OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines B&S 12HP puffing smoke

Hi folks I'm new kid on the block here and have bought an old and seeminly tired Greenfield with a 12HP B&S I/C motor. These types of engines are new to me. My experience is more motorbikes and car engines. Of recent times I have started to play with two strokes.



I bought it off a fella a week ago and suspiciously it was warm when I got there. Today after starting it for the first time ( I rewired the electrics and picked up new air cleaner, oil change etc) it ran pretty badly. It fired up fine but blew a puff of smoke accompanied by a cyclic run fine for 3 seconds, slow down then pick up again which on the rev up would blow the puff of smoke. It did this the entire time but got much better when it was warm. Once warm the cyclic stutter was far less noticable but would still blow a puff of smoke. When the idle was fine (between the stutter) the motor blew no smoke and idled fine.


In the above picture, the mechanism infront of the fuel tap which has the spring loaded screw rotates in sync with idle pattern. Is this some type of govenor?

During the week I will remove the carb, clean and replace parts. Due to the price of parts (needle and seat and two gaskets for my 5.5I/C cost me $50 last week!!) I was not wanting to spend too much on a very tired motor and put it (money) toward a replacement.

If anyone has any advice on what to check before I spend dollars I'm all ears.

Thanks

Edit Model 281707 Type 0137-01 Code 88060311

The cyclic speed variation is almost certainly hunting, and can be cured completely just by turning the mixture screw anticlockwise slightly (one eighth to one quarter turn probably, but just adjust it until it doesn't hunt and runs nicely). If the puff of smoke is black, it is just petrol smoke caused by the governor suddenly slamming the throttle all the way open (acceleration enrichment), and if so, there is nothing wrong with your engine (which is quite a nice one). If you post a picture of the carburetor we can probably point out the mixture screw to you. The spring-loaded screw in your last picture, above, is the idle speed adjustment and is on the carburetor, but not the same side as the mixture screw. The lever the idle speed screw is on is part of the throttle butterfly, and it is moved by the governor. The governor moves the throttle to control engine speed. When you adjust the mixture, watch the lever the idle screw is on: as you reduce the leanness of the mixture, the lever will move less. When the mixture is close to correct, it will move only slightly and seldom unless you put load on the engine or move the speed control.

Based on what you have posted so far there is no reason to replace anything in your engine, and it will probably be more reliable if you do not do so. However if the puffs of exhaust smoke are blue, rather than black, your piston rings are probably on the way out. I suggest you adjust the mixture which should get it running properly, then assess whether it smokes.

Hi Grumpy. The puffs definitely have a tinge if blue especially when you close the throttle to shut the motor off. I'll try for some pictures this arvy.

Going by the Briggs engine id chart you have I came up with engine having an alternator. I have a pair of wires coming out from just behind the starter motor.

Thanks.

From your first set of pictures the carburetor looks like a large one-piece flo-jet. If so it will have the mixture adjustment screw sticking down vertically from the bottom-center of the float bowl (vertically down below the idle speed adjustment). Please photograph the float bowl from lower down in front so we can see this.

Your engine was made on the third of June, 1988. According to the model number it has a 12 volt gear drive electric starter with alternator.

From your reports of blue smoke it sounds as if your engine is well on the way toward needing new rings. Meanwhile I hope you are keeping its oil clean. That has a major effect on the wear rate, and as you said, parts for your engine are expensive in Australia. It would be unfortunate if your Synchrobalance wears out.

I purchased the mower one week ago. I did run the mower on the day I purchased it. Unfortunately when I got there the owner had already started and warmed the engine up so I couldn't see what it was like on a cold start. The original air cleaner was filthy and the two screws to hold the element in place were missing even though the element top has rub marks to indicate they had been there originally. Who knows how long they had been missing. I would say the motor has had a good dusting.

The oil that was in there was clean but I changed it (B&S sae 30) prior to starting it at home. I picked up a new air cleaner and screws. The plate which the air cleaner sits on had partially separated from the main housing so I cleaned it up and resealed it. New fuel hose as the old one was badly perished and a new fuel filter.

New battery and mount plus replaced all wiring.

I'll rustle up some more pics.

Thanks for your help.

From your engine's model number it sounds as if it has the reciprocating type of Synchrobalance, not the counter-rotating shafts type. The reciprocating weights seem a bit prone to wearing out if the oil is dirty. Hopefully your engine has always had clean oil, and has just been dusted a bit, which may not be terminal with that cast iron bore. You'll probably need new rings, and you can hope the piston has survived: you'll find out for sure when you do the rings. It's mainly because of the Synchrobalance that I was concerned about the engine's history. I wouldn't be worried about the owner having pre-warmed it, given that it seems to be running very lean: you can fix the mixture in a couple of minutes. You might check how clean the fuel tank is though, in case there is some dirt in the fuel system - something has to have changed, to make the carburetor run lean, and a small amount of dirt is one possibility.

Hi Grumpy. I didn't have a chance to take any extra pictures but I will admit to an embarrassing moment. I had a good look at the carb (with aircleaner etc still attached) and the spring loaded screw in the middle of the float bowl had been touched. By me. I thought it was a fuel contaminant drain of some sort. Ahh the shame!!

I turned it out about one and a half turns and it idled smoothly from a cold start. It still did blow puffs while running but during a fairly smooth idle. Did a few laps of the yard and it seemed fine except for the very fine tinge of smoke.

I'm hoping it might sort itself with the fresh oil. I'm guessing the bloke who owned it wouldn't have been a stickler for the correct weight oil.

It does blow smoke with any advancement of the throttle position. Predominantly black but with some blue.

The black smoke indicates momentarily rich mixture during engine acceleration - nothing to worry about, but you should check your mixture adjustment. Run engine close to full speed, and adjust main mixture control (under floatbowl) richer and leaner to find rich and lean limits before it gets slightly rough, then put it halfway between. There is a separate idle mixture adjustment sticking up vertically from the top of the carburetor - adjust that one with the engine idling.

It should never blow blue smoke, regardless of what you do. Seems like your piston rings are past their best. You should think about replacing them - until you do, it will almost certainly have some blowby, which will sludge up the crankcase and cause some engine wear. It isn't an urgent issue, but if you end up with everything else working well, it seems worth getting the engine right as well.

Hi Grumpy, you asked about some carby pictures,





I fiddled with the screw I mucked up earlier and today it seemed to run OK.

Why the cyclic puff of blue smoke every 5 seconds or so accompanied by a very slight misfire? Why would it not be constant?

Valve issue?


Edit; today whilst running it I threw a multimeter across the two wires protruding behind the starter motor. At about 80% throttle it was putting out 13V and sitting there very stable. Can I connect these directly to the battery with an appropriate fuse?

That is certainly a large one-piece flo-jet carburetor, as I thought. It seems to have a good reputation as a carburetor. You can see the idle mixture adjustment screw sticking out of the top, with a spring under it.

Try watching the throttle butterfly when it does its puff-and-misfire: does the throttle move? If it does, the puff of blue smoke is just due to it suddenly working a lot harder, and generating some blowby, which probably blows a bit of oil into the breather hose, which puts it into the air intake. It shouldn't need to do those cycles. A tune-up may help: adjust both the main jet mixture (at high speed) and the idle mixture (at idle) to make it run as well as possible. Clean or replace the spark plug. Check the clearance between the ignition module and the outside of the flywheel, and set it to about 0.012" (thickness of an ordinary visiting card). Check the tappet clearances - intake 0.006", exhaust 0.010", measured with the engine cold. Let's see if it still does tricks when it is in tune.

The manual seems to think your starter-alternator has an integral regulator, and a rated maximum output of 10 Amps. It is not supposed to initiate output unless connected to the 12 Volt battery. If it is the integral regulator type, it connects directly, positive lead to positive battery terminal, negative lead to ground. To test it, connect it with a 12-15 amp fuse and start the engine. Run it slowly at first, measuring the battery terminal voltage. If all is as it should be, the battery voltage should gradually rise to 14-14.2 Volts approximately. How long that takes depends on the state of charge of the battery. If you have an ammeter with a capacity of ten amps or more, you could put it into the positive alternator lead and check that it charges the battery, and that no current flows when the engine is stopped. Do these things carefully - if the alternator does not have an integral regulator after all, or the regulator has failed, it could overload the alternator, which is why you need to start with the engine at low speed. If you accidentally put your meter across the battery while on the Amps range, you will not have a meter any more. You probably know all this.

Thanks muchly Grumpy. I'll have a play on the weekend.

I have organised a replacement motor at this stage and will cost up the fix up on this one.

Hi aldot and grumpy,have been reading this topic with great interest(and others)i too was having the same problem with my 8h/p rover.having the same carby as aldot i removed/cleaned/tuned and made adjustments to the carby with great success.i do believe that after 8 years of solid work it is performing better than any other time,so thank you grumpy for great advice.

ps;considering my engine is a little worn i put 20 mills of -no blow smoke-stuff in with the oil just to help it last a little longer...ken

Ken, I have little or no respect for oil additives as a solution to problems. Things that are worth having, have already been adopted by the oil manufacturers and have become necessary oil ingredients to pass the automotive tests (SF, SG etc.) That is not to say that new oil additives are not being invented: I'm just saying that the ones marketed as individual products are often valueless and sometimes simply frauds.

Generally, an old engine should run just as well as when it was new. If it doesn't, it can be fixed so that it does. Eventually, it may have so many worn parts that it is easier and/or cheaper to just buy a less-worn one instead. The difference between a professional and an enthusiast lies in when they think it is time to quit.

I'm glad your engine is running properly again Ken, it makes machinery much more satisfying to use when it works as it should.