Topic: 1975 CB750 Head Scratcher

[John G]

Where do you look when you run the newly rebuilt engine and after a short time the oil seems to increase in the oil bucket. I turn the fuel on and no leaks on the overflow tubes out of the carbs. The floats are set. Why is gas getting into my oil? Anybody?

[69cb750]

Fuel valves in carbs are leaking.

[Don R]

 Often, it's a case of the tank oil seeping down into the crankcase and you add a little more. Then when you re-start it the sump oil gets returned to the tank.
 Of course, if your oil does smell like gas you probably have a slow leak float valve problem. Be sure to always turn off the petcock when parked.

[Scootch]

I have been plagued with the carb overflow situation for a while... Even with turning the petcock off. When I parked the bike for the winter I ran it till it quit. I figured that would take care of it. Wrong. Gas leaked a couple times. I had suspicion that the petcock was leaking. Either simply leaks a bit or barometric pressure changes in the tank is forcing gas through the petcock. I'm not entirely sure that my tank is venting properly. I just now removed the gas lines from the petcock. They had fuel in them. I have routed a couple hose stubs from the petcock to a catch can. We will now see what happens. One way or other the petcock is letting gas through...

[John G]

When you say fuel valve, you mean the needle and seat?

[HondaMan]

I have been plagued with the carb overflow situation for a while... Even with turning the petcock off. When I parked the bike for the winter I ran it till it quit. I figured that would take care of it. Wrong. Gas leaked a couple times. I had suspicion that the petcock was leaking. Either simply leaks a bit or barometric pressure changes in the tank is forcing gas through the petcock. I'm not entirely sure that my tank is venting properly. I just now removed the gas lines from the petcock. They had fuel in them. I have routed a couple hose stubs from the petcock to a catch can. We will now see what happens. One way or other the petcock is letting gas through...

Scootch: you're on-track about how it can leak with barometric pressure changes (which here in Colorado are much more severe due to altitude), however...the petcock's seal is rated for a little over 6 PSI hold-back pressure when new. That said: mine finally wore itself thin enough that by 2008 I had to replace the [original] one because I was getting overflowing float bowls with the gas shut off during the winter(!). A new petcock seal (that 4-hole thingie) fixed it right up, though.

Mine gets exercised a lot: every time I walk away from the bike for any reason, it gets turned off.

[Scootch]

Thanks for the confirmation Mark! My petcock is definitely leaking. I ordered a new seal.

[BenelliSEI]

For winter storage I always pull off the fuel lines (plug them with a pair of clean bolts) and then loop about a 12” piece of clear tubing on the petcock outlets. It’s surprising how often they will drool and fill that bit of hose! Time for a new rubber disc.

[HondaMan]

While I don't recommend this for everyone...I store mine "wet", essentially ready-to-ride. Once a week or so I'll turn on the fuel petcock for a few minutes, make sure nothing leaks from the carbs, and shut it off again. This prevents the need to rebuild (or clean) the carbs very often, as the gas around here has a bad habit of leaving residue behind that plugs the idle jet. This method prevents the problem while letting me nab one or more of those rare winter riding days.

[Scootch]

I am conflicted about how to best store the CB. Back in the day I had terrible luck storing mowers etc with fuel in the carb. Then I started installing shut off valves and running them out of gas. That has worked well for years. Around here we do occasionally get warmer weather in winter but there is salt or salt powder on the roads so can't ride. I am glad that I finally made myself look into this fuel carb overflow issue and get it resolved.

[John G]

My petcock doesn't leak but yet it still seems to get fuel in my oil when I run it. Quite the head scratcher.

[scottly]

What do your spark plugs look like?

[Don R]

 The petcock mounting screws also have fiber washers that seal to the petcock, if they are cracked, left out or chipped they will also allow fuel to seep through the petcock. Recently, Terry mentioned using copper washers there rather than the fiber ones. Seems legit.

[newday777]

The petcock mounting screws also have fiber washers that seal to the petcock, if they are cracked, left out or chipped they will also allow fuel to seep through the petcock. Recently, Terry mentioned using copper washers there rather than the fiber ones. Seems legit.

Not on the 75&76 have different petcock. No more screw washers. This thread is 1975 head scratcher....

[John G]

Interesting stuff; no. Well I might be assuming it's gas but I'm going to drain the oil; again, and see if it burns. The gas should light well before the oil. Maybe I put to much oil in to start with but I don't see how. Stay tuned.

[willbird]

The petcock mounting screws also have fiber washers that seal to the petcock, if they are cracked, left out or chipped they will also allow fuel to seep through the petcock. Recently, Terry mentioned using copper washers there rather than the fiber ones. Seems legit.

With just random copper washers, or even perhaps re using them it might be a good idea to anneal them. The red fiber ones are not TOO salty on Ebay. The guy mails them and makes sure you actually got what you purchased.

Bill

[bryanj]

Don, i have even used 6mm hydraulic "Dowty" washers on the petcock mounting screws but try to get the alloy bodied ones not steel(these are the ones with rubber inserts)

[HondaMan]

Interesting stuff; no. Well I might be assuming it's gas but I'm going to drain the oil; again, and see if it burns. The gas should light well before the oil. Maybe I put to much oil in to start with but I don't see how. Stay tuned.

Maybe consider this sequence of events:
1. Bike sat for a while (like weeks).
2. Oil got changed, and full amount was put in oil tank, but crankcase was not drained.
3. Engine was then run. The oil that previously seeped into the crankcase got pumped into the oil tank, making the level too high.

This is common these days, as the rubber stopper (check valve) in the oil pumps are now 50 years old and hard, which lets oil slowly seep thru them and into the crankcase, overfilling it (there should be about 4-6 ounces of oil in there at any one time). You can replace the stopper with an oil pump rebuild kit. This scenario is pretty common in these pages, but most often seems to show up in Spring when the bikes are coming out of hibernation.

[Scootch]

The petcock mounting screws also have fiber washers that seal to the petcock, if they are cracked, left out or chipped they will also allow fuel to seep through the petcock. Recently, Terry mentioned using copper washers there rather than the fiber ones. Seems legit.

This is exactly what I found today. Got the petcock apart and noted that in its previous life someone left out the washers.

[newday777]

The petcock mounting screws also have fiber washers that seal to the petcock, if they are cracked, left out or chipped they will also allow fuel to seep through the petcock. Recently, Terry mentioned using copper washers there rather than the fiber ones. Seems legit.

This is exactly what I found today. Got the petcock apart and noted that in its previous life someone left out the washers.

Yes....but....
This thread is on a 1975 that doesn't have the screws and washers. The petcock is mounted using the threaded bung on the left side of the tank.
The fiber washers and screws were only on K0-K4 petcocks.

[Scootch]

Noted. On closer inspection the washers are in there. They are in there so well that they were stuck and had to be coaxed to come out. Apologies if I have added confusion...