Flat spot improved with new intake snorkel

[Luca]

Recently I was getting bothered by a flat spot in the throttle at 60mph in top gear.  I gave the 2-year-since-rebuild carbs a quick cleaning (sprayed through the bodies and jets rather than soak them) and synched them to no avail.

Rather than start messing around with the jetting, I ordered a new $6.00 snorkel for my plastic airbox top, PN: 13 72 1 337 501, which is smaller at the mouth than the two identical snorkels fitted to my stock '82.  Took a couple weeks to get here from Germany...

...installed it in the dealership parking lot and rode off.  The butt-dyno says that the flat spot is gone.  The bike sounds much better when I give her the stick at 60mph as well.

It seems that BMW issued bulletins for fixing flat spots by first changing an intake snorkel... and eventually put the bikes out from the factory with one of the 13 72 1 337 501 snorkels installed.

If you have a mid range flat spot on an early 50 horse machine, I'd suggest trying the cheap and easy snorkel swap before messing with the jetting.

[Barry]

I don't have a US service bulletin for that mod but I do have a European one which is attached.

Goes to show how much influence that tuned intake system has on those bikes. On the R45 models that intake system resulted in a change of main jet from 123 to 105 which is huge in % flow terms.  When I tried putting 105's in mine it simply wouldn't run on load above 3000 RPM.

[Tony Smith]

Yet people insist, in the name of "art" in replacing the stock intake system with those accursed "pod' filters. And then they wonder why a Honda CT-110 in reasonable fettle can show them a clean set of heels.

[arvo92]

I ordered a new $6.00 snorkel for my plastic airbox top, PN: 13 72 1 337 501, which is smaller at the mouth than the two identical snorkels fitted to my stock '82.

Any chance of taking pictures showing the difference of these two airbox tops to us uneducated ones?

online search by part number did not get me to the right place.

[steve hawkins]

I have a set of pod filters and no flat spot.  I am not saying I have improved anything, but I have certainly not ruined my riding experience.  And they look far better than a plastic box ever will.

Having said that, if you change the standard set-up in any way, expect there to be repercussions down the line to take into account.  But it ain't that difficult.

Rev Light

[Motu]

I have removed my snorkels...and have no flat spots. I can create and remove flat spots with various exhaust systems...so I think the stock exhaust system is susceptable to intake changes.   I don't really notice a flat spot...unless it's really bad. But I do notice differences in fuel consumtion - a mid range flat spot will increase fuel consumption at a 4,000 or so rpm cruising speed.  Less power and increased throttle opening will increase fuel consumption.

Currently experiencing almost the best fuel ecconomy I've ever had....but it's winter and have had a 10 to 15km increase in range to reserve since mornings have been under 10C.

[Barry]

I

Currently experiencing almost the best fuel ecconomy I've ever had....but it's winter and have had a 10 to 15km increase in range to reserve since mornings have been under 10C.

It's curious that you get best consumption when it's cold. I always get less MPG in the Winter due to cold oil and extended warm up times not to mention that there is several percent less energy in winter fuel due to the need for more volatile fractions. Heavy fractions of petroleum products always have more energy per unit volume and that holds true for all the grades up to the tar like stuff supertankers run on.

Back in the 70's we always used to think our bikes went better in the cool night air. I think that was because they were running too rich and the colder denser air leaned the mixture back to something closer to what it should have been.

The only other thing that I can think of that might cause better MPG  is evaporation losses from the carbs and tank are lower in the winter.

[Ed Miller]

I have both types of intake snorkels but I can't really tell any difference in performance.  

[Motu]

[quote author]It's curious that you get best consumption when it's cold.[/quote]

You think maybe I'm riding slower because it's so bloody cold?  Or I forgot some modification I did one day in shed when I was pissed? I recently took off the S fairing and stock bars, and have my dirttrack bars and Oxford muffs, stock seat and panniers - heavier and more drag...and still went 348km to reserve.  Denser air....and lots of wet fog - most certainly better ecconomy in cooler weather.

[Luca]

Any chance of taking pictures showing the difference of these two airbox tops to us uneducated ones?

Go have a look at the maxbmw.com microfiche.  They have pictures of the airbox top.  Mine will look like the post-9/82 style.  The difference is in the size of the bell mouths on the snorkels.

[Barry]

 

You think maybe I'm riding slower because it's so bloody cold?

... I do that but still mange to use more fuel every winter. The pattern has been consistent for the last 6 years with approx. 10 MPG swing between winter and summer MPG.

Must be the wet fog. Water injection is said to improve MPG