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Repurposing a 25cc strimmer engine

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Paul White 313/05/2018 14:44:37
109 forum posts
23 photos

In the amazing pool of information within this forum can anybody help me with setting the timing on a 25cc petrol ex strimmer engine

. The motor suffered failure of the ignition unit, a DIRECT replacement being unavailable a replacement unit has been obtained but has a different mounting thus the usual -just screw on the new one , does not work.

I have done a web search but cannot find how the cdi unit is positioned relative to the flywheel magnets on these motors.

All help gratefully received.

Thanks Paul.

vintagengineer13/05/2018 14:53:16
468 forum posts
6 photos

Without seeing the engine. I would assume the magnet acts as a switch for the spark. You need to set the piston about 10 degrees before TDC, set the cdi unit just after the magnet. You will need to play around to get the best timing position.

Paul White 313/05/2018 15:40:33
109 forum posts
23 photos

Vintagengineer, Thanks for your very fast response to my cry for help.

The problem I have is not understanding how the cdi unit is fired, at what position the flywheel magnets need to be relative to the metal core of the cdi. Does the statement of "-setting the cdi just after the magnet" mean the back end of the passing magnets aligning with the back end of the cdi core.?

Howard Lewis13/05/2018 16:42:56
6113 forum posts
14 photos

Even if you set the timing correctly, you ought to ensure that the clearance between the flywheel magnet and the CDI unit is correct, (assuming that it can be adjusted) otherwise you may not get a big enough pulse to trigger the spark.


Robert Dodds13/05/2018 17:01:06
320 forum posts
62 photos


I had a similar issue with a lawnmower that had a slightly different mounting arrangement. Do the old and new units look similar in layout of the laminations in the coil? If so the new unit needs to go as near as possible in the same orientation as the one you've taken off,to keep the spark timing right with some sort of adapter plate to facilitate screwing it down. The other important consideration is the air gap between the poles on the cdi and the magnet poles on the flywheel.My instructions were to use a piece of 0.005" perspex between them whilst tightening up. Runs like a Lotus 7 now!


Bob D

PS Howard beat me to the gap!

Edited By Robert Dodds on 13/05/2018 17:02:52

Martin Newbold13/05/2018 20:18:49
415 forum posts
240 photos

You can make an adapter out of sheet steel. If its the coil you are replacing then just line it up close the old one the most important thing is to make sure its as close to the flyweel as you can get without rubbing the flywheel. Usually the flywheel will have a electric or magnetic switch under the flywheel which will fire the coil through a capacitor to engerise the spark through the coil. Even modern systems have to have this switch capacitor and coil arangement even with cdi.

While you have it apart look at the ppipe work routing from the fuel tank to the carburettor these are usually not very refined and could do with a spring through them.


Doubletop13/05/2018 23:21:13
439 forum posts
4 photos


Try this as a starting point. Even if your unit isn't RCxcel the principles are much the same. (e.g. the units from Hobby King)





Paul White 315/05/2018 12:34:28
109 forum posts
23 photos

I would like to thank all the respondents to this thread for their time and sharing of experience.

The net of information seems to be trial and error as the method of setting. This I shall have to use as

the procedure I adopt.

Thanks to all.


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