Snorkels

[fayeslane]

Removed the airbox snorkels and went for ride on my 93 R100GSPD today -270 kms. Definitely more responsive. If it is breathing better because the air flow is improved how much effect will the have on the fuel/air ratio. Should I consider jetting changes? My main concern is that it may run too lean and cause problems $$$. Will monitor the plugs for any tell tale signs.

Has anyone looked at this in depth?

What is the role of the snorkels in the original design?

Searched this site for answers, but no luck.

Does this sound familiar; Riding today. Came around a corner, big rock in the middle of the road. Don't look at the rock!, Don't look at the rock! Looked at the rock, hit the rock, hit the ground! Slow learner.

 

[Kenny]

Unlucky with the rock

TBH I can't see how removing the snorkles ( I could be wrong !) would make any difference to performance as they are hardly performance bikes.....are you sure it wasn't just the feel good factor?

 

[Rob Farmer]

Oh Dear! you're on the slippery slope now. I've spent hundreds of pounds on dyno runs and tweaking to get my 100PD to run properly. When I first bought it it was only putting out 37 BHP it's now in the very low 50's. The standard airbox is restrictive for the 40mm carbs but spot on for 32mm's. so if you have 32mm carbs leave it alone.

to get mine to breath properly above 4k I've ended up with an early aluminium RS airbox and starter motor cover. All I wanted was a standard bike I could just ride

One of the site members spoke to HPN who told them the height of the airbox needs to be raised by 1cm to be ideal so the volume of the filter side of the airbox is probably too small for the 40's.

The snorkels control the amount of depression in the carbs when the throttle is wound open. If you take the snorkels out or fit another airbox you increase the range of the fuel/air ratio being fed into the carb with the snorkels in place the air spends more time in the high torque/ high power range.

This printout is from a gadget I bought for plotting the air/fuel ratio (I said it was a slippery slope) and shows a run through the rev range on my 100PD it's pretty much spot on now. It's not a perfect run, obviously it's done on the road so the bikes not in WOT most of the time.

I've been working to these ratios

6.0:1 Rich run limit
9.0:1 Low power, black smoke
11.5:1 Rich best torque at WOT
12.5:1 Safe best power at WOT
13.2:1 Lean best torque at WOT
14.7:1 Chemically ideal
15.5:1 Lean light load, part throttle
16.2:1 Best economy, part throttle
18-22:1 Lean run limit

WOT = Wide open throttle

If I was you I'd keep it standard, ditch the Y piece if you have one and forgot all about messing with it

 

[John Armstrong]

Probably noisier = more responsive

Rob Farmer and BamBam IIRC have done some various changes.

This magazine has a good article on why they think you can't just mess with one item on the airhead:

http://www.bmw-motorcycle-magazine.com/index.php?id=35

 

[fayeslane]

Have to concede it was a beautiful day, snow on the mountains in the distance, a decent ride after some wet weather so maybe the feel good factor had kicked in...

But I keep coming across references to improved performance with the snorkels removed and I think it went better

My rational self says that BMW designers wouldn't bung on a couple of snorkels for no good reason but I have come across additions put on other vehicles to meet various pollution, noise etc laws that weren't conducive to efficient running.

Hoping the collective intellect of this forum can show me the light (& the next rock)!

 

[fayeslane]

The "leave the bloody thing alone" advice seems to be the go. There is a GSXR 750 in the shed if I need a performance hit

 

[John Armstrong]

IIRC the thoughts are that stock gives the best overall package. But that might not suit you / how you ride. So changing things can make it better in one area, but make them much worse in another

 

[Kenny]

The "leave the bloody thing alone" advice seems to be the go. There is a GSXR 750 in the shed if I need a performance hit

Now your getting it !!!

This is why I also keep one of these things at the back of the shed....

 

[Redboots]

Now your getting it !!!

This is why I also keep one of these things at the back of the shed....

Urban Tiger... Best one they did. I had one. All the later ones got more "civilised"

John

 

[Rob Farmer]

But I keep coming across references to improved performance with the snorkels removed and I think it went better

It will have improved performance because they seem to run too rich as standard. Has your bike got a Y piece fitted or the standard collector?

The best thing you can do is book it in for a dyno run. Unless you know exactly whats happening then it's all speculation. If you listen to other peoples thoughts and ideas you'll end up spending a fortune and chasing your arse. A basic dyno run is around £30 and takes 1/2 an hour. It'll all be blindingly obvious once you have done it. once you know whats wrong you can address the issues.

 

[Beemerboff]

I got the a better throttle response when I removed the snorkels on my GS too - the snorkels are much smaller than the carb intakes.

And it is manifold depression which sets the position of the pistons in the carbs, not air flow through the venturi, so all this stuff about freer flowing intakes and filters not working with Constant velocity carbs is just wrong - the whole idea of the carb design is to maintain the air velocity constant

However , if the air flows a little bit faster and the piston doesnt open as fast/proportionally you will get more suction at the needle and a richer mixture. So more air flow on a CV carb can richen the mixture.
That is the reason for the spring above the piston - it slows down the initial piston movement and gives you a richer mixture in transition.
Your SU dealer will have a choice of 8 spring strengths if you fancy a bit of experimentation.

And remember that throttle response doesnt necessarily show on a dyno - instant response to the throttle is what you notice as you ride, but horsepower is measured at constant revs and then plotted on a graph .

 

[(RIP) maverick]

The "leave the bloody thing alone" advice seems to be the go. There is a GSXR 750 in the shed if I need a performance hit

Don't fix it until it is broken

 

[Rob Farmer]

Don't fix it until it is broken

Or

If it ain't broke fix it until it is