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Ask Doug E. Your Own Question
Doug E.
Doug E., Kawasaki Master Technician
Category: Motorcycle
Satisfied Customers: 2886
Experience:  Professional motorcycle mechanic since 1978. Vintage motorcycle restorer.
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Hello, I have a 1981 Honda CB650 with carburetor trouble.

Customer Question


I have a 1981 Honda CB650 with carburetor trouble. I recently took the bike to my mechanic to have the carbs cleaned and now they will not stop leaking. The carburetor never leaked before the cleaning. The bike is back in the shop but he cant figure out why they wont stop leaking. He replaced the the float needles but that didn't help. The leaks are irregular coming out of different carbs each time. Is there a way to fix these carbs???

If there is no way to stop the leaking are there any other carburetors that will fit on this bike (33mm)??
Submitted: 6 years ago.
Category: Motorcycle
Expert:  Doug E. replied 6 years ago.

There are a few possible solutions, but it's important that your mechanic is familiar with vintage Honda carbs.

The needles can be replaced in those, but the seat is pressed into the body and can't be replaced. You can clean the seat by using a q-tip and pearl drops toothpaste or a similar product with very fine grit. This may help the needles seat. Make sure that was done.

Check the floats themselves to see if they are damaged or leak.

The float levels must be set using a fuel level gauge and not by measuring them. This will not work on old carbs that have uneven wear. Level the carbs and supply fuel to them. Attach a clear hose to the float bowl drain fitting and open the drain. Route the hose up alongside of the carb body and when the fuel stops rising, measure from the level in the hose to the top of the float bowl. See my diagram for how that test looks. I would set each one at 2-4mm below the top of the float level.


The other problem I see is that if a real harsh cleaner was used, the overflow stand pipe in the float bowl can leak where it is pressed into the float bowl. An epoxy type sealer is used to fit those.

If you follow those fixes your carbs will work unless they are total trash. Let me know what you find.


Customer: replied 6 years ago.
I'll try this out and let you know the outcome. Thanks.
Expert:  Doug E. replied 6 years ago.
Good luck.

Customer: replied 6 years ago.

I forgot to mention one other aspect of this problem, apparently every my mechanic has tested the carbs on the bench they work perfectly but when mounted on the bike they leak after running. Would this info help narrow you analysis of the problem?
Expert:  Doug E. replied 6 years ago.
Has your tank be cleaned? It's very possible there are micro particles of rust in the tank that are messing up your carbs every time they go back on. On a bike of that age it is a good idea to seal the tank to prevent any rust problems. I use a product called POR 15 that comes in a kit with the tank acid wash, rinse and sealer. This stuff is so tough you can't even scratch it when done. Avoid using the rubbery sealers like Kream because they can come loose over time. You can also use a product called RedKote that is made for radiator sealing. I have never used it, but it is a favorite among those who restore tractors.

The other possible solution is that the float level is just a touch too high and the vibration of running the bike causes it to flood just a bit. Check the float levels on the bike and you may need to set the floats to the minimum level to correct this.

I live on a nasty dirt road and on my 750 I had to set the floats to the minimum level to keep it from flooding out before I reached the pavement. The point being is that all vintage bikes have a personality and what is perfectly set up for factory specs may not work in the real world. If the tank is sealed already then it may just be a matter of setting the float levels. Be sure to use the gauge method. That is the only thing that will work on older carbs. If he is trying to measure the float arm height, you will never get it right.

Customer: replied 6 years ago.
Thanks Doug, I actually just had the gas tank professionally cleaned and lined so there's no rust there (my mechanic's advice as well). I will tell my mechanic to set the level as low possible and try one more time. Let you know what happens
Expert:  Doug E. replied 6 years ago.
Set to the lower of the two specs and check to make sure the float arm pins aren't worn. That's not very common, but I've seen a few wear enough to cause the float arm to rock a bit when running.