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Dellorto Rebuild: Teardown

posted by Terrence Dorsey on Monday, 22 October 2012 @ 20:02

These are steps I used to tear down and rebuild my Dellorto DRLA 40 carbs. The same steps should work for DRLA 36 carbs as well.

The basic steps are outlined on STP's Garage and more detailed information, carb theory and tuning tips can be found in Bob Tomlinson's Dellorto Superformance Tech Book.

I recommend Tomlinson's book. The writing is... sorta all over the place — typos abound, for starters. There's quite a bit of repetition since every Dellorto variant gets its own almost identical install, rebuild and tuning instructions. However, the "Carburetors and How They Work" section is pure gold in its description of how the venturis, jets and butterflies work together to carefully meter fuel and air intake at various engine speeds. If you don't already grasp how carbs operate, this will open your eyes and give you a strong foundation for tuning later on.

I have added some notes about tools needed and other details you should be aware of. It's a simple procedure if you follow the steps, keep a clean, organized work area and take your time.

These same procedures should be applicable to all Dellorto DRLA 36 and 40 carbs. My research so far suggests neither size is better than the other, and both can be tuned to run well even on big-bore engines via venturi and jet sizing.

Tools

It's a good idea to have the tools you'll need at hand when you need them. Here's a list so you can get them together.

Dellorto rebuild tools.

Note that many of the fittings are soft brass, so having screwdrivers that fit well can make the difference between stripping a bolt and getting it out cleanly. Have at least one big mother screwdriver on hand. Cheap screwdrivers that you can grind down to size are handy here.

The allen keys, if you have them on hand, make useful feeler gauges for setting the floats. It can also be done with a small ruler, or you could even make ad hoc go/no-go templates from thin cardboard.

You'll want some way to organize small parts as they come off the carbs. Egg cartons, small jars... whatever is at hand and will help you keep related parts together and not falling on the floor (where they will be lost forever).

Aside from the cleaners listed, you might want to check out AircooledTech's Tools-On-The-Cheap - DIY Soda Blaster.

Set up a clean workspace with plenty of room to keep your tools and parts at hand and in order. Rags or a roll of towels on hand to clean up spills and wipe parts down as you work.

Initial Teardown

Prep: Remove breather tubes, air cleaners, throttle links and fuel lines. I use the fuel line clamp pliers to keep fuel from leaking all over the place. Just disconnect the fuel lines at the carbs. The rest of the lines can stay in place.

Before we get started, here's an overview of the carb so you'll be familiar with the various parts.

Dellorto DRLA 40 side view.

dellortoshop.com in the UK has an exploded parts diagram and parts listing for the DRLA and other Dellorto carbs. They are a good source for parts if CB Performance doesn't have what you need or you're on the other side of the Atlantic Ocean.

Let's get started.

  1. Remove nuts holding carbs on manifold studs. These are usually 12mm, but could be 13mm. Combination wrenches work best here to get at all of the bolts. Clean and retain the nuts and washers.

  2. Remove carbs from manifolds. If present, clean and set aside the thick rubber spacer. Availability of the correct rubber spacer is spotty, so keep your if it's in reusable condition. Cover the open manifolds so nothing unsavory falls down the rabbit hole.

  3. Remove the 10mm nuts holding down the velocity stacks and the stacks themselves. Clean and set aside along with washers.

  4. Remove the air cleaner base plate. If you're lucky, the gasket comes up in one piece. If not, remove the old crud carefully, making sure not to gouge the mating surfaces.

  5. Remove five screws and washers holding down the carb top plate. As noted before, use a screwdriver that fits the screw well and go slowly. Stripped screw heads are ugly, slow you down and are a drag to source new ones.

  6. Carefully remove the top of the carb, making sure to avoid damaging the floats. Set it aside upside-down (floats up) as shown in the photo below. Watch out for spilling gasoline! The float chambers may still contain a fair amount of gas. It's a good idea to work over a rag or container just in case. No smoking!

From here on, it's a good idea to work slowly, keep your parts organized and label things so you remember where they went.

Here's the carb top and body separated. At this point you can access the jets inside.

Carb top and body separated.

Jets, Venturis and Fittings

  1. Remove the main and idle jets. They just screw out with a large, flat head screwdriver.

  2. Remove the accelerator pump valve from the bottom of the float chamber. Again, a large flat head screwdriver does the trick.

  3. Remove the accelerator jets.

  4. Remove the mixture screws (flat head screwdriver or 8mm wrench or socket).

  5. Remove progression well plugs (P3 Phillips head screwdriver).

  6. Remove air bypass screws with a narrow, flat screwdriver.

    Be gentle — they may be really stuck in there and are pretty easy to strip. Modify a screwdriver if necessary to fit the slot securely. If the screws don't move or you feel like they're going to strip out, even after sitting in penetrating oil, just leave them alone.

    Also, don't throw out the o-rings on these. They're probably in decent condition and the parts in the Italian rebuild kits don't fit correctly.

  7. Remove the fuel delivery valve pump covers. Be very careful to not lose the weights and pump balls!

    Note that the cover can be very tight. This is another case where you want a screwdriver sized exactly to the slot in the cover. Customize one if necessary. Proceed slowly and try to not bung up the soft brass. That said, new parts are available from CB Performance at reasonable cost.

  8. Remove the venturi lock nut (11mm) and retaining screw.

    Pull out the auxiliary venturis, then the main venturis (see the photo below. Mains should be marked on the upper, inner surface with the size. You can also measure the narrowest internal dimension. These go in with the narrowest area toward the top and should go back in that way, but mark the top if there's any possibility of confusion.

Here are the main (rear) and auxiliary (front) venturis.

Main and auxiliary venturis.

Congratulations. You're most of the way there.

Accelerator Pump

Now turn the carb over to start dismantling the accelerator pump mechanism. The activating arm is attached to the throttle shaft and needs to be removed for cleaning and access to the rest of mechanism.

Lots of fiddly small pieces here, so note carefully the order in which they're assembled and keep all the parts together. A good tip is to take cell phone photos as you go to provide a quick reminder of how the parts go together.

Accelerator pump springs and washers.

There are several small washers throughout this assembly. Different sizes in different positions, and not interchangeable. Watch out for them and note which ones go where.

  1. Remove the clip from the pump arm under the carb body. Pull the arm out of its hole. Watch out for the washer.

    If the arm doesn't want to come out, there are two optional methods. First, you can loosen the bracket attached to the throttle shaft and move it to a better position for taking off the pump arm. This didn't work for me.

    Second, leave the arm in place and take it off later, after the other pump parts have been removed.

  2. Remove the adjusting nut (6mm). Note order of springs and washers on the pump arm.

  3. Remove the four screws holding on the pump cover.

  4. Pull off the pump cover slowly, making sure not to lose the spring underneath.

Be careful removing the accelerator pump diaghrams. If not torn or punctured, they could be useful spares. However, the rebuild kit includes new ones.

Fuel Inlet and Filter

Remove the fuel inlet fitting. The specifics seem to vary from installation to installation. Mine used Solex-style fittings and needed a 16mm wrench.

There's a filter inside the fitting. A replacement is not included in the rebuild kit. Be careful with this, clean it up and reuse it if possible.

I'm pretty sure new washers are included in the rebuild kit.

Floats

Set the carb body aside and go back to the top plate.

  1. Remove the clip and two retaning screws holding on the float. Carefully remove the float itself and set aside.

  2. Remove the fuel inlet needle and seat (9mm).

    Make sure to get out the paper gasket under the seat. If you leave it in there and accidentally put a second new on in there, you'll have a devil of a time setting the float levels correctly.

  3. Remove the paper gasket from the top plate.

That's it. You've torn down the carb and are ready to start cleaning, checking, measuring and rebuilding.

Accelerator pump springs and washers.