What do the carb vents do?

85XJ700N--in my many efforts to solve the poor mpg for my bike, I plugged the end of the carb vent tubes and took a ride. I wanted to see if I was losing gas out of them. After about 5 miles I did a chop check and sure enough the plugs (can't remember if it was all of them) were really black. Then I had a hard time getting the engine started. Rode another 5 miles and parked it for about an hour and when I got back there was a lot of gas under the bike and again I had a hard time getting it started. I then unplugged the vent tubes and had no further problems. So my question is what exactly do they vent, and why should it produce such a rich running and apparently overflow?

 

The vents are to vent the float bowls & allow them to fill properly.

If there was fuel under the bike STOP RIDING IT!!! you obviously have carb float or float needle/seat issues (as pointed to by black plugs too) and may have gas diluting your oil now... Don't want you to kill your engine!

The plugging the tubes didn't make you run rich, you were already running rich and we know that as you have crap fuel consumption that you're trying to cure... The rich condition is most likely caused by (or at least contributed to by) either badly adjusted floats or leaking needles/seats...

EDIT-: first jobs to do now are

1, drain/disconnect gas tank or otherwise ensure it's not going to dump fuel through the carbs any more

2 check oil for high level & or gas smell

3 drain & replace oil immediately if diluted (seals/chain guides don't like sitting in gas) with cheap 20/50, & run motor for a few seconds to circulate it

4 repair carb float valves/seats and properly wet-set floats & bench sync

5 refit carbs, warm up engine without riding, drop cheap oil & filter, replace with usual brand (M/C wet-clutch friendly/max SG grade). This step removes the last residues of gas flushed by the cheap oil. Ideally get OEM air filter now as it will likely have had a soaking from the overflowing carbs.

6 warm engine, running sync & colortune

7 bet you get better mileage now if you did all this

 

I did this a couple of months ago, and did do everything you mentioned above, except I still have the Uni filter--cleaned and lightly oiled. I replaced the float valve needles with the solid brass ones and they seen to be working well. Haven't had anymore gas overflows and I'll do the chop test this weekend.

 

The thing is, if you had gas under the bike chances are you have a sump full of gas now too that's what I'm getting at...

And it had to get there somehow as well, AFAIK that somehow can only be float/needle/seat related I'm afraid...

 

I changed the oil right after this just in case I had gotten gas in the oil, but I never smelled any gas, so I think I dodged a bullet. It seems to me that if the gas overflows the bowls it would either drain into the air filter box and then to the ground or into the oil--or both. So I think mine must have gone through the air filter. I don't see anyway it could have drained through the vent tubes.

 

Cool glad you got that done. If you look under the air filter box down low on the left side though you'll see a breather pipe that runs from the airbox straight to the sump via the hole in the top of the middle gear cover, so most times if carbs overflow some gas will find a way into the oil...

That's assuming the gas was coming from overflowing carbs of course, I'm 99.9% on that but wish someone else would have a read & see if I'm missing anything...

 

Question, if the floats being set too high will cause the gas to overflow what will happen if the floats are too far in the other direction and shuts the fuel off too early. Does it cause the bike to run lean/hot?

 

It certainly can yes, most times causes hard cold starting too.

 

Floats set too low can also lead to the float bowls running dry when you grab a handfull of throttle. That's not exactly the best for merging onto a freeway.

 

In order for Gas to get from the Fuel Tank and wind-up in the Crankcase, it

has to go through the Petc0ck, ... first!

Test the Petc0ck. If it is leaking-by, ... STOP the leaking.

Either install a Kit, ... or

Just replace it with a Brand New one.

 

Yes, but that's just kinda glossing over the fact that there's a float valve leaking too. If the float valves are in perfect order you should be technically able (although would be ill advised) to leave the petrooster on PRI or leaking, with impunity.

Ideally repair both, IMO the order of importance being 1-: Float valves 2-: Petrooster

 

Ideally? 8O

Yes that's the "order of importance" but it's really not necessary to install lawnmower parts on your classic Yamaha if you fix it correctly. Petroroosters can certainly be rebuilt successfully; I've done it numerous times. However your results may vary; and truth be told I have a brand new (still vac-op, but brand new) one on my '83. But I can also trust my carbs if I slip and leave it on PRI for a week. (I've seen me do it.)

FLOAT LEVELS (not just whether or not the valves actually operate) are critical to properly setting up the carbs for these motors. The specs are not more than a millimeter or two either direction; and that's not much.

Ya just gotta do both, and do them right.

 

Yes, ideally. We have to understand that not everyone will fix everything at once & if that's the case, carbs first.

Nobody actually suggested the old Briggs/Stratton check valve.... this time

 

The petcock was rebuilt earlier and I've been under the bike with a flashlight looking for leaks and if there is a leak I haven't been able to see it, feel it, or smell it.

 

There is a significant probability that a rebuilt petcock would still leaks. This has been reported in the past. Sometimes a new petcock is the best bet. Manual operated ones are less expensive.

 

Well folks, ...

The Briggs & Stratton Fuel SHUT-OFF works!
Turn it OFF, ... and NO Gas goes to the Carbs.

Say what you will, ... but ... Rebuilding a Petc0ck is a crap-shoot.
There are Rebuilt Kits that are a total waste of time and money.
I quit installing Kits because I couldn't guarantee the thing wouldn't leak WORSE than before the "Didn't work ~ didn't fit right" Kit went in.

What then, ... is the BEST advice?

Buy a NEW Petcock -- or -- Install a Shut-Off.

An Ounce of prevention is worth a pound of cure.

 

That's still avoiding the real issue here though, that is the FLOAT VALVES are/were leaking. If it happened once it'll happen again, and it may even be the cause of the high fuel consumption. Get them right & you don't even need a petrooster.

Not denying that the B/S shut off works, it does, since it is a valve that seals off flow. But that still doesn't change the fact that there's an issue with the stock fuel system. Anyway, the kits Chacal sells are not "didn't work-didn't fit" kits & a correctly rebuilt petrooster that was in a rebuildable condition WILL work fine.

OK maybe a lot of the petrooster's valve seats are past polishing by now & I do see your logic in not bothering if you've found most to be too far gone, but that shouldn't prevent anyone at least looking inside to see if it'll repair before condemning the petrooster & buying a new one.

100% agree with this statement, so the O.P. needs to pull the carbs & repair the leaking float valves before the cure becomes a bottom end rebuild... :?

 

Since the Brand New Petc0ck that came with the Bike did its job flawlessly for nearly three decades, ... we should expect another Brand New one to do the same.

I have not had success rebuilding Petc0cks with aftermarket Kits.
The time spent installing the Kit and then having to mess-around trying to get the Kit to work could have been applied to doing something else.

I appreciate the time and effort Fitz took to do the comprehensive DIY Instructional.
It's far better than the simplified instructions in the Manual.

But there is a chance that after performing all the meticulous cleaning and prep, outlined in Fitz's outstanding pictorial, that all the work could be for naught.

We also need to recognize that there are some people who, for numerous reasons, rely upon others to do their maintenance and repairs.

Having a way to guarantee that the Fuel Tank won't empty itself into the Crankcase or onto the Garage floor is easily accomplished, highly effective and very cost-effective.

 

One problem; many solutions. Such is the way of life

 

What Rick says is plain thruth. You may do the best job ever with your petcock rebuilt kit ad still have a leaky petcock. Sometimes old parts and new ones simply don't match, and there is nothing you could do about it.

The B+S fuel valve is a simple amd cheap temporary solution to stop crankcase flodding until you have the time to fix and/or set the floats properly.

 

OK I accept what you're saying here, but the real issue is STILL one of float valves leaking!

 

The recommendation is made as a safety measure for those who rely on others to do their maintenance and tuning.

I agree that the root problem needs to be addressed.
Many have already experienced finding their Crankcase full of Fuel or the Bike parked in the middle of Lake Gas.

For those who lack the skills to pull and overhaul their carbs its a stop-gap measure to keep the gas in the tank.

For others who have NOT YET had to contend with the issue of Petc0ck shut-off failure, ... it's a low-cost insurance mod that will provide peace of mind knowing their Petc0ck is nearing 30 years on-the-job.

 

90% of the time I run into a leaking fuel cock it's the rubber washer with 5 holes behind the "ON" "RES" "PRI" selector. Yamaha generally calls it VALVE, FUEL. Those can be purchased from Yamaha or aftermarket, both work equally well and I expect Chacal sells them too. They're inexpensive.

The other place I see them leak is the O-Ring in the diaphragm valve. The last one that was doing that I simply put on a new O-Ring.

I've used the aftermarket rebuild kits and agree 100% with Rick that they're more trouble than they're worth. When you finally get one to work it's often noisy to boot, making a loud clicking sound at idle.

Personally, if I had one with a good diaphragm I'd just replace the parts mentioned.

 

This is generally caused by a stuck "one-way valve" disc in the output flange. If the little disc doesn't move freely (so that it acts as a one-way valve like it should) then the pulsating manifold vacuum will hammer the valve back and forth creating the loud clicking.

But it IS true that the rebuild kits can not work on occasion, and for a number of reasons.