ET: Engine (Twin) Still Can't Start My Twin, Replaced Mag, But Not A Pop

[CarlHungness]

I know the Lucas Magneto is good, journeyman Doug Wood tested it and says it's OK. I bench tested it myself by just spinning the shaft with my fingers and got #1 to fire, couldn't spin it fast enough to fire #2, but didn' worry about it as Doug said it's sparking. I installed it at 35 degrees BTDC with ATD blocked open. Can't get as much as a pop out of the plot. Last summer, the last time I rode it it took me 30 minutes to start it with the B-TH. I ran the carbs dry as usual, drained the gas tank, ordered new coils for the B-TH, still no spark. Had it tested locally, new and old coils are OK, but B-TH won't fire them. Thus I re-installed my Lucas. I have probably put the Lucas in 4-5 times over the years and the B-TH once, so my procedure has worked in the past. I am not getting wet plugs either, so wondering what to tear apart next. Wonder if the kill button wire could be shorting out the mag. Will un-hook it tomorrow and start over. Looking for suggestions. This is the first time the machine has not started in over 30 years of ownership when I've replaced the sparking provider.

 

[LoneStar]

The Lucas specification for a good magneto is a 5.5 mm spark at 130 rpm - which seems a lot, but it's measured in open air rather than in the cylinder under compression. Does yours meet this standard? HT wires, connections and plug caps all checked for low resistance? Known good plugs? ATD free to retard for starting, with good springs?

Beyond that, the Usual Suspects:

• Adequate compression on both cylinders (valves seating?)
• Carbs delivering fuel, idle circuits clear
• Cam timing correct

 

[brian gains]

i'd choke it heavily and put raw fuel down the plug hole to at least get a 'bang' out of it. That would confirm there was a spark of some sort at least.
Had a situation on the Comet where out of the blue would not fire and plug was dry as, never figured it out but cured itself after twenty kicks or so; blocked cap vent/ vapour lock?

 

[erik]

The magneto fires 360 degrees wrong.This means it fires into the exhaust! Erik

 

[timetraveller]

If that is the case then you can check it using the method I described recently. Plugs out, tappet covers off, rotate engine with rear wheel and watch the rear inlet valve open, then close, bring piston up to TDC and then see if the points are just about to open. If not then do the same for the front cylinder. If both are wrong then it is probably a 360 degree problem. If one is correct then possibly it is timed up on the wrong cylinders. There is nothing special about any of this. Just think back to how four stroke engines work and go from there.

 

[vibrac]

Get it on the rollers On and drain the sump its surprising how a sump full of oil kills the rotation

 

[oexing]

To be open, I´d be worried if I could not get two sparks from that mag by finger spinning. In such case I´d have not much faith in having a decent spark in the cylinder with compression present.

Vic

 

[CarlHungness]

Here's the end of my mag, is the 'clip' to hold the cap on in the right place..it is about 7 ' o'clock and the
only other place it could be is about 2 o'clock. I've checked timing and my light goes out when I open the
ATD at 35 degrees. Since I have timed it many times in the past, have a spark plug stop, know how to get
it on TDC, etc. am confident it is timed just real close to 35 if not spot on.

 

[CarlHungness]

Get it on the rollers On and drain the sump its surprising how a sump full of oil kills the rotation

I drain sump before every start, even if it has been sitting for two days...'get it on the rollers..' I'm not a
racer anymore (did Midgets) so no rollers.

 

[CarlHungness]

To be open, I´d be worried if I could not get two sparks from that mag by finger spinning. In such case I´d have not much faith in having a decent spark in the cylinder with compression present.

Vic

Excellent spark on #1 with fingers...great sparks with drill turning it..can't get #2 to spark with fingers spinning
the unit. Doug Wood checked it, says it's good and he's done a lot of them.