[RTM]

Guys, I have a Fantic FM250 (125cc) which has been converted to a 156cc by using a liner. Its got a new Piston and ring fitted / new big end new mains and crank seals.

I honed the cylinder a little but didn’t check the ring gap (buried my head in the sand here) and threw it all back together.

It runs and pulls just fine but it just wont tick over.

As the revs drop to just above tick over it seems to hover there as if running weak. It will then drop a few rpm, fluff and then die.

I can get it near tick over by opening the throttle a touch just as it dies but rather than a nice clear ding ding ding I get a a whooaaaa whooooaa as if the carb is sucking lots of air and its just runs very fluffy indeed.

Ok this all sounds like weak running or an air leak but all is good. Would a worn cylinder cause these problems due to the piston porting?

The bike had a 3mm thick spacer under the cylinder originally, but I left it out as it all measured up the same as a 200 engine I have kicking about.

I have cleaned the carb, I have played with the mixture, I have retarded the ignition.

Any idea’s

[B40RT]

Clean carb again, plus filter in tank.
Compressor is good, squirt carb cleaner or similar through every orifice (on the carb!)

This post has been edited by B40RT: 05 February 2010 - 12:57 PM

[RTM]

I used carb cleaner and my wind machine last night – spotless.

However I just spoke with Bob Wright.

He has suggested that it might be the coil failing ( I have a spare) or the boss on the flywheel might need to be wound out to be closer to the pick up – yep it’s a screw apparently not a rivet as it appears.

Will try these tonight and report back.

[RTM]

Niether of the above did the trick. I tried a 200 carb and that seems to have worked. Not ridden it yet but at least it ticks over now.

[RTM]

The above cure didntwork as I thought.

The bike is nearly ticking over now but fails at the last hurdle. What are the symptoms of a worn slide? or just a knackered carb?

[RTM]

Although I am not quite sure about why this is the case it appears the fault might be to do with the 125 to 156cc conversion.

I’ll know more when I pull it all apart.

Bill Pye has advised me of the following.

As my engine has been converted the crank cases need to be machined out to accept the larger diameter skirt.

The 3mm thick spacer has something to do with this apparently, I will measure everything up later.

Apparently its common for the machining to break through into the gearbox side during this operation.

Also the 156 Piston needs to be shortened by taking 3mm of the bottom of the skirt on the induction side.

I can confirm this was done on the old piston, as its here on my desk – but I bet it’s not been done of the new piston which has been installed.

I will report back when I know more.

[bilc0]

You did'nt put your spacer back in then,if not pop it back in and ty again.

[RTM]

No I didnt. I measured the modified 125 barrel against a old 200 barrel I have kicking about. Everything appeared to be the same, I must have been wrong eh !

[bilc0]

God knows RTM,your just gonna have to play around with it.

[RTM]

I checked for cracks or leak paths through to both sides of the crank cases - nothing.

I removed 3mm from the skirt as per the old piston and installed it with the 3mm spacer in position.

It nearly killed my trying to get it to fire up and then it ran very poorly indeed.

Stripped it again removed the spacer and it runs slightly better but its still racing.

Too late now to play with it tonight.

[hewson]

Why did'nt you try it first with the 3mm base spacer , before cutting the piston down ?????

[RTM]

That is a good question. The reason or rather my logic was, it once ran with a cut down piston fitted and it must have run for a good few hours as the old cut down piston was worn. & Bill Pye told me I had to cut it down as it was a converted 125! The engine also knocked when running as it something like the piston was hitting something.

Its actually nowrunning the best it ever has and nearly ticks over. But it still fails at those low revs and other times it picks up and revs. I cannot find a constant.

I am wondering it it could be down to float level? or maybe the needle position.

Anyone know the correct float setting ?

[bilc0]

Would be worth looking into your carb abit more.....Is your air screw working ok.???

This post has been edited by bilc0: 27 February 2010 - 02:02 AM

[RTM]

After a great deal of faffing about with this fantic. I decided it must must be an air leak. So I removed the carb and simply filled the crank cases up to the inlet port height with paraffin.

It drained away quickly so now I knew there was a leak and it must be into the gearbox as it was not coming out anywhere I could see.

I removed the cluch cover and flooded the garage floor with paraffin. The leak was behind the crank pinion flywheel.

The reason simply being the crank seal installed having a 22mm bore and it should be a 20mm bore.

My excuse is – the Parts list I down loaded from frankfieldfantic.co.uk is wrong.

That shows a Part No 260.41.00.5500 on both sides, so when I initially only changed the seal on the ignition side I bought two of the same and didn’t question it.

I will install a new seal later this evening and let you know if its ok.

My concern now being that I have removed 3mm from the bottom of the piston on the inlet side.

[RTM]

We have tick over !

Apparently only the very first Fantic’s made (1978/79) had the 20mm diameter crank on the Pinion side. All of the later models were 22mm.

This post has been edited by RTM: 04 March 2010 - 08:54 AM