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This is doing my head in. Imola 185, 28mm phbh, breathe sweet filter and curly jl3. Scoot will not rev beyond 4000ish rpm, when it does it bogs down and backfires. Changed plug, cap, lead and cdi. Used internal pickup and external pickup. Used af light flywheel and Indian flywheel, used Anthony Tambs external power cdi. Checked fuel flow, petrol cap, cleaned jets and I’m now stumped. Anyone have any fresh ideas?

Change the complete stator?

Regardless of what others say ("definitely electrical") my mates always gave a backfire before going on reserve (fuel shortage)......maybe try a different carb' if all else fails.

So did it run OK before or is it a new build?

Rebuild after a seize, new piston and air leak cured.

Piston back to front?

Exhaust blockage I had the same symptoms convinced it was electrical changed everything. Only to eventually after deciding to strip the motor finding a blockage in the pipe.
The symptoms were totally electrical. Wouldn’t Rev out back firing what a Nitemare. Was a new one on me.

Is it a back fire (loud bang) or a mis-fire. Back fires are caused by un-spent fuel igniting in the exhaust which is 9 times out of 10 a timing issue where as a mis-fire could be a number of things. Has it ever ran properly or has it never ran properly?

It's not the carburettor needle retaining plate fitted in the slide beneath the circlip rather than on top, is it?

Warkton Tornado No.1 wrote:It's not the carburettor needle retaining plate fitted in the slide beneath the circlip rather than on top, is it?

no that was one of the things I checked as I had the circlip go missing on a vhsa carb fitted to my ts1 which gave the same symptoms.

missing lynx wrote:Is it a back fire (loud bang) or a mis-fire. Back fires are caused by un-spent fuel igniting in the exhaust which is 9 times out of 10 a timing issue where as a mis-fire could be a number of things. Has it ever ran properly or has it never ran properly?

it ran fine originally, its a misfire more than a backfire.

Stator pal

MickYork wrote:Change the complete stator?

Regardless of what others say ("definitely electrical") my mates always gave a backfire before going on reserve (fuel shortage)......maybe try a different carb' if all else fails.

Yep I built a TS1 motor for a mate, that's been bullet proof, but that has always backfired as it goes on reserve.

Dazts1 wrote:Exhaust blockage I had the same symptoms convinced it was electrical changed everything. Only to eventually after deciding to strip the motor finding a blockage in the pipe.
The symptoms were totally electrical. Wouldn’t Rev out back firing what a Nitemare. Was a new one on me.

I had this recently and I’d changed everything believe me. Turns out the perforated sound absorbing sheet tacked inside the clubman had partially come loose and blocked the tailpipe. Turns out it’s not as uncommon as I thought as I told a mate and the same had happens to him.

Head Gasket

so you can only get 4000rpm?
is it steadily getting worse?
when on stand hold the revs @ full, if it continues to struggle past 4000rpm then I'd say electrical, more likely the LT coil bud

hullygully wrote:so you can only get 4000rpm?
is it steadily getting worse?
when on stand hold the revs @ full, if it continues to struggle past 4000rpm then I'd say electrical, more likely the LT coil bud

that was my initial thought but trying a scootronics cdi which gets its power from the lighting circuit and an external pickup gives the same problem so I'm tending towards fuel. I'm going to drop the exhaust and strip the carb this weekend and see if I can sort it. my mind said electrical but having changed it all I've ruled that out.

I had a similar situation a couple of years ago where it just wouldn't rev past 4.8K.
Had to change the pickup coil to sort it.
Sometimes..... a worn mag bearing can also do it.

Fast n Furious wrote:I had a similar situation a couple of years ago where it just wouldn't rev past 4.8K.
Had to change the pickup coil to sort it.
Sometimes..... a worn mag bearing can also do it.

He is using an external pickup and a Scootronics DC ignition that doesnt use the LT coil so that shouldnt be the porblem BUT strobing it would remove all doubt

Sorted, changed the af flywheel for a mid weight Indian one and it seems to have cured it.
Thanks for all the suggestions, I got there in the end.

gp200ts1 wrote:Sorted, changed the af flywheel for a mid weight Indian one and it seems to have cured it.
Thanks for all the suggestions, I got there in the end.

In that case, it's almost certainly the pick up alignment. If you want to go back to the af light flywheel, you could use an external pick up. Personally I'm a big fan of them.