I have a potentially stupid question.....

[Scootertrash]

In all of my years of wrenching, I've never pondered this, I've always used the carb that was on the machine or a supplier designated replacement.

What, if any, differences are there between a 2 stroke and a 4 stroke carb? Can you take a carb off of a 2 stroke and put it on a 4 stroke? Or a 4 stroke on a 2 stroke?

:wondering

[dougspcs]

Fundamentally, yes they can be swapped back and forth..

However, how well it may run from one application to another will vary.

Since the carbs are of course jetted for a specific engine and since the displacement and working RPM of the engine is designed into the jetting design you may find the power curve is difficult to get right.

It's not a stupid question..it's actually quite a good question.

[Highhorse78]

I think the main differences would be in the 2 stroke carbs designed for oil injection. Theses carbs sometimes operate at a slightly higher velocity to help pull the oil in. I've never tried it, but I speculate tuning could be a nightmare .

[Mr. Clean]

1985 ATC250SX with 83-84 250r carb. Ran GREAT, NO issues with tuning. Had supertrapp pipe as well. Guy in FL owns it now.

http://i280.photobucket.com/albums/kk179/miked1957_photo/DSC_4300.jpg

[El Camexican]

I don’t know if it matters, but it does seem that all the 2 stroke carbs I have ever worked on had air screw adjustments (pre-main jet orifice) and ALMOST all the 4 stroke carbs had fuel screw adjustments (post-main jet orifice) I recall only one carb on a 4 stroke that had an air screw. Lectrons interchange so maybe its not that important.

[Dirtcrasher]

All I can say is the 2 stroke carb lacking a butterfly choke versus a 2 stroke enriching circuit.

I don't know about the "higher velocity deal" the pump in the motor regulates oil/gas mixture.

[jb2wheels]

Interesting about the chokes, the last few street bikes I worked on had enrichers not true chokes. All Suzuki street bikes - SV650, GSXR750, 600 Katana & I think even the GS1000 but that one's a little fuzzy now. But I've worked on older bikes with butterflies. All four strokes.

[Scootertrash]

Thanks guys!

That's kind of what I thought, I just never really dug into it. Nice info to have.

[RIDE-RED 250r]

Ya know, I have a little something I have pondered for quite a while but never really figured out as well, and it's along these same lines...


Why is it that the carbs run on bigger bore thumpers like 350x, 400ex, ds650, etc have those dual valve carbs?? Dual valve meaning a slide and a butterfly?? An no, the buterfly isn't just for choke. They are synchronized with the slide?

And along that line, is there any reason engine function-wise, not fit and cable routing-wise that say a 250r carb can't work on say a 350x??

This is something that has had me scratching my head for quite some time now...

Oh and Scooter I agree, not a stupid question at all! Very informative.... :beer

[RIDE-RED 250r]

I think the main differences would be in the 2 stroke carbs designed for oil injection. Theses carbs sometimes operate at a slightly higher velocity to help pull the oil in. I've never tried it, but I speculate tuning could be a nightmare .

I don't think thats it at all...

Many sledders eliminate oil injection on sleds all the time in favor of premix. Just have to jet accordingly and properly delete the oil injection system. Has nothing whatsoever to do with the carbs as pretty much all oil injection systems are piped directly into the crankcases. And most older systems the oil injection lines were run into the reedcage between the carbs and the crankcase...

I have never seen an oil injection system on a sled that injects oil before the carbs...

[Highhorse78]

I don't think thats it at all...

Many sledders eliminate oil injection on sleds all the time in favor of premix. Just have to jet accordingly and properly delete the oil injection system. Has nothing whatsoever to do with the carbs as pretty much all oil injection systems are piped directly into the crankcases. And most older systems the oil injection lines were run into the reedcage between the carbs and the crankcase...

I have never seen an oil injection system on a sled that injects oil before the carbs...

I was refering to the carbs with the oil inlet entering the venturi after the slide. These can be found on many early Yamaha 2 stroke enduros ect. The shape leading into the bore of the venturi aides in the velocity required to help the oil mix with the incoming fuel and air mixture. So for example, taking one of these carbs and doing major tuning/jetting with a different motor could lead to problems.

[deathman53]

there is 2 strokes that inject oil into the carb. 88?-07 yamaha blasters do that. What happens is alot of the time, when somebody messed with the carb, they forget to put the oil line back on or the oil pump fails-plastic gear is being turned by metal gear and what will fail first????

[RIDE-RED 250r]

I was refering to the carbs with the oil inlet entering the venturi after the slide. These can be found on many early Yamaha 2 stroke enduros ect. The shape leading into the bore of the venturi aides in the velocity required to help the oil mix with the incoming fuel and air mixture. So for example, taking one of these carbs and doing major tuning/jetting with a different motor could lead to problems.

Gotcha.... :beer

Guess the sleds do things a bit differently than bikes. Thanks for the info, and I stand corrected. :)

[jdm97ek4]

Scooter yes you canrun 2stroke carbs non oil inection on 4 stroke motors I do it here and there

When I getback friday I will chec that manifold for you I.firgot all about it until just now

Im going to run a yz 80 flatslide carb on my zuma 30mm but I thought about putting it.on the 200x

[Highhorse78]

Ya know, I have a little something I have pondered for quite a while but never really figured out as well, and it's along these same lines...


Why is it that the carbs run on bigger bore thumpers like 350x, 400ex, ds650, etc have those dual valve carbs?? Dual valve meaning a slide and a butterfly?? An no, the buterfly isn't just for choke. They are synchronized with the slide?

And along that line, is there any reason engine function-wise, not fit and cable routing-wise that say a 250r carb can't work on say a 350x??

This is something that has had me scratching my head for quite some time now...

Oh and Scooter I agree, not a stupid question at all! Very informative.... :beer

Those carbs you are referring to are called "constant velocity" or "CV" carbs. Heres a pretty good animation of one in action- http://www.youtube.com/watch?v=wyspAHrMbb8

[El Camexican]

Ya know, I have a little something I have pondered for quite a while but never really figured out as well, and it's along these same lines...


Why is it that the carbs run on bigger bore thumpers like 350x, 400ex, ds650, etc have those dual valve carbs?? Dual valve meaning a slide and a butterfly?? An no, the buterfly isn't just for choke. They are synchronized with the slide?

Hey Buddy! The simplest way I can think to say it is that a diaphragm slide on a CV carb smooth’s out the acceleration as well as the deceleration on an engine, especially on a 4 stroke that has more reciprocating weight than a 2 stroke. It also increases mileage dramatically. I’ve ridden my Suzuki GS1100 with the stock CV’s as well as with 36mm RS flat slides and I can tell you the flat slides are violent and use at least 25% more fuel between point A & B. Some later GSXR models came with flat slides, but they also had much lighter internals and were hard to jet. BTW it is VERY difficult to get a CV carb to work without an air box. It can be done, but it’s difficult and never seems to work as well as the stock set-up does. As far as I know there are no CV carbs that have linkage between the butterfly and the slide. The butterfly is connected to the throttle cable and can be opened and closed as you wish. The slide opens and with vacuum from the engine. Because the needle jet is moved by the slide the power comes in and goes out very smoothly.

[shovelryder]

Ive run Harley CVs on everything from air cooled VWs to a yamaha banshee......Got one on my old shovelhead now......best carb ever made in my opinion.

[LastFoolerInVA]

The main difference is the fuel "circuitry" in the carb..its bigger in a 2 stroke carb... which would make a 4 stroke run rich... but you could always compensate with lean jetting..you can make it work.

[RIDE-RED 250r]

Hey Buddy! The simplest way I can think to say it is that a diaphragm slide on a CV carb smooth’s out the acceleration as well as the deceleration on an engine, especially on a 4 stroke that has more reciprocating weight than a 2 stroke. It also increases mileage dramatically. I’ve ridden my Suzuki GS1100 with the stock CV’s as well as with 36mm RS flat slides and I can tell you the flat slides are violent and use at least 25% more fuel between point A & B. Some later GSXR models came with flat slides, but they also had much lighter internals and were hard to jet. BTW it is VERY difficult to get a CV carb to work without an air box. It can be done, but it’s difficult and never seems to work as well as the stock set-up does. As far as I know there are no CV carbs that have linkage between the butterfly and the slide. The butterfly is connected to the throttle cable and can be opened and closed as you wish. The slide opens and with vacuum from the engine. Because the needle jet is moved by the slide the power comes in and goes out very smoothly.

I see... So the 350x carbs and others like it pretty much are accomplishing the same thing, only instead of a vacuum actuated slide the throttle slide is mechanically actuated??

Thanks for clearing that up for me.. I have wondered about that for quite some time. :beer

[El Camexican]

I see... So the 350x carbs and others like it pretty much are accomplishing the same thing, only instead of a vacuum actuated slide the throttle slide is mechanically actuated??

Thanks for clearing that up for me.. I have wondered about that for quite some time. :beer

I guess so. I looked at a parts fiche of a 350X carb (I am not a Honda guy) and I don’t see a butterfly a diaphragm, or a spring it appears to just be a round slide mechanical carburetor (albeit a complicated looking contraption). I've attached a Harley CV carb image for reference The "CV" stands for constant velocity.
165702
Just get rid of the butterfly and replace the diaphragm with a cable and you would have a mechanical carb.

[RubberSalt]

**DISCLAIMER!** hasn't done extremely extensive research(yet)

Well for the interchange able carbs. For the most part, It's supposed to be a half no. Carburetors operate off vacuum. 4 strokes have a very strong vacuum. They are able to boil fuel much easier than, let’s say, a 2 stroke. 2 stroke vacuums aren't nearly as strong on the intake as a 2 stroke. The fuel in them tends to boil out much easier.

Yes, the fuel boils out. It is not sucked like some would believe.

With that being said. Some jetting adjustments, a 2 stroke carb should work on a 4 stroke. But the 4 stroke will more than likely not work on a 2 stroke.

[sledcrazyinCT]

One thing about using a carb from a sled on a trike is the float needle and seat must be changed as the orifice size are for different flow rates. This is becasue your sled carb was fed by a fuel pump and most trikes are gravity fed with your gas tank mounted above the carb in most situations.

[RubberSalt]

The needle and seat will be perfectly fine. It's the floats you need be concerned about. The more flow the better into the float bowl, never have to much. The floats will need to allow fuel to actually flow with lower pressure.

[RIDE-RED 250r]

I actually had to deal with this situation working on a Tri-Z last fall.

We replaced the stock Z carb with a stock '85 250r carb. When we hooked up the gravity feed carb from the 250r to the Tri-Z with it's fuel pump fed setup, we had overflowing issues we couldn't resolve. After looking around here on 3ww we found out that carbs meant for use with a fuel pump do in fact have a smaller inlet valve orifice so the float can adequately shut off fuel supply when needed. Conversely I learned that putting a pump fed carb on a gravity fed setup you do run the risk of running the fuel level too low on long WOT pulls.

What we did to address the overflowing issue on the Z was as per instructed by members here, was to simply plum a by-pass line between the fuel pump and carb inlet. Bingo, problem solved.....

I cannot speak based on experience about running the bowl too low using a pump set up carb on gravity feed as I have not personally experience..But others in the threads I dug up on the subject seemed to have had it happen....