Briggs/stratton

Help!! I have a B/S 18hp I/C engine that is leaking gas into the crankcase, I replaced the diaphrams and pump body of the fuel pump, it ran ok at first then it mega flooded, the airfilter is soaked with gas and there is gas again in the oil, could the crankcase vent be involved somehow?Thanks for any suggestions

 

Lost:

This engine has a problem with carbon build up in the combustion chamber. It is possible there may be a piece of carbon under a intake valve allowing gas to come back to the carb. and in to the crankcase through the vent.
Check the compression to see if it is the same on both sides. Another sign of carbon in the combustion chamber is a knock that sounds like a loose rod.

LOL

 

Thanks, ill try that i was sure the problem would be a leak in the fuel pump,i have also been told a blocked muffler might cause this kind of problem, any thoughts on that? ill pull the heads of and check it out and let u know

 

Hello lostoutwest, check the carb float, I suspect it is hanging in the open positon. If the intake valve is open when this happens the cylinder will flood and fuel can drain past the rings and get into the oil. DO change the oil before running this engine....Mike

 

mikejmerrit
havent had a chance to look yet will post as soon as i can look inside this bad boy and see if carb ect is ok

 

Hello Lost out west!

I agree...with all due respect to rogerh, don't go pulling the heads just yet. There are other more likely causes of your symptoms.

Most likely, the fuel inlet needle is leaking as Mike said. Even with the fuel pump, and the tank being lower than the carb, a leaking needle can cause a siphon effect and drain fuel into the engine. Definitely change the oil before starting it again if it is diluted with gas.

Otherwise, I would think about the fuel pump...which you have already addressed. I have never seen carbon buildup in the combustion chamber cause an engine crankcase to "fill" with gas.
Is that what is happening? Or is the engine just flooding out but not getting it into the crankcase?

 

The oil is getting into the crankcase and apparently into the air cleaner.This is a customers machine, when it came in the airfilter was dry the area around carb seemed damp with fuel, like there was leakage. I changed the pump body and the diaphrams, drained and replaced the oil and ran it for a while it seemed ok.The next day customer call and say it leaking again and this time air filter also wet with fuel, so he should bring back in, im gonna do a compression test as roger suggested.It is a new style carb with 4 bolts holding on pump not three i can change the needle ect but the seat is non removable as far as i know, I am realitively new to small engines but trying to learn fast lol, all your guys advice is greatly appreaceated, also what about clogged muffler mabe causing some of this

 

Listen to Cheese!

He is correct. The inlet needle is leaking, or the float has fuel in it
and it cannot float high enough to shut off the fuel.
Fish

 

Hmm machine came back just before closing time and air box was full of oil and gas , i can see the float or needle causing fuel in the oil but not oil in the airbox/ or am i missing something.I was thinking that mabe rings worn bad and blowby causing positive crankcase pressure and forcing oil up the breather tube into airbox, i will due compression test and leak test and see if rings are possiblely causing this problem and let u guys know what i find

 

You may find weak compression but that is not causing the oil in the air box. Did you check the float? It has to be failing to cut the flow of fuel. Check Fishers thought on the float having a tiny hole in it. This has confounded many. Look at it this way...Faulty float operation lets as much as a quart or more of gas into the oil. This thins the oil and adds to volume and is easily drawn into the carb via PVC. I think that is all the problem is. The only other thing I can think of that may cause the fuel/oil mix to get all the way into the filter is some blowback caused by an intake valve to tight. This can cause whatever in the carb venturi (throat) to be pushed backward in to the air filter. Its amazing how tight a valve can be and an engine still run OK but on a twin is even more likely because you have one good cylinder over riding the bad one.....Mike

 

Agreed...If the oil gets heavily diluted with fuel, the oil level gets too high. This high level of thin oil gets blown everywhere (up through the PCV mostly) and soaks the filter. The intake valve being too tight can soak a filter too, but it would really take some time to get a noticeable amout of fuel in the crankcase.

 

Briggs & Strattion engines are bad at gas leaking through the carb. The simplest way to correct this problem put a fuel shut off valve between tank and fuel pump or carbeurator. Turn gas off. Drain oil. Refill with oil; remove spark plugs. Tape up end off spark plug wires. Turn engine over to blow gas out of cylinders. Replace plug an plug wires. Turn fuel on. Start engine. When you stop engine turn fuel shut off off. This will prevent gas from leaking into the engine. At a cost of abotu $5.

 

You do need to check your float

 

Guys I'm still inclined to believe there is a intake valve problem. Lost hasn't said anything about gas coming out of the muffler or the crankcase being over full of oil/gas mixture.

 

Does the mower run well and cut and flood after sitting or does it flood as its running?

 

If this thing mows i am in big trouble lol, as it is in a blacktop roller any ways i wont get to it now to monday and i cant get a straight answer out of the guy that brought it in as he speaks english as a second language , i will check compression change the needle ect and try to get more info from the guy that runs it ,and go from there

 

I installed a new needle, changed the oil ran it for bout half an hour ,now letting it sit over nite and see what happens

 

Lostoutwest: I hope you have better luck than i did with the same problem,On my 17hp l-head twin in an MTD tractor. It started after the tractor set outside for about a month!
I had oil in the air cleaner had gas in the oil, wich overfilled the oil. I was able to get it started after much effort but not for long and it was smoking and flooding like crazy. I changed the oil rebuilt the carb. and tried to start it, acted the same.
After a compression check showed no pressure on the right cyl. the head came off. I re-ringed it and found a bent intake valve. The valve showed .020" runout on the face. so it was not contacting the seat but in one little spot. Am waiting on the valve now. Make sure your engine has the proper oil level in it crank it over a few times and then do a compression test.....And your correct that carb has no adjustments at all, for the float hight or fuel mixrues.

 

In this paticular case as it is in a blacktop roller it is virtueally impossible to get at heads ect , so i was really hoping for a easy fix but no luck as oil was full of gas once again , from just sitting over nite, im not quite sure i understand how a valve problem could cause this problem, but i guess i gonna have to pursue that next !!!

 

On this machine (roller) a fuel shut off would probably help a great deal but tank is in difficult place only place i could put valve would require effort from the operator every nite to access valve , and with theses guys its asking too much lol

 

If the machine is flooding overnite, and without being run, then it's not a valve problem. Does the fuel tank have a pressure relief vent? Is it open?

It might be worth checking the fuel pump again. There are two flap valves in there. They should have a spring behind each one. The spring is tiny and often gets lost without notice. The face of the pump that the little flap valve seals against should have absolutely no corrosion, buildup, or scratches. If any of this is found on the face, it must be cleaned/repaired for a positive seal. I have seen the plastic body of these fuel pumps warp and distort just enough to allow leakage as well. It is not usually visible to the naked eye, but it's enough.

 

Hi Cheese
I ran the machine for about a half hour , checked the oil it seemed ok there might have been some gas in it but not alot in the morning the oil was full of gas, as far as the fuel pump goes i replaced the plasitic pump body and both diaphrams and replaced the needle valve. Do you mean is the tank vented? when u say pressure relief, it also has a very poor idle (surges) but seems to run ok at full throttle

 

Ok...did you notice the tiny springs that I mentioned? If not, then they fell out when the pump got disassembled. (happens all the time) They are so small that you wouldn't have noticed or heard them. They should be there.

Another thing I have seen: The seat, where the needle seals in the carb sometimes gets deposits or varnish on it. It is basically non-replaceable. What I have found is either a Q-tip soaked with carb cleaner pushed up there and twirled a few times, or a Q-tip with rubbing compound on the tip done the same way will clean up the seat area. Gas is seeping somewhere in one of these areas.

The fuel tank should have a vented cap, to keep it from developing pressure or suction. It shouldn't leak even with some pressure, but the pressure could be the difference between it leaking or not leaking if that's what is happening. Usually the cap has the vent in it, unless it takes a boat-style tank. On that there will be a thumbscrew to loosen to let the tank breathe.

 

Cheese
In the pump body there is 2 valves i believe that have a spring with them is that the ones u speak of , or are there others

 

Yep...2 valve, 2 springs. Tiny brass or copper ones.

 

Cheese
I didnt get a chance to look at it today just ordered a complete carb as customer in big rush and wants a new carb put on it so thats what i will do.I did look at the gas cap it doesnt seem to be vented,mabe through the threads, but it is probably a big part of my problem

 

Is there a pinhole in the center of the cap? Do you hear pressure escaping when you open the cap? A non-vented cap usually gives problems when the unit is running. fuel that leaves the tank and gets used by the engine needs to be replaced with air. If air cannot get into the tank to replace the volume of fuel used, a vacuum develops and the engine starves for fuel. A non-vented cap will also allow pressure to develop (rather than vacuum) when the temp rises and the air inside it expands. That pressure is on the fuel at the carb end as well. If your fuel system is not good and tight, in the carb as well as fuel pump, this pressure will spring a leak somewhere.

 

Cheese
If there is a hole in this cap it is microscopic in size i was assuming that it vented through the threads .i looked for any other kind of vent or valve and could find nothing, it never starves for fuel or makes a noise when i open cap, i think ill drill a small hole in it ,see if that helps

 

Cheese
I compared cap from roller to cap from 4hp briggs it has the same rubber piece in it but there is a metal piece that moves up and down in the small cap, i am gonna assume mabe this piece is missing from the big cap, how does it work as there is no visible hole in the small cap either do they vent through the threads?

 

I think that the small metal piece you are describing is a baffle. There is a rubber piece that goes between the cap and the metal piece. The rubber has holes in it that go to an air space. The metal keeps that air space from filling with splashed gas. The air space vents through channels cut in the rubber, between it and the cap, to the thread area where it vents. If you don't have trouble with it starving or developing pressure, then I don't think the cap being vented is an issue.

Even if it were an issue, it still isn't the entire problem. The main reason I mention it is that if it were developing pressure, and whatever is leaking is a sporadic seal, then that pressure could be just the extra persuasion needed for it to leak.

 

Hello everyone
I have removed top of carb replaced needle again also gave seat a good cleaning with cutip and carb cleaner, that was 2 days ago and so far no leaks but it has only been run for short periods ,time will tell thanks alot guys for all the advice much appreaceated

 

Anytime ! I think that will be the end of your problem.