Ran well, now no power

GregValiant

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Apr 20, 2020
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I've got about 300 miles on the bike (48cc Bikeberry kit with stock carb - .75 jet). It ran pretty good until day before yesterday. I was riding at top end and as I came up to the house I backed off. There was a backfire as I hit the kill switch. I pulled into the garage and parked it. It had never backfired before.
Next morning I took it out and it would only get up to 12mph. I could peddle it up faster, but it would slowly back down to about 12.
Pulling the main jet revealed nothing. I've pulled the magneto cover but I can't see anything obviously wrong there. (I decided not to pull the magnet to check the woodruff key until I received further advice).

That's kinda it for my troubleshooting skills with 2 strokes (it's been a while since I had the 500 Kaw triple).
So I'm looking for suggestions. The engine fires right up and idles well. When I rap the throttle it winds up. It just doesn't have any power to get up to speed. Main jet is fine. It's getting fuel.
Thoughts?
 

Tom from Rubicon

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Apr 4, 2016
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Welcome Greg,
The engine fires right up and idles well. When I rap the throttle it winds up. It just doesn't have any power to get up to speed. Main jet is fine. It's getting fuel.
Thoughts?

Back fire, one of my favorites, but I think you are running real rich. Like trying to run with the choke on.
The two smoke boys will be along presently.

Something you could do back in the early seventies and before was to coast into town on your Harley, close the throttle and retard the ignition. Pop Pop Pop!
I was a bad boy.
Good luck Greg,
Tom
 

Tom from Rubicon

Well-Known Member
Apr 4, 2016
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Rubicon, Wisconsin
Welcome Greg,
The engine fires right up and idles well. When I rap the throttle it winds up. It just doesn't have any power to get up to speed. Main jet is fine. It's getting fuel.
Thoughts?

Back fire, one of my favorites, but I think you are running real rich. Like trying to run with the choke on.
The two smoke boys will be along presently.

Something you could do back in the early seventies and before was to coast into town on your Harley, close the throttle and retard the ignition. Pop Pop Pop!
I was a bad boy.
Good luck Greg,
Tom
 

GregValiant

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Apr 20, 2020
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Venice, Florida
Thanks Tom.
I'm on my last gallon of oil at 16:1. The plugs are black but they're a bit oily too which is what I expected. So reading them is a little tough. It was running out of oomph at WOT but ran well at 3/4 so figured I go bigger on the jet (and I had a .75 drill handy). I had put a velocity stack on it (for looks ya know) and made an air filter from foam I picked up at O'Reillys Auto. So my thought was it could use more fuel. I'm looking for some jets (they are actually cheaper than the drills) and I'm guessing that I'll end up at 68.

Since riding it is useless right now maybe I'll take a look at the woodruff key. Is that a left handed thread on the nut the holds the magnet in?

Shovelheads taught my whole neighborhood how to wrench.
 
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Greg58

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Greg if it popped or backfired you may have broken a ring. On removing the magnet if you don't see fuel in the area there's no need, the seal is working. My 48cc bikes run best with a #72 or #73 drilled jet. Most stock jets are rich to keep the engine cooler, leaner is faster but much hotter. I would check the rings, if you are careful you can remove the head and cylinder without breaking a gasket and reuse everything. The head gasket will reseal.
 

GregValiant

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Now I have to skirt my BikeBerry rant...
There WAS a bit oil in the magneto area (5 or 6 drops worth). The cover has a gasket so the oil/fuel must have come from the crankcase. The petcock doesn't always close completely so along with the drippage I see there could be drippage I don't see.
I'm not going to start tearing it apart until I make a couple of rides with the 70 jet back in. Funny though...it ran well for about a week with the jet at 75. Mind you, it didn't run as fast, but it wasn't breaking up at high RPM either. Goes along with my "what did you do last" philosophy of motorcycle repair as well.
 
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GregValiant

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I think Tom gets the prize. Ran up to Home Depot, about 6 mile round trip. Much better with the jet at 70 but needs to go leaner. After soldering and re-drilling it's probably on the big side now. I think I'll order a couple of drills since there seems to be issues with jet sizes when ordered as a set.
What is considered to be a "step" in jet size? I see them advertised at .01 increments and at .02 increments.
 

Greg58

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Don’t confuse jet number and drill bit number they are opposite. With drills the larger the number the smaller the hole, with jets the larger the number the larger the hole.
 

GregValiant

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I understand Greg. I'm not talking wire size but rather the actual diameter of the drill. So I'll be ordering .68 and .65mm drill bits. They're about $3 each.
I built this about 10 years ago. I still have it. The Webers are exceedingly easy to jet. Before buying real jets I soldered and drilled out the jets I had while I was tweaking it. That's why I had the micro-drills handy.


IMG_0017.JPG
 
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Greg58

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No problem, we see a lot of different skill sets here on the forum, I didn’t want to mislead with what I recommend. I remember you said you found fuel in the magneto area, that seal is the one that has gave me problems on my oldest engine. It’s thinner than the right side and will blow out, sometimes you can use 3m glue to keep it in.
 
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GregValiant

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Yeah Greg, I wasn't happy about seeing the oil in the magneto area. Is the nut that holds the magnet on the shaft a Left Hand thread? That would be good to know before I start reefing on it.
I took a 17 mile jaunt yesterday. Eventually it does get up to 22-23 mph. I ordered a couple more sizes of micro-drills as well.
Tom, the front end comes up when I bang second gear. I found a GEM gearset at 3.7:1 ratio for the final drive (stock is 3.4:1) and swapped that in. Just 8% difference, but for a 580# cafe bike, it made a significant difference in quickness. I put it on a dyno to complete the jetting. It made 78 horses at 61ft-lb. Stock was 61-61.
 

GregValiant

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Thank you sir.
And on another topic, on the other end of the crank is the pinion gear. The one in this engine is only engaged with the big clutch gear about 1/2 it's width. The rest of the gear is just hanging in space and not doing anything. It needs to move inboard about 3mm to fully engage. Since I'll have one cover off I'll probably pull the other as well. When I first looked at it I figured typical Chinese assembly and good enough. It's starting to bother me though.
 
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Greg58

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On most engines correcting that will involve splitting the cases, some have shims on one or both sides of the journal. Some will have a lot of run out too, my best running 48cc has what I think is too much movement but it keeps humming on with no problems.
 
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GregValiant

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The drills showed up today so I turned the jet into a 65. I didn't get to test it much. The cheesy plastic throttle handle broke inside and no longer grabs the lead. So now the clutch perch doesn't lock and the throttle is broken. Almost everything in the Bikeberry kit has broken at one point or another and I've only had the bike about a month. Junk.
 
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