No solid Spark on one 787 engine in Challenger 1800...cranks but won't start !

[Cronullalad]

Hi all,
A bit of a story, starboard ending was bogging down between low and high revs so rebuilt that engines carbs. All went well engine was very responsive after carb rebuild so a good job done.
Next day tried to start but engine would just turn over with the occasional splutter but not start. Swapped plugs from bad engine to good engine (very wet) and it started up ok. Swapped the Ignition Coil pack from bad engine to good engine to good engine and it started up ok.
So on the bad engine, what tells the Ignition Coil to send power to the spark plugs ?? There must be a loose connection some where in that part of the circuit ???
Any advise on how to track down this issue ???
Thanks in advance.
Geoff

 

[Dr Honda]

The 800 engine's ignition is different than most of the others. It uses a DC powered (12v) CDI unit, and a standard style pick-up coil. (other Rotax engines use a MAG)

So...

1) There is a issue with the pick-up coil breaking away from the mount. It normally just kills the spark... but if it's close to the flywheel... you will get some random sparks. So... pop the MAG cover off, and check the coil. (and the wires/plugs associated with it)

2) The CDI unit is relatively reliable... but there is a "Cascade" circuit in it. That drives the 12v DC up to the 300v needed to drive the ignition coil. Unfortunately... I don't know if that's in your MPEM on that boat. (I'll have to look) If it's external... then swap the CDI's, and see if the problem goes away. If it does... I have a CDI from a '96 XP I'll sell you for less than "eBay".



******EDIT****


Lets hope it's a pick-up coil. I don't know the year of your boat... but it's looking like the CDI's are internal to the MPEM. from 97~99.

 

[Cronullalad]

Thanks Tony, my boat is a 1998 model. To be sure it is the pick-up coil can i check its output before taking the mag cover off ?? What would i expect to see on the multi meter when checking the wires coming from the pickup coil connecting to the ignition coil pack ??

 

[racerxxx]

Your MPEM is one piece, Your boat being after 97 has the updated pick up coil bracket so I would almost 100% rule out a broken bracket. The boat just doesn't take the pounding like the skis do. IIRC if you check the pick up you will check resistance between the yellow/white wire and the black/yellow on the 6 pin connector on the front of the mag and I think it needs to be plugged in. I think you want to see between 190 and 300 ohms. Back probe the connector with paper clips

 

[Cronullalad]

Thanks Racer, will check the readings out on the weekend.
Geoff

 

[Cronullalad]

Hi all,
Well I checked the 6 pin plug and have a reading of 260ohms so I guess there is no problem with the mag. So next stop is the MPEM ???
How do I check if this is where the Ignition problem may be ??

 

[Cronullalad]

Hi guys,
Any other thoughts on this issue ????

 

[racerxxx]

I would start checking the grounds. I'm not positive if the ground the the coil box hours to the mag connector or but you should trace that. If the mag/coil pick up doesn't have Ground it won't make spark but will turn over. This is based off how a ski works so the boats may be different. How many grounds are hooked up to your battery?

 

[Cronullalad]

Have had limited success.......
Can now confirm that Trigger Coil is active and shows 0.3Vac when turning engine over, also noted that the rev counter is registering when turning over (helps to confirm that the trigger coil is in place). Also now have spark to both plugs on the bad engine but when trying to start it only coughs and still won't start. Was running the other (good) engine earlier in the day just to confirm it was still ok then an hour later tried to start it again and now has the same symptoms as the bad engine.......
Every thing seems to be in place ...spark and fuel but both won't start.......any more ideas ???

 

[racerxxx]

How old is the battery and what does the voltage drop to when you're cranking the engine over?

 

[Cronullalad]

The battery is only 6 months old but it was at only 50% of charge after spending several hours of intermittent cranking so have removed and have it on charge now. Given the low charge in the battery would that have affected the spark strength when turning over ??? Will check the voltage drop when I reinstall the battery next Saturday.

 

[racerxxx]

Yes. On a 787 if the voltage drops below 10.8 (iirc) the spark will/can drop out.

 

[ialonso]

So....maybe unhelpful, but needs to be asked...

Have you tried placing some fuel down the carburetor as a test?

I know you are now looking for an electrical solution....but no fuel can cause these symptoms. Are you *sure* it's not fuel related?

 

[Cronullalad]

Hi all,
Success !!!!!!!
Its marvellous what a battery with a full charge will do for you. Following installation of a battery with a full 12.7 volts, both engines roared to life and were charging at 13.5 volts.
Fuel, although a concern in the early stages, was not the problem.....well other than having way too much after turning both engines over for extended periods.
Thank you all for your assistance and advice.
Geoff