'79 GS850 Gas Leak

[TMannion87]

I have a 1979 GS850 that is leaking gas from the carbs 3 and 4. After searching the forums here, it looks like it is either a petcock or float problem. My question is how do you determine which is the cause? I removed the carbs and cleaned them. The bike ran last season. It will run if I give it throttle, but it dies shortly after. Any help would be great. Thank you.

[TheCafeKid]

Well, for starters, i would suggest a full tear down and cleaning of the carbs. The fuel leak may or may not be your only problem, but since you have to get in there to adjust the floats, and im assuming you havent done it yourself, or it hasnt been done in a while, it might be a good idea. Baring that, the best way to tell if its a float issue, is to simply pull the carbs and check the float hight measurment. On that bike, with VM carbs, I believe its supposed to be 26mm from the gasket mating surface, plus or minus 1mm. (tho in some manuals, it calls for no plus or minus, however, i simply go with that, as its generally an across the board adjustment) Fortunately, the VM carbs were equiped with overflow tubes, before the EPA got their hands in everything and said that wasnt allowed anymore, so you SHOULDNT have much fuel in the crankcase, although i would not let this problem go unchecked for very long. Why tempt fate... Fuel in the crank case can lead to a VERY short life for your crank bearings.

Now, to see if you have a PETCOCK issue, its much simpler. These petcocks are vaccum operated. Which means it needs the vaccum pull of the motor to draw fuel from the tank when the petcock is in the ON (or RUN) or RES (reserve) position. If the petcock is in the PRIME position, fuel will flow FREELY, whether the motor is turning or not. There has been some argument among us, some say that the float valves SHOULD hold back the gas once the float bowls are full, but I and others simply disagree. If you leave it on prime long enough, and there is enough gas in the tank, it will eventually push passed the float valves. They are not a dam, merely a flow control valve. Anyway, the best way to TEST your petcock is to unhook the fuel line from it, run a piece of temp line on it, then unhook the vaccum line (the small line) from the petcock, install a temp one (this prevents you from having to try to finagle them back on the carbs...its a little tight in there) and give a suck on the vac line in both the On and Res positions...if fuel flows when you suck...you're likely in good shape. If it flows with no suction...the petcock is shot...ditch it, and buy a new one...DO NOT try a rebuild kit...GIANT waste of time and money...

[robertbarr]

Here's a work in progress about petcocks:

http://www.cycleorings.com/petcock

Next is the following canned response:

Let me point out the obvious: If a carb (or carbs) are overflowing when the bike is not running, you have TWO problems. First, your petcock should prevent ANY flow; second, the needle valve should allow flow only up to a fixed height in the bowl (ie, not overflow). Excess fuel has to make its way past both of these parts.

(A third possibility on old carbs is for fuel to flow past the inlet seat O-ring. [n/a for VM]. That doesn't apply to newly rebuilt carbs, since the owner has just installed brand new. Right??)

I'm not 100% sure why some needle valves leak and others don't, but I'm starting to lean with Keith Kraus' suggestion, which is to use only OEM valves and seats. Simply because it works. The question why is a separate matter. (I have a theory, but no guinea pig to test it on at present).

For the petcock, you'll hear a number of suggestions. I think that the vacuum operated petcock should be kept operational, for a few safety reasons.

Several of the folks here are convinced that petcock rebuild kits are useless, and your only real option is to buy a new OEM petcock.

For now, for people who are 100% sick of carb overflow & the ensuing problems, the consensus then is to go with OEM petcock, seats & needles. it's expensive, but it seems to be the only certain way to correct this BS once and for all.

(I would add: no doubt there are people whose overflow problems began when they installed carb rebuild kit(s). The machining on the parts is often abhorrent and inexcusably amateurish, and the f&^%#ing O-rings don't fit correctly. The consensus for a while now has been to buy gaskets IF you need them, valve seats & needles IF you need them, and an O-ring kit.)


The petcock does not, in any way, REGULATE the rate of flow. It should be either on or off. If you have overflow on a running bike (and it's not inhaling fuel through the petcock diaphragm via the petcock vacuum line) then there's a problem with the inlet valve.

Either:

1) the float is no longer buoyant enough or is badly adjusted, or

2)the valve is bad, or is being held open by some foreign matter.

And finally is the standard BWRINGER's lecture:

One of the secrets of the GS850 engine and carburetion is its outstanding low-speed manners. This, in turn, allows you to much more easily exploit the corners of its handling envelope, which leads to more giddy fun than you've ever had with your clothes on. As most racers eventually find out, smooth is fast.

If you had a freer-flowing aftermarket exhaust, a good set of K&N pods, a dynamometer, exhaust gas analyzers, a degree in chemical engineering, no neighbors to bother, a few weeks off work, a large box of Keihin jets, and several fifths of good tequila so you can sleep at night, you might be able to extract a few more horsepower from the 850 engine at certain RPM. And if you're really good, you might be able to get the low end half as smooth as it was stock. Maybe.

With the stock exhaust, forget it -- all you're going to do is screw up everything else quite badly to get maybe 5 more horsepower at 7,500 rpm or something like that. These things have a certain balance, and more intake flow must be balanced by more exhaust flow.


What I have noticed at the rallies is that very, very few 850Gs are actually running right. Make VERY sure it's actually running the way it's supposed to before busting out the modifications.

Brian's EZ-and fun plan for GS850 happiness:

1) Seal the airbox and air filter with weatherstripping.

2) Ensure no intake leaks. Spraying WD-40 or water doesn't tell you much, since very small air leaks can cause problems even though they won't suck in enough WD-40 to make a difference. Replace your intake boot o-rings and boots if needed, and seriously consider spending the lousy $28 for new airbox/carb boots.

3) Ensure clean carbs with correct settings, new o-rings, and original OEM jets. No, not just squirted with something. I mean completely disassembled.

4) Check/adjust valve clearances (Manual calls for every 4,000 miles. This is not optional.)

5) Ensure healthy electrical system.

6) Seriously consider upgrading coils and plug wires.

7) Install new, stock NGK B8-ES plugs gapped to .031".

8 ) Fine-tune float height and idle mixture screw to ensure best off-idle transition.

9) Clean air filter and reinstall with only the lightest oil mist -- over-oiling and/or letting the filter get dirty is a common and critical mistake, and will make the bike run funny at low speeds and run rich. This may take a few tries.

10) Make sure the exhaust seals are sealing.

11) Ooh, much better now, huh? You're gonna need upgraded suspension - Progressive or better fork springs and shocks. Set suspension sag appropriately.

12) Upgrade brakes with new pads and stainless lines to deal with all that extra speed.

13) Install new petcock, since I'm going to head to the roof with a rifle if I have to read about yet another #2 plug fouling and failed hillbilly attempts to rebuild the petcock and/or deny there's a problem.

14) Oh yeah -- check compression somewhere in there to ensure the valves and rings are reasonably healthy.

15) You'll probably need new OEM clutch springs -- the clutches last forever, but the springs get tired after 20 years or so under pressure. E-Z and cheap.

[BassCliff]

Greetings and Salutations!!

Hi Mr. TMannion87,

Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers. Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

[TMannion87]

I checked out the petcock today and gas flows from the fuel line in the prime and reserve positions but not in the on position. Does this mean there is a problem with the vacuum line or the petcock? Thanks for your help.

[BassCliff]

Hi,

Check out this page from my website...

Where Do These Hoses Go?

It tries to explain the operation of these vacuum operated petcocks. Basically, in the "ON" and "RES" positions the petcock will not allow fuel to flow unless there is a vacuum applied to the petcock which actuates a diaphram in the valve. In the "PRI" (Prime) position the fuel is allowed to flow via gravity, without vacuum being applied.

If your petcock allows fuel to flow in the "RES" position without a vacuum being applied, the petcock is bad. Hoses are cheap. Just replace them.


Thank you for your indulgence,

BassCliff

[TMannion87]

Bad petcock it is then. I plan on ordering one from PartShark.com tonight. Thanks for the help and I'll let you know how it works out.

[SqDancerLynn1]

What every one is saying is don't take shortcuts. Do the job once and go ridingClean the carbs, Remove the intake boots and check them while you replace the Lg O rings