85 Marathon Carburator issues

[elt13]

Hello. I just got a 85 Marathon for a couple hundered dollars. The carb was in a box and the guy saif it needed a new one. I bought a new one and its dumping fuel in the exhaust. A few things i did are listed here

It does run, i can get it to fire on starting fluid. In fact its with run a long time with very little starting fluid. It has 110 psi compression. I have a new carb right out of the box. Not sure what the mixture setting shoud be but i played with it and got nothing to change. I have it at one turn out right now. It has good fuel pressure. The engine is a Robin 244cc

As soon as i hook the fuel line up, right after i had it running in starting fluid it fails to run and loads the muffler up. Is this a float setting problem maybe? I cant find any carb settings anywhere.

Any help would be great thanks. Dave

[Dave Box]

How do you know that it is dumping fuel into the exhaust? Is it because you have fuel dripping from the joint between the cylinder and the exhaust pipe?

If so it is more likely that your fuel pump has failed. What happens is that the diaphragm springs a pinhole and allows fuel to siphon down the vacuum line into the crankcase. There it will leak past the rings and out of the exhaust port.
If your issue was the carburetor bowl overflowing you would see fuel weeping from the breather pipe on the top of the bowl rather than the exhaust pipe.

There is no mixture adjustment per se for your BV 18 carburetor. The only screw is the low speed bypass adjustment. You should undo the locknut and gently wind it fully in, then wind it out two full turns. From this start point you can adjust half a turn in either direction until you hit the sweet spot. For me this was 1/2 turn back in. Don't forget to tighten the locknut each time. Once you have it adjusted dribble a little Locktite on there to hold the locknut in. If it becomes lose there is a chance the screw will wind out and be lost forever.

I assume that you are still running the oil pump. If not you need to go steady with that starter fluid as it contains no lubricant.

Dave

[elt13]

I actually have a ton of fuel coming out of the muffler onto the ground. The fuel is mixed on this thing, but yeah i thought about the starting fluid burning the motor up.

I mess it with it a little more yesterday and found some things that just dont seem right. I can get it running and keep it running for quit awhile without the fuel line going into the carb. I put an extension on the pressure side of the carb and let it dump back into the fuel tank. It ran like that for maybe 4 minutes.

Still the pump you think?

[Dave Box]

Still think it's the pump, even more so now as there really is no viable path for fuel to get from the carburetor to the exhaust pipe. With the engine running the pump will continue to work with a pinhole and it will charge the carburetor bowl which will contain enough fuel to run for several minutes even with the inlet removed. Here are a couple of things that you could do.

Remove the pump and break it down. You may be able to see a hole. Quite frequently they crack on the point where the body clamps down on the diaphragm. Sometimes it is very hard to see the crack but you might feel it with your fingertips. Here is a picture of mine which had the same problem



If you do this then draw a line down the outside of the pump to ensure that you get the top and bottom halves back together in the correct orientation otherwise the pump will not clear the engine mount. (Ask me how I know this).

The other thing you could to is to jury rig a small fuel tank and gravity feed the carburetor thus eliminating the pump. You will need to clamp off both sides of the pump to preserve the crankcase pressure (and to prevent fuel spraying out everywhere).

The repair kit is almost the same price as a new pump ~$33 so if you do have a split I recommend a new pump.

Dave

[elt13]

That was it. I made up a gravity feed tank out of a water bottle and it ran great..thanks a bunch you saved me some headaches and a little cash. I was close to dragging it to the shop.

[Dave Box]

You're welcome, enjoy the toy.
Dave