Going for stock 26mm to Mikuni 28mm (stock 85 yz80)

[deeeebeeeeee]

Picked up a 28mm Mikuni, not sure if it's a VM, that has been previously jetted for a ported blaster, I will be putting this carb in place of the stock carb.

Edit:
Well its not going to matter because I am going to be going from a dg pipe, to a toomey B1 at the same time.
If i remember correctly the toomey doesn't need as big of a jet as the dg, something about the toomey does a better job of pulling exhaust out and stuffing mixture back in at the right times?? Who knows I already have 290 to 320 covered I just need to get 330 to 360 or 370 covered as well the genuine ones cost me $4 at the local yamaha shop.
Edit


The stock carb has stock pilot and needle jets as well as stock needle as well as a 310 main jet, and is set safely rich across the board except at possibly half throttle where it seems a little less rich (have run around half throttle for the better part of 45minutes with no I'll effects, besides being a little warmer than usual)
Anyway stock 26mm on 310 main
Since a larger inlet area will reduce the velocity of the air, then my bet is it will need a bigger jet. But i don't know the differences in the the air track length of the carb, inlet to outlet taper etc. between the two csrbs.

Here is my thinking and explanation of such, if it is a gross miscalculation, or I got it all wrong please someone steer me right. Either through what you learned by making a similar change. Or by fixing my math to give a more accurate result.
Thanks in advance and I apologize if the following explanation seems a little different buy it is really how I have to think through things.
26^2 x3.14 is 2122 and for 28 is 2461 this is just the area at the opening to the reed Valve 2461/2122 is 1.159 or 16% larger Here is where I get shaky, because I know I am leaving out venturi effect calculations and such, but since the change is small I should be roughly able to get away with a linear figure for flow so the Jets I am using are rated by fuel flow rate.

So I am thinking 310 x 1.159 or about 360 might be a good starting point.
Since I know 310 on the 26mm is a tiny bit richer than needed even when playing it safe I might start at 350 and give it a go.
I am hoping the pilot and needle, used on the previous owners blaster are close enough to be set with the air screw and needle height, if not I have the stock parts from the 26mm that may be closer??

Anyhow if anyone can give me a idea what to expect,it would be greatly appreciated.
I have forgot a good part of my basic physics courses and I never took a dynamics class.
Nor have I ever made such a change on any engine 2 stroke single cyl or not.
I can guess all I want but would prefer to hear from some one who knows.

 

[joeak47]

I would stick with the factory Mikuni VM26 carb with the Toomey. Great combo and it fits properly. Porting the cylinder and re-chambering the head on this combo is pretty wild.

 

[pudvah]

Toomey calls for a 310 main jet. I plug chopped a 310 and could go 1 size larger if I wanted to be a lil fat but i'm perfectly comfortable where i'm at. I'm interested to see the difference in main jets this winter when it is 20 degrees compared to the 95 degrees it has been.

 

[jvb]

The 28 mm is not going to be a big difference, but all small things add up!
If i am not mistaken, the jet size shouldn't be to far apart, or maybe even a smaller jet!
(My vm 32 Mikuni had a 190 main jet installed, too small but didn't need to go as big as in the stock 26mm)
I switched the main jet from the 28 to the 32mm and it ran to rich
The 28 mm maybe has a better venturi?
Really got to try this yourself, and see how the Blaster reacts with jet changes.
Nice pipe by the way, and have fun fiddling with your Blaster

 

[Fast6hand]

I'm running a complete Toomey setup as well. I have the stock 26 on it and love it. Very easy to jet. Currently running a 340 main w/ a 32.5 pilot on the middles clip. Does very well. It is a little rich, and could probably go to a 330. I do have a rechambered head, CFM air box/unit filter, and VF4 reeds though.

Being Toomey is a mid/high pipe, going to the 28 is only going to favor the top. Although when I got the rechambered head, it really smoothed out the power, and gave me just enough down low grunt for my needs.

 

[deeeebeeeeee]

Ok so the 28m 85 yz80 carb arrived today and is even more temptng than i had imagined, on top of the standard idle screw and tors free top. It also has the quick change bowl. With all the temptation of the 28mm, besides an increased charge flow at high rpm, I am having tube convincing myself that it is going to be sooo large (16% more flow area) to have a an undesired effect, especially above 3 or 4k rpm, especially since it is said to have a more effective venturi.

It just one of those things I will have to try so I can see exactly how much and where the power band is affected. Its not really that big of a deal to re-Jet the stock 26mm carb for the Toomey pipe and ride it that way. At least until I get a throttle cable that fit's without the tors. I also need to check to see if the bowls will interchange, as well as the carb caps I kinda doubt they will.
When I get the throttle cable I will set the tuned ready to go 26mm to the side and pop on the 28mm and see how it does/re-jet it.
Worst case scenario I keep the 28mm and revisit it after a rechamber, then again when I do the top end and add the ported jug.

I hope being a mid -top end pipe that the toomey will support my eventual plans for a mildly ported cylinder, chambered head and 28mm carb along with dual stage reeds, and timing mod (which I might need tweak a little if I am going to be running more overall rpm.


I could well be wrong, as I am certainly not a scholar of the two stroke, but my cobbled together thoufhts on this, we're basically if ad 85 yz80 pulls enough air to run properly on a 28mm carb then the blaster which pulls more air across the board should do well.
The toomey being a bit more of a rpm pipe should happy with the improved intake flow.


Now that I have heard what others have experienced and know, tackling this two stroke has turned my world upsidedown. I got the 28mm for almost nothing. It had a standard idle screw and top cap as well as a quick change jet and was what I figured to be a more properly sized carb. A yz80 w/ about the same peak hp uses a 28mm, a yz 125 uses around a 36mm. Now I hear that i probably won't like what it does for/to my bike. It seems that it could give me a short peaky power band that is happier at running an rpm range that the stock porting won't quite support.

That and/or its expected that i may loose too much in the 2-4k rpm range and maybe even some in the 4-6k range to make it require even more shifting as well as uncomfortable in tighter situations requiring me to constantly slip the clutch to keep moving without jumping on the pipe and blowing the tires off at the worst time, only to be rewarded by running out of steam and being forced into the next gear just as its really coming on good.

This example is obviously an exaggeration, but is what I have run into more than once, where a guy drops a couple grand or more in a high power drag motor, +6 inch swingarm, gears it to do 90 on the pavement, then spend 90% of their time on old logging roads and technical woods trails. Then decidesmits too much and throws a fatty pipe on it. Lol

 

[jvb]

I use a Honda cr 125 throttle cable for my Blaster,but probably any 125 dirtbike throttle cable will work.
In my opinion the 28mm should work great with your set-up.

 

[Awk08]

you'll be fine with the 28 with a little added top end.
give it a shot, remember...... 1 full throttle run plug chop all jets and maybe even the needle setting before running it too long and frying it.

if you don't feel confident guessing a starting point, call the dude @ carb parts warehouse, he knows his shiz and gladly helps out, but be prepared to at least order a few pilot/main jets and maybe a different needle from him for his knowledge, they're similarly priced to the others.................

http://www.carbparts.com/mikuni/mikuni_dirt.htm



edit: check the one size bigger main listed for the 28, versus a 26 (stock numbers, not for our blasters, but does show an increase for the 28).................

http://www.carbparts.com/mikuni/vm__round_slide_standard_specifi.htm

 

[deeeebeeeeee]

OK so I talked to the seller of the 28mm carb, said he had it on a basically stock blaster, boyesen reeds, and the carb?.?
Didn't make much sense to me, but who knows.

Took it apart this morn, and everything seemed ok gaskets were soft, various small orifices were clear. I did notice that the seller has been running it with a screwed up float height , the line on the carb showing the bowl was running 1/3 of the way full. Sure enough the float was way....way out of spec. Simple enough to fix but it puts me back to square one except I know, that the carb will be set quite rich for a stock machine now that the float is fixed.

I was able to get a couple more jets today, and now have 280-350 plus two drilled jets, one almost the same as the 350 maybe a hair bigger, and the other falls somewhere in the sizes I have already.
I will probably drill it to 400 or so and throw it in the onboard toolkit in case I run into someone with an air leak on the trail....