Topic: CB750 petcock question.....?

[bjatwood]

What causes a CB750K3 petcock to leak fuel into the carbs?
I’m thinking mine is leaking fuel into the carbs when it is in the off position….
I say this cause after running the bike all is OK, and I run the bike till it starts to pop
with the gas OFF. Then on day 3 of just sitting there I get a gas drip right in front of the rear wheel the size of a quarter. ..?
But then I think, shouldn’t the carb fuel valve keep fuel from entering the float bowl.
And, how does one rebuild a Petcock?

[Shane72]

Here is a link to a rebuild kit for that petcock (unless you have a different one in there):

http://www.ebay.com/itm/Honda-CB-750-K-CB750-750K-CB750K-CB550-550-petcock-rebuild-mount-kit-/230989541353?pt=Motorcycles_Parts_Accessories&fits=Make%3AHonda|Model%3ACB750&hash=item35c80cafe9&vxp=mtr

The large bowl on the bottom of the unit is threaded on to the body above--note the nut head molded into the bottom of the bowl.  This is the first thing to remove to get it off and apart.

The parts in the picture are all laid out in a row the way they go in, with the exception of the 4-hole rubber washer on the far right.  That 4-hole washer goes behind the selection lever, and is retained (along with the facing plate and a wavy steel washer) by two tiny Phillips head screws.

My guess is that your leak involves that 4-hole washer at least, but may also be made worse by a leaking float needle and seat in one or more of the carburetors.

The petcock is an easy dis-assembly/clean/replace parts/reassemble job--the carburetors are a bit more involved.

All the best,

Shane

[bjatwood]

Thanks,
Think I will take the float bowl off tonight and do a drip test to see what is happening with the petcock and how "much" it is leaking.
I'm Starting with the easy stuff first since I have already "played" with the carbs twice in the last 2 years I've owned this classic

[bjatwood]

OK guys, looks like the longevity of Honda petcocks is 40 years! Jeez... thought it should of gone at least 50! LOL
Petcock is definitely leaking, new Honda Part #16950-300-020 is heading on it's way to Nebraska. No more smelly gas garage for me 

[drones76]

Part #25 below.  The two washers.  I just put the same petcock(brand new) on a Kawasaki KZ400 and it still had a leak when turned off. I took the two washers(copper) and annealed them with a torch.  Put it back together and the leak stopped.  If yours are not copper get a new set.  Also could not hurt to put a sealer on the threads of the bolts that go into the tank.

The washers were not included with a new petcock.

http://www.cmsnl.com/honda-cb750-four-k3-usa_model481/partslist/F++11.html#results

[Tugboat]

^Good tip.

[Old Scrambler]

If your tank is clean, the petcock body should be good. Old fuel and cleaning fluids can 'eat' some of the metal. The 'biscuit' washer can be cleaned (soaked) in dish-soap and re-installed like new. These are the same basic units derived from the CL72 model in '62. I have a few and all are working fine. When you replace the body I use the halves of a sliced grommet to seal the cap-screws to the tank.  My original K3 is still working just fine.

[bjatwood]

My original K3 is still working just fine.

Your about at the end of its life expectancy.....