topless air intake

[twist]

Has anyone thought of running the flat filter with the top cover off for improved air flow? I've been thinking that the top cover could come off and the filter could be held in place by a pair of mini bungee cords. Not very elegant but could work. Or maybe cutting the top out of the cover and using the standard clips to hold the flat filter in place but without the intake tubes running over the starter cover, just wide open with no cover. There has been a lot of talk in the past about the virtues of the round over the flat for air volume. This would be a nice solution and done right would look good, too.

[ME 109]

Will a particular engine benefit from increased air intake............or will it make no difference at all.

Why not just cut the top right out of the air box cover, leaving the original outer edges and use the original clamps?

[dwire]

Or, you could put a super or turbo charger on it... lol
I'm not knocking you for doing what would be the sorts of things that are second nature when building open wheel racers so long as the changes were within the rule books, but I would expect any modification to the intake system such as this is more of a rsik of getting road grime, bugs, dirt and crap into your engine than that extra little bit you may OR MAY NOT SEE AT ALL out of your bike after doing it. Both intake and exhausts are far more complex than is generally realied... And our race motors only got a couple hundred miles at best out of them before being torn down to nothing and ANYTHING out of spec replaced. Do you plan on getting more than 250 miles out of your bike before its next bottom and top end rebuild?

Just food for thought; you may be getting a bike ready for vintage motorcycle days here at Mid-Ohio in Lexington, OH - where I as a young dumb and full of "uhm" young man did all my training for licensing and such to race Indy/CART cars. Then, due to knot knowing any of the right sorts or crowds to hang with was never able to make the step up into professional cars; at that time likely would have been CART - before the IRL started making a big mess of things - we now have an entire role reversal! I sure hope they can get it together as the roots of open wheel racing, a great deal of them trace right to the US and of them many right here to my native state of OH... Good luck and if you do this, see if you can get the bike up on a rear wheel dyno before and after - I am sure we woud all like to see how these sorts of mods truly perform. Have a great weekend.

Regards,
Douglas

[twist]

the story behind this is a conversation here on round vs flat intake boxes. I bought the flat set up fairly cheap and got it installed on the bike. In the first ride the bike ran fine enough during the warm up but once the bike was good and hot I whipped the throttle open and got a near stall, it really felt as if it weren't getting enough air. I am running Mikuni TM 38 flat slides with a 190 main jet, (can't remember the needle jet and pilot jet size). I recently put on a set of dunstall mufflers and had to re-jet. At that time I was using the round air cleaner with stock drilled air box. That set up was VERY responsive to throttle. Still it didn't feel like it was getting enough air and a riding friend said he could slightly smell unburnt fuel when we were out romping around in the hills. I figured a filter open like that would allow the motor to get enough air. Maybe I'm going about this all wrong and need to rethink my approach.

[Deleted User 72]

I don't think fiddling with the intake tract will address the issue you cite. It works pretty good as stock. I would look for other causes. Timing, adv mech, diaphram(s), air leak?

[dwire]

Wow - a lot of that must have been brought from a different thread, so my apologies as the modifications you describe, while I don't even know what vintage or model bike this is, well I would not be the one to ask. There are some racers here that may have insights and if you are anywhere near OH - or any other state with a wonderful road course that runs vintage bike races - I sure would go speak with those sorts of crowds as they surely have done it all and all on a rear wheel dyno (the open classes at least...)

If you search my worthless banter on the thread about modifications to for instance a bike like my 71 R75/5 and I hear "will doing all of this stuff make my bike faster?" or "I really hate my charging system, I want a 30,000 watt one..." I am usually a bit down on such topics as my bike when it was running and the identical bike of my ex's I rode daily NEVER DISAPPOINTED ON ANY LEVEL for what it is. If it were (my bike) a cafe' racer intended for offroad/course riding I would be pissed, but while it's one heck of a performer considering the age and to me a beautiful piece of design and engineering, I never think, "man if I only could get this much more HP out of this..." Maybe gas milage - that's about it...

Sorry I was unaware that a portion of the topic was elsewhere. I will stand by the claim that you should be very certain whatever you use is not going to leak any. The intake pipes that go on my 71 R75, well you could use them and a chain hoist to lift the bike, so I think they are going nowhere, but I did used to routinely be able to go to the motorcycle races when I was REALLY YOUNG and pick up enough K&N style filters that had fallen off M/C's individual carbs to make it so growing up through the ranks of go-karts, I only once bought one and that was because the way we had to jet and set up the carb, for dumping just about two times as much alcohol in the only way to really choke the engine and get it to start was to slide a rubber Furnco plumbing adapter clear over the proper sized filter and make it suck a big gulp of fuel to start. Sort of funny now that I think of it... But that was a "stock" Briggs&Stratton 5HP horizontal engine that saw nearly 6,000 RPM and the added safety of alcohol was rather clear, even after having to jump in a few of the track's ponds or get hosed down by a fire truck because someone was dumping fuel everywhere in front of you and it ignited... lol

A lot of fun racing those Karts at speeds that should NOT be legal. (some over 120MPH...) Put me back in a carbon fiber tub - or heck an aluminum one - anything than sitting in a yard kart at those speeds - it DID motivate you to get out front and stay there I can say that much, but I am way OT... Thanks for clearing things up. Love to hear how you solve your problem(s) and if the end results are what you'd expected particularly if you had a baseline dyno and can run one once you are done. The seat of the pants method usually lies and a stop watch is not of much more use for a lot of reasons too. Best of luck Sir!

Kind regards,
Douglas

[Major Softie]

In the first ride the bike ran fine enough during the warm up but once the bike was good and hot I whipped the throttle open and got a near stall, it really felt as if it weren't getting enough air.

You are not only mistaken, you have it exactly backwards. What you experienced was a lack of velocity through the carbs, resulting in a shortage of vacuum, and thus fuel. You were getting plenty of air, but no fuel. This is what CV carbs, and accelerator pumps, are designed to prevent. Large carbs with no accelerator pumps will always exhibit that symptom. The solutions are: smaller carbs, CV carbs, accelerator pumps, or a more judicious throttle hand at low rpm.

[dwire]

Funny you mentioned that Major, as his description remind me of the same thing - something I always called the "Double Pumper Effect" (Thanks Holley Carb for this...) All the kids that wanted to work on cars and went off to vocational school had not so hot Hot-Rods, well they'd take one look at the stock cars and look in all the automotive engine porn Hot Rod magazines and decide that their car just had to have a zillion CFM double pumper from Holley. Because surely those two much tinier holes in their vacuum secondary carbs could not flow enough air into their engine for it to breathe right - and besides, look at those stock cars going (at that time) near 180-200MPH around a big track?

So off with the appropriate carb and on with a shiny new mechanical secondary carb with bores as big as beer cans. Most of these adolescents were the jock ego type that would pull up with their burned out glass packed mufflers and brag and want to go out to "the road" and run a little 1/4 mile outside our no account town. More than a handful of giant loud V8's that should have just flown got their lunch handed to them (quite immature, I know) by myself in an old ratty F100 with a 300 straight six (my parents' hardware work truck no less) with a single barrel carb. Most still today don't know why that was; especially the very upset Mustang Cobra driver that darn near ripped me out of my truck for doing so... Not having any experience with what this poster is riding, I'd have still not know to make a bold statement like that as it wold have been a mere guess and I still have missed where that info was given somehow. I also frankly am no BMW of any make or model expert, but can build you a winning race engine if you give me the tools money and don't restrict my gray matter any...

I hate to inform the OP I surely would agree with the Major and was going to ask why a person would ever put Mikuni's on any of these bikes anyway, but again, I have very little specific knowledge of BMW's I know how and why they work and know other than a few safety, prudence and quality improvements (manufacturing defect, or "oops" that might have been fixed on a later model...) They seem to run best darn near bone stock - and I never want for any more - ON A ROADWAY that is and frankly on the raceway, I'd far prefer have four wheels beneath my seat. It's OK y'all can call me a puss now...

[twist]

the bike is a 77 R100, but it has been modestly modified. My goal is to improve intake and exhaust, reduce weight, improve handling, and make it fun to ride as well as unique. I'm thinking of going back to the round filter since it ran amazing then. The carburetors have been on the bike for a few years giving me excellent service. I don't like the bing set up. I must be missing something, this is not testosterone driven teenage hot rod magazine I want that. I posed the topic looking for input on the mechanical nature, (thank you major). I would be more than happy to benefit from the mechanical knowledge you have. Is the comparison necessary?

[Chuey]

Here on Boxerworks, I've read that the square airbox was "tuned" to work just right on the engine and with the Bing carbs. Considering that the square airbox came after the round one, and at a time when the engines and carbs got bigger, my guess would be that they are actually admitting more air to the intake.

Some thoughts: There is such a thing as too much carburetion for a given engine.

The original system was designed by engineers who know more than all of us combined about intake systems.

That could mean that significant gains in smooth power may take more than simply adding bigger carbs and more noise out the back.

Modifying can be fun but it probably isn't as simple as it seems it would be.

If you want to rule out airbox restriction, you could always do a short run without the airbox hooked up or with the element out.

Chuey