49cc big loss in power

Desertrider19

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Dec 21, 2019
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I have had this motorized Bicycle for about 2 1/2 years. It has worked great all the way up till a few weeks ago when it suddenly lost power. Still starts and runs fine, but hardy any power when on level terrain.
I tried taking apart the carburetor and cleaning it, but that didn't help much.
I've replaced the sparkplug once since I've had it. Maybe it's time to change it again. I don't know.
 

Desertrider19

New Member
Dec 21, 2019
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Could be a compression leak, it's time for a new head gasket and resurface the head
I was kinda thinking that it was a compression issue.
Something else it's doing is coughing/choking when I am holding down the throttle all the way. The bike doesn't speed up any, just slowly lugs and dies.
It seems to have the most power right after it has warmed up.
 

wrench

Well-Known Member
Aug 20, 2019
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So Cal
I was kinda thinking that it was a compression issue.
Something else it's doing is coughing/choking when I am holding down the throttle all the way. The bike doesn't speed up any, just slowly lugs and dies.
It seems to have the most power right after it has warmed up.
Sounds like a compression leak to me
 
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Desertrider19

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Dec 21, 2019
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Sounds like a compression leak to me
Okay! I'll replace the head gasket.
And by resurfacing the head, do you mean just cleaning it well? I was just planning to get a new head if need be, but if I could just use the one I have, I think that would be great.
 

Desertrider19

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Dec 21, 2019
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Okay. I haven't gotten new gaskets yet, but I did take off the cylinder head and clean and inspect it. There was TONS of carbon caked on the head. Got that all cleaned off, and I noticed a tiny increase in compression while trying to start the bike. The top end gasket looked fine to me.

I did notice that there was a bit of oil right below the bottom end gasket. So I think that maybe my problem.
I attached some pictures that might be helpful.
0422212238.jpg
0422212213b.jpg
0422212213a.jpg
 

wrench

Well-Known Member
Aug 20, 2019
345
312
63
So Cal
Yea, looks like the cyl base gasket is leaking.
Clean the carbon off from the inside of the head and top of piston with drill and wire wheel or Dremel and wire wheel. Be careful not to use the wire wheel on gasket sealing surfaces
Before reinstalling the head, resurface it with some 220 grit sand paper on plate glass to make sure it's flat, same with the cly deck. Scrap off as much of the carbon with a razor blade then resurface it with 220 grit on plate glass too.

Now is the time to make sure the cyl head and cyl deck are perfectly flat before reassembly
Tip:
Mark up the surface to be resurfaced with a black sharpie pen. When all the black sharpie marks are gone the surface is flat

Spray brake clean or carb cleaner on the cyl and case to clean all that oil off
 
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Desertrider19

New Member
Dec 21, 2019
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16
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19
Yea, looks like the cyl base gasket is leaking.
Clean the carbon off from the inside of the head and top of piston with drill and wire wheel or Dremel and wire wheel. Be careful not to use the wire wheel on gasket sealing surfaces
Before reinstalling the head, resurface it with some 220 grit sand paper on plate glass to make sure it's flat, same with the cly deck. Scrap off as much of the carbon with a razor blade then resurface it with 220 grit on plate glass too.

Now is the time to make sure the cyl head and cyl deck are perfectly flat before reassembly
Tip:
Mark up the surface to be resurfaced with a black sharpie pen. When all the black sharpie marks are gone the surface is flat

Spray brake clean or carb cleaner on the cyl and case to clean all that oil off
Hey thanks for the information. I was debating on whether or not to use a wire brush on the cylinder head/piston, but if that won't damage it, I'll get that cleaned up.
Thanks for the help.
 
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