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 – I bought them online for $16/kit.
Cleaning supplies: Simple Green, a can of WD40, at least two cans of carb cleaner, an old tooth brush, a small brass wire brush, and fine 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.
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 float 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 need to be replaced, you’ll find a replacement gasket in your kit (first photo).
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.
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.
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 Philips 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.
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.
If the needle jet 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.
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).
I mentioned earlier that I had already removed the drain bolt from the float 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).
So, now that you’re done disassembling the carb, it should look something like this (plus carburetor 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 on the float chamber cover and the o-ring on the drain bolt (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 (click to enlarge).
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 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. I’ll explain that in my next post.
*What is the pilot screw for anyway? It changes the air-fuel mixture – it’s used to adjust the carbs to higher (or lower) altitude – unless you’re permanently moving to Laramie, Wyoming, or some other place in the Rocky Mountains, don’t bother messing with this screw.