Carburetors

Thursday, February 2, 2012

Carburetors

Old bikes usually come with old carburetors. Most of the machines I brought home over the years did not run, and 99% of the time the culprits were neglected carburetors. The set on my CX500 project was no exception. As you can see from this picture, the carburetors were leaking from the crossover pipe, underneath the cups, and down by the float bowls.

Fortunately, rebuilding old carburetors is not a rocket science. All it requires is time, patience, and a little bit of money. Most old carbs have dried gaskets and clogged jets/passages, due to time and modern gasoline. The ethanol we are forced to consume does a number on carburetors, as the gasoline turns into something that doesn't even resemble gasoline very quickly.

First order of business was to take the carburetors apart for thorough cleaning.

As you can see, even the vacuum cups were covered in a gummy residue.

In the past, I always used solvents to clean these, but the carb cleaner is not the most innocent substance to play with. On my last carburetor rebuild, I tried using Pinesol as a cleaning agent, and while countless users and believers in its potency claim excellent results, my attempt did not go as well as I was hoping. So, this time I tried something different - baking soda blasting. I do not own a blasting gun, and my compressor is rather small, but I used a two-feet length of vinyl hose and my duster gun. There are instructions online, so I'll spare you the details, but the simple setup essentially creates negative pressure to draw the baking soda up, where it mixes with the fast exiting compressed air. This is not the most efficient setup, but an easy, inexpensive one that actually works. I bought a 5lb box of baking soda at a local grocery store, attached my duster outfitted with the vinyl hose, and started at it. I quickly discovered that with my setup 80psi was just about ideal pressure. One word of caution: the soda gets everywhere, so I did this outside. The soda was gentle enough that I could hold the parts in my hand, yet potent enough to remove the nasty residue rather quickly. The next rain washed just about all of the excess off my yard. After blasting, I simply washed the parts in a stream of hot water and blew all passages with compressed air.
The results were excellent.

The soda removed everything except for some old liquid sealant and the aged factory finish, which was peeling off all over the float bowls and cups. These I lightly sanded and painted, to protect the exposed aluminum from future corrosion.

Then I took over the living room floor, and spread out all I needed to complete the task: New O-rings for the crossover pipe, new jets, float needles, gaskets, needles, O-rings and pilot screws, and air cut-off valves. Essentially almost everything that can be removed except for the butterflies and pressed-in pieces.

I also replaced all fasteners with new, stainless steel, buttonhead fasteners. Yes, I used split washers and anti-seize to prevent the future seizing of the stainless steel fasteners in the aluminum bodies.

First came rack assembly, along with fitting the crossover pipe with the new o-rings. That parts is a little tricky as everything has to align at the same time, but it was much easier on this set than on the CB750 carbs I did last time (unlike the CB, on this set there is actually ample room to play with the various parts). After that, I fitted new jets (I stuck with stock sizes for the time being) and float needles, checked the float level, and continued with the assembly.