Yamaha SR250 - A clean standard build

Again, I have no idea if this will work or help, but I figure its gotta be better than an open carb - just look at some of the details from the stock SR set up - as said, the engineers are not stupid - notice the curved/radiusd mouth on both the carb-airbox intake boot and the airbox - atmosphere boot. Plus, if one wants to run the pod filter 'look' then performance shouldn't have to suffer.

I think what happens with pods is that too much air becomes a problem, you can change jetting and mix ratios all day, but if the venturi(s) are just too large for the pump you will never tune your way out.

Someone needs to make a set of foam filtered velocity stacks that have an ability to squish the venturi at both ends, then you can run clamp on filters, but still need shielding from cross flow air like wind or even deflection from your legs so you wind up with an air box... almost and in some instances the vacuum and air box provides is needed like in CV carbs but those are another set of rules all together.

Those images/articles I mentioned I had as inspiration/influence...

http://mgaguru.com/mgtech/power/pp104a.htm
http://mgaguru.com/mgtech/power/pp104b.htm

http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

This led to the final design I have which has an elliptical intake curve leading (with G2 curvature) into a 6mm (1/4'' ~) full 270° radius. Hope it fecken does something!

Tune-A-Fish said:

I think what happens with pods is that too much air becomes a problem, you can change jetting and mix ratios all day, but if the venturi(s) are just too large for the pump you will never tune your way out.

Someone needs to make a set of foam filtered velocity stacks that have an ability to squish the venturi at both ends, then you can run clamp on filters, but still need shielding from cross flow air like wind or even deflection from your legs so you wind up with an air box... almost and in some instances the vacuum and air box provides is needed like in CV carbs but those are another set of rules all together.

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Yeah I had a feeling the open carb made things harder to get the jetting right. Thats what that article about the MG said too - that it cleaned up the throttle. I can only imagine that this would make the fuel metering slightly easier/smoother - ie. easier to get the jetting right. This guy said the same thing about adding a velocity stack to his SR500 on the dyno:

https://www.youtube.com/watch?v=VUupiGwjBdw

Not 100% sure what you mean about a velocity stack with a venturi at both ends? I know Harley guys have discussed the effects of cross winds on velocity stacks a lot and how detrimental they can be. Thats why some choose the double stack if I'm right. I am hoping with the carb behind the engine in this case, it will not be as much of an issue?

Tune-A-Fish said:

I think what happens with pods is that too much air becomes a problem

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Too much air is not the problem. Velocity is the problem. Even with 'good' pods. The only correct application of pod intakes is on a race bike that will be up in the rpms the majority of the time. It's almost like, science or something.

One way to demonstrate this is to take a straw, cut the each end at an angle and tape it to an air gun so one end is in the air path and the other can be placed in a cup of water.



Shoot away playing with air pressure to see how it affects the atomization of the water. The same concept makes a paint sprayer work and many other applications. The volume of air through gun remains the same, but the velocity changes with air pressure.

deviant said:

One way to demonstrate this is to take a straw, cut the each end at an angle and tape it to an air gun so one end is in the air path and the other can be placed in a cup of water.

Shoot away playing with air pressure to see how it affects the atomization of the water. The same concept makes a paint sprayer work and many other applications. The volume of air through gun remains the same, but the velocity changes with air pressure.

Click to expand...

That mang is a sweet way to make a makeshift bedliner sprayer

a member over at KZRider.com played around with several iterations for these velocity stacks with filters over them he ended up having them made in delrin. He claimed to have gotten them dialed in pretty good. I picked up a set myself to try. here is the thread if you are interested...
kzrider.com/forum/3-carburetor/408059-ve...thbores?limitstart=0

I think the important take away from that thread is that carbs that are set up for more performance will invariably have to make compromises with "streetability"

doc_rot said:

a member over at KZRider.com played around with several iterations for these velocity stacks with filters over them he ended up having them made in delrin. He claimed to have gotten them dialed in pretty good. I picked up a set myself to try. here is the thread if you are interested...
kzrider.com/forum/3-carburetor/408059-ve...thbores?limitstart=0

I think the important take away from that thread is that carbs that are set up for more performance will invariably have to make compromises with "streetability"

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That is outstanding! Exactly the kind of info I needed to read. Looks like he solved all the issues really nicely for that carb set up. I think there is a need for more 'in-filter-velocity-stacks' for bikes. They're quite common on cars. I saw a really nice solution some guy had come up with over on the SR500 forum, but it was quite customized: http://www.sr500forum.com/forum/viewtopic.php?t=11792&p=81059

Yeah I think whenever running this type of setup you will loose some bottom end, unless of coarse you go for a really long intake trumpet!

Check out this sweet, very practical article: http://www.emeraldm3d.com/articles/emr-adj-length-intake/

So depending on packaging, you could potentially make a tractor of a screamer according to those guys ;D Anyone got room on their bike for a 300mm intake trumpet? Haha

This was also an amazing read! Learnt a hell of a lot reading through all the mods to this engine:

http://www.sr500forum.com/forum/viewtopic.php?f=9&t=10452

http://www.sr500forum.com/forum/viewtopic.php?f=15&t=8458

Dont know why there are two threads, seems like the same build but one thread has double the pages...

"I think the important take away from that thread is that carbs that are set up for more performance will invariably have to make compromises with 'streetability'"

"Yeah I think whenever running this type of setup you will loose some bottom end, unless of coarse you go for a really long intake trumpet!"

I guess it depends on your definition of "performance". If you are talking about increasing average lap times at a track, that is one thing. If you are talking about riding on secondary public roads (these are 250cc bikes after all), then that is another. I think that riding on secondary public roads (which all have speed limits that we should be observing so as not to portray ourselves as adolescent squid riders) demands more low-rpm torque than high-end rpms (which govern hp). Torque is what gets you started from a stop and gives you roll-on power to modulate speed in a turn. If the engine mod does not increases low-end torque, I would argue that it is NOT enhancing performance for riding on public roads.

JadusMotorcycleParts said:

I think there is a need for more 'in-filter-velocity-stacks' for bikes. They're quite common on cars.

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They're quite common on motorcycles, too. Open up a Honda airbox and you find rubber velocity stacks connecting the box to the mouth of the carb.

deviant said:

They're quite common on motorcycles, too. Open up a Honda airbox and you find rubber velocity stacks connecting the box to the mouth of the carb.

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I pulled mine out and boiled them in water then soaked them while hot in linseed oil to soften and it worked great, Just need to commit to cutting open the UNI's and fit them... may need a larger filter to start with.

Over analyzing. This stuff is not new or difficult to understand. What we need is not more bikes with filtered velocity stacks, but instead we need less people removing them. Redesign your airbox for cosmetic reasons but don't expect to consistently gain performance over any stock setup. Trust me they wanted their bikes to be as fast and reliable as possible because otherwise they wouldn't take their share of the market.

^ like!

DohcBikes said:

Over analyzing. This stuff is not new or difficult to understand. What we need is not more bikes with filtered velocity stacks, but instead we need less people removing them. Redesign your airbox for cosmetic reasons but don't expect to consistently gain performance over any stock setup. Trust me they wanted their bikes to be as fast and reliable as possible because otherwise they wouldn't take their share of the market.

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I get it, the cover of Cycle World meant more than anything and I remember reading old articles where the setup from road to track meant major changes. What I strive for is a little of both worlds and to just stop trying would mean a slow death on the couch no? We also know in the early 70's everything was under the all new EPA and smog rules so these bikes were indeed made to squeeze all they could from what they were allowed to burn.

Is the air box really that ugly? No but it's not simple and certainly not sexy

DohcBikes said:

Over analyzing. This stuff is not new or difficult to understand. What we need is not more bikes with filtered velocity stacks, but instead we need less people removing them. Redesign your airbox for cosmetic reasons but don't expect to consistently gain performance over any stock setup. Trust me they wanted their bikes to be as fast and reliable as possible because otherwise they wouldn't take their share of the market.

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This is true to an extent but a bit oversimplified IMHO. If your engine is stock than I would generally agree with you, pods and stacks aren't going to benefit you. What about an engine that has been overbored, increased compression, port and polish, and bigger cams? surely the intake demands have changed so it doesn't make sense to use the stock one.

Speed and reliability were/are the focus of some manufacturers on certain bikes but there are many other factors that play a significant a role in engine design. Smooth throttle response, EPA restrictions, noise limit restrictions, and gas consumption are all things the manufacturers also consider. If you are modding a bike to go faster It stands to reason you don't give a shit about most of those, with an exception to throttle response and perhaps reliability, you don't need to compromise as much.

For example my Aprilia has a aftermarket exhaust and has been chipped. The throttle is kinda twitchy under 2500 now, but I don't care because I don't do much city riding on it and rarely let it dip below 3000 anyway, The performance gains were very noticeable, low and midrange got a huge boost. The cons are my gas mileage dropped, throttle response is not as good in the extreme low end, and it is a loud SOB now. Those are all deal breakers for manufacturers, but inconsequential to me.

Good discussion. Time for some further reading... Just got this in the mail today

Total fluke, but I literally opened the book and turned to this page. Clearly shows how much flow is gained over a stock set up with an intake trumpet. No comment on air velocity however. I understand this is for high performance engines operating near the top of their rpm ceiling, but it is always nice to see some concrete numbers.

I think in most cases, like Zap and DohcBikes say, it is better to leave the stock set up alone. But like tune-a-fish says, the stock set up is far from sexy. So if it has to go (because the stock SR set up IS butt ugly), why not replace it with a proper filtered intake trumpet rather than just a clamp on filter - you might get back some of the power lost and maybe even (with a wish and a prayer) even get a small gain (up top only)

The problem is the bottom end can be downright awful. Ever put too big of a power valve on a Holley carb or run the wrong size squirters? Hit the gas and watch it stall. Or if it does take off, it has an awful flat spot and shows little power until you jump up above 4000 rpm. What's the point in building a motor with torque, if you have no way to use it? I for one am not interested in having a moped beat me off the line or out run me to the next light.