OK in desperation I turn once again to you guys!

the story so far.....

I have a Sym 50 Moped (dont laugh!) that i use solely to get to the railways station in the mornings....

exhaust was completely shot so i bought a new one from tecnigas

when i went to fit it i found that the mounting flange, built into the cylinder, was cracked off...

so i removed the cylinder and welded the flange back on, and went to refit the cylider, and broke a piston ring

so i got a new set of rings and fitted them at the weekend. got it all re-assembled and it just wont start...

checked i had sparks, checked i had fuel, checked i get compression.... all good

bearing in mind it was running fine until i tried to change the exhaust... but still i tried stripping and cleaning the carb and the removedchecked he reed valves... all good from what i can see......

so help guys! where do i go from here

could it be the CDI unit?
kids have scooters and once or twice the CDI units have failed.

also the middle one thinks that his scooter can run with no oil and burned out his yamaha 125 engine. a very very expensive lesson to learn.

good luck

Ditch

take the engine off the hedge trimmer and fit it to the chicken chaser.

sorry couldn't resist

2 stroke I take it? if you removed the piston, did you refit it the right way round? I would look at what you have done rather than look else where! for what it takes I would pull the cylinder off and double check that, make sure the rings are in the grooves correctly, check the ring gap!

my Moped struggles to clean the tiles nowadays oops sorry thats mop head

quote:

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Originally posted by ditchlewis
could it be the CDI unit?
kids have scooters and once or twice the CDI units have failed.

also the middle one thinks that his scooter can run with no oil and burned out his yamaha 125 engine. a very very expensive lesson to learn.

good luck

Ditch

---

If the moped was running fine before you changed the exhaust and removed the cylinder why look elsewhere?

desperation.....

the head and cylinder are pretty simple and i replaced the gaskets. if i hold my thumb over the spark plug hole i can feel what seems like ok compression as it pushes past my thumb....

i can feel air coming from the exhaust when turning over so dont think there is a restriction in the new exhaust ....

cant see what else to check there...

is the plug getting wet?

yup.. but not very.... it hasnt once even had a glimmer of trying to start

did you remove the piston to change the rings?

no.. did it in place

this is a cdi unit for a jinlun 50cc scooter.

this unit controls the spark and some 50cc have their speed limited by this unit.


Description

they are usually located very near the engine but i am not sure where on your bike.

i have usually found the cdi to be at fault when the things wont run properly even when you have fuel and a spark.

hope you get it fixed soon.

good luck

Ditch

[Edited on 8/11/10 by ditchlewis]

Warm the spark plug, put a squirt of neat petrol straight down the spark plug hole and replace the plug sharpish then try to start it. Have had to do this with brand new ones straight from the dealer never been run.

First thing, check for a spark.

If there is a spark, squirt some easy start down the carb

It has got fuel ? and the tap is on ?

tried throwing fuel straight down the venturi of the carb... still not a glimmer of a rumble from the engine..

given the history of whats been done i am now thinking i may have dislodged a wire... maybe contact with the CDI unit is crap....

going to crawl over it with a fine toothcomb later and follow all wires i can find, unfortunately a couple disapear below plastic mouldings around the engine which are a bitch to get off but will give it a go....

cheers for the help guys

well a couple of weeks on and still no joy....

have pulled the damn thing apart and put it back together again...

can't find a CDI unit anywhere! - have found a small coil hidden away under the frame and it looks ok but not a lot wrong with it i can see

beginning to think i will have to do the unmentionable and take it to a professional!

New spark plug.... no joy
cold start into intake - nothing
spark plug out and cold start straight into cylinder, plug back in and............... nothing! not even a whimper!

how can i have fuel in the cylinder and a proven spark and not even get a single bang out of it?

Hi Dave,

I have a very limited knowledge of two strokes and none whatsoever of your particular model
so this may be complete bull.


In principle, when the piston is on the up stroke this creates a partial vacuum in the sump.
This opens the reed valve and air/fuel mix is sucked into the sump.

On the down stroke the sump pressurized, and as the piston reaches the inlet port the mix enters
the cylinder, is then compressed on the up stroke, and hopefully, BANG.


So if you are sure you have a spark, fuel and compression, check the reed valve for cracks and grit stuck behind it.

Check also the crank shaft oil seals as pressure will be lost through these.

Check crank cases for cracks or gasket leaks.


Hope this helps.
Paul G

p.s. Buy a Honda C90. They go on forever.

well at last i have found something wrong....

not sure if and why it has caused the effect i have but i have found a small crack in the piston... when 'tested' with a pair of pliers half the piston skirt came away in my hands!

so new piston is now on order and i wait to so if it makes any difference........

fingers crossed!

She runs!

I guess it was the crack in the piston skirt... still cant see exactly how a hairline crack in the skirt could cause my issues ut there you go! it runs now

thanks to all for advice

[Edited on 27-11-10 by DaveFJ]

Lol the exhaust on mine came loose, somehow it then managed to melt the spark plug!

Hmmm

she runs ..... for a while.....

seems I have introduced a new problem

It runs for about a minute and then cuts out. after a few seconds it restarts fine then runs for another minute.... etc etc

made the journey to work this morning bloody slow!

Am thinking its the electric choke playing up but not sure

Sounds like a fuelling problem, mine did the same thing when nearly empty and about a day after I filled it (loosening the cap sorted that)