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Honda CB350f Engine troubles and leaking carburettors

3234 Views 8 Replies 6 Participants Last post by  hillsy
Hey Guys,

My Dad and I are in need for a bit of advice regarding our current project. We have been turning a 1972 Honda CB350/4 into Cafe style racer and have hit a snag in the final stages of the rebuild. When we first got the bike we managed to get it started without too much hassle.

Since then we have only changed the following on the engine:

  • Rebuilt the Carbs with a Carb rebuild kit from 4into1 (Using preformed O rings which fit properly more)
  • Installed a new contact breaker assembly unit including new condensers and points etc. We attempted to install the assembly in as close as possible to the same position as the previous one.
The problem at this point is that, while the engine turns over fine, it won’t fire and the carbs are overflowing petrol. The overflowing is occurring both out of the overflow tubes as well as the air intakes.

This is what we are doing/know:

  • We are siphoning petrol from a bottle rather than using the tank at this point
  • There is fuel in the float bowls of the carbs
  • The spark plugs are sparking but are not wet - we don’t think we can smell any petrol on them
  • The point gaps are correct
  • We have not got any air filter pods attached at this point
  • We haven’t changed anything on the engine apart from replacing the circuit breaker assembly and doing a general service of the clutch etc.

I’ve attached a picture of the bike so far for interests sake

Land vehicle Vehicle Motorcycle Motor vehicle Automotive tire

Any suggestions of what could be going wrong and how to fix it would be greatly appreciated,


Tim and Chris (NZ)
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Good tip from Trials Rider - except I'd put an empty bowl under the carb to catch the leaking fuel.

I don't know if you have the same issue as here in the USA but we have alcohol in our fuel and it raises holy hell with gaskets in carbs where the fuel will leak out of the side of the carbs. We here have to ensure to only use the highest quality carb rubber gaskets. If you don't have alcohol in your fuel and/or your fuel leak is not in between the two half of the carb then you have either a set float level problem or the fuel is not shutting off when the float level reaches it's max height.
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