Rebuilding Carburetors – 1999 Nighthawk 750

Part 3: Carburetor Disassembly

The carburetors are off the bike and separated – what next? In this post, I will explain how to disassemble the carbs so they can be cleaned.

Before you get started, make sure you have the following items:

  • Four carburetor rebuild kits (one for each carb) – I ordered mine here.
Carburetor rebuild kits
  • Cleaning supplies
Simple Green, WD40, carb cleaner, brushes, steel wool

I recommend Simple Green, a can of WD40, carb cleaner, an old tooth brush, a small brass wire brush, and some steel wool.
If your carbs are really old, grimy and/or rusty, generously cover bolts and exposed threads in penetrating oil (I use a brand called Liquid Wrench) and let them sit for at least one night. Your patience will pay off, I promise.

Once separated, carbs 1 – 4 all look more or less the same: the disassembly process is the exact same for each of them.

Nighthawk Carburetor

Let’s start with the float chamber – to remove the float chamber cover, remove all four bolts (grey arrows).

Unless you have a very good reason to mess with the pilot screw* (red arrow), leave it alone. It’s supposed to stay exactly in the position it is in – and for the love of all things holy, don’t tighten it! If you do have to remove it, as you screw it out, carefully count the turns before it seats lightly. When you reinstall it, turn it the exact same number of turns.

You might notice that I already removed the drain bolt in the center of the  oat chamber cover – it’s a good idea to drain your carbs before you remove them from the bike.

Once you remove the cover, you can push the float pin (white arrow) out to either side and remove the float. Pay attention to the float valve that’s attached to the float (right by the pin) – your carburetor kit should include a replacement valve. The float chamber cover’s rubber gasket will also need to be replaced, you’ll find a replacement gasket in your kit.

Carburetor float chamber

As you can see in the photo, floats can get discolored by gasoline – I’ve seen them in every shade between white and brown. It’s nothing to worry about.

Now that the float (A), pin (B) and valve (C) are out, you can move on to the jets. It’s not important to know their names but here they are anyway: main jet (blue arrow), needle jet holder (green arrow), and slow jet (yellow arrow). A flathead screw driver will do the trick for the main jet and the slow jet, and if you don’t have a miniature wrench for the needle jet holder (green), just use an adjustable wrench.

Carburetor guts: float (A), pin (B), and valve (C); main jet (blue arrow), needle jet holder (green arrow), and slow jet (yellow arrow)

It’s highly likely that the needle jet holder and the slow jet are fused together and will come out together. That’s totally fine. By the way, the actual needle jet sits inside the guts of the carb and may not fall out when you remove the holder (green arrow) – that’s okay, you can push it out when you disassemble the vacuum chamber.

Remove the four bolts (grey arrow) that hold the vacuum chamber cover in place – be warned: underneath that cover is a large spring that will jump out at you like a jack-in-a-box.

Vacuum chamber cover

These particular bolts have given me a lot of grief in the past – they tend to get very rusty. To avoid stripping their heads, make sure the bolts have been soaked in penetrating oil and you’re using the right-sized screw driver. If the bolts are seized, soak them in more oil and use a manual impact driver. Patience is key here. I almost ruined a carb once because I had to drill out these bolts after I stripped the heads …

So, this is what the inside of the vacuum chamber looks like.

Carburetor vacuum chamber

Carefully pull out the diaphragm/vacuum piston. The jet needle is attached to the piston – make sure not to bend it.

Carb cleaner should not be used on plastic and rubber, so the best way to clean the jet needle is to stick a Philips head screw driver down the piston and twist it. The needle holder is spring loaded and will jump out easily. The needle will just fall out when you turn the piston upside down.

Diaphragm, vacuum piston, jet needle

If the needle jet (yellow circle) didn’t fall out earlier, you can remove it now by carefully sticking a long, thin object (a small screw driver, for example) down the carb and pushing the jet through the jet shaft.

Needle jet

Almost done – the last step in this disassembly is the removal of the starting enrichment valve (blue arrow) with a wrench.

Remove the valve nut (A) with a wrench and pull out the spring (B) and the valve (C).

Valve nut (A), spring (B) and valve (C)

I mentioned earlier that I had already removed the drain bolt from the  oat chamber cover – if you haven’t removed the drain bolt yet, don’t forget to do so now. There’s a small o-ring that will need to be replaced on the bolt (it’s included in the kit).

Drain bolt

So, now that you’re done disassembling the carb, it should look something like this (plus carburetor kit):

Disassembled Nighthawk carburetor and rebuild kit

Give all metal parts a thorough carb cleaner bath. The brass wire brush is great to clean the outside of the carb – use the tooth brush inside the carb, the jets and valves. Rubber and plastic parts should be cleaned with Simple Green (or dish soap).

Once everything is clean and dry, install the replacement gasket (the big square-ish one) on the float chamber cover and the tiny o-ring on the drain bolt (see E in the schematic below; it should be a snug fit – if it’s too loose, you selected the wrong o-ring from the kit).

Assembly is in the reverse order of disassembly (surprise, surprise!). If you’re anything like me and scrolling backwards through this page seems like too much work, check out this schematic from the repair manual – I numbered the parts in the order of assembly.

If you didn’t touch the pilot screw (I really hope you didn’t), ignore A – D.

Aaaaand you’re done! Except that you have to do this three more times…

For carb #2, remember to install the bracket for the throttle cables (blue) when you re-install the vacuum chamber cover, as well as the idle adjustment screw (green):

Carb #2

Carb rebuilding is not rocket science but it does have elements of brain surgery: be patient, go slow and use good tools to avoid frustration and damage to the carbs.

As you can probably tell from the photos, I don’t rebuild carbs in the garage – I do it on the dinner table. I have separate plastic containers for each carb so I can make sure not to accidentally mix up the parts from different carbs. Cleaning is best done outside (or in a garage), because it can get messy and carb cleaner is not everyone’s favorite scent.

Once all four carbs have been cleaned and re-assembled, they’re ready to be put back together and installed in the bike.

Next steps: Part 4 – Carburetor Installation


* What is the pilot screw for anyway? It changes the air-fuel mixture – if your bike is running too lean or rich, this is where you’d adjust that. But be warned: it’s more art than science with these old carbs.

Rebuilding Carburetors – 1999 Nighthawk 750

Part 2: Carburetor Separation

The carburetors are off the bike – what next? In this post, I will explain how to separate the four carbs from each other. FYI, I took the photos while I put them back together – that explains why the carbs are so clean and shiny. They were much (much!) dirtier when I took them apart.

Before you do anything else, put a piece of painters tape on the top of each of the carburetors and number them like this – it’ll save you a lot of headache later:

Nighthawk carburetor assembly

The separation process begins by separating carburetors 3+4 from carburetors 1+2. Here’s how you do that:

1. Loosen the four bystarter arm screws and pull out the shaft:

Four bystarter arm screws
Bystarter arm shaft

2. The spring and three bystarter arms will come off once you pull out the shaft:

Bystarter arms, shaft, screws and spring

3. Remove the three carburetor synchronization screws and (upper) springs:

Carburetor synchronization screws and springs

4. Take off the front bracket and the rear bracket.
In order to ensure perfect carburetor alignment, the brackets are installed with quite a bit of tension on the bolts – I wasn’t aware of this and it took an impact driver to get the bolts out:

Impact driver to the rescue (clearly, this is not my arm …)

The impact driver finally got the bolts out – not without stripping two of the bolts.

The bolts are installed in a very specific order – taking them off in reverse order might make things a lot easier – particularly if you don’t own an impact driver (my manual impact driver is one of my favorite tools – you can get one made by Tekton for about $20). And make sure you use a screw driver with a large head (#3 works best).
This is the reverse order (start at 1):

Loosen front bracket bolts in this order (starting at 1)

Same goes for the rear bracket:

Loosen rear bracket bolts in this order (starting at 1)

5. Once the brackets are off, you can easily pull carburetors 1+2 and carburetors 3+4 apart.
You will end up with two pieces: the carburetor 3 synchronization spring (left arrow) and the three-way fuel joint pipe (right arrow).

Left: carburetor 3 synchronization spring; right: three-way fuel joint pipe

This completes the first stage of the separation process. Next, separate carburetor 1 from 2, and carburetor 3 from 4.

6. Gently pull each pair apart – you’ll end up with the carburetor synchronization spring (arrow), the thrust spring, the air vent pipe, and the two-way fuel joint pipe.

Carburetor synchronization spring (arrow), thrust spring, air vent pipe, and two-way fuel joint pipe

7. That’s it – the four carbs are separated and ready for disassembly. Make sure to keep all those ittybitty parts and pieces in one safe place. I use plastic containers (with a lid) or Ziploc bags. If you loose any pieces, even just a tiny bolt, you might end up paying an annoying amount of money to replace them.

Here’s where I usually order OEM replacement parts for the Honda CB750:
BikeBandit
PartZilla
I ended up have to replace some of the big bolts I stripped when I tried to remove them in the wrong order.

This useful schematic from the service manual helped me keep track of where all the little parts belong (and, coincidentally, they are numbered in the order of disassembly – No. 1 and 2 are for California models only, by the way).

Next steps: Part 3 – Carburetor Disassembly and Reassembly

Rebuilding Carburetors – 1999 Nighthawk 750

Part 1: Carburetor Removal

Most modern bikes are fuel injected – our Triumph Speed Triples are. But the 1999 Honda Nighthawk and the 2004 Honda Hornet are carbureted. Both bikes feature four carburetors (one for each cylinder) … delicate little devils that don’t require any of your attention – until they do.

Carbs like to be synchronized, and eventually, they will demand to be rebuilt. This four-post series explains how to remove, separate, disassemble, and rebuild the Nighthawk carburetors. Note that no matter how ambitious you are, this usually turns into a multi-day project … though it could be done in a day, I suppose. Make sure to order a rebuild kit (which is basically just a set of replacement gaskets) before you get started. And when I say “a rebuild kit”, I mean: one for each carburetor. So, in the Nighthawk’s case, I needed four.

In case you’re not sure – the carburetors are tucked in between the air cleaner and the cylinders:

Carburetors on a 1999 Honda CB750 Nighthawk

Before you get started, make sure to
a) turn the fuel valve OFF:

Fuel petcock on the Nighthawk (sorry, this pic was taken after I removed the tank from the bike – that’s why the background doesn’t look right)

b) and drain the carburetors:

Draining the carburetors

When you open the drain bolts (white arrows) about 1/2 to 1 turn, the fuel will start running out of the nozzles (grey arrows) – make sure to catch the fuel with a rag, or better, in a container. The fourth carburetor’s drain screw is on the other side of the bike.

In order to remove the Nighthawk’s carburetors, you’ll have to remove a few other parts first:

  • The seat
  • The side panels
  • The tank
    To remove the fuel tank mounting bolt (under the seat) and lift the fuel tank up, disconnect the vacuum tube and the fuel tube, and remove the fuel tank.

Okay, now you’re in business. This is how you remove the carburetors:

1. Loosen all four connecting tube band screws that connect the carburetors with the air cleaner housing:

Connecting tube band screw

When the screws are loose, wiggle the tube bands to make sure they’re loose, too.

2. Remove the four air cleaner housing mounting bolts:

Air cleaner case mounting bolts – right side (battery side)
Air cleaner case mounting bolts – top
Air cleaner case mounting bolts – left side (air filter side)

3. Once these four mounting bolts have been removed, move the air cleaner housing rearward. You might need to wiggle it up and down until the connecting tubes from the air cleaner disconnect from the carburetors.

4. Loosen the four insulator band screws that connect the carburetors with the cylinders:

Insulator band screws

So far, so easy. The next step is where it starts getting a little trickier.

5. You need to disconnect the throttle cables and the choke cable from the carburetor. In order to do that, you have to first loosen the two nuts and the screw that attach the cables to the carburetor brackets:

Throttle cables and choke cable

Once the cables are no longer attached to the bracket (you can pull the throttle cables out sideways after the nuts have been loosened), you should be able to unhook all three cables from the carburetor. (Disclaimer: I always struggle with this – sometimes it’s easier to leave them attached and disconnect them during step 7)

6. Pull the carburetors rearward while wiggling them up and down until they disconnect from the cylinders.

7. Now that the carburetors are disconnected from the motorcycle, you can pull them out sideways – it’s easier to pull them out to the left side:

Pull out the carburetor to the left side of the bike

It might require an extra set of hands to push tubes and cables out of the way while you thread the carbs out.

8. Congratulations, your motorcycle just gave birth to a 5-pound carburetor assembly!

Carburetor assembly

One last thing, before you start taking the carburetors apart – stick some rags into the air cleaner tubes and tape up the cylinder openings. You really don’t want any dust to get in there, let alone have a mouse move in and make itself at home.

1999 Honda Nighthawk without carburetors

Next steps: Part 2 – Carburetor Separation and Disassembly