Topic: DCOE40 Weber stacks on stock carbs?

[slikwilli420]

I have a set of old school stacks for stock 750 round top cabs but all they have is a brass screen. Wanting to use a better filter like the BBR units for dcoe carbs I thought about using the whole stack. The ID would fit perfectly on stock carbs with a thin rubber sealing gasket for an air tight seal. These are known high performing stacks and BBR filters snap on outside the venturi which is a good thing too. Thoughts?

[scottly]

What are BBR filters? If they are only foam caps the OD of the stacks, then no, I don't think they are a good idea. The Uni filters I use have an ID of 2.5", and an internal length of 3", so they have a much greater surface area than a 2.5" disc, and would have less pressure differential for the same filter medium. The less the pressure differential between the carb and atmosphere, the more air the motor can inhale, and the more power the motor can make.

[slikwilli420]

These are a pretty widely used filter. Not sure why there would be any issue.

http://www.carburetion.com/Products/ProductDetails.aspx?part=CUSF57

I probably see these on more Weber carbs than anything.

Aside from filtration, of the stacks will fit, what is the thought on how well it might work?

[scottly]

Those filters look to be pretty restrictive, as well as expensive. What do the old school stacks look like? I'm not sure how you would fasten the Weber stacks, unless you cut slits in them and used clamps?

[754]

Those look like what we used to say..
 Keeps out big rocks and low flying birds...

[slikwilli420]

Alright,I have the stacks fitted to the carbs. I find a set of aluminum ones on the Czech Republic for a great price. I cut them down to the flange and used 3 set screws to hold them on. Do I need a good seal at the carb? I'm guessing yes but many other stacks just clamp on and are not air tight.

[slikwilli420]

Anyone have ideas on the need to seal the stacks at the carb mouth? I have a set of Ramair filters coming from the UK that I will be using. This setup will allow the air to be properly filtered and let the stacks do their job properly. Really looking forward to getting this setup running.

[turboguzzi]

that tight mesh will surely kill flow.
the Czech stacks are surely better than none, but if you want to know whats the really best shape, read here
http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf
essentially the radius at the entry needs to do a full 180 degs, not just 90
you can find this shape in car stacks, seen very few made that way for older bikes.

[slikwilli420]

TG, how do you figure an air filter made for such duty will kill air flow? The filters I am going to use fit over the straight portion of the velocity stack, leaving the entrance to the bellmouth unobstructed.

I read through the article, but must admit that I am confused why you are so against a stack design that has been proven time and again in arguably one of the best automotive carbs ever. Is there a better design out there? I am sure there is, but none is available to me at the price point I paid or in a design that is already proven.

Based on the design considerations of the original air box and the fact that everyone says it is the benchmark for our bikes, I would say this is one of a very few options outside the original that should and can work better. It maintains the velocity stack which is known to be the best intake shape for the CB750, as well as adding filtration that is away from the bellmouth as the original airbox did.

On paper this will be superior to the stock box since it incorporates the strengths but none of the weaknesses.

My original question still stands, do I need to seal the area where the stack attaches to the carb?

[turboguzzi]

i'm not against anything, it's simply that when someone like professor blair says there are better designs than the previous thought best, i personally pay attention. see pics, seems that others too.
why do i figure the mesh will kill flow? calculate the reduction in cross section caused by the mesh and tell me the result.   
i dont see an issue with no sealing of the stack, it will have little effect on the beneficial inlet tuned wave action IMHO.
surely the availability at a price point is important. like you, i often find myself having to compromise between $$$, time and the need to get the bike ready for next race.
if it helps to know, my just pensioned race bike was run for 5 seasons / 30 races without any air filtering, compression and oil consumption are still great.

[Tintop]

I'd say to check my thread for the sidecar rebuild but....the pics are / were hosted by PB.   That said ITG in UK make a 180 bell mouth stack for Webers (various ID's).  I used a 55mm (long) on my 40DCOE.  They do work better because the rolled edge promotes smoother air flow at the entrance.  That said, the hp gain @ WOT is likely marginal over traditional stacks in a bike engine.

[turboguzzi]

nice TT, seems like the rolled 180 lip thing is catching

http://www.med-engineering.co.uk/products/ancillaries/ram-air-kits-all/med-stub-stack-kit-for-the-45dcoe

[slikwilli420]

Thanks for the input guys. The article TG posted concluded that while the design stack I am using isn't the BEST, it is within a couple percent of the ideal version and that's enough for this amateur racer.

My only concern with using filtration is if (when)I get off track I could suck some nasty things in that with a filter would have been a non issue. Getting off track is no good anyway but sucking crap into the engine just salts the wound.

[Greg H]

Not quit "on post" I agree ,but reading this reminds me of a funny incident many years ago (probably 1980 ) involving yours truly  .
I was down to race a guy in the semis at York Raceway with a chance of a  slot on Yorkshire TV if I made it to the final .
I ran a 970 sohc motor back then in the Street class ,which ran bored out stock carbs and open Bellmouths .
We used to stuff rolled up tissues into them to stop anything nasty getting in there when we had the bike in the pits .
We were all gathered at the start up area (along with some Top Fuel guys ) I'm pretty sure Sam Wills was there as well as  some other visiting US racers .
When it came time to start up my motor it wouldn't start ,much kicking ensued then suddenly it fired up with all sorts of tissue coming out of the exhaust like confeti
Obviously , we (or should I say I )forgot to take the tissues out of the Bellmouths .
Everybody had a good laugh at that ,including the eventual winner ( Pip Higham) .
I lost by a wheel in the semi to Mick Fancy on a 16 valve Suzuki .
Hey Ho! Good Days   

[turboguzzi]

agree that sucking stuff from rear wheel when "off roading" is a concern. all my bikes have a polypropylene screen that covers the whole width between the rear frame tubes coming from the shock mounts, goes up all the way to the seat and downwards, it ends just above the swingarm cross member. 

cant remember if you fitted one too

[slikwilli420]

agree that sucking stuff from rear wheel when "off roading" is a concern. all my bikes have a polypropylene screen that covers the whole width between the rear frame tubes coming from the shock mounts, goes up all the way to the seat and downwards, it ends just above the swingarm cross member. 

cant remember if you fitted one too

I do not have one per se, but my oil tank does a half decent job with a stock box. Now that I am running the stacks (filters or not) I will need to come up with something a little better. I also added back a cut down version of the sprocket cover, simply to keep chain goop from getting everywhere as it has done in my last few outings.

Now I need to think about my jetting changes. With my open 4-1 and stack air box with K&N filter, I was running 115 mains. I am guessing even with filters I will be in the 120-125 range now. Thoughts?

[Tintop]

For a filter you can run one that covers all the stacks (not individually) like a Weber one.  ITG, Piper, and others make them.  Mine on the sidecar had a foam back with holes for the stacks that slipped over the bells.  Rule of thumb is that the clear distance from the end of the stack bell to the inner face of the filter should be equal, or greater than the length of the stack for the stack to work properly.

[slikwilli420]

Here is what the mounted stacks with filters look like. I believe I have the clearance as prescribed or pretty close.

[scottly]

That filter looks much better than those dense screens.
While my Uni filters aren't ideal, without them my motor would have been destroyed after riding across the Mojave desert in a windstorm.