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Small Air Cooled Gasoline Engines

Briggs Model I won't run


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  #1  
Old 12-14-2016, 01:40:04 AM
Ken Karrow Ken Karrow is offline
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Default Briggs Model I won't run

Hate to admit this but I have an engine I can't make run. It is a Model I Briggs on a PE77D generator, 1944 vintage. So far on ignition I have replace points and condensor from NAPA and replace condensor with another as well. 2 magnet flywheel with good magnatisim, coil is .5 ohm primary, 4.5K secondary, good flywheel key. Set points at .020 and .018 to retard a little. Tried 2 plugs Champion J8C and NGK both new and both nonresistor and have set at .025 and in desperation .020. Ohmed kill switch, open. Have nice blue spark with plug out of engine. Have lapped and set valves. Have pressurized combustion chamber with air at 100 psi and no hissing at carb, exhaust, breather, head gasket, or oil fill. This should eliminate rings and valves. Feels like good compression and I read 50psi cranking with rope. Have had carb apart twice run all passages and cleaned and checked fuel filter. Have fuel in bowl of carb. Tank and bowl have nonethanol fuel less than 30 days old. Greased both manifold gaskets even though they looked good to prevent air leaks. Have set idle mix at 3/4 turn and main jet at 1/2, 3/4, 1, 1 1/4, 1 1/2 turns. Have dropped pan to verify oil pump operation. If I add a little fuel in the plug hole and reinstall plug and try to start I get it to fire a little but won't pick it up and run. The sound I get now is a little more authoritive than when I started but not great. Mag to flywheel clearance is under .010. Any chance this is suppose to have a 3 magnet flywheel, it is a 2 post mag? Any more ideas? I am embarassed to even ask. Thank you all.
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  #2  
Old 12-14-2016, 08:40:30 AM
I like oldstuff I like oldstuff is online now
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Default Re: Briggs Model I won't run

Illogical but is the cam timing off? With the head off turn it over and the valves should rock between intake and exhaust opening at TDC.

If the flywheel magnets are weak you'll have a weaker spark under compression. Try setting the points about .008 and see if it will fire up.
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Old 12-14-2016, 09:33:17 AM
Ken Karrow Ken Karrow is offline
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Default Re: Briggs Model I won't run

Sorry miss type, on original post. Has 3 legged mag and 2 magnet flywheel. Have checked cam timing as suggested and flywheel magnets seem strong, grab a skrewdriver pretty good.
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Old 12-14-2016, 10:37:05 AM
termat termat is offline
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Default Re: Briggs Model I won't run

I'd try swapping the coil with a known good one. Same thing has happened to me in the past more than once with those Briggs,nice spark, won't run. I switch coils and it would run. Drives you crazy!
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Old 12-14-2016, 03:27:12 PM
sprkplug sprkplug is offline
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Default Re: Briggs Model I won't run

The first tool anyone who works on small engines should reach for whenever there's a problem is an in line spark tester. I prefer the mechanical type with a spark gap. Without it, you're just guessing.
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Old 12-14-2016, 04:25:39 PM
Doug Tallman Doug Tallman is offline
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Default Re: Briggs Model I won't run

.5 ohm on the primary side is low. Should be around 1 ohm. I figure you need to be able to jump at least 5X the plug gap outside the cylinder to have good spark.
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Old 12-14-2016, 07:44:29 PM
Andrew Mackey Andrew Mackey is offline
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Default Re: Briggs Model I won't run

Only 2 magnets on the 3 leg mag flywheel.
Loosen mag mount bolts and lift mag. Tighten hand tight. Take a 3x5 index or a thin white paper business card and place between mag and flywheel, with magnets facing the magneto. Loosen the mag mount bolts and let it drop onto the flywheel. Lift up on the flywheel and tighten the mag mount bolts, while holding the flywheel 'up'. Let go of the flywheel and see if the card drops out, or pulls out with no resistance. If the card is loose, then the crank bearing is bad. This will affect both the mag performance and change the point timing. If there is resistance in pulling out the card, next thing to check - with the spark plug out, hold the high tension wire 1/4" off the head and pull the engine over. If you don't get a 1/4" minimum of spark, then you have a mag problem. You should get a fat medium blue spark. Weak reddish or purplish spark may mean a bad condenser or resistance (corrosion). Weak thready pale blue, bad mag. Is the high tension wire soldered to the coil? If so, it probably has shorted the windings! The wire should only be crimped to the loop on the coil. Make sure the points are absolutely clean. Put a white business card between the points and pull out slightly, then open the points and remove. The card should come out absolutely clean. Any grey or black residue, repeat until it comes out clean. Point gap should be .020", plug gap .025". Closing the points WILL affect spark. I would not change from factory spec.
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Old 12-14-2016, 10:45:52 PM
YellowLister YellowLister is offline
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Default Re: Briggs Model I won't run

Will it fire on starting fluid?
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Old 12-15-2016, 11:47:05 AM
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Default Re: Briggs Model I won't run

Check your clearance on the mag to flywheel, if had some of those mags that were finicky on the gap there. If I remember correctly it should be 6 or 7 thousands. But if you are getting good spark and compression squirt a little gas in the cylinder and see what it does.
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Old 12-15-2016, 12:57:47 PM
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Default Re: Briggs Model I won't run

I always used a business card.. no fancy ones to gap the coil.
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Old 12-26-2016, 12:55:32 PM
Ken Karrow Ken Karrow is offline
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Default Re: Briggs Model I won't run

Thanks to every one, I am here to let you know where I'm at. The mag was weak, didn't have a known good one but on the third one I found a good one. Don't have the Briggs spark check but it shows .166 gap should jump. Found that with original mag, at .080 spark would jump to metal wrap on plug wire (this is on a PE77D generator), stripped some of the wrap back and fixed that. Anyway on the third mag it would jump .166 easily and would even jump .125 to top of plug with plug installed and having compression so it was jumping the plug gap plus .125. Took head off to double check cam and ignition timing. The valves work in time with piston and at TDC of piston the magnets on the flywheel are just past centered on the mag and the book says that the engine fires as the magnets get just past center on the mag. It does not seem to have excessive drag in the generator end. Have primed with gas in plug hole and have tried starter fluid. Engine seemed to get stiff after repeated tries so lubed clynder walls with oil since I though maybe I had washed everything off with priming. Have tried with several different spark plugs, some known good. Have rechecked carb and all seems clean and open. Have even assembled with exhaust valve open and applied air to clynder and it does vent through muffler. Though maybe muddauppler nest in muffler. Have suction through carb. Will fire weakly but not enough to run, will warm head slightly. Just will not fire with authority necessary to run. Am using a starter rope that will give at least 5 revolutions plus coast so should have at least 2 or 3 power cycles. I'm open to more ideas, hate to give up on it but I do have other projects that I need to get on to. Never had one that wouldn't run or at least knew why it wasn't practical to fix. Thanks again.
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Old 12-26-2016, 01:52:25 PM
Kris Golden Kris Golden is offline
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Default Re: Briggs Model I won't run

I had wondered about the generator drag too.
I built a small gas powered air compressor with a Reo slant cylinder lawn mower engine and since its low horsepower it now won't start with a rope due to the drag of the compressor. Another factor is the weakling pulling the rope.
A cordless drill on the crankshaft nut will spin that Reo with some attitude and it fires right up.
Have you tried belting it to one of your other engines to see if it will take off and run with more "umph" than the rope offers?
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Old 12-30-2016, 12:24:23 PM
Ken Karrow Ken Karrow is offline
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Default Re: Briggs Model I won't run

Made an adapter to fit the rope pulley that I can turn with a heavy duty 1/2" drill. Fires just enough to kick the adapter out, still no run. I'm going to try taking the brushes out of the gennie end and if not successful will set aside and go on to something else for a while. Kris, thanks but I can still pull the rope pretty good even though my back is giving problems. Hope to see you at Adrian or Lathrop.
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Old 12-30-2016, 12:59:42 PM
Andrew Mackey Andrew Mackey is offline
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Default Re: Briggs Model I won't run

The 3 leg mag needs at least 500 RPM to make a good spark. Factory says 700. The later 2 leg mags needed 700 minimum. One of the reasons B&S went to the electric starter was the fact that older people could not turn the engines fast enough to get spark. You would pull your brains out, no fire. take the mower to a shop, young kid comes out - one pull and off it goes!

If the engine was recently rebuilt, the rings may not be seated. You may have to give the engine an assist to get the rings to seat well enough to get the power needed to turn the generator. Pulling the brushes may help, but I think you need to put an electric motor on a belt and turn the engine until it gets up to speed. I have used a 1/3 HP dryer motor with a 3" pulley, and an old V belt that runs on the wrap a rope starter cup, to turn the engine. That should get the engine up to a speed where it can continue on its own.
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Old 12-30-2016, 03:13:17 PM
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Default Re: Briggs Model I won't run

Just looked at your first post and 50 psi is way too low for that engine to run. You need at least 80-100psi spinning it over fast by rope. Does the engine feel like its binding when you pull it over? Along with a good spark tester made for small engines I would suggest working on whats causing low compression. Did you try a few squirts of SAE30 oil in the cylinder to try and build up compression? Also, Ive found just because an engine has good spark with the plug out of the engine when its installed back in the spark may be poor because compression causes resistance around the electrodes.
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Old 12-30-2016, 05:41:43 PM
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Default Re: Briggs Model I won't run

I also cast a vote for compression. Just had one act like yours. Verified spark about 10 different times, tried half a dozen different carb settings, then realized the crucial third component. Even though it felt like good compression, squirting oil down the cylinder a couple times improved it greatly. Started right up. Get that oil around the ENTIRE perimeter of the piston at bdc, work it through some cycles back and forth by hand, and even add more oil if necessary. On mine i could feel when the rings started to seal and i knew it would start.
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Old 12-31-2016, 11:35:07 AM
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Everett Hayden Everett Hayden is offline
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Default Re: Briggs Model I won't run

Why not take the engine clear off of the genset, put it on your bench and go from there. Perhaps you already have and I just missed it, but I like to isolate.
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