Need to do my BX, but ... A CLUE !!!

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AndersDK
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Need to do my BX, but ... A CLUE !!!

Post by AndersDK »

... wife wants me to do up the mower first [8D]
So please help me on this off-topic, to free my hands for my BX project [:I]
YES - I know, some of you guys helped me last year with my Briggs&Stratton 12Hp IC engine.
I never made it to run as it should, it still 8-strokes (misses every second) and smokes a lot.
I tried "bore down" the jet in the carb, by inserting a wee-thin wire down the jet. This makes the engine barely run unless full throttle, - and it still smokes and 8-strokes [:D]
It does not matter wether air filter unit is fitted or not, it's the same.
The float in the carb is perfect as is the needle valve. I even tried offset the needle valve a bit, to lower the level in the float chamber. Drilled a small hole to check level too - it's downright perfect, even too low, but the engine doesn't seem to care [:)]
The ignition produces a heavy blue spark of some 10-11mm in free air.
At least 5 different (2 brand new) spark plugs have been tried, 3 of these the original B&S. Engine doesn't care [;)]
Now I finally tried stroboscoping the ignition, and was quite astonished that a rock steady ignition occurs at 18' BTDC.
I checked my method and ref marks over & over again, and the conclusion is firm : an average of 18' BTDC.
Engine revs has no influence, same timing any rpms tried.
Anyone have an idea if this is a sensible ignition timing ??
I thought it to be approx half that (8-10) for a simple 1Cyl engine with non-adjustable ignition ?
BTW : all engine spec (magnetic coil to flywheel gap, valve clearance/timing, plug gap) is perfect within spec from B&S downloaded service manual.
PS : these engines uses a magnetic ignition coil system. Do these devices suffer from (permanent) magnetism, i.e. needs a de-mag now & then ?
PPS : anyone out there with a Haynes "Small engines repair" manual ?
NiSk
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Post by NiSk »

Hej Anders!
Most of my problems with B&S motors has been due to the flywheel not sitting correctly on the crankshaft, i.e. sheered or worn woodruff key allowing the flywheel to be offset to the correct ignition timing. But I guess you've already had the flywheel off so you would have noticed something like that.
//NiSk
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Post by jeremy »

Anders
I know this may sound daft but is your fuel fresh. A couple of years ago after the winter lay up we couldn't start our B & S engined mower which I couldn't understand as it had been ok when we last used it and I had done nothing at all in between. I was just about to go through the usual strip-down routine but superficially all seemed ok but the fuel looked rather green and oily. I drained it and put in fresher and it started normally.
I think unleaded contains some parts which evaporate very readily and the B & S tanks are always use that cap with the 3 vent holes which allow good breathing for the tank and allow the volatile elements to dissappear. My cure is now to run the thing on minimum fuel so that we regularily add fresh.
Probably totally irrelevant I know.
I agree that 18 degrees advance would seem excessive. I could undersatnd points opening at 18 degrees to allow time for a spark to build up and produce say 8 degrees on a strobe. For the likely speed range of these engines I would have expected timing of only a couple of degrees BTDC. This magneto will produce a 'spare' spark at the end of the exhaust stroke.
Is the ignition controlled by points (or hall effect device etc) or is it timed by the generation of the spark by the flywheel magnet passing the coil? Does it use a condenser?
Sorry my magneto theory is a bit hazy as I haven't had to deal with one ofr years - I suppose an indication as to how reliable they are!
jeremy
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Post by AndersDK »

Tx guys [:)]
1) the flywheel key is visible under the retaining nut, and 100% sure is not dislodged or crunched. It's VERY clear to see both flywheel & axle key recesses are opposite each other. This key is the original part (soft metal), to ensure it wont damage engine, in case the cutter board stuffs up with grass or hits a stone.
2) the engine has been in use for 3-4 mnths last autumn (after repair) in this condition. The engine was/is weak of course and smokes, but otherwise it functions.
Fuel (95UNL) and engine oil are fresh, as is the plug. This is my usual spring service [:I]
3) This engine is dead simple, using a magneto device triggered by a magnet inside the flywheel. The magneto to flywheel cleararnce is pr spec (0.3mm). Clearly the spark gets too weak if clearance is set too narrow. No CB points.
But came to think of it, I've NOT tried a larger clearance.
4) the magneto device has a stop wire, which grounds the coil to kill ignition. This wire is disconnected to make SURE [8)]
W/o engine cowling fitted this wire is visible all it's run. It rests in a grommet otherwise it's dangling free not touching anything.
5) the compression has been tested to 4.5bar, which seems very fine on this engine type, as the inlet valve is timed to close very late for the engine to be easy start cranked. This is the usual design on these engine types.
Also the compression feels nice & good when slowly cranking engine by hand.
My own best guess is the ignition TDC timing.
I'm puzzled how the timing can get set too high, as no means of adjusting exists, other than ofsetting the flywheel on the crank axle.
That's why my thoughts on the magneto device somehow triggering on the first "claw" passed by the magnet.
BTW : the ignition is rock steady on strboscope, not a hint of in-stability or missing spark.
Thing is it just won't run clean [xx(]
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Post by oilyspanner »

Mixture too rich? the carburation on these things is primitive, does it have a throttle or is it operated by a load governor?
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Post by AndersDK »

Tx Stewart -
Both a throttle (manual setting) and governor, as it's a garden tractor.
Positively not too rich. If you read up my first post, I tried lots of testing for a flooding engine. Leaning it by any means just makes it a hard starter and it will barely run on full throttle, AND THEN IT STILL MISSES AND SMOKES [:(!]
The smoking is positively caused by the missings.
It's no doubt the odd 18' BTDC ignition timing that's the problem, I just came in writing this answer after removing the cylinder head (once again). Then I strobo tested once again to be sure I'm NOT wrong with ref marks etc.
I found out that the inlet valve is just closing a wee bit AFTER sparking occurs, i.e. the sparking occurs just barely when the inlet valve is stil a wee bit open.
This explains the odd back pressure on the intake, which in turn totally upsets the air/fuel flow and the idle mix, which has no influence at all.
It does not backfire though.
Even blocking off the idle mix with a rubber piece under the screw has no influence on engine running.
I'm pretty sure now that the ignition timing of 18' BTDC is the problem. It's way too "high" an ignition setting.
The valve timing is spot on pr factory manual spec. These engine types have the odd inlet valve timing to ease cranking for starting the engine, thus the inlet valve closes very "late" (starter motor is a small permanent magnet motor).
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Post by AndersDK »

At last : <font color="red"><font size="6">A CLUE</font id="size6"></font id="red"> to the problem [:0]
By sheer coincedence I found a couple of bits from the old flywheel key stored in my box of odd bits for mowers. This key crumbled when removing flywheel for engine repair.
This key CLEARLY have a very odd profile, like a Z. It's DEFO not a simple square profile key, like the "original part" delivered from my dealer, as correctly found pr the parts list. Also this new original part is stamped B&S.
I DEFO believe this is the answer to my problem, as this is the only variable (or part) different between good running engine BEFORE, and bad running engine AFTER - repair.
It appears the original key would slightly offset the flywheel related to crank axle, thus giving at least some 4-6 degrees offset in ignition timing.
This flywheel key is not shown in parts list for my specified engine type & serial no.
Still I need the information of a likely normal BTDC setting of the ignition on this engine type !
I realise I have to do either of one :
1) try source the correct flywheel key
2) try fabricate a key
3) try offset the magneto module mountings
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Post by oilyspanner »

Anders, are there holes in the holding down screws securing the baseplate that can be filed out to give the ignition retardation?
Stewart
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Post by AndersDK »

Unfortunately not Stewart [:(]
The magneto ignition module are mounted on 2 studs reaching up from the underlying (horizontal) cylinder casing.
but ... [:)]
I spent some time taking as precise as possible the flywheel radius, and the tangential distance between TDC and ignition spark markings on the flywheel edge.
Then I applied my good old school trigonometry calculations, to calculate the precise ignition BTDC occurrence.
The result was pretty accurately 11.5' BTDC, which is still way too high (early) ignition for such a simple engine type.
Then I applied the same methods to calculate the needed offset on the flywheel key.
For an ignition retarding of 5.5' (reaching a more sanely 6' BTDC ignition setting), the flywheel key just needs an offset of approx 1.25mm.
This is in fact highly suitable, as the flywheel key is 4.6mm wide [:D]
Just takes some fiddling with my good old set of small key-files [:p]
BTW : A swedish owner with a Honda powered appliance just lately confirmed, that a too early ignition gives exactly the symptoms my engine has.
This Honda engine is also a side valve engine, and the owner forgot to torque the center nut. After a sudden stop the engine was 8-stroking and smoking, because the flywheel had pushed itself against the key, settling at a higher BTDC ignition [;)]
But I sure wish that someone had the precise BTDC setting for these engine types. I'm only assuming a 6' BTDC setting is sensible, as my "backbone" is trying to tell me something from my youth fiddling with mopeds ...
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Post by jeremy »

Anders
I've had a look at the manual for my 1946 Austin 8 HP side valve (900cc) and I see that the instructions are for the ignition is timed at TDC. I see this is the same as a 1952 Morris Cowley van (side valve) with the comment that it can be advanced to get best results after a road test. More sophistication takes us to the MG TD which was pushrod OHV and again was timed on TDC.
I think 6 degrees is too much for this engine which I suspect is in a much lower state of tune than the Austin.
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Post by AndersDK »

Jeremy - this is pure gold [:p]
I remember these earlier LO-rpm engines have nigh on a BTDC setting on the ignition [8D]
Of course this B&S is no modern engine, it's indeed very sensible [:)]
Tx a lot Jeremy [:X]
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Post by NiSk »

Hej Anders,
Do you have the engine type number? If so you can download the manual from B&S at.
http://www.briggsandstratton.com/displa ... ocid=66450
//NiSk
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Post by AndersDK »

Which I already did together with parts list - last year - tx NiSk [8D]
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Post by AndersDK »

<font color="red"><font size="4">News flash on this on-going engine saga :</font id="size4"></font id="red">
It appears these simple engines indeed runs with a rather high BTDC setting [:D][:D]
Lately I received a reply on a small engines Q&A from a B&S specialist telling me I'm out of my mind trying to set the the ignition at 4-6' BTDC [:I]
They usually runs at approx 23' BTDC he said [:D]
As my latest findings was a BTDC setting of approx 12', confirmed by stroboscope, it's no wonder this engine won't run clean [:)]
Also it appears that ignition parts (including flywheel) had modifications in production from '88 production year. My engine is a '91.
Further investigation shows that the flywheel key resess is approx 0.6mm wider than the the original parts key. The original parts key matches perfect on the crank recess.
As this mower tractor was bought from a farming machinery dealer, not a small engines specialist, and furthermore had a major repair by this same dealer before I bought it (7 y's ago now), I now suspect a wrong type flywheel fitted.
Tomorrow my small engines specialist will have a look on the flywheel, for a compare against stocked items ...
The saga continues ... [:D][:p][^]
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Post by DoubleChevron »

Hi Anders,
did you ever figure this one out ??? We'd all love to know what turned out to be the problem.
seeya,
Shane L.
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