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   Author  Topic: Carb Theory  (Read 5478 times)
dooitvintage
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Carb Theory
« on: January 30, 2013, 02:37:27 AM »
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In trying to determine whether to use a VM26 or 30 on an Elan dual carb setup I decided to do a little research on carbs and operation. This is probably more useful to us novices but a good read nevertheless. Part of the reason for this is I wanted to be able to use the 2 carbs I purchased in single operation on a twin if I didn't like the dual setup.
Ignore the 4 stroke and Rochester sections.
I was trying to work it through in my head why the jetting specs for a single carb setup couldn't be cut in half for a dual setup. That's not how it works. And why if a single Til,or Mikuni, could be used on a twin, why couldn't the 26's be used as a good single carb on a twin when it's air dimension and spigot dimension are actually larger than an HR Tillotson. In fact, the Mikuni used on the single Elans in the 80's was a 28, which has the same air/spigot size as the 26. Jetting then comes into play. The pages on carb/jetting specs from Vintagesnow show the dual specs for the Citation SS and you get an idea what the factory came up with in comparison to the Citation 4500. I'm posting this so those of us who have not done much with this may be a little less intimidated by it all.

  http://www.carbkitsource.com/carbs/tech/articles/Carburetion/Carburetion.html

http://vintagesnow.com/Ski-Doo_files/1980_Shop_Manual.pdf    (http://www.vintagesnow.com/Ski-Doo.html 1980 service manual pages 19-20)

http://www.motorcyclecarbs.com/About_Mikuni_VM_Carburetors_W143C146.cfm

fyi  http://www.sudco.com/flange.html
« Last Edit: January 30, 2013, 03:09:58 AM by dooitvintage » Report to moderator Logged
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Re:Carb Theory
« Reply #1 on: January 30, 2013, 03:40:15 AM »
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I ran into the same problem with my NOS 247 I have, VM28's are hard to come by so I got a VM 30 but like you say the spigots dimension are different between the 30 and 28, but you can find cab boots to work (contact goose), but you run into the problem that the studs for carb mounting on the singles are so close that the nuts will contact the cab boot, I found using long cap screws will not contact the carb boot, then there is the jetting problem, with the VM30 I got the jets same as the 28 and 30, have not tried it yet but have heard the stock VM30 works fine, will see once I put the motor in the Elan.
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dooitvintage
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Re:Carb Theory
« Reply #2 on: February 1, 2013, 02:05:32 PM »
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some good info...
http://www.jetsrus.com/a_jets_by_carburetor_type/needle_mikuni.html
http://www.jetsrus.com/carburetor/carb_mikuni_VM_series_throttle_valve.html
http://www.jetsrus.com/carburetor/carb_mikuni_VM_spigot_mount_carb_exploded_view_parts.html
http://www.jetsrus.com/needle_valve_float_bowl_gasket/needle_valve_and_float_bowl_gasket_mikuni.html
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dooitvintage
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Re:Carb Theory
« Reply #3 on: February 2, 2013, 06:36:49 AM »
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dooitvintage
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Re:Carb Theory
« Reply #4 on: February 8, 2013, 04:55:10 AM »
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Anyone ever figure out CFM requirements for their machine? A lot of what works has been the result of someone's trial and error.
I ran across this the other day on how to properly size a carburetor. The requirement is airflow capacity, measured in CFM, cubic feet per minute. Here's the formula:
1. multiply engine displacement in cubic inches, by the maximum engine speed, in revolutions per minute. Conversion 1 cc = .0610237441 cubic inches
http://www.asknumbers.com/CubicCentimeterToCubicInch.aspx
Max engine speed in this case, a 250 Elan twin, is approx 7800 rpm.
2. Divide by 3456(not sure where this comes from).
3. Then divide by .85, which is an engine's typical (yours may be less, or more) volumetric efficiency  (it's ability to use what you give it).
For a 250 (248,249) it translates to 40.5 CFM required at 7800 rpm. Still looking for a Mikuni chart listing CFM for their products and haven't looked yet for Tillotson. Keep in mind that this number is at max rpm and unless you're racing, you probably won't need all of that.

FYI, 47.64 CFM on a 294. I've decided to put the VM26's on my 294SS after I pulled the intake manifold off and found the plastic adapter that covers both ports was warped. I have the o-rings for the side facing the manifold but it was the engine side that couldn't possibly be sealing 100% with one gasket.  I'll post a pic later.
« Last Edit: February 8, 2013, 11:42:23 AM by dooitvintage » Report to moderator Logged
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Re:Carb Theory
« Reply #5 on: February 8, 2013, 07:29:29 AM »
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dooitvintage.....You got me on the edge of my seat and breathless waiting for you to come up with calculations to figure out a "new" carb set up. Actually I am turning a little blue from holding breath so I hope you hurry.
So if you can figure out calculatiuons to give about 12-15 grams of air to 1 gram of fuel at all throttle positions you`ll have it 
Obviously the OEM guys know how to calculate this so it can be done.
Bones
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Re:Carb Theory
« Reply #6 on: February 8, 2013, 09:36:07 AM »
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BONES CAN'T THIS INFO BE FOUND IN THE MIK VM SUPER TUNING MANUAL???
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dooitvintage
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Re:Carb Theory
« Reply #7 on: February 8, 2013, 11:35:39 AM »
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have you read the VM SuperTuning Manual?lol
Don't get me wrong, it's great information. They tend to generalize. Lots of graphs with abbreviations and variables. It's great if you have a carb that's been working on something and you want to fine tune it or your driving habits have changed or you're racing in different altitudes. But I don't know anywhere you can go and say.... I want to put dual Mikuni's on a 294SS. What size carb will do? What size is really too much? What jets should I start with?

I thought it'd be interesting to figure out exactly what my engine NEEDED for carburetion.  I don't know if I'll come up with anything useful. And what I want is an awesome daily driver type, runs great in all throttle ranges. I'm not interested in running different altitudes at WOT.

I don't know if I listed it above yet, but I found this to be a great read to help understand function:

http://www.carbkitsource.com/carbs/tech/articles/Carburetion/Carburetion.html
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charlie73
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Re:Carb Theory
« Reply #8 on: February 10, 2013, 07:57:35 AM »
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I used your formula for a 318 single and got 39.63 cfm at 6000 rpm.
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dooitvintage
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Re:Carb Theory
« Reply #9 on: February 12, 2013, 09:59:26 AM »
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I'm going to post some info on jet needles later. First, there are 2 types of needles. Single taper and double taper. I managed to spend a little over $100 on jets today to continue this experiment with the 26's. I suspect the most difficulty in putting together a great setup is the midrange, with way too many choices!
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dooitvintage
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Re:Carb Theory
« Reply #10 on: February 12, 2013, 10:05:32 AM »
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Quote from: charlie73 on February 10, 2013, 07:57:35 AM   

I used your formula for a 318 single and got 39.63 cfm at 6000 rpm.

Keep in mind the .85 efficiency rating is probably high for almost everyone. And your cfm number is figured at max rpm. I'm fine tuning it but the cfm ratings on Mikunis seem to differ approx 5 cfm per 1 mm in carb size difference.  Ex: 32 mm= 114, 34 mm=124, 35 = 129 etc.
Examples only at this point!
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Elannut
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Re:Carb Theory
« Reply #11 on: February 17, 2013, 08:24:19 AM »
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Awesome thread !!!!!!!!! I've been searching for this type of info. for a while !!!!
Keep up the great work, I have a pair of 26's to put on a Type 249.

Steve
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Elannut
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Re:Carb Theory
« Reply #12 on: February 17, 2013, 08:31:11 AM »
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BTW, where did you find the cfm ratings on the Mikunis ?
I have a VM 34 on a Type 248 & it doen't seem to have the "top speed" of my Tilly carbed 76' Deluxe twin, maybe too much carb or jetting ??

Steve
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Re:Carb Theory
« Reply #13 on: February 17, 2013, 09:05:46 AM »
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TRY TO FIND THE SUPER TUNERS GUIDE TO MIK'S. IT IS ABOUT 20-30 PAGES LONG.
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dooitvintage
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Re:Carb Theory
« Reply #14 on: February 17, 2013, 10:45:25 AM »
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here it is

http://vintagesnow.com/Carburetors_files/vmmanual.pdf

I'm not trying to rewrite the Mikuni manual. There's plenty of info NOT in there and what is there is basically for existing setups. It's important to read and try to understand what's in there. But it's not an answer to all.
The VM26-8074 carbs I bought had these
Main: 4/042  #190
Pilot: VM22/210  #60
Needle Jet: #182 N-8
Jet Needle: 5F21
Cutaway: VM26/56  2.0
Air: BS30/97  0.5
Float Height: 15-17MM

Example 1980 Elan 250 single
VM28-242
Main: #160
Pilot: #30
Needle Jet: 182 O-8
Jet Needle: 6DP1-3
Cutaway: 2.0
This set of specs was still in place in 1989.

Example 1979 Citation 300 (294)
VM30-104
Main: #220
Pilot: #30
Needle Jet: 159 P-2
Jet Needle: 6DH4-3
Cutaway: 2.0

Factory specs were taken directly from the service manuals on Vintagesnow.com. AWESOME resource.

Now, take a look here. Just for informational purposes, at the ESTIMATED CFM of a couple Mikuni carbs.
http://mossbargerracing.com/tuning.htm

« Last Edit: February 20, 2013, 11:08:16 AM by dooitvintage » Report to moderator Logged
dooitvintage
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Re:Carb Theory
« Reply #15 on: February 17, 2013, 10:55:00 AM »
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For a good idea on how the jet and needle numbering translates to lean/rich, check out Pat's. I'm not plugging anyone but they were good enough to provide the info for those looking to make changes.
http://www.psep.biz/store/mikuni_carburetor_part.htm

And a couple more useful links.
http://www.motorcyclecarbs.com/About_Mikuni_VM_Carburetors_W143C146.cfm

http://www.motorcyclecarbs.com/VM26-VM44_Spigot_Mount_Parts_W192C1420.cfm

http://www.bayshorecad.com/Sleds/Literature/Carbs/MikuniVM/Mik_Conversion_Specs.pdf
« Last Edit: February 17, 2013, 11:15:23 AM by dooitvintage » Report to moderator Logged
dooitvintage
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Re:Carb Theory
« Reply #16 on: February 17, 2013, 02:55:10 PM »
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So, based on what Mossbarger racing came up with for estimated CFM for popular Mikuni carbs, the VM26 could expect to provide approx 69 CFM per carb. IF the formula is as simple as 5 cfm difference per mm of carb.
« Last Edit: February 17, 2013, 11:43:41 PM by dooitvintage » Report to moderator Logged
dooitvintage
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Re:Carb Theory
« Reply #17 on: February 20, 2013, 11:05:57 AM »
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Here is the best info on jet needles I've found so far....

http://www.sudco.com/vol33/56-59.pdf

Understanding the function of the needle is important.  As it turns out, understanding the taper dimensions is equally important when choosing one for a specific application. Sudco has an excellent chart to choose a needle based on competition, racing and overall (overall is like saying little of everything, but essentially full range).
The smaller the diameter close to the top, or clip, will be richer, and a needle can start rich but another could be considered richer overall if it has more taper dimension points that are smaller diameter. I'm not sure how much of a difference we could notice in needle numbers in close sequence. The differences are sometimes in thousandths of mm's.
I understand the reason now for the double taper needle!


So, VM26-8074. First setup.
Pilot: 30
Needle Jet: 182N8
Main: thinking I'll start with the 190 in them and try it.
Leaning toward 5DP7.
Working on the rest.


Comparison:
6DP1: 62.3/28.9/2.511/2.511/2.476/2.312/1.748/1.075
6DH4: 62.3/25.5/2.52 /2.52  /2.44  / 2.258/1.915/1.575
Overall the 6DP1 is a richer needle.
I would LOVE to know how Bombardier chose exactly which needle to use!
« Last Edit: February 20, 2013, 11:25:46 AM by dooitvintage » Report to moderator Logged
dooitvintage
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Re:Carb Theory
« Reply #18 on: February 20, 2013, 11:56:10 AM »
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And to understand the clip position: moving the clip down, or effectively raising the needle, gets us to a smaller diameter on the needle quicker. Moving the other direction, or leaner, means while there will be more air with the raising of the throttle valve there will also be less fuel when it's needed. This is where many get into trouble and burn up a motor. Including my Dad many years ago trying to beat a friends 294SS with his 440!
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dooitvintage
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Re:Carb Theory
« Reply #19 on: February 20, 2013, 12:08:29 PM »
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And I'm thinking, could be incorrectly, that I can break down a factory carb setup using the needle dimensions and calculate the cfm requirements at various throttle positions(rpms) , and recalculate based on my engine's requirements to come up with the equivalent needle for my application. All the while maintaining 13-15:1?lol
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