L head 18 briggs wont start when the motor is hot.

homefried1 · Jul 11, 2020

tomw0 said:
Well, phooey. Pretty sure I meant to say governor. No idea why 'magneto' was typed. Brain fade due to overworked somethings.
I have a few thoughts... of course. The blowby you mentioned in the air filter housing... does the crankcase vent feed into that housing and that is where you noted the blowby? A little is normal.
The hand control should pull strongly on the throttle spring when moved to 'fast'. The butterfly should open wide even with the engine at rest. It will sit wide open if the control is at fast. The governor will do nothing until the engine starts, and then will pull against the spring. If the linkage is not set properly, the governor can overcome the spring real quickly, and never allow the throttle to open as it should. If the spring or links are in the wrong holes, as in someone attempted carb repairs and didn't get them back properly, it may limit engine output.
If you don't 'know' the engine & history, you may want to look for 'witness' marks on the linkage indicating things are hooked and connected properly. Or not.
tom

Good tips and will try them all! Thanks for the help.

And yes, there are the two tubes that go up and connect into the filter housing? That is where you see blowby. As for the history of that engine, it has been worked on by other people than myself so I dont know the total history of it.

homefried1 · Jul 13, 2020

RedOctobyr said:
When suspecting a weak spark, I've tried closing down the gap on the spark plug, to make it easier for the spark to jump.

Also, if you have an inline spark tester, that will show the result much more directly. They're a helpful tool, under $10 at Harbor Freight, I think.

It helped me confirm my coil problem, I left the tester connected. It was flashing happily when cranking, or when running. But when trying to restart, it wasn't flashing, showing I had lost spark.

Now I have got another one with a similar problem, this one is a Kohler 16 hp single lunger. My inline tester shows spark, I am getting fuel, and have compression but the engine wont fire up. What could I be missing? Slipped flywheel key maybe?

RedOctobyr · Jul 13, 2020

Yup, a sheared flywheel key could make it not start. Especially if you don't know the engine's history. If it was running fine and this started randomly, the flywheel key is less likely. But if it also hit a rock or something, that makes it more likely to shear the key.

If you remove just the flywheel nut, you might be able to look down the crankshaft and see if the keyway slots are lined up. This isn't a perfect test, as even a very-slight misalignment can make it not run. But if the slots are clearly misaligned, you know the key is sheared. Impact tools are great for removing flywheel nuts, but you'll need a torque wrench to reinstall it.

tomw0 · Jul 13, 2020

Ummh. "Getting fuel" means ??? It could be getting TO the carb, and INTO the float bowl, but internal blockages may prevent it from getting into the engine.
Slurp a bit into the carb air inlet(filter removed) and give the starter a twirl. If the engine is lacking combustibles, it may fire off and pop a bit on that small slurp. Most engines with spark and compression will pop and snort when fed gasoline(if you can call it that still) in the form of liquid. Many will run for a few seconds if they fire up at all.
If you get that response, you likely have a carburetion problem, such as blocked jets or passages.
If not, have you pulled and inspected the spark plug electrodes for possible fouling? Schmutz on the outer and/or center electrode can cause the sparky to take a shortcut direct to the cylinder head instead of jumping through a nice fuel:air mix (and causing it to fire...).
Won't hurt to take a look.
tom

homefried1 · Jul 19, 2020

tomw0 said:
Ummh. "Getting fuel" means ??? It could be getting TO the carb, and INTO the float bowl, but internal blockages may prevent it from getting into the engine.
Slurp a bit into the carb air inlet(filter removed) and give the starter a twirl. If the engine is lacking combustibles, it may fire off and pop a bit on that small slurp. Most engines with spark and compression will pop and snort when fed gasoline(if you can call it that still) in the form of liquid. Many will run for a few seconds if they fire up at all.
If you get that response, you likely have a carburetion problem, such as blocked jets or passages.
If not, have you pulled and inspected the spark plug electrodes for possible fouling? Schmutz on the outer and/or center electrode can cause the sparky to take a shortcut direct to the cylinder head instead of jumping through a nice fuel:air mix (and causing it to fire...).
Won't hurt to take a look.
tom

I shot ether directly into the plug hole. Plus, before I did that I had pulled the plug out and it was wet on the electrode with gas, after I had poured gas through the carb, I have not poured any gas in the tank yet. I pulled the bowl off and it was clean as could be, and the needle moved freely with the float.

I have heard it bust one time out of many revolutions of the engine, but did not fire up.

tomw0 · Jul 19, 2020

Sometimes 'compression' may not be enough compression. To do a quick check, add some oil down the plug hole to seal the rings for a short bit. Check compression then to compare. If it goes up a bunch, put some fuel directly into the cylinder, and attempt a start. If you now get pops and snorts, you may have a ring problem causing low compression, making starting hard. Sorry but I don't remember if this is an OHV or flathead, but you may have another case of inadequate valve clearance if the latter, again causing low compression. If the former, it is possible, but not common, for the lifters to get varnished(?) and sticky such that they act more like solid lifters than hydraulic, and again tending to hold the valve off the seat slightly.
If the engine is foreign to you and you've never heard it run, the possibility also exists that someone in its past did some 'work' and didn't get things back together timed so valves work at the right time.
Either design can be checked visually(sort of) to see that the valves are doing as expected relative to the piston cycle. You just have to 'walk' the crankshaft through 2 turns, noting valve and piston activity. Ditto on the spark but it's a lot harder to note when it occurs. Perhaps a daub of paint on the flywheel noting TDC would make noting spark timing more observable.
tom

homefried1 · Jul 22, 2020

RedOctobyr said:
Yup, a sheared flywheel key could make it not start. Especially if you don't know the engine's history. If it was running fine and this started randomly, the flywheel key is less likely. But if it also hit a rock or something, that makes it more likely to shear the key.

If you remove just the flywheel nut, you might be able to look down the crankshaft and see if the keyway slots are lined up. This isn't a perfect test, as even a very-slight misalignment can make it not run. But if the slots are clearly misaligned, you know the key is sheared. Impact tools are great for removing flywheel nuts, but you'll need a torque wrench to reinstall it.

I looked at the key by removing the nut that holds the flywheel, it LOOKS OK, but I am going to go ahead and pull the flywheel anyway. Its so hot out here that its hard for me to get anything done!

homefried1 · Jul 22, 2020

tomw0 said:
Sometimes 'compression' may not be enough compression. To do a quick check, add some oil down the plug hole to seal the rings for a short bit. Check compression then to compare. If it goes up a bunch, put some fuel directly into the cylinder, and attempt a start. If you now get pops and snorts, you may have a ring problem causing low compression, making starting hard. Sorry but I don't remember if this is an OHV or flathead, but you may have another case of inadequate valve clearance if the latter, again causing low compression. If the former, it is possible, but not common, for the lifters to get varnished(?) and sticky such that they act more like solid lifters than hydraulic, and again tending to hold the valve off the seat slightly.
If the engine is foreign to you and you've never heard it run, the possibility also exists that someone in its past did some 'work' and didn't get things back together timed so valves work at the right time.
Either design can be checked visually(sort of) to see that the valves are doing as expected relative to the piston cycle. You just have to 'walk' the crankshaft through 2 turns, noting valve and piston activity. Ditto on the spark but it's a lot harder to note when it occurs. Perhaps a daub of paint on the flywheel noting TDC would make noting spark timing more observable.
tom

I BELIEVE the compression is OK, I am going to replace the flywheel key if I can stay out in this heat long enough to get it done.

flyin blonde · Jul 22, 2020

I had a similar problem with a mower and the same engine, seemed to struggle when I was mowing, but fine when the blades were disengaged. The deck drive belt would also squeal when I engaged it, and then it started squealing every now and again when I was mowing thick grass. I found the spindle bearings were starting to drag and the deck belt tensioner was stuck, so now I'm in the process of a deck overhaul, hopefully that will solve it.
It also won't start when its hot, but runs pretty good otherwise. The p.o. told me he overhauled the carb, so I suspect he either didn't get it clean or didn't reassembly it correctly.

RedOctobyr · Jul 22, 2020

If it won't start when hot, it might have a failing ignition coil, which stops working when it heats up.

I had this last year on a Briggs opposed twin. It would restart fine if I only shut it down for a minute or two. But shut it down for 5-10 minutes, giving the coil a chance to heat up (no air flowing across it), and it would just crank and crank, but not fire. The inline spark tester confirmed I'd lost spark.

A new coil for $15 got it working properly again.

flyin blonde · Jul 22, 2020

Thanks for the tip, I just bought a coil tester from HF, just for this reason. That thing is worth its weight in gold, but only cost me $7.99! I had coil issues on another engine, working intermittently, and was able to pinpoint it and replaced the coil, problem solved. Don't know why I waited so long to get one!

L head 18 briggs wont start when the motor is hot.

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