Lack of power/bursts of power

D.Knight

Member
Apr 18, 2017
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Finally got my bike running yesterday so I've been riding it around trying to get the kinks out, when today I was riding and out of no where I get a huge boost of power. Now it makes me think I wasn't getting all of the potential power. It idles perfectly fine and the power boost is very spotty. Any ideas?

Didn't mention that I'm running 80cc 2 stroke with upgraded carb
 
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Greg58

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May 1, 2011
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A engine will do a lot of weird things when new, if its something you can deal with for awhile it may get better after break in. Somewhere around 3 full tanks it should start feeling stronger. Just make sure the head bolts, carb and exhaust stay tight.
 

D.Knight

Member
Apr 18, 2017
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CA
I thought it might have been due to it being a new motor, but Im going to change out the stock jet on the hp carb with a 69. I have a 65 through 69 set and I heard 70 works good around sea level so I'll try that.

Also had a little leak in my exhaust, it's an expansion chamber with a muffler and it bolts on to a pipe that wraps around the frame. It uses 2 copper gaskets to make the seal but I don't really trust it so I'm going to try sealing a little better. Did notice the power kick more often after I torqued the nut that connects the two parts of the exhaust.
 

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crassius

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Sep 30, 2012
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could be it's a bit rich and you are 4-stroking sometimes - does exhaust note seem to change a lot when power kicks in?
 

D.Knight

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Apr 18, 2017
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could be it's a bit rich and you are 4-stroking sometimes - does exhaust note seem to change a lot when power kicks in?
I'm running 16:1 (yes i know thats way too much but its all i got atm) but yes I do hear a difference in the exhaust note
 

D.Knight

Member
Apr 18, 2017
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Sounds like crassius nailed it. That fuel/oil mix is very heavy. You can just add straight gas to your jerry can to thin it down to 32:1 and be safe.
And you think that will help with the random power kicks?

I put the 69 jet on and I definitely don't feel a kick on flat or downhill but still get it on an incline
 

Chaz

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Jun 3, 2012
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Also, as I think crassius mentioned, don't get too hung up on fine tuning until you've broken the engine in a bit, like 3 or 4 tankfulls. I would say even a few more. As she breaks in her manners and power will improve and stabilize.
 

D.Knight

Member
Apr 18, 2017
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Yea you guys are absolutely correct on diagnosing the issue, after reading up and looking into 2 stoke 4 stroking, that is exactly what is happening.
 

MotorBicycleRacing

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Jul 28, 2010
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A 69 jet is smaller than a 70 jet.
The jet size is measured in mm so it is the same as the Decimal Equivalent Column (MM) on the right.

You must be confusing the drill bit left column as the jet size.

Also the difference between a 69 jet and a 70 jet would be hard to tell in real life.

Here is a jet size guide. As the jet number gets larger the actual opening of the jet gets smaller. So, if you are 4 stroking rich with the stock 70 jet and then go with the 69 jet you are making your fuel/air even richer.


 
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Chaz

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Jun 3, 2012
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Ha ha , I actually do need glasses. My apologies. I wasn't reading too carefully and your avatars are pretty similar at a glance through tired old ape eyes. I'm overdue for a nap.
 

D.Knight

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Apr 18, 2017
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CA
So now I have another issue. I put about 25 miles on the bike with no big issues. I rode 5 miles to work the other morning and it ran like a champ. At the end of the day, I get on and start pedaling, and she just doesn't want to get fully going. The motor starts but there is zero power, almost as if she's starving. I'm getting plenty of fuel (24:1), as when I take the carb off (hp carb) and take the bowl off, when I let the float down plenty of fuel starts flowing, and replaced the jet for good measure. My buddy took out the spark plug and hit it with some sand paper, put it back it, and she fired up. I did a lap around the block and came up to my shop, put in the clutch and she died. Since then I have tried 4 different plugs (3 brand new), all gapped differently and still not getting going. Any ideas?
 

Chaz

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Jun 3, 2012
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Vancouver, British Columbia
I wonder if you have a small leak at the head gasket. I see from the pics that you still have the stock acorn head nuts. They can cause a false torque reading because the bolts can top out inside. You can replace them or just cut the domes off. Mine came off real easy with a pair of pliers as they were so thin but you may have to saw them off. It's a simple thing so worth a try.
 

crassius

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Sep 30, 2012
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if it wasn't smoking a whole lot when it ran (means motor is full of fuel) then I'd first check head gasket
 

D.Knight

Member
Apr 18, 2017
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CA
Took the head off and replaced the gasket, also doubled up on washers so there's no way they could bottom out.