First build- extreme loss of power/four stroking only under load

alec

New Member
Apr 26, 2016
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0
0
Maine, USA
Hey everyone, I'm looking to get another opinion on the issue my motorized bike is having. This is a chinese 80/66 cc "flying horse" engine and is my first build. It didn't take me too long to get it up and running, and for the first few months (despite some minor troubles to work out) it steadily ran smoother and faster over time as it should. I got probably at least 100 miles on it.

However, recently after tightening down several components I observed a considerable loss of power, and behavior that I identified as 4 stroking after doing a little research. I tinkered and experimented with it and thought I nailed the problem down to be either the CDI or the carburator, so I ordered replacements for both, a stock CDI and a "high performance" carb. When the parts arrived the CDI wouldn't spark, so I left the same original CDI (which produces what looks to me like a nice spark) on the bike and tried out the new carb. A replacement CDI is in the mail. The bike still 4 strokes heavily while riding. It hardly goes faster than 12 mph on the road I'd say, and the motor produces a very improper sound in relation to the throttle position.
When idling however, the engine sounds great though. Proper 2-stroke noise and pretty good throttle response. Even when I lift the back wheel and let go of the clutch, the engine runs greats and spins the back wheel mighty fast. Very little/no four stroking with the exception of some occasional problems at WOT. So the engine really only has problems under load/under the weight of a 140 pound person.

My gas is mixed 32:1 maybe closer to 34:1. I broke it in with 20:1 There's a little bit of smoking, but nothing excessive. I worry that the wires coming out of the engine (from the magneto, right?) are pretty close to the hot muffler, and may have even touched at some points in this bike's career. Also one of the mounting studs for the muffler is pretty loose (spins easily), but I don't think this is a problem? I don't believe the loss of power is compression related, because the bike provides a lot of resistence before it starts right up.

What do you all think the issue might be? Let me know what other information I should add.
Thanks!
 

crassius

Well-Known Member
Sep 30, 2012
4,032
156
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USA
first, if that loose stud rips the threads out, it will become a problem, so fix it while thinking about the rest

reading spark plug should tell a lot about richness/leaness

checking that motor is tight and gaskets are not leaking is easy

checking seals is a bit harder
 

Tyler6357

Well-Known Member
Mar 15, 2012
1,143
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Santa Barbara, CA
Check for air leaks around the head and where the carb connects. If you don't find any leaks I would recommend that you get a smaller jet, if you are 4 stroking you are running a jet that is too big.
 

alec

New Member
Apr 26, 2016
8
0
0
Maine, USA
A recent plug check (about a week back) indicated a dark choclatey like 70% cacao plug. I didn't know what that meant in terms of the ratio or whatever.
I'm gonna give that muffler screw stud some loctite.

And how does one adjust jet size? It does seem like I might need to do that.

I'll also see if I can do some compression/airleak checking.
 

crassius

Well-Known Member
Sep 30, 2012
4,032
156
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USA
if I thought it was rich, I'd try adjusting needle before doing anything with the main
 

alec

New Member
Apr 26, 2016
8
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Maine, USA
I had tried moving the position of the c-clip up on my original carburator, but it made no difference as far as I can tell. What other adjustements should I try?
 

crassius

Well-Known Member
Sep 30, 2012
4,032
156
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USA
if there is no diff when moving needle, then something else other than carb should be checked - whenever everything else is working well, changing carb setting will change the way it runs
 

alec

New Member
Apr 26, 2016
8
0
0
Maine, USA
Yeah, that's what's making me think that it might be an electrical issue and or a heat related problem. I don't really know though, I'm pretty inexperienced at this.
 
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Tyler6357

Well-Known Member
Mar 15, 2012
1,143
170
63
Santa Barbara, CA
If it is 4 stroking than that means it running rich, therefore not a heat issue, if anything you are getting too much oil. I would recommend getting some cheap jets and changing to a smaller one. To change the jet just remove the float and unscrew the old jet and screw in a new one. I think the .70 size jet is the standard carb jet that you get, it's often times too rich. But before doing that, make absolutely sure there is no air leaks, particularly around the head. If it was running good and then stopped running good it's probably an air leak from what you described. Start the engine and see if you can feel hot air shooting out from around the head, where the head connects to the cylinder (head gasket). If you decide to change the jet, start rich and move down until you dial it in.

http://www.ebay.com/itm/231458900645?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

http://www.ebay.com/itm/271748133936?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT
 
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Tyler6357

Well-Known Member
Mar 15, 2012
1,143
170
63
Santa Barbara, CA
Thanks, that's some very useful advice. I'll try to get the air/fuel mixture right.
Okay, when I say "rich" I'm not talking about the percentage of gas/oil mix you are using, I mean, the larger the jet is the larger the hole is that sucks gas from the float and into the intake and therefore the richer you are running, the smaller the jet is the leaner you are running.