Merc 300 no spark

[puddleduck]

One of those annoying episodes - form a motor running perfectly 3 weeks ago to zilch. Only got a very weak spark on HT lead #1, nothing on the others. Had the distributor off yesterday, gapped and cleaned the points again, got a belt off the dynamo (!) checked the timing, put it all back together an still nothing. I know it's something obvious but what? this engine stated third pull after a major rebuild now nothing - I appreciate it's been outdoors under a cover which does boats and engines no good at all - ideas gratefully accepted!

miserable in Southampton

[haventaclue]

A wild guess,something is earthing her out,I would start looking for a short/bare wire

[gfgarland]

Remove the wire on the side of the mag going to the harness and try again. If the spark comes back look for a bad key switch, harness, or mercury tilt switch, the little condenser looking part on the side of the engine that is wired into the mag. Also, make sure the mag is grounded to the block OK and the carbon brush in the cap is in good shape. If that fails, report back and i will walk you through it.

[puddleduck]

many thanks, abroad at the moment - will attack motor upon my return!

[puddleduck]

Merc 300 has spark! following a frustrating pulling apart of magneto, checking points, etc, etc, then getting the occasional kick-back (still have the scars on my right leg from my villiers motorbike days doing the same thing!) finally decided the problem must lay with timing. Most old Mercs are factory set and there are two marks on the flywheel to line up with the magneto drive pulley. One at TDC, the other at 0.235" BTDC. The manual was a bit amibiguous which mark is correct, then Rapier came up with an inspired suggestion "come and lift the lid off my 400 and take a look". Low and behold brains here had set his motor on the BTDC mark instead of TDC. RESULT! Bloody thing started third pull. Stiil don't understand how it started beforehand though, maybe the garage gremlims were getting at the timing when I was away?

Thunderbolt III flew at Chertsey - very happy bunny! Knees still recovering!

[Fastjeff]

You still need to time it correctly or holes can mysteriously appear in the pistons.

Jeff