QuestionEric, Thank you again for another comprehensive answer. Certainly if I was paying for labor, I would have junked the leaf blower much earlier in the troubleshooting process. I agree; replacing pistons or rings on small engines is probably not very cost effective. But my labor is free – I am doing this as a mini-hobby! I have always enjoyed working with engines of all sizes.
Now, let’s get down to my last goof. I falsely identified the crack in the head. There was a clear crack, but it was in a black deposit that was on the surface of the head. I removed the head in anticipation of replacing it, but the crack did not extend through the block and washed away when I cleaned the exterior. But then I found the REAL problem; the reason the engine would misfire when it got hot…
On the head of the piston I found thick, black carbon deposits. Some was thick enough and heavy enough so the carbon had lifted slightly from the metal. I suspect that when the engine ran fast for a while, the lifted edges of the carbon deposit got hot could pre-ignite the incoming fuel mixture causing the malfunction. I scraped away the carbon deposit and reassembled the engine. It runs fine.
The carbon deposit is probably due to excess oil that I used. I thought I would “treat” the engine nicely with a little extra oil since I was only running it in the fall of the year and it sat unused for the other 11 months. Rather than the 25 mix called for, I mixed for around 18 - 20. The plug insulator ran pretty black and the engine smoked a little while warming up, but it always started and ran well – until this incident.
With 20:20 hindsight, I wish I had inspected the piston head through the sparkplug hole. I might have seen the carbon deposit and torn down the engine earlier. Perhaps there is a “Carbon Out” goop I could have used – is there? But I got hung up thinking that it was loosing fuel or ignition – I never thought about pre-ignition from hot carbon edges.
Eric, I want to thank you and this forum for the help in locating and fixing this problem. I wanted to post the final answer in case anyone was following this case.
Ken
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The text above is a follow-up to ...
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Eric, Thank you for your comprehensive reply with details on how to perform the crankcase leak test.
I started by checking compression. Measured exactly 100 psi on the cold engine. The compression dropped to 85 psi with the engine hot.
Finally, getting my mind off carboration and ignition, I did a careful visual inspection and found a radial crack in the cast aluminum cylinder head. The metal around the crack is coated with black combustion products so I should have seen it before! It is located about ½ way through the top of the piston travel. The head is cooled by a blast of air coming from the fan. The crack is on the opposite side where there would be less cooling.
Now I have a new question. Is it worthwhile to attempt to replace the head? Sears quotes the part [5401016000 (EC04-0125)] at $90.63 – call it $100 with shipping. A new leaf blower will be around $400.
Not knowing any better, I would reassemble by carefully compressing the old rings with a couple of screwdrivers while slipping the old piston into the new head and run in the new head with a little extra oil in the mix.
But, is this a foolish repair? I would hope that this would give me another 5 years (40 hrs of run time).
Is there a less expensive way to get a new head?
Eric, what does your experience say?
Ken
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We rarely replace cylinder, pistons or rings on small 2 cycle engines these days. My own old Echo trimmer is in desperate need of a cylinder, piston and rings. I too am in your shoes. Do I spend about $100 on parts of put that money to a new trimmer.
My advice would be put the money to a new blower. I prefer Stihl. They are reasonably priced, run well and we rarely have the come back to the shop. We repair a lot of Sears, our best customer for repairs, Weed Eater and Lowe's or Home Depot blowers.
If you decide to fix your old one a hose clamp, the screw type, works well to compress the rings to install the cylinder. Place the hose clamp on piston/rings, tighten it up then loosen just a little. Install the cyclinder and the hose clamp should slide down the piston onto the connecting rod. After you are done just un-screw and remove the hose clamp.
Let me know what you decide to do.
Eric
AnswerThanks for the update. I sometime forget about carbon build-up because we rarely see it these days with the unleaded fuel and 50:1 two cycle mix. I had similar carbon build up on a 4 cycle 12 hp mower engine. It was a little eaiser to find because the engine would shear the flywheel key every now and then. There was so much carbon on top of the piston that is kept compressing it into the cylinder head. When the carbon deposit became too compressed the engine would shear the flywheel key. It took me a couple of trys to figure out what was going on.
Again, thanks for the update.
Eric
