QuestionI am proud owner of a 1981 Honda CB 650 Custom since it was new and the bike has been dependable, but as everything gets older it is need of repair which includes the carburetors. I have experience in carburetor repair from rebuilding automotive and small engines so it is not like I am a novice.
My first attempt was earlier this year which I pulled the carbs from the bike dissembled completely and cleaned every part in carburetor cleaner and used wire to clean out any debris in the ports and used an air compressor for further cleaning. All “O” rings, seals, jets, float values, etc. was replaced as part of the rebuilding process.
Carburetors were reinstalled and the bike started fine and the carb’s were synchronized and everything was fine for several weeks of riding until I noticed a small gas leak from the bottom bowl of the #3 carb. Seeing this is an inside carb and to access the fuel bowl is next to impossible, I had to remove the carb’s from the bike.
I found that the rubber seal for the float bowl had a cut which caused the leak and was replaced. The carb’s were once again reassembled to the bike. The bike started fine with no leak and took it for a ride and everything seemed to be fine until I tried to start it later in the day and this is where the problem starts that I am having.
With the choke pulled out the bike starts fine and runs for about 10 seconds and then quits. Even with no choke the bike will not start without increasing the throttle. I tried opening the throttle fully and the bike will start for about 5 seconds and sounds like it is running on one cylinder rather than four and then quits. Leaving the bike sit for several hours and repeating the process the same happens.
From investigation the carb’s are getting gas into the bowls as found by opening the drain screws at the bowls and gas pours out easily. Thinking it maybe a fuel filter problem, I replaced the fuel tank petcock. One thing I need to mention is that because of problems in earlier years, I have disconnected the automatic fuel shut off.
Thinking I may have done something wrong with the rebuilt, I sent the carb’s to an individual that specialize in rebuilding vintage carburetors. It has been over two months and no update when my carb’s will be looked at.
Being impatient and hoping to ride this year, I found a nice set of carb’s from a salvage yard and rebuilt them as I did earlier and everything worked fine until I tried to restart the bike again and the same problems have reappeared.
Hopefully you may be able to have a recommendation of what I should look at. Your help would be truly appreciated.
AnswerJeff, thanks for sending in detailed notes on what you have done and the outcomes.
I know there are a number of vacuum hoses connected to the carbs and intake manifold, so check those for damage and look at the intake manifolds, themselves. Especially with taking carb racks off and on a few times, they have been known to crack and suck air.
Check the gas cap for proper venting. Years of fuel vapors can plug up the venting system and cause a fuel lock in the system. What can happen is that the gas will flow slowly to fill the bowls, but not quickly enough to keep them full all the time.
You might backtrack and establish a known baseline for the motor... adjust valves and check compression. Then check for a good strong spark at all plugs. If spark is weak on either the 1-4 set or the 2-3 set, swap the spark modules, located behind the battery and see if the problem changes from one set to the other. Test the spark plug caps for resistance values around 5k ohms.
After you start it and it dies, pull the plugs to see what they are telling you... wet/fouled or dry lacking fuel supply. Try removing the drain screws again on the bowls, and then turning the fuel petcock back on to verify that there is a strong fuel flow feeding the bowls. If weak, crack the gas cap open to see if the cap is interfering with the fuel flow. Having a new petcock/screen eliminates that often overlooked area of flow restrictions.
Make sure that the choke cable adjustment is fully closing all four butterflies at full choke.
Bill Silver
