Dirty, Dirty Carbs

The battle of the century seems to be keeping this 28 year old Honda CM400 running. Actually it's not been that bad, but she does take up quite a bit of time every couple of weeks. Right now I'm trying to figure out why my engine is getting so hot, and I mean like skizzle hot. The crankcase is averaging in the 230-250 degree range as far as I can tell, which is a bit on the hot side. Now, there are a list of culprits that could be causing this, the most likely are or were a dirty carburetor, valves out of adjustment, or the timing is off. I don't have a timing light or feeler gauges, so that kinda pushes me into checking the carbs first.
This didn't bother me since I had been looking at the blow up diagram from bikebandit.com for something like two months now. I was able to find some replacement screws for ones that had been stripped before she was mine, so I was ready to tear into them. First thing first, I gotta get them off the bike first. This is not super hard, but does require you to take off the gas tank to get to the throttle and choke cables. After taking off the tank, drain the float bowls, and loosen and detach all cables, except the choke, WHILE KEEPING TRACK OF WHERE THEY WERE!!!! I dunno, take a photo or something, I had 3 cables and marked them 1,2, and 3 to keep em in order. Worked for me, why not you? After you detach cables 1&2, pull the choke out( at the handlebars), and loosen the clamps around the boots. At this point it's kinda by feel to get them out. I straddle the bike, and pull the carbs back to get them out of the boots connecting to the engine block. Then I'm able to kinda wiggle and force them out to the left. If you pulled the choke, that little tab won't get hung on the rear boot, making life a lot easier. With a little finesse, they should pop out. Each time I take them out, it varies in difficulty, sometimes are easier than others? After the carbs are free, remove the throttle cable and viola! A set of carburetors in hand!

After freeing them, be careful with them. Don't shake em or do stupid crap like that. Now you want a CLEAN AND ORGANIZED working area. I can't reinforce this enough, there are lots of tiny parts in there. I lay a cheap towel down and put all my tools and needs on it before I start, this tends to save a lot of time in the long run. All I really use is a can of carb cleaner, a can of compressed air, a phillips head screwdriver, two flat head scew drivers(one large, one small), and maybe some pliers and tweezers. Along with that, I'll use tin cans to soak jets and other parts in and a larger container to soak other parts in. Now it's time to open them up. I start with the bowls, seems logical and it will tell you what you can expect in the rest of the carb. A preview of this is seen when you drain your bowls before taking the carbs off. In my case, lots of particulate rust from a rusty gas tank and a crappy wire fuel filter. This is not a good thing to see. Before you start to spray everything with carb cleaner, remove all rubber pieces, carb cleaner will destroy them. Replace them if necessary, be careful with them if you're not! After cleaning out the bowls, start taking the jets out and looking for clogs and whatnot. It's not recommended that you stick wire through the jet holes, but in the case of heavy varnish, it might be necessary. I like to use the wire out of the center of a twisty tie. They're super thin, so there's no threat of damaging the needle. After checking the jets, throw them into a can to soak in cleaner, and continue to disassemble the body. Remove the floats and the float needle. Check the float and needle for defects.

My cabs are cv type with pistons, so I flip them over next to take the pistons apart. After removing the main screws, slip the body apart to expose the slider and carb interior. Remove the needle jet and toss it into the can with the other jets. The whole body is almost apart at this point. Parts are everywhere, things could get hectic. All that's left is removing the accelerator pump and air cut off valve. The air cut off valve is on the side of the body. Take the cap off slowly, there is a spring underneath it that can be lost easily. The pump itself is a brass piece with a rubber gasket. Check to make sure there are no tears or cracks in the gasket and remove the small o-ring in the small hole to the bottom left of the pump. Next is the accelerator pump. Similar to the air cut off valve except its a brass rod with a rubber gasket. Same drill on this one, look for defects and clean gently. At this point, the carb body is completely apart and its time to clean it. Clean it good, blow cleaner into all it's little holes, nooks, and crannies. Make sure all the paths are clear of obstructions and then blow all the cleaner out with compressed air. The insides should be shiny clean!

Time to reassemble. I put it back together the opposite of how I took it apart. Makes it easier for me to get it back together with all the parts in the right order. I tend to lay pieces out in the order I take them out, so I just have to move down the line to get it back together. After reassembling the first carb, do the exact same thing to the other one. It's best to clean them separately. If you forget how to put a piece together, you can look at the one that's still together, plus, they tend to not like having parts swapped between the two of them. I hope this helps someone who wants to work on their carbs but is hesitant to. It's not that hard if you have some mechanical ability and can stay somewhat organized. In the end, I pulled some nasty crap out of my carbs. LOTS of rust. I plan on resealing my tank this winter. Unfortunately, she is still overheating. I'm becoming more suspect of a small hole in my lower head gasket. It might be blowing back into the cylinder head causing my mixture to lean out, thus causing it to run hot. Sounds like there will be a blow by blow of how to replace most of the gaskets and seals in the engine this winter.