Blow by. . .

[jakobgoodnessgracious]

Hello everyone, I found some help at this link:
http://forums.sohc4.net/index.php?topic=116447.0 . However, I am still a little confused.

I finally stopped a leak I was having from one of my petcocks and after a test ride and my joy of a successful run I forgot to turn them off. After I went back outside I noticed fuel dripping from the same black tube in the picture in the link. I know of the two carb overflow tubes and I am pretty certain from where the splash on the ground was that it was the black tube between the two carb overflow valve tubes. I tried to trace its origin with my hand but got stopped at the engine where it goes under a piece of metal or something. What I don't understand is that it was mentioned in the link up top that if one forgets to turn off his/her petcocks fuel could come from this blow by valve? How is this the case unless one has a faulty float needle or carb to even be letting fuel get into the crankcase (I think I'm understanding this right)? Shouldn't the overflow be taken care of by the carb overflow tube letting the excess gas out? Or is this just normal to have fuel come out of your blow by tube if you forget to turn off your petcocks?

Just making sure I don't have to fix something else. . .

 

[retiredgentleman]

"Blow by" is not the correct terminology to use. If you have gasoline dripping from the bike, its almost certainly coming from the float bowls. The float bowls must have a vent on the top to allow air to enter. If the level in the float bowl becomes too high, fuel will come out of the vent lines and cause a drip.

So why does the level in the float bowls become too high? Something to take note of; float valves in carbs are not a 100% isolation valve. They are a flow/level control valve, and they do a good job of controlling the flow into the carbs, as long as the engine is running and consuming fuel. When the engine is off, they are considered to leak slightly, (sometimes they are leak free but not to be trusted) and that is normal for them to leak a small amount.
That's where the petcock comes in. Always shut off the petcock when you turn off the engine. With the petcock off, there is 100% isolation of the gasoline. If you have a vacuum operated petcock, they are supposed to be 100% isolation, and may do the job for you. When they get old they can start to leak. I like to use manual petcock(s), and I have never had any gas leakage on my bike.

 

[MaxPete]

Agreed. Back in the day, every knowledgable biker shut the petcocks off before leaving his/her bike. When vacuum petcocks came along in the late 1970s, people got out of that habit, but the danged things so often leak, that leaving them turned on when parking a motorcycle is not a good idea. It's even worse when they're 40-odd years old.

In my cafe built I am swapping over to an XS750 fuel tank as many others have done and I will be using the non-vacuum variant petcock rather than the stock vacuum units that came on the '77 XS750. These are readily available at MikesXS or elsewhere and are intended specifically to eliminate the problems that are endemic with vacuum petcocks.

Pete

 

[jakobgoodnessgracious]

Ah okay well thanks guys. I'll just make sure I always turn them off. They are indeed old. I decided to buy some rebuild kits for them and so I'm hoping they continue to last a great many years without issues... They seem like relatively simple devices and that is a great thing. They're still the manual ones that came with my 74. Oddly mikes lists 74 as having three screws. Mine have two and are listed as such in the parts manual. Luckily I didn't listen to mikes on that one.

Anyhow, the bike is running, doesn't die, and the speedo, tach, and lights function. The petcocks aren't leaking now. I am about ready to take it to get inspected so I can get it registered.

No big deal but if anyone reads this and has any advice: I've been wondering if I'll get in trouble for riding it without plates to get inspected. Seems counterintuitive that I should get in trouble just because I don't own a trailer to take it to be inspected. I mean it has insurance and the title has been signed over and notarized and I have my permit...

Thanks again for the info on everything.

Jakob

 

[weekendrider]

In MO you are allowed to take the vehicle to the closest inspection station and return home without having the plates. You might call your local police dept., sheriffs or highway patrol office for clarification.

 

[jakobgoodnessgracious]

Sounds good I'll give them a call to be sure.