OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines B&S 850 Series

I have a b&s 850 Series engine . Not used for 2 years, drained gas, removed tank, rinsed tank with fresh gas, removed carb. bowl, scraped out crud, and guess what still won't start, checked compression, 47lbs. is this low? and thinking i need to rebuild carb.?

Hi snookwon, welcome to Outdoorking.

It would help us get started on this if you post the engine's Model, Type and Code numbers, which will be either stamped or on a sticker on the engine. Often the information is on the top cowl, but it should not be difficult to find.

For the moment I'll guess that your engine is a Model 121Q02, until you correct me. It has been out of service for a couple of years, and now won't start. I take it it was running when last in service - please tell me if that is incorrect. I'm guessing it was put away with fuel in the tank, and no stabiliser in the fuel. I see you are in Virginia, and I don't know whether "reformulated" gasoline is used in Virginia, but I'll guess that it is. If all of those guesses are correct, you probably found a lot of yellow gunk, almost like aspic, in the fuel tank and float bowl. This material used to be gasoline components before they decomposed. The stuff is probably in the fuel line, fuel stop tap, fuel filter, main jet, and perhaps some other parts of the fuel system, plugging them up.

You might start by removing the float bowl, putting a cup or something underneath the carburetor, and turning on the fuel tap. Does fuel flow out of the carburetor in a steady stream, into the cup? If it does, there is no need to dismantle the fuel system parts upstream of the main jet. If it does not flow, remove the fuel tank cap and see if fuel then flows. If the tank cap makes a difference, you will need to clean its vent. If it isn't the cap, the other fuel system parts upstream of the carburetor need to be cleaned or replaced.

Once you have fuel flowing properly into the carburetor, the next step is to see whether the carburetor is going to need to be investigated. To do this, remove the spark plug and put a spoonful of fuel into the combustion chamber through the spark plug mounting hole. Reinstall the plug and connect its wire, set all controls into the starting position, and crank the engine. If it fires a few times, and probably runs for a couple of seconds, then stops, you will know that your problem is that the carburetor is not passing fuel, and it will have to be dismantled and cleaned, or the fuel pump has failed. However if the engine does not fire with fuel in its combustion chamber, the problem is not with the carburetor, or not only with the carburetor. The remaining possibilities are that the ignition is not working, or the compression is inadequate.

To check the ignition, first remove the spark plug again, clamp it to an unpainted metal part of the engine, and connect the plug lead to it. Then set all controls in the starting position and crank the engine, looking closely at the spark plug gap. If you see a series of strong, even sparks across the gap, the ignition system is working and it will be necessary to look for possible compression issues.

Briggs does not specify compression pressures these days - like some other engine manufacturers, they focus on leakage tests. However the 47 psi pressure you quoted is rather low. The engine has a decompressor, so you should expect something like 75 psi rather than the 120 or more you would get without a decompressor, and furthermore the engine should probably start with 47 psi, but nevertheless something does not seem to be sealing properly. This is not unexpected after standing for 2 years, however. I suggest you should check the valve clearances, and push the rockers with your thumb to ensure the valves are moving freely, then recheck the pressure.

If you come back here with the model information, and the results of those tests, we can move forward in a more specific direction. That is quite a decent engine you have, and it will probably not be difficult to resolve this and get it running well again.

ok, i've got some info. 120202 3780B8 and 0710199044713
Tried gas in the cylinder and got it to fire up.
Got a carb. rebuild kit from a local dealer. He did understand the numbers.
I have rebuilt my outboard motor carbs before, anything special with this one? does the silver knock-outon the side need replacing? looking close at the float niddle seat looks difficult to change.
thanks for your reply

Here is the Illustrated Parts List for your engine:
http://bsintek.basco.com/BriggsDocumentDisplay/18koxFXLnfBhU7y.pdf

Did you verify that fuel will flow through the fuel line and out through the carburetor's needle and seat, when you remove the float bowl? I usually recommend that people do not dismantle carburetors unless there is reason to believe it is the carburetor, not the upstream fuel system, that is at fault.

If you go to Page 4 of the IPL, you will see there are two basic versions of the carburetor which might be fitted. From the date code, yours seems to be the earlier type: 125, not 125D. It appears to be a Walbro LMS carburetor. Here are the disassembly and reassembly instructions from the shop manual:





The overhaul procedure Briggs specifies is very comprehensive, but probably unnecessary in most cases, especially yours where all you have is some jelly and perhaps gum in the carburetor, rather than a wear problem. Personally I would just clean everything as specified, but would not remove Welch plugs, throttle and choke shafts, and the float chamber seat. You will notice that replacing the float chamber seat is relatively difficult. Otherwise I would follow the manual and install the easy parts of the kit. If you have questions along the way, or if there are still issues after you follow the process, let's try to work them out in this thread. If, on the other hand, all is well and it works perfectly first pull, please tell us in this thread so we can close the case.