Carb Leaking Fuel from Overflow

jensou

jensou

New Member
Hello CT70'ers, I've been trying to get this '71 CT70 K0 to start for awhile now. I rebuilt the carb. I've got spark. I just rebuilt the throttle and every single time I've tried to start this thing fuel starts leaking out of the overflow. I can't even get it to pop. I've checked the float height (20mm?) multiple times and checked to make sure the float needle is moving freely. All seems correct until fuel starts leaking :frusty:

I'm at a lost as to what I am overlooking. Any help would be immensely appreciated. Thanks!
 
Drain screw is as tight as I can get it and then some. Once you start kicking over the engine all the carb does is completely fill with gas. I've got it apart right now and once again checking the float needle.
 
Hi, I know I'm new here and this is my first post, but I couldn't help reply. I recently had this same problem and could not figure out what was going on either, tightened the drain, replaced the O-ring on the drain, checked float level. Finally I did just an old school "blow/suck" test. I took the float bowl off, put water in the bowl, plugged the overflow brass tube with my finger and blew into the drain tube. It ended up bubbling, it was a hairline crack that you could barely see, the crack ran down the length of the brass tube. To fix, this is what I did, I couldn't find that brass tube anywhere and didn't want to replace the whole carb because of that. So I brazed it using solder, took a soldering gun and applied a thin layer of solder down the crack. Then (prob over kill) attached a vacuum gauge to end of drain tube, plugged tube at top, and applied vacuum for 5 min to make sure was good. Problem solved, as long as you know how to solder, which is not that hard, you will be fine. Good luck
 
when you rebuilt the carb Im assuming you removed the needle seat (brass nut the needle fits inside) Its almost never fully seated, and if you didnt count how many turns it took to take it out, it would throw you float height off a bit
 
Are you certain it is leaking at the overflow? I recently had to replace the two screw vertical pot metal cap on an ST90 carb opposite the fuel valve because of corrosion at the o-ring ridge. I had fuel everywhere and also thought I had a bad float. But I carefully cleaned the old dried fuel from the carb body with some Brake-Kleen and eventually figured it out. Now I'm looking for a replacement cap for the carb that I robbed this cap off of.

Rick
 
I agree with greenug about the split standpipe for the overflow. Had one on a CB500-4 that did exactly the same thing- good call new guy- J/K
 
Thanks for all the replies! I'll definitely look for hairline fractures. That's not something I would have thought of. I was stuck thinking something mechanically was wrong. I did, however, do a compression test today and found out the motor is only registering around 60.
 
You will need perhaps 100 PSI to have a good chance at getting it started. But definitely give it a try before you tear things apart as the rings might free up for you with a little luck. A good, freshly rebuilt engine will have 140 -160 PSI of compression just to let you know how far off you are. But please do not get discouraged, a top end rebuild is not that difficult if you follow our advice on machine shop work, buying a piston, valves, seals, etc. ASK LOTS OF QUESTIONS FIRST before you buy anything. That is what we are here for . . . to provide advice, guidance and encouragement. We have all been at the same place as you with dead engines.

Rick
 
Lots of good info in this thread.

In my humble experience, I've used the following technique to resolve issues with overflow leaks.

Be forewarned, If you've painted parts around the carburetor or engine with spray bombs, spilled gas will lift the paint.

Fit a piece of transparent fuel line to the drain port on the bottom of the carb bowl. This must be a snug fit! The drain port will most likely be tapered, a correctly sized line will not require the use of a clamp.
Turn the fuel line so the opposing end is pointed up and in line with the throttle valve (should be J shaped).
Tie the line in place, try to keep the fuel line as close to the carburetor body as possible.
Level the bike front to back, side to side.
Put a drain pan under the bike.
Open the drain screw 2 full turns.
Turn fuel on.

Fuel will enter the line you just installed. You WILL see a dribble of gas from the drain screw; disregard. If your float height is correct / needle & seat functioning and your float is air tight, the fuel should settle in the fuel line at approximately the same position as the mating surface between the fuel bowl and the base of the carburetor; it should not rise higher. If it does rise higher, fuel will eventually leak out the over flow.

Turn off fuel, allow bowl to drain.

Remove carburetor bowl (it may be easier to remove carb assembly from manifold to remove the bowl, but you will have reinstall the carb without the bowl).

With carburetor installed on intake manifold, examine needle. BE CAREFUL and GENTLE with next step EXCESSIVE PRESSURE applied to the float will effect your float height or possibly damage your float / seat!

Press lightly upward on the float assembly, once the needle contacts the seat, the spring inside the needle will collapse; the little 'tit' on the base of the needle will sink into the needle body.

Having a helper would be handy. Turn on fuel while retaining light pressure on float. Does fuel leak out of carb? If so, your needle or seat is fouled.
If not, turn off fuel and release pressure on float (a small amount of gas will now dribble out).
Remove float ass'y and put it into a cup filled with water. Submerge the float using a popsicle stick or other light tool; do air bubbles come out of the floats? If yes, the float is porous and must be replaced.

A few useful tips for performing compression tests:

Adjust valve lash (.002" to .004" intake and exhaust) with engine cold prior to test.
Be sure to have the throttle wide open or carburetor removed before performing a compression test.
Bump (compression stroke) the engine 5 times and record reading.

Poor a small amount of engine oil into the cylinder via the spark plug hole (a few cc's is more than enough) and repeat test. If the value of the reading is 15 percent or more than the original reading the result is a fail, regardless of the original reading.
 
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Squeebles said:
is the float needle in the right way? the cone-shaped end goes up, pointy end rests on the float.
Click to expand...
I know this thread is really old but Squeebles, thank you for that! I made the rookies mistake and had the float needle on my bike in upside down after a carb clean out. Now I don't have gas leaking all over the place when I turn my Fuel valve to the "ON" position! Thanks again!
 
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