the case.

Quote:
But there is an issue remaining which definitely rings bells imo. Those 117.5 main jets. There is no way they should be appropriate without a very leaky airbox (or one drilled for at least double the intake area).
You could be right on the money there Mike because I just had a look on worldoftriumph, they only show the 3yclinder model airbox and it has two inlets, maybe the 4cyl is the same? That might account for the bigger mains.

So theoretically if I made a 2nd hole in my standard airbox, the Trophy carbs might run perfectly .

....yeah I know but I can dream can't I?

Quote:
But the main jet mainly matters at the top end only, with some lower interaction with the needle. Maybe what's been going on with those oversize mains is some serious suppression of needle travel by the Trophy (1 hole) slides to compensate. Go back to original slides & the great gob of fuel as the slide opens too fast completely bogs the motor.
This is where the backwards thinking bit comes into play - bogging occurs not from too much fuel, but from too much air! This is the part that I didn't understand either. As the slide opens wide, it does not allow more fuel in - rather more air. Fuel can only be sucked out of the emulsion tubes if there is enough air velocity (speed), and as the slide rises the orifice gets bigger, creating lower air velocity and less fuel!!

The purpose of the slide is not to allow more air in, exactly the opposite, it's there to keep the opening as small as possible, in order to create the right velocity of air that can keep the supply of fuel going, and the air/fuel mix at the correct balance.

This is of course regulated also by the taper of the needle. It's all a matter of timing and I guess the needles are designed to start allowing more fuel to escape at a given slide height.

You've got an oxygen sensor permanently mounted haven't you? Try removing your airbox and see what it reads as the engine starts to bog, my bet is that it will read lean, not rich.

Quote:
I once raised the DJ needles in mine by one notch (out of five) to see what happened. Didn't get more than 10 metres or 1st gear. Opening the throttle by even an 1/8 or so resulted in a momentary shove (good!) & then it bogged completely & would go no further. Not really possible to even get 2nd gear & hold it. (Fortunately needles are accessed without carb removal)
Yeah that sounds like a "too much fuel" bog down, because if everything else was setup as it should be, then that would allow more fuel out of the emulsion tubes for a given slide height.

Quote:
In theory, at such low throttle (& revs) it should have made little difference - supposedly still on the straight part of the needle, not the taper. In practice however, it seems obvious from this that the transient response of slide (& needle) must be pretty important.
Yeah, the timing is critical.

Quote:
ps have you read thru' the 'Hinkley faqs' collated from numerous T3 triple tuners? (they are around the forum somewhere I think)
I've only seen the "proven setups" thread, I like to do things the hard way lol.
ArferBrick is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options
All posts must adhere to Forum Rules

Register Now



In order to be able to post messages on the Triumph Forum: Triumph Rat Motorcycle Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter your valid email address, that can receive an automated confirmation message. Otherwise, you won't be able to gain full access.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Similar Threads
Thread Thread Starter Forum Replies Last Post
Mikuni carb rebuild - taking out slide guide gamsa T3 Sport / Touring Forum 3 01-07-2011 10:18 PM
'95 T-bird Mikuni slide guides gliderace Hinckley Classic Triples 4 07-22-2010 06:53 PM
Mikuni low vacuum djw T3 Sport / Touring Forum 6 04-21-2009 02:54 PM
Mikuni Flat slide HOPOIL Classic, Vintage & Veteran 2 04-14-2009 02:16 PM
cv carb vacuum slide mod Gohm Maintenance & Workshop Talk 1 06-21-2005 10:29 AM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome