Best carb size?

[griff]

Randy, exactly which model 38 Mikuni is that? VM series?

If its a real round-slide, Jason is right. Primarily SM application, or old-school 2-stroke off-road motorcycles (like from the 60's and 70's).

If thats the case, its really the wrong carb for a Raptor/Warrior. The vacuum curve this engine generates isnt exactly compatible with the way the fuel circuits are setup on a round-slide.

Which wide-band setup are you using?

 

[jasonp]

Randy, exactly which model 38 Mikuni is that? VM series?

If its a real round-slide, Jason is right. Primarily SM application, or old-school 2-stroke off-road motorcycles (like from the 60's and 70's).

If thats the case, its really the wrong carb for a Raptor/Warrior. The vacuum curve this engine generates isnt exactly compatible with the way the fuel circuits are setup on a round-slide.

Which wide-band setup are you using?

Correct me if I'm wrong Griff but doesn't a 2 stroke create less vaccum then a four stroke?

 

[griff]

Correct me if I'm wrong Griff but doesn't a 2 stroke create less vaccum then a four stroke?

I would say very generally speaking, yes. Since theres no head-valves and no cam(s), vacuum is fairly fixed based on displacement, stroke, and port position. Thats engine side only of course, not including the external intake tract (boot, carb, box/filter). 2-strokes also use reeds to manage aspects of that as well.

 

[justrandy]

Yeah, its a VM38 roundslide brand new from bike bandit. Came with a primary type needle-jet which mikuni says is meant for 2-stroke generally. I also have a VM32 for the quadsport that Sudco recommended and tuned for me (not quite right I'll add, but close)... The only real difference is the addition of a bleed type needle-jet instead of the primary type. Came from Sudco so.....

I'm using a DVM to read the voltage of the 02 sensor. This particular sensor seems to read around .7 volts when the engine is running well. I have another sensor that reads .4 volts on the same jetting. I've set up 2 engines this way.... It works so well I'm planning to analyze the jetting on all my other quads just to see.

I put the sensor in the joint right before the muffler. It heats up in about 1 minute and reads consistantly.

Can you guys explain more about the term "wide-band" and why this carb doesn't work well with the Warrior?

 

[griff]

Wide-band refers to an o2 sensor setup used with software or meter system to determine A/F ratio based on exhaust. Its basically what youre doing with your o2 sensor, though your setup using a voltmeter is a bit primitive

Like I said, 2-stroke carb applications are a different animal compared to carbs intended for 4-stroke engines. The big issue is the overall fuel circuit design, its intended for a specific set of parameters... Everything from fuel delivery volume, vacuum signal, vacuum ramp-up, the list goes on.

 

[justrandy]

Does Sudco know what they're doing?

 

[griff]

Does Sudco know what they're doing?

As far as carb parts, you cant beat'em really. Now for applications, they know some, but they certainly dont know all what carbs will work with what vehicles.

 

[justrandy]

Well, their recommendation for a 270cc, 4-stroke, 2 valve head engine that is very similar to the warrior was a VM32 with a 5L1 needle in a bleed type needle-jet. I find the combo works pretty well once I got the main jet right, but I have a dial-a-jet also. Its hard to imagine how or what could be better. I used to have a TM28 (flatslide mikuni). It worked pretty well also. Hard to tell the difference really,,, except I can rev faster with the 32 it seems.

Now, the VM38 has been a lot of trouble to tune, but it wasn't set up by Sudco either. And I think the primary needle-jet isn't best of choices. Even so, its seems to be working and I'm curious what I'm missing in terms of power, response, etc compared to another carb. I'd like to know more about vacuum curves, carb circuitry, and carb selection.

My current understanding is a flatslide has more velocity; hence, better lowend throttle response and probably a topend restriction. And a cv has the advantage of being altitude independent at a sacrifice of throttle response. A round slide is just an old design, but flows pretty well at WOT... Below that, it sucks. That's what I've learned from doing a LOT of reading online. But I'm far from knowing it all, so I'd like to learn some more.

 

[jasonp]

Well, their recommendation for a 270cc, 4-stroke, 2 valve head engine that is very similar to the warrior was a VM32 with a 5L1 needle in a bleed type needle-jet. I find the combo works pretty well once I got the main jet right, but I have a dial-a-jet also. Its hard to imagine how or what could be better. I used to have a TM28 (flatslide mikuni). It worked pretty well also. Hard to tell the difference really,,, except I can rev faster with the 32 it seems.

Now, the VM38 has been a lot of trouble to tune, but it wasn't set up by Sudco either. And I think the primary needle-jet isn't best of choices. Even so, its seems to be working and I'm curious what I'm missing in terms of power, response, etc compared to another carb. I'd like to know more about vacuum curves, carb circuitry, and carb selection.

My current understanding is a flatslide has more velocity; hence, better lowend throttle response and probably a topend restriction. And a cv has the advantage of being altitude independent at a sacrifice of throttle response. A round slide is just an old design, but flows pretty well at WOT... Below that, it sucks. That's what I've learned from doing a LOT of reading online. But I'm far from knowing it all, so I'd like to learn some more.

1 all carbs have to be tuned to correct alitude.
2 dial a jet basically to me is a band aid for improper tuning.
3 wide band o2 sensors basically use 5 volts instead of 0-1 volt which most your o2 sensors are and are inaccurate to tune from. The wide band was basically developed by a car manufacture to get better fuel economy as this sensor can now tell the cars computer more accurately fuel mixture per air content in the exhaust stream. Now all dyno shops will carry these sensors because how accurate they can be in telling you what is the correct fuel mixture not just by exhaust temp. You have to understand as well, every engine is different some like 12.5:1 mixture and some like to be more leaned out. This is why I could care less about what the fuel mixture actually is, as you really don't know what mixture will make the most power unless you on the dyno. Go ride it and find out what jet makes the most power.

For me sudco recomended a TM36 flat slide with accel pump or kehin 38mm round slide. When I first got mine it was very close. Had to come down on the main 2 sizes then bigger pilot jet but I had no air box and a pipe as well. Spot on with the needle. They even sent me a different accel pump nozzle to test out which works great.
Flat slides will flow more air because there is less obstruction in the carb itself. No butterfly valves or vacuum operated slide to get in the way of airflow, hence more vacuum at the jet blocks. The tend to make a little less power until the mid to top end of the power band. They like 4 valve, high compression engines that are quick reving to take advantage of the single slide controlling the airflow. This is why you see this type carb on every 450 4 stroke bike and 4 wheeler (Kehin FZR), for Maxim performance and response. Since the slide is basically attached right to the throttle and not vacuum operated the velocity will drop off at lower rpms with the throttle wide open causing the engine to loose some torque. This is where a CV style works but has more restriction in the upper end as it has to forms of valves to control the airflow. But you trade a little response for a very broad even range of power this carb will put out. Also this carb will work better for slow reving 2 valve engines.
The round slide carbs are low dollar replacements that work pretty good. The upper forward part of the slide will creates a restriction and even at full throttle there still will be a slight obstruction in the air flow path. This part is designed into the carb to help the jets work properly also helps response in trade off for actual air flow.

 

[jasonp]

so really it depends on the engine (air pump in a basic form) on how to look at choosing the correct carb. Yamaha did a great job with the raptor 350's carb. And I agree with everything griff said in that link. I sugest you read more info from actual carb sites and take it in. Don't let opinions fog your eyes on the facts. I love to learn and am far from knowing it all.. you are really making me think.

 

[griff]

Flat slides will flow more air because there is less obstruction in the carb itself. No butterfly valves or vacuum operated slide to get in the way of airflow, hence more vacuum at the jet blocks. The tend to make a little less power until the mid to top end of the power band.

Remember, more air flow only at the very top-end compared to a CV of the same size. Gotta be up there in the power-band.

If by jet block you mean the physical orifices that fuel comes out of, theres less vacuum with a flatslide at those points mid-range and below when the throttle is snapped open. Thats why flatslides often have accel' pumps.

You must be tired, hehe...

 

[justrandy]

My ISP (Charter - they suck!) is all screwed up and I'm using dial up at the moment... So, I can't risk typing a lot and losing it, but the first thing I want to get to the bottom of is the deal with flatslides. Everywhere I've read online says flatslides are for bottomend. Mikuni says flatslides flow air faster than roundslides. Since air is moving faster, that implies restriction. So, how can a flatslide flow more air (than a roundslide) if its restricted due to velocity? The way I understand it, a roundslide flows more air at WOT than a flatslide does. But a flatslide flows air faster at, say, 1/4 throttle than a roundslide does... Hence, better throttle response.

When charter gets their crap together, I'll post some links n quotes.

 

[Fugazipythons]

I have a 99 Yamaha warrior.. upgraded cam and air box...I have to replace my carb..I bought a 34mm mikuni...I had I believe a 36 I know nothing about this...and I better with a bigger carb or is the 34 ok