OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs & Stratton 21hp

Hi,I am stuck with a problem regarding a ride on. It is a John Deere D120 with a 21hp B&S motor. Problem, will not turn over on compression stroke,but when I turn the motor back a revolution by hand, hit the key and it will start,plus backfire(only once though). Have been told it could be the internal decompression unit or the valves, or both, having never worked on an engine this big not sure of valve clearence. And would the decompression unit make it backfire, can understand how it mite stop it turning over on compression stroke. Any help or advice is appreciated.

Hi Paul, is that a Vanguard or Intek engine - that is, a Briggs overhead valve engine? If so the symptoms are generally consistent with a very common problem.

The decompressor on those engines opens the exhaust valve momentarily about halfway through the compression stroke. This is similar to most other decompressors. However the Briggs one is very sensitive to exhaust valve tappet clearance adjustment, and it also may wear out after a while. If the clearance is even slightly more than specified, or the decompressor is worn out, the decompressor stops working and the engine won't turn over on the starter motor. So, first tell me what engine is in the machine, and I can tell you what the tappet clearances should be. Most likely the exhaust clearance of the engine you are looking at is more than it should be, and the engine will behave normally once you adjust it. However if the decompressor will not work even when the tappet clearance is at minimum, it will be time for a new camshaft.

If it turns out that the engine is a 33M677/0149-B1, the required valve clearances are 0.003-0.005" for both intake and exhaust, but this is different for each different engine size, so let's make sure we are talking about the right engine.

I haven't heard of the engine actually backfiring, we'd have to have more information to figure out what is going on there. If the engine is just about making it to the ignition timing point and stops rotation just as it sparks, strange things could happen - most of them not good for the engine.

Hi Grumpy, whats an easy way to tell the difference. This motor has got over head valves, the number I have found is 331877/0240-B5. Have had a quick look through the " operators manual" and it states that if engine backfires then you have a faulty spark plug,could this be part of the problem or is it their way of making you buy more stuff until you get peeved of and take it to a mechanic.

Paul, that engine is Model 331877, Type 0240, Trim B5. Because it is a 33 cubic inch Intek, the tappet clearances I quoted (0.003 - 0.005") are correct. The usual practice is to aim for 0.004", and that is what I suggest you do.

You lost the lottery: it is an Intek, not a Vanguard (which costs more and seems to have a substantially better reliability record). Essentially, Briggs commissioned Daihatsu to design the Vanguard with the intention of making at least the single cylinder version itself. However it found it too difficult to make and redesigned it to overcome this, renaming it Intek, while continuing to sell the Daihatsu-built twin Vanguard, and commissioning Mitsubishi to build the single Vanguard. The Mitsubishi deal quickly fell through and Briggs subsequently manufactured that engine itself. Unfortunately the Briggs redesign from Vanguard to Intek had some unfortunate effects in a couple of areas. On the single cylinder Intek, the main problem is a propensity to blow head gaskets. The twin has that problem but also had until recently a problem with the air cleaner box leaking and dusting the engine.

Here is the Illustrated Parts List for your engine:
http://bsintek.basco.com/BriggsDocumentDisplay/18kvuDXSnfBhU7y.pdf

The Briggs Operator's Manuals tend to focus on simple faults the average operator can fix personally, hence the reference to spark plug failures. Unfortunately most owners cannot distinguish between backfires (fuel explosions in the muffler) and several other faults such as spits, and in the case of LPG engines, fuel explosions in the intake manifold or air cleaner. Because you have an Intek that won't crank, the odds are rather strongly in favour of the exhaust tappet being a bit loose or the camshaft worn out (the former being much more likely). On the Intek and Vanguard engines it is essential to adjust the tappets at least once per year. If you do that, you probably won't have the failure-to-crank problem until the camshaft wears out, which is likely to be a long time.

Hi Grumpy,thanks for that. Lost the lottery, think my bad week is turning out to be a coulpe of bad weeks now. But chin up keep going. Curious question,i am unable to find my good set of feeler guages ( think some one has "borrowed" them ) but have found an old set but they are in mm. for example 0.15mm. Have used my digital vernia calipas and worked out that 0.06mm + 0.04mm add up to 0.004" . Just want to make sure this is correct before i go ahead. Please correct me if i'm wrong. Thanks Paul.

Paul, 0.1mm is indeed 0.004" (or 0.00394" anyway) so, yes, that is the correct metric equivalent. Please let us know what the clearance is before you adjust them though - in mm is fine, there would be more people using mm than inches by now.

Hi Grumpy, have checked valve clearance, the exhaust was spot on but the inlet was slightly tighter ( 0.05+ 0.04 =0.09mm ), done slight adjustment. Gave the mower a little try, wouldnt kick over on compression, so turned motor back by hand tried again, kicked over and "backfired" at same time. It is actually coming from the aircleaner region ( felt it through my hand as me dad was kicking mower over ).

Hi Paul, there seem to be two main possibilities remaining: the decompressor is worn out or stuck, or there is a different problem not associated with the decompressor. Let's start with the easiest thing to check, despite it seeming unlikely as a cause: extremely premature ignition timing. This might explain both the refusal to crank and the tendency to eject burning gas through the carburetor, if it is kicking back on the starter. To check this possibility, just disconnect the spark plug lead from the spark plug but leave the spark plug installed, and try to crank the engine. If it will crank properly when there is no spark, but not when there is spark, the problem will be due to substantially advanced spark timing or perhaps incorrect camshaft timing. Just report the results of this test, please, rather than investigate this possibility further at this stage.

The other possibility is that the decompressor has stuck in the full compression position, or worn out completely.

Here is an illustration (from the manual) of the compression release on the Vanguard and Intek engines:


As you can see, the compression release consists of a stamped piece of steel, sitting on-edge, which protrudes slightly higher than the cam lobe at a point halfway through the compression stroke. Briggs indicates that there is a possibility of this component wearing out or sticking. Here is the instruction for inspecting the camshaft:


Before considering dismantling the engine to replace the camshaft, you should perform a simple check on the decompressor. Look closely at the rocker arms while slowly turning the engine by hand, with the spark plug removed so it is easy to turn. Watch for one of the arms moving momentarily during the compression stroke. If the clearance is correct and neither arm moves, the decompressor is kaput.

Hi Grumpy and Paul,
I have had a similar problem with the exaust valve coming loose and caused it to backfire and wouldn't turn over sometimes. There was another time I found the carburetor butterfly had come loose and found it's way wedged into the elbow of the manifold and the two screws from the butterfly had worked their way past the exaust valve and in to the exaust itself (muffler). Just an idea though probably not the cause of your problem.

Can you clarify the problem with the exhaust valve Rob? What had come loose? Engines sometimes "drop valves", meaning the valve stem cotter falls off, but that usually results in a smashed engine. The valve seat insert might come loose, but that would not prevent the engine from turning over normally. Sometimes with overheating, the exhaust valve guide comes loose and slides upward, stopping the engine from rotating before the exhaust valve can open fully - maybe that's a possibility, though I don't see how a jammed exhaust valve would cause it to pop through the carburetor, especially since it would happen during the exhaust stroke, nowhere near the firing position.

Hi Grumpy, have done as you suggested. Took spark plug lead off, still wont start on compression stroke, any were else it will. Carefully watched rocker arms while rotating the motor, slowly through compression stroke, no movement at all. Done it a couple of time just to be sure. If it is the decompressor that is no good how could that be causing the "backfire" (am i getting to ahead of things at the moment).

It is timed to fire somewhat before top dead center. If it stops rotating just about the time it fires, it will try awfully hard to start backwards, and it might manage to get back far enough for the valve to be opening a tiny amount if the decompressor is worn out but still lifting the exhaust valve just enough to make it leak into the muffler. You would then get a genuine backfire: an explosion in the muffler.

If you back it up a turn from compression, put the spark plug lead on, and hit the starter, does it start and then run normally? No funny noises and you can't fault its performance after starting?

If it runs properly when you do that, it is pretty clear that there is nothing wrong with the engine except that the decompressor isn't working. At that point, I think it will be necessary to remove and inspect the camshaft and tappets.

Grumpy , sorry to cut in but it was the valve seat, the ring which the top of the valve closes down on. The problem was with a side valve and just realized that your on ohv sorry for that. I did have a very similar problem with a 17 hp kohler courage ohv engine and that was simply? a valve adjustment and till I got it right it would not turn over. I think I should just observe .

Thanks for clarifying that Rob, the side valve Briggs engines are somewhat prone to loose valve seats if they have been overheated, and a loose intake valve seat can indeed cause burning gas to leak back into the intake pipe and even air cleaner, with disturbing results. A tight tappet or leaky valve can have the same result. Also, your Kohler sounds as if it had a tight exhaust tappet until you adjusted it correctly. I haven't heard of that engine being as prone to wearing out its decompressor as the large Briggs OHV engines are though, which makes regular tappet adjustment on the Briggs absolutely essential. Many, many people replace Briggs starter motors and batteries, sometimes repeatedly, before they find out what the real problem is and adjust the tappets. As the Briggs decompressor wears down, tappet adjustment becomes more and more critical, until even with the minimum specified clearance, the decompressor does not work and the camshaft has to be replaced.

Hi Grumpy, turning it back off from the compression stroke it will start and run properly but still puzzled about the "backfire" (thats what i'm calling it) which seems to be coming from the air filter carby region, as there was a small puff of smoke that came out of air filter just before it acctually kicks over. For example, whirl whirl bang then starts as if nothing happened.(the whirl is the motor turning over). So other then that acctual bang it is running perfectly.

As long as there is no backfire if you back up the engine from compression before starting it Paul, it seems reasonably certain that the backfire is being caused by the failure to crank. When we fix the failure to crank, the backfire should be gone.

There are ways to investigate, for instance, how far the engine rotates backward when it fires just as it comes to rest, about 20 degrees before Top Dead Center. To do this without modifying the engine for the experiment, however, would require high speed movie cameras or a similar technique. If it were done, it would probably tell us with certainty which of two things is happening. The first possibility is that, when the engine is rotated backwards by the force of combustion pressure to a point midway through compression, the worn-out decompressor opens the exhaust valve by a tiny amount, causing a backfire. The second possibility is that the backward rotation extends a bit further, to the first 20 per cent of the compression stroke, when the intake valve is still slightly open, causing a pop in the air intake and air cleaner. If I were there to see and hear you crank your engine, I could probably tell whether it rotates backward when it pops, and if so, how far. I could probably also tell whether the pop is produced in the muffler or the air cleaner. However I'm not there, and it doesn't matter anyway, as long as it is cured by repairing your engine, which has to be done anyway. Producing those pops is damaging your engine, and you should stop doing it.

Hi Grumpy, thanks for that. Will get hold of the gentleman that owns the mower and try and talk him into repairing it, he told me he not interested in spending to much on it, its a shame really as the mower isnt that old. Thinking if i explain it the way you have and whats happening and long term what will happen hopefully i can change his mind. Will let you know how i go.

Thanks for the update, Paul. If it were me I'd replace the rings at the same time, so that won't need to be done for another 500 hours or so - but there are arguments against doing the extra work. One is that if you replace the rings you'll have to take the head off, and with an Intek, that might get it started on blowing head gaskets. Another reason is that we don't know how many hours it has run now, so we can't really guess how close it is to needing new rings. Essentially though, I tend to replace rings when I'm opening the crankcase if an engine has wear anywhere, provided I can get factory-quality new rings very cheaply. If it's never been dismantled before, that should pretty much make it into a new engine again. That rule seems to work for ex-contractor Hondas that have never been in the hands of amateur owners, anyway. (I've had a couple of Hondas that had passed through multiple hands, ending up with them running on just a splash of oil in a sump-full of swarf, or in one case, no oil at all but a dollop of thick molybdenum disulphide oil additive. There is nothing you can save from those engines except bolt-on external parts, and some assorted fasteners.)

Hi Grumpy, have some bad news. The bloke that owns the mower is not interested in repairing it in any way, bit silly if you ask me considering it is only about 2 yrs old if i'm not mistaken, and only done approx 200hrs work. Would of liked to sink me teeth into it, but each to their own. Explained everthing to him as you have to me but couldnt persuade him, no doubt i'll see him when it is in worse condition then it is now. ( have dealt with a few Hondas like you have, the best one had water in sump, two-stroke fuel and a spark plug from what looked to be a chainsaw which they had somehow got running, needless to say it was cheaper to buy a new mower then try to repair) Thanks heaps for your advice and help Grumpy, much appreciated.

Thanks for the final report, Paul. I guess the owner can keep using that ride-on if he hand-rotates the engine back from compression before each time he starts it, so it gets a run at compression before it hits it. I suppose that might work for quite a long time.

I'll close this thread.