Carburetor problems
One of the things most CT90 owners deal with at some time or another are carburetor problems. While often, they're identified in bikes that have sat for years, they can also occur to bikes that are driven every day. Before going through this section, have a look at this CT-110 carb diagram. The CT90 is similar but distinct, and early CTs are different yet again.

Leaks
Leaks are one of the more common findings in bikes that have sat for a long time. You put fuel in, and the stuff starts leaking out of the carb. Leaks usually occur at one of three places, and while two of them you can see, the third is actually the most dangerous.
  1. Overflow tube: if you have fuel coming out the overflow tube, which leads down below the engine, or at the place the tube would be if someone hadn't taken the tube off, the float bowl is overfilling. This is usually caused by a stuck float needle, and this is common in bikes which sat for a long time. The reason is, the gas gradually dries out of the float bowl, and as the level falls, the float needle opens, and then the fuel dries out, leaving resins behind which glue the needle in the wide-open position. You put new fuel in, the float bowl fills, but the needle never completely closes, causing overflow out the tube. This is fairly easy to correct, provided there isn't a lot of corrosion in the area of the needle and jet. Take all the brass parts out, clean them with brass wool and machine oil, make sure they all fit smoothly, then put it all back in. If they still bind and leak (you can test this without fuel, by reinstalling the float and blowing air in the fuel inlet tube. It should be airtight with only a little upward pressure on the float). If it still binds, get a rebuild kit and replace everything.
  2. Fuel shutoff valve: the valve on the side of the carb that controls the main, reserve and OFF positions has a seal in it which can crack or dry out, and it will leak around the lever. If you can make the leak stop or slow down by applying inward pressure on the center of the valve, take it apart and replace the seal in there.
  3. Leaking into the head: this is the most dangerous, and occurs only if the overflow is blocked. In this case, because the carb is higher than the head, overflowing fuel will go down the intake into the head, and eventually wash all the oil out of the cylinder and gradually fill the crankcase with gasoline. If you try to start such a bike, several bad things can happen. The head could go into hydraulic lock (the combustion chamber is filled with incompressible liquid) and you could break all kinds of things. The bike could start, but without oil in the bore and with diluted oil in the crankcase, you will trash the engine. Worst of all you could detonate the gasoline vapors in the crankcase, basically blowing the engine apart while it's under you. Overflow tubes can get pinched off by being installed incorrectly or kinked. It could get plugged by insects or dirt or grease. Either way, you need to clear it out and make sure it works. If you cannot be sure it works, drain the fuel out of the tank so that you don't trash it. If you find that the crankcase is full of fuel, drain it into a large pan, then take the pan safely away from any source of ignition (out into the middle of a field is good) and let it sit for a couple of days to let the gasoline evaporate, then put the oil in a container and recycle it. Before you kick the bike over the first time after this, squirt a little Marvel Mystery Oil into the spark plug hole to lube the bore until the new engine oil gets to it.

Idling
If the bike will start on choke but will not idle without having the throttle open a little, the idle jet is either clogged or misadjusted. If it will start on choke but dies when you've warmed the bike up and flip the choke open, then the idle jet is partly clogged or misadjusted. Open the carb up and clean it out, and if it refuses to respond, replace the idle jet.

Dies when you hit the gas
If the bike idles normally but won't rev up, it's likely the idle jet is fine but the main jet is plugged. Open it up and clean it out. Often, a bit of dirt or resin will get in there and block the orifice. Hit it with some carb cleaner and a very fine piece of wire, such as off a wire wheel. Do not ream it or poke anything harder in there. If you can't unclog it, replace it. There's also a chance the the jet needle (the long, pointy thing on the end of the throttle cable) has come loose and isn't pulling far enough up out of the jet orifice.

Won't run right wide-open
If you get good idle and good acceleration through midspeed, but the bike starts to sound like it's running "heavily," with effort, when you're wide open over 35 or 40 mph, and the sound improves when you back off the throttle, you may have either a jet that's too small, or the jet is partially clogged and not letting full fuel flow through. It's important that you not try to force such a bike to run too fast, because it's running too lean. Running too lean increases engine temperature and in extreme cases can put holes in the piston. It may also be time to re-jet the bike.

Re-jetting is a process of trial-and-error. Too small a jet won't let enough fuel through at high speed. Too large a jet causes the bike to run rich. Jetting needs can change over the life of the bike and depend on its usage. For example, as the bike loses compression over time, it can need a slightly richer mixture to maintain similar performance. This mean, "bigger main jet." If you've overbored the bike, resulting in a larger combustion chamber, it will need more fuel to maintain the same performance. If a bike was jetted for use in the mountains, where the air is thinner, and you take the bike to Delaware or Florida, where the air is denser, you may find that you need to re-jet.

Most Honda dealers will still carry a jet kit. In general, this is a selection of jets you can choose from and possibly try out, then go back and get a different one if you don't get the results you need. There's no one jet that's best for a CT... the early ones used large main jets that on the order of a #75, but later CTs used smaller jets. The relationship between main jet, the size and placement of the needle, and the idle jet, is somewhat mystic and hard to nail down. Basically, whatever works for you, works. If you don't know what jet you have in the bike now, take it out and look on the rim. The jet size number will be stamped into the brass there.

Runs for a while, then dies. Let it sit, it will run for a while again
This usually means that the thing is sucking the float bowl dry, and that the float needle is stuck partway closed, such that it cannot replenish the bowl as fast as the engine is burning it. When you let it sit, fuel is trickling into the bowl and thus, you can start and run again after a while. Clean or replace the float needle. Also, make sure the float has no leaks in it. If it will float in water without taking in any, it's fine. The older floats are brass; the newer ones are plastic.

Extremely rarely, this problem can be caused by blockage of the vent holes in the gas cap. Remember, as fuel gets used out of the tank, air has to come in to replace it, otherwise a partial vacuum will form and the fuel won't flow. Make sure the gas cap vent holes are not blocked. If you've had your tank relined, this could be why.

IMPORTANT: CARB PROBLEMS ARE RARELY TO BLAME FOR SUDDEN BIKE SHUTDOWN AT SPEED!
If the bike was running and died as if you shut it off with the key, it's almost always an electrical failure, usually the points or the ground. If it died in fits, like it was choking or running out of gas, it's more likely to be a carb or fuel problem. And remember, the leading fuel problem is, "the bike is out of gas." Even if you think it should have fuel, check.

Last updated: 09/23/2001