Are HT coils generally the same?

[Blah blah]

The teenagers' Mito is spluttering like a bad 'un over 8000 rpm, it will get to the 12k redline some of the time with some popping and banging, other times it'll fly round to 10k with no problems then spluttery splut.

The carb is clean, fuel is fresh, plug is new and it still does it so I'm looking at the ignition side as it feels likes it's electrical.

I'm thinking it's time for a new coil however, being a rare, old Italian bike I can't pop down the road to buy a new coil off the shelf but I can pick up ones for Suzukis and the like that look the same and if it's ok for an RG125 / RGV250 or similar then presumably it should be ok for us.

Or am I missing something?

Anyone know?

[Nobby the Bastard]

Typically a coil failure either is completely dead or fails when it gets hot. Either way it works or doesn't work, no middle ground.

I had the same on my rg, turned out the carb was worn.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Blah blah]

I initially thought 'carb' which is why I cleaned it and the tank as it was fine one day and not the next.

And full throttle or half throttle didn't make a difference and upto 8k (before the power valve opens and it takes off like a scallded cat) it pulls well. It still starts with no problems and happily ticks over

If it was a worn carb, then I'd expect it to be spluttering at full throttle openings at low RPM, or be different when I changed the needle height but it doesn't and wasn;t.

A new plug improved it for a couple of runs but then we were back to popping and banging.

[Nobby the Bastard]

On my rg the oy fault was it wouldn't pull above 80 any more, no carburation faults before then. The problem went away when I swapped the carb.

As you say just at the point that the power valve opens I think you've identified your problem.....
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Blah blah]

It's on the list of things to strip and clean...

[Robby]

Ignition coils are not all the same. That said, they do all work in a similar fashion and I doubt the requirements on that bike are anything unusual, so I would give it a go. The wrong coil will be better than a broken coil, and will at least work diagnosis. If the problem stays the same, the problem is not the coil.

Mitos are fairly old now, and are pretty much the definition of a highly strung 125 2 stroke. They will be sensitive to having the right mixture, and unfortunately for you, there are a lot of things that can affect this and will be impacted by age and in particular spending a long time sitting in a shed.

Easy side first - you've cleaned the carb. Also worth making sure you have fresh fuel in the tank, and no water. Drain off whatever is sitting down by the reserve. Then new fuel filter. That will give you some confidence that the carb is doing what it should be.

Then down into the engine, inlet rubber could be cracked, reeds broken/chipped/knackered. Crank seals if you're really unlucky, but that's a last ditch thing after checking everything else.

Down to the exhaust, powervalve as mentioned probably needs cleaning and checking the exhaust itself for clogging.

Finally, is it due a top end rebuild? From memory these required a fairly frequent piston ring replacement.

[stevo123]

Agree with Robby, try a compression test and see if it is in spec before splashing out on new parts buddy.

[Bhud]

No, they aren't all the same. The crucial way in which they differ is in the resistance across the primary winding. The ignition system will be affected to a greater or lesser degree depending on the type of ignition system used by the bike and the extent to which the resistance differs from stock.

[Blah blah]

It seems like a single spade connector, 55mm hole center coil fits an awful lot of bikes including various older strokers (from TS50s to RGV250s and various other manufacturers) and so for £15, I've bought one.

I'll stick another fresh plug in when the coil turns up, check the wiring and then see what happens. If nothing else it'll rule that out. Last time it was fine for a couple of runs out after fitting a new plug but then it was back to spluttery spluttery hence suspecting electrical rather than mechanical.

[Nobby the Bastard]

It's not the connectors that are an issue.

You haven't considered...

The number of windings on the primary coil
The resistance by unit length of the primary coil
The number of windings in the secondary coil
The resistance by unit length of the secondary coil

They all make a difference to 1) the output to the spark and 2) how hot the coil gets. You don't even know what the max voltage and current that would be hitting the input side to even consider whether or not a random coil will do the job.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Nobby the Bastard]

[sickpup]

Before looking at the coil take the flywheel off and look at the state of the woodruff key, they snap and the timing slips.

Sounds ridiculous I know but it is surprisingly common and people spend a fortune trying to fix a £2 problem.

[Blah blah]

To close this down, we fitted a new this-one-fits-lots-of-different-bikes HT coil, lead, cap and plug and the Mito is now back to its glorious 2 stroke self.

Checking the resitence on the old coil and the secondary was around 6000 ohms when a good one should apparently be 11000 ohms

So to answer my own question, it would seem like a lot of HT coils are the same (apart from those that break !)