Broken down... but fixed itself!

matto · Crazy Courier Joined: 18 Apr 2012 Karma :

On my way to work this morning I had to stop off and pick up some bits from one site before heading to another for some training. I left the first site and was going reasonable slowly through traffic. Got out onto a more main road open the throttle and had a little backfire. The bike then proceeded to back fire four or five times and then cut out. Got the clutch in and coasted for a bit, tried to restart but nothing, so pulled over the side of the road.

Tried starting the bike a couple of times and it was very relucatant to turn over and was certainly not going to start, so I started to try and call ahead to let work know I was going to be late. Due to not having the number and the ladies that work on the main switchboard being utterly useless this took me 10-15mins. Once I'd finished the call I had another go at starting the bike...

...and it worked fine, in fact after the traning I've done about 30miles on it today and have taken it for a MOT (almost passed, just little things) Anyone got any idea what could've caused this randomness? and how I can prevent it happening again? It's a '96 Aprilia Pegaso 650, with a new sprag/starter clutch and recently rebuilt carbs.

Aff · World Chat Champion Joined: 05 May 2011 Karma :

Sticking/badly set up carb float causing it to flood?
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map · Mr Calendar Joined: 14 Jun 2004 Karma :

Had a hiccup with one of my old bikes similar symptoms.
Found out a little later it was the coil on its way out.
This may or may not be the same but something to poke at.

Usually though with intermittent faults they're a right PITA. It's actually easier to diagnose and fix something that's totally broken rather than just failing.
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matto · Crazy Courier Joined: 18 Apr 2012 Karma :

Don't think it could be the carb float, but the coil is a distinct possibility, as it would explain the reluctance to start... but how would a coil cause a bunch of backfires?

map · Mr Calendar Joined: 14 Jun 2004 Karma :

andym · World Chat Champion Joined: 16 Nov 2010 Karma :

map · Mr Calendar Joined: 14 Jun 2004 Karma :

andym · World Chat Champion Joined: 16 Nov 2010 Karma :

matto · Crazy Courier Joined: 18 Apr 2012 Karma :

I've taken some advice and it appears that upgrading to a Dyna-coil is a great idea on an old big single regardless of if the current one is slowly breaking.

However I have no idea how to work out which dyna coil I should get, beyond being a single output one, how do I find out what primary and secondary resistances my old coil had?

matto · Crazy Courier Joined: 18 Apr 2012 Karma :

Ignore me, I found an rotax manual.

primary 0.2 to 0.5 ohm
secondary 6 to 13 ohm

so I guess I should aim to get something with figures between those values?

Islander · World Chat Champion Joined: 05 Aug 2012 Karma :

Yes, those are limits so anything that's between or those figures will pass.

matto · Crazy Courier Joined: 18 Apr 2012 Karma :

One last stupid question! Would a coil with a 14k secondary resistance be a terrible thing if the limit in the book is 13k?

cb1rocket · World Chat Champion Joined: 30 Jan 2010 Karma :

matto · Crazy Courier Joined: 18 Apr 2012 Karma :

Well I've ordered a new plug cap, leads and a mini Dyna Coil DC12.

It has a 0.5ohm primary and 11.7k ohm secondary, unfortunately the new plug cap is 5k ohm rather than the 1k ohm original but I think it should work... I guess I'll try anyway.

Cheers for the help, I will update when all the parts arrive.

matto · Crazy Courier Joined: 18 Apr 2012 Karma :

Well I've wired it all up and had it started, might be a placebo effect but it sounded crisper (can't think of a better way of describing it) and I was able to take it off the choke after about 30sec of running...

...however it's way bigger than the original coil and I really can't see an obvious way of attaching it to the bike and fitting the fuel tank over it. The further I move it from it's original position the longer the sparkplug lead has to be, which can't be good for resistance.

Going to have to go stare at it and scratch my head a bit more Very Happy

cb1rocket · World Chat Champion Joined: 30 Jan 2010 Karma :

good news but you should of got one the same size Laughing

picture of original? I'm sure you can find one to match
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map · Mr Calendar Joined: 14 Jun 2004 Karma :

matto · Crazy Courier Joined: 18 Apr 2012 Karma :

The new dyna coil was only £75 so getting a second hand one seems a bit silly considering the original is broken. It's a minature dyna coil too, so the smallest they do!

Anyway after much thinking I have made a little bracket and have managed to attach it at such an angle that you can just squeeze the tank back on, will take it for a run at some point in the next couple of days and see what improvement there is.