Leaky carbs - gas in oil

[fxrer]

I’ve read frequently in this forum of one adverse consequence of leaking carbs, i.e. gas contaminating the engine oil. My ’80 SG carbs leak slightly, as evidenced by minor gas discoloration on the float bowl of the #1 carb, and the engine case below it. I’m not sure if other carbs leak too. I’m guessing the leak must from the float needle and seat, or its sealing o-ring. I think I also have a leak at the octopus, which allows gas to continue to flow to the carbs; I now turn both petcocks to “OFF” when the engine is stopped. When I restart, after sitting a day, I turn the petcocks to Prime, and can hear air whistling through the gas cap vent for a few seconds, as fuel refills the lines and float bowl(s). So, it’s time for new float needles and seats. A few questions first.

1. I see PartsNmore has them, $12 per kit, with sealing o-ring. Are these good quality? If not, are there other sources? (my carbs have the press in seat, secured by a tab and screw)

2. How and where does gas find its way into the crankcase? How much is too much?

3. How can you tell if gas is in the oil? My level is normal, color and viscosity seem normal, and I can’t smell any gas, although it’s hard to be sure.

4. With the carbs still installed, is there a way to test whether a float needle and seat is leaking on a specific carb (interior ones especially)? I can remove the bowls, and replace seat and needles on the outer carbs without removing the entire bank, but the interior ones require removing the bank, which I’d rather avoid if I can.

Thanks for any suggestions

[rrago]

Rick,

To check if there's gas in your oil,

Dip a wooden stick into the oil.

Step away from the garage.

Light it. If it supports combustion, you've got gas.


P.S. Everything I've bought from Parts n More has been good quality.

[TopCatGr58]

Hey Rick,

To test for leaky carbs, you'll need to pull the air cleaner off/out, and the put the bike on center stand and perhaps even put something under the front wheel to make it lean back as much as possible, then turn the petcocks to PRIME, and then wait and watch.

Your leaky #1 should start showing the fuel flowing out the back of the carb(inlet bell side), and if any others leak, they too will run out the back of the carb throat. Keeping it tilted back will help to prevent the fuel from flowing INTO the engine. And then only if the intake valve for that cylinder is open, will the fuel actually leak down into the crank. But without pulling off the valve cover, it's difficult to tell which cylinder's intake valve is open!

The #1 carb is more likely to develop leaks due to the bike leaning towards that side when on sidestand, and all the crud in the tank flows more easily to the left side, flowing into the left carb bank. Also fuel sits in that one longer, evaporates slower, and can gum up when parked for a while that way!

Yes, PNM parts are okay!
T.C.

[fxrer]

Hi again T.C., a.k.a "Carb Guru"!!!

Your test is very logical: raising the front of the bike above "level", induces any float bowl overflow to run backward out the inlet bell.

But, as the inlet valves (I assume you refer to the ones in the cylinder head, next to the exhaust valves?) are higher than the carbs, how can they allow gas to get into the engine oilpan/resevoir?

[pggg]

Fx, it's a close call but the valves are about 4 inches long and just sneak in under the float levels. Also if an air-cooled motor ain't using a bit of oil between changes then the motor ain't trustworthy. If the oil level dont gradually drop, then chances are the oils getting fuel in it.

[TopCatGr58]

Hey FXRer,

I'm no carb guru, but thanks! Like PGGG said, and it's really not close but actually below the height of the carb throat!!Remember, the carbs are mounted to the head, the valves are IN the head, they stick down towards the pistons, and really sit just a few mm's above them...the bottom of the head! And then sit right in the path of the fuel air mixture going into the head. SO...when the carbs overflow, it flows thru the carb throat into the intake mounts, and into the head, and IF the intake valve is open, it then flows DOWNWARD into the combustion chamber....and then oozes around the rings, and down into the crankcase!!
T.C. simple shadetree survivalist!