I went to fire up my Ofna Force 21 engine over the weekend and the pull start would not engage properly. I took the pull start off and turned the spindle by hand. There is some small movement in this spindle, pushing it does seem to engage the crank and pulling it out disengages it. I removed the end cover and removed the rest of the assembly, however, I am now unsure as to how this assembly actually works. I can see how the slotted drive plate engages with the crank pin for starting, but how does this engage and disengage? Any help would be welcome.

Should be a OWB (One Way Bearing) in there.....If your Pull Starts Not turning the Engine over .....Chances are the OWB's Slipping.....Clean the OWB and Try Starting it a gain....(Cleaning the Bearing Works most of the time)


One Way Bearings

Hi
there is a small spring behind the pin that sits in the crank conrod shaft.This maybe gummed up or broken stopping the pin from properly engaging in the cutout
of the starter shaft.
Wozza

Thanks, that would explain it. I felt sure that there must be a spring loaded component somewhere and it is obviously the pin in the conrod shaft. I take it that this runs on the plate once the engine is running. I will check to see if this is broken or seized, it does seem a bit short I must admit.

Yeah which means that little pin is banging in and out every revolution of the motor .The Idea behind it was to allow the backs to be changed easier,
on some motor's you have to grind the pin of the crank or get a alt crank to fit the bump start back plate.Meh at least its easy to replace
Wozza

That pin must take a bashing. If that is the spring loaded pin inside the big end bearing then it is stuck fast and there is not enough protruding to get a pair of long nose pliers on it. Is there any way of getting this out without a strip down?

Hi
Have you got the pin out? if not the pin will push a small way inside the conrod bigend shaft before the spring full compresses. Shoot some oil down there and give the pin a light push in hopefully that will loosen it up.It only takes a few min to remove the crank shaft so it may pay to do that anyways.

Wozza

No, seems to be stuck pretty good. A strip down wouldn't be a bad thing anyway and a good chance to give it a bit of a clean out. Can you point me in the direction of some instructions for stripping this down? Thanks

http://www.ofna.com/manuals.html
towards the bottom "motor manual" "Force .21 P4, Black T8"
Basically pull every thing of the forward part of the crank (clutch collet and brass bush and the carb ) undo the 4 head bolts remove the head and glow button,
put the piston at the bottom of the liner and stick a finger down there pull up the liner,put some pressure on the exhaust port with your finger and give it a wiggle,Should just slide out 80% of the time. If its stuck you will have to heat up the crank case and use some sort of soft plastic lever (I use an old tooth brush handle with the bristles cut off) to lever it up either from the bottom of the crank case or one of the ports,be very careful as the slightest bur on any of the ports will stuff the piston when its put back together.Once you have the liner out you be able to slide the piston/conrod back and of the crank pin.Then put a nut on the cranks thread(where the clutch normal go's) and using a large screw driver handle give the nut a sharp tap to get the crank out of the bearings.
If your going to replace the bearings your going to have to heat the crank case and using a suitable drift knock them out.To replace the bearing put them in the freezer and heat the case they should then drop right in with little or no force
Hope this helps
Wozza

Thanks very much for that Wozza. I'll let you know how I get on. Probably have to wait until next week now as I have been told I am visiting the in laws this weekend []

Well, after a long time, I have actually got around to looking at this. Got the liner and piston out ok but cannot get the flywheel off. Is there a secret to this? The nut seems to have a groove around it, this isn't a locking nut is it? One problem is holding the flywheel while I turn the nut. I have tried holding it in a small vice between two bits of softwood but it slips round between these.

One other question, when replacing the liner, should it go back into the block dry or should anything be smeared on the outside before replacing it in the block?

coming in a little late but reading last post.
when i reassemble the engine i usually put a couple of drops of after run oil in the front bearing and a couple of drops in the inside/rear bearing and place the crank in there and spin it to spread the oil in well.
and also i use after run kinda like assembly lube on con rod bushing, wrist pin, piston/sleeve before sliding into block, and pull start pin in the crank to kinda keep it from seizing along with a drop or two in the one way bearing so it doesn't stick and slip.

Job done. I found that a 19mm ring spanner fits perfectly over the three pins on the flywheel and this held it tight while I loosened the nut. The sprung loaded pin was stuck pretty tight in the crank but a couple on taps on a small punch did the trick. All cleaned up and free as a bird now. Now its time to get a cuppa, sit down at the kitchen table (when the wifes at work of course) and get it all back together again.