2-to-1 carb

I also rode mx for many years. only time the throttle got stuck for me was after a heavy crash. But that was because the throttle grip was smashed to pieces and wouldn't move. :)
so +1 here as well.
 
while we are discussing carbs...I pulled mine apart to clean them...I had 135 mains in both carbs...and the pilots I couldn't tell ...what are the jets in the air inlet near the filter flange for..they were marked 155?
I'm going with pods and hopefully a 2-1 exhaust....at 300 feet above sea level I'm seeing alot of varying opinions on jetting.
So...the million dollar question is ...whats a baseline setting for the rebuild.
main jet size?
pilot?
air jet?
how many turns on the screws?
I know its been asked before...but humor me if you would...I'd like to get the necessary stuff when I buy the rebuild gaskets etc,..and not get hung up by waiting for little tiny things that will hold the build up longer...time is a hot commodity nowadays.
I have a buddy thats good with tumning..but I need to get it in the right ballpark so we're not wasting time.
 
I started building a single carb intake for my xs400 g then abandoned the project after reading a post about it working for a 180 degree crank (xs650) and not a 360 degree crank (xs400) because the intakes are partially open at the same time. Has anyone successfully ran a single carb on an xs400?
 
Any two stroke carb that comes from around a 200cc engine should be fine. The two strokes need more CFM than the four strokes. Like mentioned before, I ran a 1991 CR250 two stroke carb on my old KZ440, and it ran like a dream. I would keep around a 34mm-35mm carb though.....definitely nothing bigger. I started with a 36mm Keihin the first time around on my KZ and it was way too big. I switched to the 34mm Keihin and never looked back.

I am not a scientist but I do not understand this statement that a two stroke needs more CFM than a four stroke. This is completely new to me and I have done a little drag racing over the last 25 years and I just want to learn more and understand this. Is it because there may be better volumetric efficiency with a two stroke? I mean most factory applications, operate at about 80 percent of volumetric efficiency to achieve the best all round power, performance and reliability. Now a full tilt high performance engine can operate at a much higher volumetric efficiency, and when supercharged or turbocharged can even run over 115 percent volumetric efficiency. But here is where I am, lets say we have a 400 cc 2 stroke and a 400 cc 4 stroke. Booth running at 7500 RPM. Why does, or how come, the 2 stroke need more CFM? How actually can the 2 stroke have more CFM when both are equal in cc and RPM. I am not trying to be a smart ass or know it all, just trying to learn and I have just never heard this before and I want to know why is all. And this just trips me out that a carb from a 200 cc would work or be proper for a engine with twice the cc. Does any of this make scene?
This is the math I have always used...Cubic In. Displacement/2 X Maximum RPM/1728 = Maximum CFM
 
I started building a single carb intake for my xs400 g then abandoned the project after reading a post about it working for a 180 degree crank (xs650) and not a 360 degree crank (xs400) because the intakes are partially open at the same time. Has anyone successfully ran a single carb on an xs400?

The xs650 is a 360 degree crank and the xs400 is a 180 degree crank...
 
Any two stroke carb that comes from around a 200cc engine should be fine. The two strokes need more CFM than the four strokes. Like mentioned before, I ran a 1991 CR250 two stroke carb on my old KZ440, and it ran like a dream. I would keep around a 34mm-35mm carb though.....definitely nothing bigger. I started with a 36mm Keihin the first time around on my KZ and it was way too big. I switched to the 34mm Keihin and never looked back.

I am not a scientist but I do not understand this statement that a two stroke needs more CFM than a four stroke. This is completely new to me and I have done a little drag racing over the last 25 years and I just want to learn more and understand this. Is it because there may be better volumetric efficiency with a two stroke? I mean most factory applications, operate at about 80 percent of volumetric efficiency to achieve the best all round power, performance and reliability. Now a full tilt high performance engine can operate at a much higher volumetric efficiency, and when supercharged or turbocharged can even run over 115 percent volumetric efficiency. But here is where I am, lets say we have a 400 cc 2 stroke and a 400 cc 4 stroke. Booth running at 7500 RPM. Why does, or how come, the 2 stroke need more CFM? How actually can the 2 stroke have more CFM when both are equal in cc and RPM. I am not trying to be a smart ass or know it all, just trying to learn and I have just never heard this before and I want to know why is all. And this just trips me out that a carb from a 200 cc would work or be proper for a engine with twice the cc. Does any of this make scene?
This is the math I have always used...Cubic In. Displacement/2 X Maximum RPM/1728 = Maximum CFM

2 strokes have a power stroke twice that of a 4 stroke, so I guess, by default it doubles its volumetric efficiency.
the crank area is pressurised by the downward piston, forcing the mix into the top via ports in the side of the barrel.
 
You mean every other stroke is taking in fuel as opposed to only 1 out of 4 strokes, I mean that kind of makes sense. Would the CFM formula be different for a two stroke? I am only familiar with the 4 stroke formula. Thank you for explaining some of this. I find it very interesting.
 
Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost. Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.

Here is a simple, to-the-point, comparison between 2 and 4 stroke engines: http://www.deepscience.com/articles/engines.html

Formulas: http://www.nightrider.com/biketech/calc_formulas_page.htm

I think the CFM formula might be doubled. It is hard to search for in google because of the terms being used.
 
OK I am starting to understand this a little better thank you. Never really messed with the 2 strokes except for chain saws and never hot rodded one out.
 
The xs650 is a 360 degree crank and the xs400 is a 180 degree crank...

Right right, i had it backwards. So you can put a single carb intake on a 180 degree crank. Im going to try to finish it and give it a try with a vm34.:thumbsup:
 
A single carb will work on both...With the 180 degree crank, I suppose theoretically the second cylinder to fire, could be leaner than the first cylinder...
 
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Mine on my DOHC 400. Made by choppersprocket with my input...
manivac001.jpg

Interesting Thread

DCopp, I would like to see a little more info on what you did on your 2to1 manifold. Looks like you made it out of aluminum. Were those pre-made flanges you used? and what diameter is that??

Anyone else have any success? maybe w/stock carb?
 
Well, after a rebuild of my intake to something more permanent, and frigging around with a few carbs, ended up with the stock carb with a larger main jet and adjustment of the needle position.

I tried a VS34 a guy gave me off a snomobile, and even after changing jets, pilots, needles, it ran way too rich. I am sure I could have figured it out with enough time and an endless supply of jets, but anything out of the ordinary is tough to come by in my area.

So, back to stock, and with the jetting, everything seems to work pretty well.

I changed the intake so it has less of an angle for the left cylinder by installing a gusset. I also installed some rings that sit inside the lip of the intake boots.

The air filter is familiar to some, just an assembly designed for a 1 or 2 bbl carb. I had to build a flange for it that bolted to the carb.

The choke is a headlight knob off a Ford for the time being. I didn't want to cut the original down.

I still need to paint everything and clean up the carb.

I'll post a video sometime.

What did you end up running for your main jet size on this single carb setup with the stock carb?
 
I just road tested my 81 xs400 with a single carb and to my disappointment it runs way to lean on the left side and to rich on the right, since there's only one mixture screw i don't know how I'm suppost to fix this. Anybody have any ideas. Also like someone else may of asked already has anyone actually successufully done a single carb on a xs400?
 
I just road tested my 81 xs400 with a single carb and to my disappointment it runs way to lean on the left side and to rich on the right, since there's only one mixture screw i don't know how I'm suppost to fix this. Anybody have any ideas. Also like someone else may of asked already has anyone actually successufully done a single carb on a xs400?

Sounds like you have an air/vacuum leak on the the lean cylinder side. One carb will provide an equal fuel mixture to both cylinders. Check for the leak by spraying carb cleaner, WS40 around that side of the intake.
 
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