YD100 stopped, barely runs now

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Marmalade1

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Apr 15, 2020
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YD100 (78cc). Maybe 2 months old. Bought new. My second motorized bicycle. Used for 4 mile round trip commuting daily. 67 jet. 1:40 gas/Lucas oil since new. 10/36t. Champion 811 plug. Ran wonderfully. Comfortable 37MPH top speed. Calm 30+MPH cruising. Good times. Started easily in hot weather, and with a few seconds of choke in colder (40-50F weather).

Last week I was cycling home and I heard a small clattering metal sound. I thought it was my front mud guard coming loose. It wasn't. I continued. The sound came back and then the engine died instantly. Almost full tank. Petcock open. I couldn't get it to run again. Cycled home.

Cleaned carb last night, resealed with RTV again. Put 70 jet in (colder weather, thought it may help in the winter). Tested spark, looks strong and blue. Still, changed plug for clearer reading of rich/lean plug chop examination. Crankcase bolts are tight. Some oil leaking from bottom of magneto area, but dry inside magneto housing. No air leaks noticed before failure. Without sparkplug engine turned over easily.

Ran it today. Hard to start, but ran, no clanging noise. Bogged down severely. Only ran with WOT, which produced perhaps 20MPH and no torque. Instantly stopped at less than perhaps 1/3 throttle. It ran, but barely and needed WOT coaxing to keep it going even after a few minutes at WOT. It didn't 'clear out' and settle down the way I'd hoped. With clutch disengaged, it was capable of revving well, but had little torque with clutch engaged and required a lot of throttle to keep it alive.

Clutch is strong and removing the clutch cover reveals it does not slip at all when engaged.

Is it simply too rich? Thanks for your help.
 
Greg58

Greg58

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May 1, 2011
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I hate to say it but it sounds like you broke a ring or lost compression, I would pull the head and rotate the engine to BDC and look at the cylinder, a lot of people have had the plating flake off.
 
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Marmalade1

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Apr 15, 2020
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Greg58 said:
I hate to say it but it sounds like you broke a ring or lost compression, I would pull the head and rotate the engine to BDC and look at the cylinder, a lot of people have had the plating flake off.
Click to expand...
Hummm, that would explain a lot of the issue. I'll take a look...
 
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Marmalade1

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Apr 15, 2020
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Greg58 said:
I hate to say it but it sounds like you broke a ring or lost compression, I would pull the head and rotate the engine to BDC and look at the cylinder, a lot of people have had the plating flake off.
Click to expand...
Well yes, it is just as you suggested. Pity.

Piston scored, cylinder scored, rings... look ok. It seems something got chewed up in there, dings all over the cylinder head.

Replacements ordered.

I want to be sure whatever it was is out of there and not blasted back up from the crankcase.
 

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Greg58

Greg58

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I would wash the crankcase out with fuel, stretch a towel or old tee shirt over a bucket and pour the fuel through the cloth to catch the debris. You will probably find a lot of fragments.
 
Greg58

Greg58

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Are both cir clips still in the piston? That’s what the piece of metal looks like, if you have both intact the person assembling it at the factory may have dropped one.
 
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Marmalade1

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Apr 15, 2020
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Greg58 said:
Are both cir clips still in the piston? That’s what the piece of metal looks like, if you have both intact the person assembling it at the factory may have dropped one.
Click to expand...

One wrist pin clip missing. And looking at the piston, it was clearly clip shaped damage. Surprised me as I thought the pin clips were not directly under stress (unlike the wrist pin itself).

Washed out the crankcase with fuel, agitated it, poured out three times.

I received the replacements. Ported and smoothed as best I could. However, I noticed it's a windowed piston. I have a regular carb.

Will this work at all?
 
Last edited:
Greg58

Greg58

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May 1, 2011
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Probably not, the down stroke of the piston will blow back through the carb. That is a reed valve piston, reed valves allow air in but seal when back pressure is present.
 
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Marmalade1

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Last edited:
Greg58

Greg58

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May 1, 2011
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I haven't used a reed set up on any of mine, from others here if you add a reed valve you'll have to have a larger jet.
 
Greg58

Greg58

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May 1, 2011
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On most you'll also have to add an offset intake because of the added length of the reed system so the carb can clear the frame.
 
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Marmalade1

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Apr 15, 2020
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Ugh. I just want this to work again.

The kit I got on ebay had the windowed piston. So now -to use it - I need an offset, reed valve, and according to some new carb and jets.

CDH power on Amazon suggests that a reed ported cylinder is needed.

Did I miss anything? Thinking of just getting a regular piston.

Thanks for your advice.
 
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Marmalade1

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Apr 15, 2020
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Greg58 said:
I haven't used a reed set up on any of mine, from others here if you add a reed valve you'll have to have a larger jet.
Click to expand...
Ordered series up 70, 75, 80 etc. Ought to be tunable from there even if i have to solder/ drill.
 
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Marmalade1

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Apr 15, 2020
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Got a CNC reed valve from Amazon. Looks great. Figured if I've got a window piston now, I may as well try to upgrade my power output.

Installed reed with regular Speed carb (70 jet still). Started with some choke. Ran...ok. Low torque until it eventually got to mid/high throttle. Remember, new rings/ piston too. So a new break in.

Several short runs yesterday. Power improved. Top speed improved a bit to 40 mph (gps), more vibrations. Still lacks the low end torque it had with stock set up.

22 miles today. Getting better. Torque still sub-par vs stock piston/no reed.

So, I ordered a mz65 clone exhaust. It'll be loud, but I want my low end torque back. Most agree that the mz65 in particular adds power with the clones somewhere close.

Any concerns? Did i miss anything?

YD100, window piston, reed valve, Speed carb, mz65 clone exhaust, larger jets,

Ought to be good, right?.
 
Greg58

Greg58

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May 1, 2011
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The longer head pipe should help low end, it did on mine. I used a stock muffler as a silencer, I welded mine on.
 
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Marmalade1

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Apr 15, 2020
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Ok. Results are in.

YD100. Speed carb (19mm). 70-75 jet (in-between size). Mz65 clone exhaust (amazon). CNC reed valve (amazon). Windowed piston.

What a cracking combination. The mz65 clone really opened up the low-end torque, good Mid-range and top was still good. Very loud. But frankly, a fun loud with cracks and pops. Works great.

Plug looks happy. Jet is opened up from a 70 to a bit less than a 75.

May benefit a bit from moving the needle clip, jetting more carefully. But I'm thrilled.

Thanks for your help random internet people.
 
U

user54508

Member
Feb 3, 2021
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Best carb for the yd100 with 4 prong reed and windowed piston is a pz20 with 1mm sized jet. Easy 1.5hp boost. Take your piston clips and toss them directly in the garbage and use good quality od type snap rings. Wrist pin will not contact the clips and saw the removal pins off the stock ones 500miles of riding, you can set a watch to it every time!
 
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