Derbi Senda 50cc wont start - has fuel, spark, compression

[orac69]

Hi guys,

Im a bit stumped - could do with some advice please. Searched for hours, still not sure what to do..

My son has a 2006 Senda X-Race 50cc. Its got a pipe, derestricted, changed jet, needle, sprocket etc. and its been running perfectly.

On his way home it just died. Cant kick start it, cant bump start it - there is no even attempt at firing up.

Ive checked / changed the plug - its got a nice strong blue/white spark - which would rule our the CDI / HT / electrics?

There is fuel getting through Im pretty sure - had the carb off, float wasnt stuck, all pipes look good and tight, had a blow through.

There is compression - a good thud when putting a finger on the plug hole..

The fuse is good.

Battery voltage was a little low @ 12.5v, so ive put that on charge - but I thought the Senda should start without a good battery on a bump..

Actually I didnt check the main jet for blockage - but shouldnt be - its only done about 500 miles since re-jetting.

advice appreciated, thanks.

[bikenut]

:D hi dude,

so it has all the ingredients of life but just will not start ( on the electric start ), it has sparks when the engine is whizzed over on the starter, even when the plug is in and you use a spare plug to check for sparks??

so, has the flywheel key sheared, sparks work etc but at the wrong time.........

i presume you have added some fule down the plug hole and tried it then, starting i mean??

check that first, flywheel marks align correctly when the piston is at tdc ish??
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[bladerunner]

Exactly as above! Teaspoon of fuel down the plug hole first.if it runs for a few seconds deffo fuel related...failing that woodruff key etc
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Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[orac69]

Thanks - dont know anything about and have never changed or inspected a flywheel - will try the petrol down the hole first and report back

[orac69]

btw - its a manual kick start, not electric start.

[reed]

I had this issue with my brothers 50cc bike, turned out to be bad fuel, if it runs with the fuel from the tank put directly 'down the hole' then it may still be bad fuel, as I found, eventually.

Confused me for a while, as it started and ran for a second or two when fuel was placed directly into the head, I think there must have been water sitting at the bottom of the tank.
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Generic XOR 50, Gilera SMT 50, GSXR 400 GK73A, Ducati Monster 600, Honda Hornet 600.
Sold. Stolen. Written off. Sold. Current.

[orac69]

Ok, so it sparks nicely with a new plug in, and holding a spare plug to the side.

Nothing happens at all when I put a teaspoon of fuel down the hole - not even the hint of a start. It's not bad fuel I'm sure - brand new.

Battery is fully charged close to 14v

So then guys it must be the magneto? I see a line on it - but what am I looking for / how to test?

Should I just replace it?

Thx

[bikenut]

so, good sparks, fresh fuel down the plug hole and still no sign..........pics needed for timijng marks but........

with plug out, and piston seen to rise to tdc ish, does mark appear close to a reference mark on the engine??

if its close then may be ok, by close i mean within a few mm, ign fire point usually a few degrees before tdc max piston movement upwards.

if the marks are way out then suspect flywheel key ( woodruff ).

if flywheel nut removed you can probably see the key "slot" which probably lines up with tdc, pointing up in line with the cylinder.........

pic would help.

it could also be a blocked esxhaust but check the key first.

if the key has sheaered, then flywheel will move relative to the crank and end up at any position, even the correct one for an instant.

pics pics pics.........
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[_Iain_]

+1 on the woodruff key.

Have seen a trials bike do this & caused myself and Paddy many hours of headaches searching for the issue.
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Please be aware that the above post may be full of complete nonsense.
Riding: '07 KTM Duke II, Baotian BT49QT-20 Driving: '88 Volvo 340

[bikenut]

its real fun sometimes isnt it!!!!!!!
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[P.]

FWIW I had zero spark when woodruff key finally gave up the 2nd time. PITA

[orac69]

Nope, its not the woodruff key - looks intact and fine. See pics.

So I guess next step is to replace the entire flywheel / magneto - perhaps the spark I was seeing is intermittent..

[P.]

I'd still be looking at the spark plug and fuel delivery.. so carb and spark plug. I have had new plugs just not work...

[orac69]

Thanks Paddy,

Ive tried 3 plugs so pretty sure its not that.

I do have a spare 21mm carb - I'll stick that on first tonight before replacing the expensive flywheel assembly or cdi.

[-]

When my other half's NSR died shortly after de-restriction, I checked for spark, fuel and it seemed to have compression with my finger over the plug hole. It wouldn't fire though.

In the end the piston ring pin had come out taking a tiny chunk of piston and ring with it and scoring the bore. It gave enough compression to push my finger off the spark plug hole but not enough to ignite the fuel.

I'd pop the exhaust off and shine a torch up the exhaust port to have a quick look if all the other checks come back OK.

[bikenut]

also, the staor has been off by the look of the screws, and it seem to be at an extreme of the adjusting slot.............has it been put back on in the right place and is it timed correctly, but check rings thru exhaust port as a missing ring will have an effect.

however, this bike was running till it concked on the way home......blocked exhaust or fuel.........or dondgy connection on the ign??

how is air filter??
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[orac69]

Thanks for the advice guys - mpd72 - you were right... Piston knackered, barrel scored, so obviously had some compression but not enough.

Top end rebuild time.

Petrol is in the crank - is it ok just to flush it out?