Fuel System – Airheads Beemer Club

Inspecting Dell’Orto Carburetors

If you are thinking of changing to Dell’Orto carbs, there are a few inspection items that you want to be aware of. These are:

Head Spigots and Connections to Carbs

1. Be aware that you must have the appropriate sized head spigots in order to mount the 38mm Dell’Ortos. Most R-bike heads either have 32mm or 40mm intake spigots threaded into the intake side of the cylinder head. In order to mount the 38mm Dell’Ortos you must replace these spigots with the appropriate sized 38mm spigots, which were used on the R90S bikes and are still available from BMW. Remove the existing spigots with a spanner wrench after soaking them with good penetrating oil and leaving them overnight.

2. Sometimes the airbox connections need to be modified in order to securely fasten the Dell’Orto carbs. For an inexpensive custom fit, go to your local Napa auto parts store or hydraulic hose supplier and purchase a 6″ section of reinforced rubber hose (2 or 2 1/4″ I think but measure yours to be sure). Cost is cheap and you can get several sections cut from 6″ of the rubber hose.

Fuel Connections

3. When you get the Dell’Ortos, remove the fuel connection on the outside of each carb and inspect/replace the fuel filter inside. Filters normally cost $1.20 each, which is cheap insurance. Also, don’t forget to check and clean the BOTTOM fuel filters on your straight BMW petcocks, if you have them. This can be done without removing the fuel tank by simply turning off both petcocks and unscrewing the lower connection. Inside you will find a fine mesh screen that captures most of the contaminants in the fuel tank feeding to the carbs.

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Poor Fuel Mileage, horsepower, etc.

Why do motorcycles get such poor fuel mileage compared
to many cars, which are vastly bigger and heavier?
How much horsepower and/or torque increase does it take
to improve performance? What about RAM AIR?
What is the best cruising RPM?
Premium fuels versus mileage and combustion temperatures.

All objects …..cars, motorcycles, bicycles …..and you! …..when moving through air …..create friction and drag. On Earth, the weight of the atmosphere, held to the Earth by gravity, causes a higher pressure at sea level, than it does on a mountain top. The higher you go with your motorcycle, the faster you should be able to go, theoretically, due to less friction with the air ….or; conversely, the less power from your engine you would need for the same speed as at sea level; due to the reduced air drag. While friction and drag are not the same, you can regard them as the same for this particular bit of reasoning, which, other things not considered, is true about the speed, etc. If your motorcycle (and you!) are not moving, but if you are exposed to the wind, that creates friction and drag. Of course, if the wind is moving towards you while you are riding, that effectively also creates drag.

As you ride higher in altitude, the oxygen content of the air is less (by weight), and your engine will have less power as you go up in altitude, which will reduce performance increases from less air drag and less friction. In some instances, fuel mileage will increase with an altitude increase, especially so on fuel mixture compensating engines, such as those with fuel-injection. Mileage may decrease or increase, especially in hilly or mountainous riding. Sometimes supercharging or turbocharging can improve fuel mileage, particularly with small engines with smaller combustion areas (cylinders and heads). Mileage can also go down, if you use more and more throttle. Many factors enter into fuel mileage.

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Petcocks, Fuel Filters, Choke, etc.

Each petcock is fastened to the tank by both right hand and left hand threads. As you rotate the large “nut” CW (facing from below), that tightens the nut to the tank while at the same time it brings up the petcock. The petcock large nut has to be FIRST engaged the proper amount onto the petcock BEFORE screwing it upwards to begin to engage the proper amount of tank threads. If you have never done this before you will have to fiddle with this until you get the right amount of petcock and nut threads assembled …to start …and finish…the petcock-to-tank fastening process. Once you learn, it is easy. You want approximately the same number of threads engaged on both the petcock and the tank after fully tightening.

BMW has used numerous petcocks over the Airhead production years. There are five types of petcocks normally seen on our Airhead motorcycles. All except the /5 bullet types are easily re-buildable. Usually this means that you unscrew some knurled or slot “nut” at the handle area (after removing a black trim cup, if it is there on your version), and you can replace a gasket that might be bad, or clean and very-faintly-lightly silicon grease the moving parts. You then have a much smoother operating petcock. On some there are one or two dimpled discs, which can be repaired with a shaped punch, lightly hit, for a better detent action; I usually don’t bother. These discs have a locating tab. They can confuse, so take notice of how they were assembled as you take things apart. You cam figure it out, if you forgot to take notice.

Our petcocks have a Reserve function. Except for the bullet type, photo below, the handle long portion upwards towards the tank is the reserve position. Horizontal (either way) is always off. German “AUF” printed on the body/cap means on, not off. Handle lever downwards means on for the main part of the tank.

Early /5 Bullet Style Everbest Petcocks were different in how the handle indicated fuel flow, & the rotating handle affecting gas flow from was in the opposite way from standard piping flow.

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Fuel Caps

Removing/replacing/rekeying other locks in the later Airhead (and Classic K bike), luggage, etc., will be found in Snowbum’s article 75A on his website, http://bmwmotorcycletech.info.

WARNING! ….BMW SCREW CAPS SHOULD NOT BE OPERATED LIKE AUTOMOTIVE CAPS! AVOID spinning these motorcycle fuel caps to the ratcheting point when tightening them! The ratcheting parts WILL eventually wear out, and WILL eventually cause you problems in trying to remove the cap. BMW Airhead fuel caps are not constructed like car caps. On most modern car fuel caps, the cap is designed to be rotated to the ratcheting point. That is done because the car caps must fully seal a complicated fuel fumes venting system; if a car cap is not rotated enough, the cap can leak fumes, which will likely be detected by the car’s computer monitoring system, and a “Check Engine” light will illuminate on the car’s dashboard. On our Airhead BMW’s, the cap for the earliest Airheads is simply designed to keep liquid fuel from sloshing out. Later models had cap vents and even later ones had simple venting to direct fumes to the crankcase. These last versions had a more complicated fuel cap, called a SHED cap, but it is best not to rotate them to the ratcheting position; even though that is often done by those locking their caps. If you do tighten to the ratcheting point, try to ratchet only one notch. Much more on these various caps with the ratcheting mechanisms further down this article!

Fuel caps and venting methods for the BMW fuel tanks vary by year & model. For information on the Pulse-Air system, the fuel tank venting system, & other associated items, please read: http://bmwmotorcycletech.info/pulseair.htm

It is common to hear about an Airhead “running out of fuel”….stumbling, loses power, etc. The problem will often be the tank vent …or the fuel cap. Both problems can be identified by loosening the fuel cap and usually hearing a whoosh of air entering. Within 15 seconds, the engine will then run OK. Engine stumbling from this venting problem usually happens much more quickly with a more-filled tank and also from high speeds. While this fuel delivery problem can occur on the models with the fuel solenoid mounted on the underside of the starter motor cover, that is less likely.

There can be other types of problems, such as fuel leaks at the caps, etc.

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Fuel system. Tank cleaning & protection. Premium vs. Regular. Fuel additives. Fitting other tanks/seats. Rusted/frozen seat hinge screws. Throttle & choke cables. Fuel hoses. Tank sealants/liners.

Tank cleaning & protection methods. Premium vs. regular. Fuel additives.
Fitting other tanks…seats & fitment with various tanks. Pesky
rusted/frozen screws on seat hinges. Throttle & choke cables.
FUEL HOSES. Tank sealants & liners.

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