User Tag List

Results 1 to 12 of 12

Thread: Dual carb conversion?

  1. #1
    Join Date
    Oct 2018
    Location
    Wellington
    Posts
    5
    New Zealand
    NI
    Post Thanks / Like
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Default Dual carb conversion?

    Hi all,

    I've searched and read various single carb conversion threads on the forum, however can't seem to find any that discuss a dual carb conversion.

    I'm fairly new to engine performance modifications, but well versed in general custom work, so please forgive me if this is the dumbest question ever for some reason

    Basically what I'm looking to do is put one single carb on either side of the bike (so two total) to free up the fake tank/airbox space, and use some aftermarket/custom air intake for similar or better performance overall.

    Currently I'm looking at some off-brand S&S Super E Shorty carbs for this, which are 47mm I believe - would that be too big?

    My bike is a '98 Royal Star Tour Deluxe, XVZ1300/XVZ13ATK

    Any thoughts/suggestions much appreciated,

    cheers
    Last edited by VV4l7; 01-11-2020 at 05:23 PM.

  2. #2
    Join Date
    Jul 2013
    Location
    Calgary
    Posts
    1,142
    Canada
    AB
    Post Thanks / Like
    Mentioned
    7 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    How do you plan to use or ride the bike? Short or long haul?

  3. #3
    Join Date
    Oct 2018
    Location
    Wellington
    Posts
    5
    New Zealand
    NI
    Post Thanks / Like
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    Both, as I use the bike in town and for short rides around (not a commuter though), as well as for longer missions like a recently completed 5000km tour of the South Island here in NZ...

  4. #4
    Join Date
    Jul 2013
    Location
    Calgary
    Posts
    1,142
    Canada
    AB
    Post Thanks / Like
    Mentioned
    7 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    Quote Originally Posted by VV4l7 View Post
    Hi all,

    I've searched and read various single carb conversion threads on the forum, however can't seem to find any that discuss a dual carb conversion.

    I'm fairly new to engine performance modifications, but well versed in general custom work, so please forgive me if this is the dumbest question ever for some reason

    Basically what I'm looking to do is put one single carb on either side of the bike (so two total) to free up the fake tank/airbox space, and use some aftermarket/custom air intake for similar or better performance overall.

    Currently I'm looking at some off-brand S&S Super E Shorty carbs for this, which are 47mm I believe - would that be too big?

    My bike is a '98 Royal Star Tour Deluxe, XVZ1300/XVZ13ATK

    Any thoughts/suggestions much appreciated,

    cheers

    The setup above is not a "both" type of setup.

    I would however like to know what the specs are on the "custom intake"?

    Now if you read thru our posts that cover these conversions then you have read that you can not overlook the physics of volumes and velocity.

    The 1st problem I foresee is the flow stall after the T.plate: you must take into consideration port design!

    You are at the moment viewing the engine as a set of twins; but it is not the case: think about it and break it down into share components!

    I can go deeper into this if you want, but for this post let me ask of you this: How will you know if both inputs are equal thru the complete range? At the moment I can only see a dyno tune for measuring total achievable output!?

    And I'm still holding back on the obvious....
    However I like the nature of the thought, likely I'll chew on it a bit more
    Last edited by Patch; 01-11-2020 at 06:37 PM.

  5. #5
    Join Date
    Jul 2013
    Location
    Calgary
    Posts
    1,142
    Canada
    AB
    Post Thanks / Like
    Mentioned
    7 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    Have you read thru this thread by Du-Ron? There is quit a bit of theory around intake sequencing and effects.

    Note: I just reread that post and correct it (I hope) Man I can't wait for Flyingfool to finish the escape dish project in the basement Tho I'm not convinced we can launch from down there but, Jeff probably has a secret he hasn't share yet; like maybe the house is hinged? Then he'll build my new intergalactic translator! Right Jeff, you did reserve a seat for me right?

    https://www.venturerider.org/forum/s...37#post1073237

  6. #6
    Join Date
    Mar 2007
    Location
    West Coast
    Posts
    18,772
    United States
    MI
    Post Thanks / Like
    Mentioned
    629 Post(s)
    Tagged
    0 Thread(s)

    Talking Re: Dual carb conversion?

    I did a fair amount of goofing around with carb conversions in a different part of life back in my bike/Chopper building days and found that following the total cubic centimeter parameters of volume being moved compared to other bikes/cars of those parameters to yield the best results.. I know I am very difficult to follow sometimes so to simplify: your scoot is 1300cc total for 4 pots (cylinders) currently designed by the experts to function on 4 individual carbs (being the non balanced shaft, lower redlined detuned/tuned for torque mid 90's RSTD I am guessing your stock carb size is 26 or 28mm?).. Divide by 2 on the CC's = 650cc per carb giving you 1300cc of volume passing thru 2 carbs instead of four = right?.. That agreed upon, and assuming you are planning on passing a 2 into 1 intake out each side so you end up using a basic side draft as they are wayyyy more plentiful to play with (which I would do too = stock, our V-4's use downdrafts).. Going from memory here, I believe the 650 carbed scoots of yesteryear ran best on 40mm total and/r maybe even under that amount. From my backyard/untrained experience, 47mm would be a tad on the high side (that carb you mention is probably/possibly associated with a HD EVO Sportster or Big Twin?). Because I HATE flat spots and hesitations when nailing the throttle and/or having to spend a ton of time trying to tune out such awkwardness = if I were doing the job I would do some investigation and find out by comparing bikes of the 650cc/single carbed versions and pirate that info from the experts who designed those bikes.
    Another consideration is: are you thinking of staying with CV Carburation? If I were doing it I would try to steer clear of the CV carbs (as on our OEM bikes) as the slide diaphrams are always another issue in maintenance. I would go name brand like Mikuni so a vast array of jetting would be readily available (I dont like painting myself in a corner by having to depend on only a couple sources for parts). A simple cable pull slide, horizontal mount (like found on 95 % of the scoots out there), press down choke lever 38 or 40mm Mik is where I would be looking. I also like Spigot mount instead of flange as they are much easier to, leak free, mount/adapt. Are you thinking Pods for breathers or you gonna run Velocity stacks? If I were gonna run stacks I would probably opt for one size down on carb size from whatever parameters proofed valid from my investigation..

    Any of that make any sense at all?

    Also, all of the above is null and void (again, IMHO) if we are not talking a stock valve timing/cammed scoot.. Our scoots are OEM no valve overlap so no scavenging at high R's is taking place.. Drop a cam grind in that shows valve overlap in its profile and we are talking a whole different animal = IMHO of course..
    Gone long winded,, sorry bout that... Still though, did I earn some brownie points to be applied to some pictures of your project??
    Remembering my brother and friend:
    "I'm not Yammer Dan" - Yammer Dan
    Proverbs 18:24 - "There is a friend that sticketh closer than a brother"..
    Miss you bro - Puc
    Life is just one step after another..on a rocky hill side-Sly

  7. Likes videoarizona liked this post
  8. #7
    Join Date
    Oct 2018
    Location
    Wellington
    Posts
    5
    New Zealand
    NI
    Post Thanks / Like
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    hey thanks Puc & Patch for the prompt replies, very awesome getting two experts like yourselves to respond to this so quickly - much appreciated!
    Patch I am assuming I will be designing and 3D-printing/casting an aluminium custom intake from scratch, which obviously would be tailored according to expert advice and common sense while assuming I'll not likely 'get it right' the first time round (might print some plastic prototypes first).
    With regard to your "not a set of twins" comment; are you suggesting the manifold could/should be a 2-into-4 type? I'm guessing this would also relate to Puc's comments on CV vs slider carb, as the CV types are more fussy when it comes to laminar airflow and pressure?

    Thanks for the link to that other thread as well, am learning a lot from reading and re-reading lol
    If I'm getting several of your posts there and in other threads correctly the gist of what your saying is basically to get everything else right first, valve clearances, compression test, carb sync, stock jets etc etc., as anything beyond that will only amplify already existing problems... - hope that makes sense?

    Also you completely lost me on the Flyingfool escape dish project comment haha

    @Puc many thanks for the notes on CV carbs; I was looking at 40mm Mikuni's earlier as they are indeed sold as specced for 300-400cc engines, but wasn't sure if that size is still adequate when splitting the output into a 650cc V2? I'm guessing due to the timing actually everything stays the same but the carb throughput is simply doubled?

    In terms of breathers I think I would wanna stick to the more 'sensible' option for this setup, if there is any , stacks with a little screen or filter would be cool, but pods could be nice too.
    Just FYI I'm not really looking to get some massive performance gain out of this, just figuring out if this is possible at all while maintaining current performance so I can work on more mods up and around the tank area later on.

    Also will likely be a winter project for me, so starting June/July I presume (NZ winter...)
    If it does go ahead the both of you have already made it into full credits and acknowledgement list indeed

    cheers

  9. Thanks cowpuc gave thanks for this post
    Likes cowpuc liked this post
  10. #8
    Join Date
    Jul 2013
    Location
    Calgary
    Posts
    1,142
    Canada
    AB
    Post Thanks / Like
    Mentioned
    7 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    Logged on to answer your post but like magic it has disappeared.

    Puc shared the obvious problem.

    This engine won't take advantage of this purposed setup, the rpms will still be low. But even if it could I would express doubts around velocity.

    The idea of larger carbs is about volume but volume without velocity will not perform well, especially at low rpm for that you'd likely be better off with CV carbs!

    The ideal for what you mentioned as reasons would port injection. The next would be the shorties you mentioned and what I call direct slide.
    Personally I wouldn't go higher than 32mm. Tho I know some have gone to 36mm.

    Now if you went with a 4 pack linkage would be a challenge but doable.

    Then I would likely shape a radios to form the intake tubing and at a right angle to pods or a basket filter that would take 2 jugs like the old sportsters I think: you'll need to draw air from outside the V.

    Now lets take a quick look at why the majority of conversion aren't working well : It is really quite simple, they can't handle the volume at higher rpms.

    Always bare in mind that vacuum pressure fluctuates due to cam lobe lift and duration as well at throttle plate angles; what doesn't change is the port geometry, it is the same throughout weather its been modified or not. The ports have velocity tricks designed into them which takes some understanding of fluid dynamics to understand what is going on and why.
    If as in the cases I have seen you over accelerate the air at the top of the carb then, not only do you choke the flow but you end up with to much bounce pressures or pulse waves which have no place to go but back up; that happens at the end of every lobe duration "the slamming of the intake valve"!
    If your carb bore is to large then your volume potential goes up but velocity drops off!

    So we see and hear the experiments start well but as they approach 1/4 throttle the rpms first seem to tighten up then begins to slow the climb, that can be referred to as resistance or drag; where does that come from? Well the answer is in the other thread I linked to

    Food for thought
    Last edited by Patch; 01-12-2020 at 06:26 AM.

  11. #9
    Join Date
    Oct 2018
    Location
    Wellington
    Posts
    5
    New Zealand
    NI
    Post Thanks / Like
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    Thanks both!

    Apologies for this brief reply but I wrote a longer one earlier that seems to be held in some void for moderator approval?

    Possibly some unlawful use of the '@' sign?

  12. Thanks cowpuc gave thanks for this post
    Likes cowpuc liked this post
  13. #10
    Join Date
    Oct 2018
    Location
    Wellington
    Posts
    5
    New Zealand
    NI
    Post Thanks / Like
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    Quote Originally Posted by VV4l7 View Post
    Thanks both!

    Apologies for this brief reply but I wrote a longer one earlier that seems to be held in some void for moderator approval?

    Possibly some unlawful use of the '@' sign?
    UPDATE: earlier post released now, please see above

    cheers

  14. #11
    Join Date
    Mar 2007
    Location
    West Coast
    Posts
    18,772
    United States
    MI
    Post Thanks / Like
    Mentioned
    629 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    Just did a quick check for you VV,,, the early Triumph Tigers were twin cylinders = 650cc of volum pulling fuel/air mix thru a single 30mm carb and developing mid 40's hp off that arrangement. Stock, they had no valve overlap and, as such, the cams and air flow seemed to be ground more for torque (low R's) similar to your scoot. That being said and thinking of what I would do if in your shoes,,, I think I would search Ebay for new aftermarket 32mm carbs (x2 = 1 for each side). I just did that and noticed that 20 dollars per carb (Chinese nock-offs) is not unthinkable. Before buying them though I would double check to see jetting availability for what ever I bought - dont paint yourself in a corner by not being able to get jet sizes needed. I know that a lot of those Chinese carbs are exact copies of OEM stuff like Mik, Kien or PKW and the jets of such are direct replacements for OEM.. I also know that you can get Spigot manifold to carb adaptors very cheap off ebay (couple bucks each).
    For cabling down to the carbs. You will probably need a splitter. Honda used a cable splitter on the twins = CB/CL/SL 175 -350's - 450's and so on - I am positive other brands did too = it was common back in the old days. I would probably pick up a similar cable arrangement off ebay to cover that. Just make sure what ever you find has length to cover throttle to slide. I made my own cables for a while but found that when needed and/or in a bind = bicycle cables are workable for the lite pull of throttle and you can get them in all kinds of lengths.. If you had a Honda 175 throttle cable assembly and it was to short = open the splitter and replace the splitter to throttle cable to add length with the bicycle cable = works great.
    Dont forget to add a port into each intake manifold for syncing carbs. Drill and tap (or just smudge em in) then screw em in, slip rubber plugs over the nipples when not using for syncing.
    My son in law in California thought my grandsons go-cart was having carb issues (it wasnt, it turned out to be a valve had gone tight) so he bought a 100% brand new complete replacement carb for it for about 15 dollars (Chinese stuff) and THAT was for a complete CV CARB!! After fixing the valve issue and getting it fired back up, I slid the new carb on it - worked PERFECT!! Something I was impressed with on that carb was that the metering rod was actually cut for the "E" clip final tune adjustment which is wayyyy cool cause this makes a carb tuneable to perfection.
    Here is video of me playing with that puppy that shows the carb at about 10 minutes in if you want a pic..


    You said thank you,,, you are more than welcome,, glad to help! Best of luck on your exciting project! Gonna be watching for the pics/video brother!!
    Puc
    Remembering my brother and friend:
    "I'm not Yammer Dan" - Yammer Dan
    Proverbs 18:24 - "There is a friend that sticketh closer than a brother"..
    Miss you bro - Puc
    Life is just one step after another..on a rocky hill side-Sly

  15. Thanks videoarizona gave thanks for this post
    Likes Patch liked this post
  16. #12
    Join Date
    Jul 2013
    Location
    Calgary
    Posts
    1,142
    Canada
    AB
    Post Thanks / Like
    Mentioned
    7 Post(s)
    Tagged
    0 Thread(s)

    Default Re: Dual carb conversion?

    Quote Originally Posted by VV4l7 View Post
    Thanks both!

    Apologies for this brief reply but I wrote a longer one earlier that seems to be held in some void for moderator approval?

    Possibly some unlawful use of the '@' sign?

    Could be or Dad & Uncle Carl were napping?

    Most people just get to the point. Puc and I ramble lots; have our reasons tho, its about getting past the nonstick surfaces!
    Puc tho being the forum fav gets away with it, me not so much. I don't take it personal tho after all Puc is several miles older than me

    If you think there's a problem with a post then reach out to freebird or marcarl, or pm Puc or myself and we'll ramble for you!

    enjoyed your thread, you did well

Similar Threads

  1. Replies: 0
    Last Post: 01-05-2020, 10:20 PM
  2. 83 venture single carb conversion
    By Venturezach in forum General Tech Talk
    Replies: 29
    Last Post: 07-24-2019, 05:28 PM
  3. single or dual
    By TheCoffsKid in forum GPS, Audio, Electronics
    Replies: 3
    Last Post: 11-08-2013, 05:21 PM
  4. for those who have done 28mm to 32mm carb conversion..
    By DarrinGT in forum Royal Star and Royal Star Tour Deluxe Tech Talk
    Replies: 2
    Last Post: 10-23-2013, 11:38 AM
  5. dual purpose
    By eagleeye in forum Watering Hole
    Replies: 9
    Last Post: 03-29-2008, 11:21 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •