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Discussion Starter #1
I have Mac trimmer power head that I picked up to tinker with - I believe it's a 28 cc (Roadrunner). I've got it running, but at full throttle it makes this loud knocking sound that I'm having trouble figuring out - despite taking it down a couple of times. It will occasionally backfire too. The only thing I haven't done is go into the crankcase - but everything feels tight by hand.This engine does have one thing unusual about it - someone stripped out the upper spark plug threads and when I got it, it had some type of adapter that I've never seen before. It screws down into the lower threads and then you screw the plug into the top of it. It creates its own little chamber and has about a 1/4 " opening at the bottom. Could this be the cause of the knocking? How do I correct it? Is there a longer plug that could do away with this adapter? It currently uses a CJ7Y. Thanks....
 

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If the adapter extends too far into the cylinder head the piston may be hitting it..., I am not sure that the adapter would have that affect on it otherwise. A build up of carbon on the top of the piston or cylinder head could also cause a knocking sound. Check the flywheel and make sure the key is not sheared, could be ignition timing also...
 

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Discussion Starter #3
Thanks 30year - I was beginning to think no one liked me.... There is a thin layer of carbon on the piston that I can see through the plug hole - but not unusual looking. Also, the carbon is intact under the plug hole - so I don't believe the piston is hitting the adapter. The flywheel key isn't sheared - in fact, I had swapped out the flywheel with another one to see if that made any difference - it didn't. I'm thinking that the plug adapter may be altering the burn pattern somehow - perhaps like knocking in a car engine. I'm going to try a helicoil and see if that helps. BTW - is there an easy way to remove carbon with pulling the piston??
 

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well you could try sea foam. The best way however is to dissassemble the engine and clean off the carbon. Carbon if very hard and if it flakes off of a 2-cycle engine it can get trapped in the ports causing damage to the piston and then the cylinder...
 

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Discussion Starter #5 (Edited)
30year - Thanks for the heads up on Sea Foam, it's great stuff!. I had never heard of it until you mentioned it and found some at O'Reilly's. It didn't clear the knocking sound in my trimmer - but it did do a pretty good job on the carbon. I treated 7 other 2-cycles and 3 additional 4-cycles. All now run noticeably better. One 4-cycle chipper-vac I hadn't used in years and the carb was pretty fouled. Sea Foam (and some elbow grease) did a number on the varnish buildup and it runs so good now that I'm think about giving my wife a shot of this stuff! ;) Are there any other 'wonder' products you recommend. What 2-cycle oil have you had the best luck with? Thanks....
 

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Let me know how it works on your wife, I never thought of using it like that!!!

I like Tanak, Echo & Shindiawa 2-cycle oil mix, they all have stabilizer added in to help your fuel last longer and prevent build up in the carburetor.
 
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