Hi. I have a Craftsman lawn tractor with a B&S Intek 18hp 31H777 engine.

At the end of last season, I put it away as usual at the end of last season and now this season it won't start. I have no spark at all. I replaced the plug and tested again. No spark. I removed the kill switch connection from the armature to see if that was the problem, and also ran jumper cables from the chassis and straight to the positive teminal on the starter. It will turn over no problem, but still no spark. What's the first thing to try? Replace the armature? How big should the gap between the flywheel and the armature be?

Please assist!

Thanks

I would suspect the coil. The Electronic Ignition is Built Inside the Coil

YouTube - Lawn Mower Repair Ignition Coil Test (http://www.youtube.com/watch?v=AsRoSeyJ38o)

You didn't post the complete No# for me to find the exact gap settings but here's the general No# Armature Gap .010-.014


Need

Model
Type
Code

I would suspect the coil. The Electronic Ignition is Built Inside the Coil

YouTube - Lawn Mower Repair Ignition Coil Test (http://www.youtube.com/watch?v=AsRoSeyJ38o)

You didn't post the complete No# for me to find the exact gap settings but heres the general No# Armature Gap .010-.014


Need

Model
Type
Code

Hey thanks 21boat!

Watched the vid and can rule out that I don't get a spark at all not even a weak/orange one, with the kill switch disconnected.

Model 31H777
Type 0297E1
Code 040202ZD

Thanks

I found the manual and the parts. Posting some basics here for future ref.

http://www.briggsandstratton.com/maint_repair/manual_and_more/docdetails.aspx?showpdf=MS5174_A_LO.pdf

http://www.briggsandstratton.com/maint_repair/manual_and_more/docdetails.aspx?showpdf=276344TRI_C_LO.pdf

If your mower is less than 15 years old, there is an excellent chance that if uses an electronic ignition system. These are very reliable as there are no points or condenser to go bad and no need for routine tune-ups.

Testing the magneto

As the magnets spin past the pole pieces of the magneto core, the points are closed and current builds up in the low voltage winding (and flux builds up in the core). At or slightly before Top Dead Center (TDC), the current (and flux) should be maximum and at this instant the points open. The flux then collapses (and the condenser (capacitor) across the points acts as a snubber allowing the current to bypass the open points and preventing arcing at the point contacts). This rapid decrease in flux results in coupling of the stored energy to the turn high voltage winding and results in up to 10,000 V or more at the spark plug.

(For EE types, this is somewhat similar in basic operation to the flyback converter in a switch-mode power supply except that the moving magnet supplies the input power instead of the rectified AC line and the points act as the switch instead of a power transistor.)

The secondary will always be accessible for testing but the primary of an electronic ignition may be not be due to the electronic components:


Secondary: 3 K ohms (maybe a little higher but not open). Much lower would indicate a shorted winding.

Primary (if non-electronic and accessible): very low - guessing less than an ohm.

Thanks, I'll have a look, replace the armature and report back...

I have the new armature, but haven't installed just yet. I notice quite a bit of rust buildup on the flywheel and the old armature contacts. Will this make a differenc, or is it a good idea to brush that off? Stupid question?

I have the new armature, but haven't installed just yet. I notice quite a bit of rust buildup on the flywheel and the old armature contacts. Will this make a differenc, or is it a good idea to brush that off? Stupid question?

Definitely. You can use some fine sand paper and clean both the flywheel and the armature. That might be all of your problem.

Thanks. Now I'm frustrated. Installed the armature. Checked for spark. Bingo! Put it all back together, cranked the engine, started quite easily.

Ran it for a few minutes, burned off some old rot and such and it ran well.

Shut it off, (it was getting late)

Today put all the housing back on and it won't start again at all. Been trying for half an hour. Pulled the plug and checked for spark. Spark is good.

Where did I go wrong?!

Please help!

Sorry to go on... I also replaced the fuel filter that was white previously (75 microns) with a red one (150 microns) this was done before it was running during the test yesterday. Just wondered if it might make a difference...

Now its to fuel and carb. Usually the fuel filter is not a very big deal but can make a difference. If they were so touchy then the engines wouldn't run long because of some Micron clogs.

I need you to check and see how much fuel is in the bowl and check the carb adjustments of the manual I sent you. The needle and seat is the second failure after filter and fuel pump must be working period.

Expose bowl and clean with carb cleaner. Be carefully of the tiny spring/clip for the needle valve the carb float operates

Now it gets more weird 21boat...

Checked for dry spark plug. Put gas in spark plug hole. Replaced plug, cranked engine and off it went. But then continued to run for a good ten minutes until I shut it off. Couldn't run any longer, it's after 10pm and I don't want to upset the neighbors!

Ran smooth no evidence of poor mixture or fuel starvation... perhaps it was just flooded and cleared up while I was away for an hour?.

Thanks Candan.

If you have any more problems get back. There's some really good people here to answers any of your questions you have on all home problems.

Just saw the next page post.

If it floods after you shut it off for a bit then the seat and needle valve is not working properly. The mower can run and yet the carb gets over filled and can run gas into the oil and then that oil needs changed. Just a heads up on the "flooding" if it continues. That can happen after the engine is shutoff, be aware.

Another thing if the cylinder needs primed with gas and then it stays running that's a sign also of bad seat and needle and or more a fuel pump. Its runs after you 'Primed" the cylinder and that's a borderline pump if all else is good.

Let me give you the skinny on fuel pumps
There are mechanical and electric fuel pumps Cars and trucks use either or. A 2 cycles outboard boat motor uses it Higher compression to operate its fuel delivery.
A larger mower uses a diaphragm fuel pump. If that has a hairline crack in it then no gas. If the rubber looks good but is to stiff and old it won't vibrate to deliver enough fuel for carb.