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B&S 18.5 twin cylinder not firing at all

1.2K views 10 replies 6 participants last post by  gerjoi52  
#1 ·
HELP!B&S Model 42A707 was running great.
Now it turns over normally with good compression, but has no spark on either cylinder - tested with inline spark tester (spark tester is good, checked on tiller, and indicates spark ok).
Has new plugs set at proper gap,
new solid state magneto set at proper air gap.
Flywheel magnet has strong pull on screwdriver at one inch, and is not rusty - key is not sheared.
battery has full charge and solenoid checks good
New fuel line and filter - gas getting to carb, and carb clean
Air filter is clean, oil has been changed and filled to correct capacity.
Engine is mounted on a 1996 Murray riding mower, all safety switches have been temporarily bypassed for testing - but I still get no spark whatsoever on either cylinder!

Any ideas/suggestions? Could be possibly be the ignition switch which is a 6 tab?

I could definitely use some help on this, please!
 
#3 ·
Yes, new coil is plainly marked and installed correctly. I even pulled the new plugs, and since my tiller uses the same type - tested them both in my tiller one at a time, got plenty of spark - reinstalled them back in mower, still no spark...
 
#4 ·
Remove the kill switch lead wire at the coil, this will remove the key switch from the equation. Test for spark, if you still have no spark, then thoroughly clean the mounting posts on the cylinder and reinstall the coil. Still no spark, you may have a faulty ignition module.

You must be generating around 600 rpms when cranking to generate a spark. If your engine does not seem to be cranking over fast, try it without the plugs installed and see if you can get an ignition spark.
 
#5 ·
You've confused me with your nomenclature here. You have tested and have spark but no spark? Do you mean you have spark but no firing in the cylinders?

As I understand it, you have pulled each plug and laid the plug body touching the engine and you can see the spark jump the gap of the plug. If that is the case then you have spark although it should be a deep blue spark. But if I read your post right about testing the plugs on another engine, then you haven't done this test. So you need to do that. Prior to that, stick a screwdriver or needle nose pliers into the spark plug wires and hold it close to the end of the plug (still in engine) to see if you get spark across that gap. If you don't have this spark, then you have to look at the wire end and boot and then back to the coil. I assume you took a business card or a piece of breadfast cereal box to gap your coil.

If your plugs are sparking, then you have to look at other issues such as air, fuel, & compression such as a sticky valve.

Will it fire if you give it short shot of starting fluid?
 
#6 ·
You've confused me with your nomenclature here. You have tested and have spark but no spark?

If that is the case then you have spark although it should be a deep blue spark.
He stated that he tested for spark with an inline spark tester, that works as he checked the tester on his tiller. But did not indicate if it was a gap type tester or bulb type tester.

The color of spark means absolutely nothing, the hottest spark is ultraviolet and invisible to the naked eye. You cannot judge the intensity of spark by the color!
 
#7 ·
Hi,

this might sound dumb, but I once had the lead from the magneto not touch the spark plug properly.
The rubber boot was over the spark plug (so from the outside, it looked like the lead was on the spark plug, but the wire metal wire contact was dislodged (inside the insulation), hence, no proper contact. In that case, I just pulled the wire contact into position.

Out of interest, what happened to your old magneto?
Does it still work and if so, what happens if you put it back on?

Kind Regards
Al
 
#8 ·
Ok , to clarify - I have NO spark at all using any of the methods to test for spark. In line spark tester is the bulb type.

Spark plug wires are connected - I'm "old school" - I connect the wires first, then slide rubber boot over the connection.

Old magneto did not generate spark either, which is main reason I bought new magneto.
 
#9 ·
You may want to clean the mounting post with a wire brush and make sure the mounting screws are good and clean as well. The most common reason a magnetron will not spark is a bad ground to the engine block.

The bulb type testers will usually indicate current, but will not indicate intensity. A gap type tester is better for this.

Best of Luck... :thumbsup:
 
#11 ·
After checking EVERY wire for continuity, bypassing ALL safety switches, checking for voltage to/thru new ignition switch, etc.
and STILL not getting spark on either cylinder, I gave up :confused: and had a local repair shop come pick up the mower a couple of days ago. Hopefully they'll figure out the problem without it costing me an arm and a leg.

Meanwhile, I've pulled out the trusty rusty but reliable push mower. Oh well - I need the exercise....:(