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Motorcycle Repair/Honda CB900 Carb problems


Hi Bill, I have a Problem With my Honda CB900C 1982 Carburetors.
I put a big bore Wisseco pistons and i try to start my bike but it was really hard , i clean all the carburetors an i got it "running" but seems to have a problem with the carburetors , check them and i am not getting any gas in the carb # 4  (Great in Carb # 1 and 2 , fair in #3 but none in # 4) . Do you have any idea of what can i do?? Also i was checking online and some sites recommend to upgrade my carburetors jets . What do you think?
Any advice it will be helpful .
Thank you very much for your time.

Alejandro,  If you only put the big bore kit in without changing the camshafts or other components, re-jetting should be minimal, but probably necessary. If you have changed the intake system (air filters) and/or the exhaust system, generally carb jetting is necessary. It really shouldn't change the way the bike starts and runs at idle/low speeds. If you are not getting fuel to the #4 carburetor, either the fuel passage connector to that carbuetor is blocked or the needle/seat for the float valve is blocked up or the needle is sticking in the seat for some reason.

I would hope that you accurately timed the camshafts on reassembly, as that will certainly cause all kinds of tuning issues if the cams are out of time, even one tooth.

The 1981-82 bikes have a vacuum operated fuel valve system which can cause problems if the vacuum diaphragm has failed, causing reduced fuel flow to the carburetors. If everything was dry when you tried to start the engine, it is easy to run the battery down cranking the motor over while the fuel valve tries to refill the carburetors. You should apply vacuum to the diaphragm line externally, to allow the carburetors to refill from the fuel tank before trying to start the engine.

You have air cut diaphragms on the carb rack which need to be checked for good diaphragms as well.

Note there are interconnecting tubes with o-rings on each end, which function to:
1. Fuel to all 4 float bowls
2. Vent all 4 carb float bowls
3. Feed fuel from the accelerator pump diaphragm system to all 4 carbs.

I suspect that your carb cleaning was restricted to just cleaning jets and the bowls, not a complete disassembly of the entire carb rack to replace all the o-rings between the tubing connectors. Any dirt, debris, varnish can work its way down the passages and block fuel flow.

Apply vacuum using a vacuum tool or even just sucking on the vacuum hose with your mouth long enough to allow all 4 carbs to refill with fuel. Hold vacuum for about 1-2 minutes, then check each float bowl drain screw to see if there is fuel present when you loosen the drain screws on the float bowls. If all are full, then you didn't give the system enough time to refill itself using the engine vacuum signals. If you don't have an equal amount of fuel in each carburetor, then you have to remove the carbs and determine where the blocks are to feeding fuel equally.

Old gasoline deposits can cause all kinds of fuel metering issues, plus the new gasoline with alcohol causes new problems for these carburetors which were not designed for this kind of fuel mix. I am finding that even with stock engines and intake/exhaust systems in place, the alcohol in the fuel is causing the engines to run lean, so I have had to re-jet even stock engines to recover their original performance again.  A quick look at the online microfiche pages shows the CB900C models to have #105 mains and CB1000C editions running #110. You might wind up having to use either #110 or #115 main jets to correct fuel issues related to the increased bore and the fuel types now. Always use premium fuels in any air-cooled Honda engine from the 1960s-80s.

Also related are spark timing, condition of the spark plug caps, coil wires, spark units and spark advancer system, all of which can cause poor performance and issues with fuel metering.

Bill Silver  

Motorcycle Repair

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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