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Honda CB350f Engine troubles and leaking carburettors

3231 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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  • There is fuel in the float bowls of the carbs"

Way too much fuel in one or more float bowls I would say. The float bowls are meant to maintain a relatively fixed level of fuel inside them, they operate pretty much the same as a toilet tank and your toilet is over-flowing.

Things to watch for: Carburetor floats on some bikes can be accidentally installed upside-down. If all of your carbs are exhibiting the same symptom, this is possible. I once owned a 1972 CB350/4, but it was too long ago to remember if putting the floats in upside-down was a possibility on that bike, sorry about that. You are going to need to take them apart again and look.
Next possibility is that the needle and seat that the floats are attached to are installed wrong or leaking … you still need to take them apart again to know that.
Least likely possibility, your floats are sinking, have holes in them and leaking fuel into them or somehow water-logged.
Yet another possibility, your carb rebuild "kit" is crap and not doing it's job of stopping the flow of fuel from the tank when the floats are trying to close the needle and seat stop valve.

She won't fire if the float bowls are over-filled, because the carbs can't produce a venturi effect and atomize the fuel mixture on it's way to the cylinders.

Nice bike, hope you get it going, they sound awesome at high revs. Yours seems to be missing a few more parts then mine did, like fenders.
Fenders are kind of important if you plan to ride it in the rain or on anything other than totally clean roads.
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Has one thing going for them, they are Not CV carbs ;)

Took a real quick look at a crap parts schematic, don't think the floats can be put in upside-down, so it's probably just leaking past the needle & seat valve.
Only reason to need to adjust a float height is if something changed, in this case that might be missing, malfunctioning or non-original parts.

OEM parts is great advice and probably only available one piece at a time, so called "kits" always raise a red flag with me unless it says Honda on the package. You might be better returning to all the original parts as long as all the original rubber bits are intact and not turned to mush or powder.

Anybody who actually Rides their motorcycles, knows the value of fenders all too well.

Start here: -> There are 2 service manuals there that might possibly be of help, if they aren't in english or for your exact model bike just deal with it, the price is right.
One thing you might try, if you can remove the float bowl from one of the outside carbs that is currently leaking, even while the carb is still mounted on the bike, you might be able to gently raise the float with your finger and observe if that stops the gravity fuel flow, if it doesn't you definitely found a problem. :/ Obviously take all necessary precautions to not blow yourself up or anything while you are spilling gasoline around in the shop.
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