Topic: CB750 K1 - #4 CARB OVERFLOW: Drpping/Leaking

[ZTatZAU]

I posted a while ago about my #4 carb dripping out the overflow spigot on the bottom of the carb.  1 drip in about 10 seconds.  The problem was compounded by a petcock that wasn't shutting off completely.

I rebuilt the petcock which then functioned as designed.  A close look at the float valve needle revealed a very slightly worn circumferential ring around its conical sealing surface.  Very minor wear which I thought could account for the 1 drop/10 second leak.

I resolved to replace the float valve set (in due time) but continued to ride the K1 750 as the weather permitted.  Before parking, I would let the motor run with the petcock off for a few minutes before shutting down.  I would then open the petcock when preparing for the next ride with no apparent leaks through the attached overflow tubes.  That is, until yesterday!

I opened the petcock to run and rolled the bike outside to start.  I walked back into the shop to get some gloves or something and when I got back to the bike there was a large puddle of gas in front of the back wheel.  So I shut off the petcock and rolled the bike back inside.

Today, I removed the #4 carb bowl and installed one of the four new Honda Float Valve Set I had ordered.  Bottom line is, when I put it all back together, (you guessed it), 1 drop every 10 seconds or so, out of the un-hosed #4 overflow drain nipple.

I did not re-set the float level after installing the new float valve but did hook up a clear tube to verify the fluid level wasn't to high and running through the overflow tube from the top.  Here's how it looked...



Does this fuel level look about right?  To my untrained eye, it doesn't look high enough to run out the overflow tube.  And to the best of my recollection is the about same fuel level I noted on all four carbs when the bike was first re-assembled.

I did consider cracks in the overflow tube but did not detect any cracks when I inspected the bowl under a lighted magnifier glass when the bowl was removed to replace the needle valve.  So I don't think cracks in the tube are the problem.  But I did consider the possibility of leaks around the circumference of the overflow tube where it is pressed into the body of the bowl.  Is this common... or likely?

Any other suggestions appreciated.
ZT

[denward17]

Sounds like it could be leaking around the circumference of the drain tube.

Test it by removing float bowl and filling it up with gas and let sit for an hour or so.

[scottly]

The fuel level looks to be a little bit low, if anything.

[ZTatZAU]

Sounds like it could be leaking around the circumference of the drain tube.

Test it by removing float bowl and filling it up with gas and let sit for an hour or so.

Great minds think alike.  That's what I did right after my original post.  I have the bowl sitting vertically in the mouth of a small jar.  It's probably been two hours and the level IS slightly down but nothing has collected in the jar.  I'm thinking one or both could be the result of evaporation.  i also do not see any bubbles in the bowl... but don't know if I would with a very small seep.  I just filled the bowl again, right to the brim, and will check it again in a little while.

If the fuel is seeping through the base of the overflow tube, wouldn't the fuel be coming down the outside of the tube rather than through the inside of the tube?  I haven't looked that closely yet to see where the fuel actually exits the overflow tube;  but does that make sense?

I'm thinking of some fuel proof epoxy (or something?) to seal up the base of the tube inside the bowl or even outside the bowl if the fuel is seeping down around the pressed fit.

The fuel level looks to be a little bit low, if anything.

I thought so too!

[scottly]

If the fuel is seeping through the base of the overflow tube, wouldn't the fuel be coming down the outside of the tube rather than through the inside of the tube?  I haven't looked that closely yet to see where the fuel actually exits the overflow tube;  but does that make sense?

Yes, it makes sense to me. If you have a hose on the tube, any fuel leaking from the outside of the tube won't drain from the hose.

[Don R]

 I've seen the bowl gasket swell into the float chamber and hang up the float. While the float valve is removed, I always turn on the fuel tap for a few seconds to flush it. Catch what comes out in a clean container to check for debris or sediment that may have gotten into the fuel path during repairs, from inside fuel hose or etc.

[ZTatZAU]

If the fuel is seeping through the base of the overflow tube, wouldn't the fuel be coming down the outside of the tube rather than through the inside of the tube?  I haven't looked that closely yet to see where the fuel actually exits the overflow tube;  but does that make sense?

Yes, it makes sense to me. If you have a hose on the tube, any fuel leaking from the outside of the tube won't drain from the hose.

Thanks Scott!  That makes perfect sense too!  When the drain hose is attached, the dripping fuel is going into the hose... not down around the hose and onto the case.  And the puddle of gas the other day definitely came through the drain hose.  So that would point away from a leak at the base of the tube... would it not?  Maybe there is a very small crack in the overflow "standpipe" tube that I can't see.

Probably wouldn't hurt to rig up a little hydrostatic test with air into the drain end of the overflow nipple... a finger over the hole in the top end... all done in a bucket of water and watch for bubbles coming through a crack.  Or, any other ideas, theories, or suggestions?

Thanks ZT

[ZTatZAU]

I've seen the bowl gasket swell into the float chamber and hang up the float. While the float valve is removed, I always turn on the fuel tap for a few seconds to flush it. Catch what comes out in a clean container to check for debris or sediment that may have gotten into the fuel path during repairs, from inside fuel hose or etc.

Thanks Don!  I did open the petcock to flush out the valve seat passage way before installing the new valve set.  Bowl gasket looks to be tucked away nicely in its groove.
ZT

[OPKP]

I have a similar problem that I've been dealing with.  Rather than start a new thread, I'll post it here and don't mean to hijack.

I have a CB750 K4, got it in 2023.  Prior owner had carbs rebuilt in 2014 and only original brass when I got it were the main jets and emulsion tubes.  Everything else was Keyster kits.

I rebuilt carbs again with Keyster kits when I got the bike.  Float valves leaked so I went back to the 2014s, which mostly dealt with it.  I got occasional leaking from 1 and 2 last year, sometimes bad (obvious valve not seated, fixed by a tap) sometimes a weep/few drops here and there.

I blamed it on the Keyster valves and ordered Honda float valves and new float bowl gaskets and replaced over the winter.  I verified float level using a gauge on the bench and again with the clear tube.  Once we got these seated, they didn't leak thru the overflow tube.  The bike runs fine, no idle problems, no signs of excessive richness (plug looks normal, no performance bogging).

Except 1 and 2 still leak.  It's a weep/few drops, but noticeable when left overnight--even with the petcock off but full bowls, I get a little bit of gas puddle up near the starter cover.  I can't see if I'm getting a drop once in awhile thru the overflow or if it's leaking thru the gasket, but I suspect the latter.  And did I mention that it only leaks when it's on the side stand (never the center stand and never when its running), with 1 and 2 being the closest to the stand. 

My side stand may be a little bent and the bike may be leaning over a bit too far to the left, with the fuel running over the side of the bowl and weeping a bit around the gasket.  Could that be the issue here?  Would the gasket, if properly seated, contain that??  I will triple check float level.

[ZTatZAU]

I have a similar problem that I've been dealing with.  Rather than start a new thread, I'll post it here and don't mean to hijack...

No problem OPKP!  Interesting post!  My Brass is all original Honda.  And a new Honda float valve assy was installed in the still offending carb.  I have the problem bowl setup in a jig on the bench and it's definitely loosing fuel.  My goal today is to try to nail down where it is leaking.

BTW: My K1, throughout this overflowing problem, has always been on the center stand.

Except 1 and 2 still leak.  ... And did I mention that it only leaks when it's on the side stand (never the center stand and never when its running), with 1 and 2 being the closest to the stand. 

My side stand may be a little bent and the bike may be leaning over a bit too far to the left, with the fuel running over the side of the bowl and weeping a bit around the gasket.  Could that be the issue here?  Would the gasket, if properly seated, contain that??  I will triple check float level.

First, I do think the bowl gasket, if properly seated would eliminate any leaks at the top edge of the bowl.  As I ponder your #1/#2 leak while on the side side, I'm wondering if the lean angle might be causing the #2 bowl to cross feed into the #1 bowl... thereby overfilling #1 and the excess fuel, might not be much, exits through the overflow tube.

Just a thought!  Hope it helps!
ZT

[denward17]

Don't overlook the orings between the carbs.

[seanbarney41]

The way I see it, the presence of the overflow hoses tells us that Honda knew there was gonna be frequent splashes and overflows under certain conditions, side stand useage, fuel warming and expanding in the sun, vibration, bumps, shakes, atmospheric pressure changes...  The hole in the top of the overflow pipe is only 2 or 3 mm above the float bowl level.  It seems outrageous to expect ultimate fuel tight performance from such a system.  I can't count the number of times I have sat down after a ride, to admire my bike and after a few minutes?..."ohh, would you look at that she is gonna take a little pee" and then it stops.  Usually that's the reminder to turn off the petcock off, but not always.  If you don't turn the petcock off and don't prop the bike upright, whether on the center stand or otherwise, you are gonna see leakage.

[seanbarney41]

Most of the above is for OPKP.  ZT, try swapping float bowls with the one next to it.  If the leak moves, the gun is smoking, just get a new float bowl.

[ZTatZAU]

The way I see it, the presence of the overflow hoses tells us that Honda knew there was gonna be frequent splashes and overflows under certain conditions, side stand useage, fuel warming and expanding in the sun, vibration, bumps, shakes, atmospheric pressure changes...  The hole in the top of the overflow pipe is only 2 or 3 mm above the float bowl level.  It seems outrageous to expect ultimate fuel tight performance from such a system.  I can't count the number of times I have sat down after a ride, to admire my bike and after a few minutes?..."ohh, would you look at that she is gonna take a little pee" and then it stops.  Usually that's the reminder to turn off the petcock off, but not always.  If you don't turn the petcock off and don't prop the bike upright, whether on the center stand or otherwise, you are gonna see leakage.

I wouldn't disagree with any of that.
Thanks Sean!
ZT

[ZTatZAU]

Most of the above is for OPKP.  ZT, try swapping float bowls with the one next to it.  If the leak moves, the gun is smoking, just get a new float bowl.

HA HA!  Didn't se this until after I posted my previous.

But as you advised Sean, I was willing to accept the slow drip I saw in my test jig; the fuel is definitely coming through the tube rather than around the tube... so I already have the smoking gun on this bowl.  I put it back together and reminded myself to always shut off the petcock until I decide on an epoxy repair or a new bowl.

Unfortunately, my first petcock on test, revealed the same drip on #4 with a steady overflow stream from #3 carb.  A couple of taps cleared that up.

ZT

[OPKP]

Great answer Sean. 

Turn off the petcock, rev her a few times and put it in the center stand.  Problem solved!

[ZTatZAU]

Most of the above is for OPKP.  ZT, try swapping float bowls with the one next to it.  If the leak moves, the gun is smoking, just get a new float bowl.

As I mentioned above, everything is back together (along with the drip) for now; and I'll try to control the dripping with the petcock.  But after everything else I've done resurrecting this bike, I really should replace the bowl rather than slobbering up the overflow tube with JB weld or similar fuel proof epoxy.

Unfortunately, the parts book doesn't list individual carb bowls so I have no part number.  And with the cross feed fuel tubes it looks like each carb 1, 2, 3 & 4 are all unique.  But are the #3 & #4 carb bowls the same with the only distinguishing feature being both drains facing to the right?

Any tips or advice on how to search for and source a #4 carb bowl (without leaks) for a K1 750, would be much appreciated!
ZT

[newday777]

Most of the above is for OPKP.  ZT, try swapping float bowls with the one next to it.  If the leak moves, the gun is smoking, just get a new float bowl.

As I mentioned above, everything is back together (along with the drip) for now; and I'll try to control the dripping with the petcock.  But after everything else I've done resurrecting this bike, I really should replace the bowl rather than slobbering up the overflow tube with JB weld or similar fuel proof epoxy.

Unfortunately, the parts book doesn't list individual carb bowls so I have no part number.  And with the cross feed fuel tubes it looks like each carb 1, 2, 3 & 4 are all unique.  But are the #3 & #4 carb bowls the same with the only distinguishing feature being both drains facing to the right?

Any tips or advice on how to search for and source a #4 carb bowl (without leaks) for a K1 750, would be much appreciated!
ZT

#3 & #4 carb bowls are the same bowl and are interchangeable
The same applies to the #1 & #2 bowls.