Airbox delete —> new carb jets?

[Mallo81]

Hello everyone,

I’m new to this forum, so I introduce myself first: my name is Marco, I’m from Italy, I recently bought a 1989 R100RT monolever that I’m restoring and transforming in a Cafe Racer.
One of the things I want to do is to delete the airbox and replace it with filter pods/conic airfilters.

Question help I need: what main jet size do I need when switching to conic filters on my Bmw R100RT?

Thank you in advance for all your help

[SteveD]

Welcommen.

More air, more fuel. What size MJ do you have now? Buy each of the next two bigger ones and experiment. Start with new needle jets too. They're a wear item anyhow. Maybe just raise the needle one notch first? One thing at a time and do some plug chops to assess.

[Zombie Master]

I'd stay away from K&N filters they let a lot of dirt by.

[daz]

Those pods get dirty quickly because of very small surface area. Then they restrict air flow. You'll jet more more air and end up with less air. Stick with the real air cleaner for best results. Unless you're just going for show in which case why bother resetting in the first place.

[melville]

Those pods get dirty quickly because of very small surface area. Then they restrict air flow. You'll jet more more air and end up with less air. Stick with the real air cleaner for best results. Unless you're just going for show in which case why bother resetting in the first place.

Here's how big the separate filters need to be to actually flow consistently:



That's a racebike that can get serviced (cleaned) every 50 miles if needed. It works on the XR750 because it's behind the rider's knee. On an Airhead there's just not room unless you've got an extreme set of rearsets.

But I know what you're going for here-my son has a Mono R80RT cafe project that he wants to do the airbox delete on. I'm still thinking about how to get a good look and some function.

[Rob Frankham]

There are two reasons why you shouldn't delete the stock filter. 1) is that cone filters are not efficient and you will get a lot of dirt into your engine. 2) is that the airhead carbs are supported by the connection to the airbox. Without that connection the carbs will vibrate up and down of the flexible head joints and very possibly fall off. 3)... sorry, there are three reasons why you shouldn't delete the stock air filter... 3) As has already been said, the filter is too small and is unlikely to flow enough air. 4) OK... so there are four reasons why you shouldn't delete the stock air filter...4) You will do away with the carefully designed inlet tracts which will affect the engine balance and running. Merely changing the main jet isn't going to make it run right. Unless you are going to pay to have the machine put on a Dyno and tuned by an expert, you are most unlikely to get a good result.

Rob

[Zombie Master]

Those pods get dirty quickly because of very small surface area. Then they restrict air flow. You'll jet more more air and end up with less air. Stick with the real air cleaner for best results. Unless you're just going for show in which case why bother resetting in the first place.

You are correct sir!

[gspd]

I also recommend against using separate pods, except for maybe on a rarely used short distance show bike.
The carbs will constantly fall off unless a new mounting system is devised.
The stock jetting, although not optimal, will work ok and not cause any major performance issues.

Nobody mentioned anything about the crankcase breather system.
The stock system recycles blow-by vapours back into the intake flow where they are harmlessly combusted.
With pods, you'll have to run the breather into a vented catch can, or on to the ground, preferably well behind your rear tire for obvious reasons. Either way, your bike will always be surrounded by the stink aroma of burning oil.

[SteveD]

That's the thing with cafe racers...the mods aren't necessarily about performance.

[Zombie Master]

That's the thing with cafe racers...the mods aren't necessarily about performance.

Then why call them racers?