1981 Merc 90 in-line 6 - random no spark...

[Bannister 6905]

OK, so the tach issue was a loose connection at the control - here comes the next opportunity...

Verified with a timing light, I am getting random "no spark" when the engine is cold. Sometimes, #1 and #2 are dead. Other times, #5 and/or #6 are dead. This is running on the hose, both at idle and and 1500rpm. #3 and #4 are always firing.

However, after warming up, all cylinders are getting fire and the motor runs great.

On the water, I get that misfire again when cold, but after warming up, the motor runs well -

Compression measured 140+/- 5 psi; carbs rebuilt, new battery, new water pump (strong stream from discharge), new NGK plugs, timing set and carbs synched...FYI only.

I am looking for experienced opinions as to where to look next...appreciate your input.

Bannister

 

[Bannister 6905]

Appreciate knowledgeable input!

 

[Faztbullet]

When #2 is not firing check #5 and see if its firing also if not its trigger, if its firing check bia readins on both switchboxes and if off replace both..

 

[Bannister 6905]

OK - when 2 is not firing, 5 always is. It's either 1 and 2 not firing, or it's 5 and 6, never a mix...strange, isn't it?

 

[Faztbullet]

Either bias circuit bad on one of the boxes or low speed coils on stator voltage weak...my bet is boxes.

 

[jimmbo]

Check all the connections/grounds for tightness and corrosion

 

[Bannister 6905]

Thanks for the advice. Both boxes checked bad - replaced with new Prufrex boxes and she runs like a champ again!