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.

Inside the Dell’Orto Bowl

4. Remove the Dell’Orto bowl and inspect all jets. Look carefully at all aluminum bosses where brass jets screw into the aluminum carburetor body. Because Dell’Ortos are considered by many as “racing” carbs, the jets are often removed and replaced, sometimes putting stress on the aluminum carb body which causes hair line cracks. While these are usually easy to repair with products like J B Weld gas resistant epoxy, it is something you should be aware of whenever you inspect a carburetor with the intent of purchasing it. Look for any excessive “crud” build up in the bowl which is an indication of long periods of the bike remaining idle and/or water in the gas. This can be easily cleaned from the bowl but you want to remember that whatever contamination entered the bowl may have migrated to other, less accessible parts.

Inspecting the Top of the Dell’Orto

5. Before removing the top of the carburetor, look to see if you have the straight or 45 degree angled cable feeds to the carburetor. The original R90S Dell’Orto carbs were fitted with 45 degree angled cable feeders, but these were subsequently found to cause stress on the carburetor cables and resulted in shredded and fatigued cables. The straight throttle cable feeders are the “preferred” choice and are inexpensive to obtain and install.

6. Remove the carb top and look inside. The first item to inspect is the rubber “O” ring that seals the top of the carb. Many times people disregard this gasket because it can be difficult to fit and impedes the fitting of the carb top. If this rubber “O” ring ever gives you trouble, replace it because it has lost it’s resilantcy and will no longer provide an adequate seal.

7. Remove the slide and inspect the following:

A. Are the surfaces of the slide smooth and free of nicks and imperfections? If the imperfections are not excessive, you can remedy this condition by gently buffing the slide on a buffing wheel with light polishing compound.

B. Does the slide glide smoothly up and down the slide cavity?

C. Is the slide spring that holds the slide needle in place straight , free of kinks, bends and is strong?

D. Is the needle retention clip in good shape?

E. Is the needle itself true and without any wear of bends? Replacement needles are cheap and readily available. Replace whenever in doubt.

F. Carefully inspect the “slop” that exists between the slide and the carburetor body. If excessive and there is not a tight, though free-sliding fit, the carb is worn beyond specs and will never perform adequately.

G. Look at the accelerator pump black plastic arm that faces to the rear of the carb. Is it in good condition and free sliding (good) or is the plastic “knob” worn down (not-so-good)? It is this arm that actuates the accelerator pump which is critical for quick acceleration response. If worn, replace it. Also, there is a small spring that maintains pressure on the accelerator pump arm. Make sure this spring is fitted correctly and provides adequate pressure against the arm to maintain contact with the carb slide walls and the accelerator pump.

Inspecting the Accelerator Pump Assembly

8. On the top back of the Dell’Orto carburetor is a triangular shaped accelerator pump assembly. Remove the three (3) screws that secure it and check the following:

A. Remember the water in the bowl? Check to see if any migrated and settled here in the bottom half of the accelerator pump assembly. If it did, there will be crud deposits ( actually oxidation of the aluminum caused by the oxygen in the water). If you have this condition, you need to continue tracing how far it migrated.

B. Check to see that the accelerator pump diaphragm is not cracked and is still flexible. So not, replace the diaphragm. Even if it is not cracked and still flexible, replace it. It’s cheap and easy to do.

C. Remove the brass, straight slotted fitting on the top of the accelerator pump. This valve should be free of debris and freely operate to allow air in but no air out. If this valve does not work properly, no air will enter the carburetor, and hence, no accelerator pump fuel will reach the carburetor throat.

The Accelerator Pump Jets

9. Note that here is where the action begins ….

A. Remove the air intake plumbing to the intake side of the Dell’Orto carb.

B. Get a flashlight and look inside the air intake of the Dell’Orto carb.

C. With the float bowls full of fuel, briskly crack the throttle open and see if the accelerator injectors squirt raw fuel into the throat of the carburetor. If so, great. If not, go to step D.

D. Remove the accelerator jets by unscrewing them from the carb body. These jets have a keyed cut-out in the side that aligns the jet to the proper angle with the carburetor throat so don’t worry about remembering which way they come out or go in.

E. Inspect the neoprene “O” ring that seals the accelerator jets to the carb body. If they are worn or cracked, replace them. If they look good, replace them. These “o” rings cost $.10 at any auto or Ace hardware store and it is worth $.20 to replace them both.

F. Gently run a fine copper wire that you can get from a multi-strand piece of electrical wire through the small opening in the accelerator pump jet that extends into the carburetor throat.

G. Install the accelerator pump jet and recheck item C above until the jet squirts adequate raw fuel into the carburetor throat.

H. To adjust the accelerator jet volume: loosen the retaining nut on the back of the accelerator pump and rotate the screw counter-clockwise. This will increase the stroke of the accelerator diaphragm and the amount of fuel injected into the carburetor throat. Keep increasing the amount injected until the motor starts to bog down on acceleration, then back off just a hair.

Dell’Orto Choke Assemblies

10. Chances are your Dell’Orto carb chokes are either operated individually by pressing down on the red choke covers (“tickling the carbs”) or by lowering the conventional choke lever that the Dell’Ortos have been adapted to. In either case, these assemblies are virtually fail proof, as all they do is hold the float down and flood the carb bowl with fuel, raising the fuel level and decreasing the effort required to draw the fuel from the fuel bowl to the throat of the carburetor.

That’s it. 10 easy steps to inspect a Dell’Orto carb system. Despite what many have written, the Dell’Ortos are not temperamental and difficult to run. Actually, they are fairly accurate carbs that provide improved performance and they stay in adjustment longer than the standard Bings.

For further information on tuning Dell’Orto carberators, go to: www.morini-riders-club.com/technical/balance.html

Roy Truelsen #4363

A very complete article, with a high number of sketches, on the Dell’Orto carburetors, with information on tuning, etc., is here:
http://bmwmotorcycletech.info/dell.htm