"Coughing" Engine

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TheE

New Member
Jun 26, 2009
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Canada
I just finished building my first motorized bicycle and am having a great time with it. I'm using a Limited Edition 49cc motor from Powerking. Great stuff!

Anywho, I noticed the engine tends to "cough" at low speeds, wherin it basically snags and I feel the jerk. This dissappears as I open up the throttle. Now I was replacing the cheapo Chinese spark plug with a better one today, and I noticed that the spark contacts were black. I've been reading the forums a bit, and apparently, that's not right.

So I think I might be having a fuel mixture issue, which would also explain why the engine is "coughing" at low throttle. However, I'm also using a 16:1 oil fuel ratio (that's what the manufacturer reccomends for early break-in), so I was thinking mabye that was the problem.

So I put the question out to the experts here: Is the coughing normal, or is there something I should be doing to smooth it out? It isn't really that bad, and it lessens as the engine warms up, but if I could reduce it that would be cool.
 
Norman

Norman

LORD VADER Moderator
Jan 16, 2008
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What is the spark plug gaped at? try 0.020 gap.
you might go to a mix of 20 to 1 then go to 32 to1 the more oil the harder it is to fire off the mix.
stock carb setting are
#2 notch on the needle from the top of the needle or the fat end. main jet dia is 0.70mm with it missing at low rpm it might be a little rich. this slight miss should go away as it get broken in and you go to a mix of less oil to gas.
The needle measures about 0.078 up by the notches and tapers to about 0.068 at the pointed end.
Float setting, with the carb upside down and the float bowl off the float should set about level on the tangs.

Norman
 
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Shadeslay

Shadeslay

New Member
Feb 25, 2009
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Arroyo Grande, Ca
I agree with Norman,

Running that rich they run rough. If your plug is black, you'll want to raise your needle clip a notch or two, which may help. 16:1 is pretty rich even using cheap oil, you could get away with 20:1 and still be on the safe side.
 
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T

TheE

New Member
Jun 26, 2009
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Canada
Ok, I'll try using 20:1 on my next fill-up. I'm using pretty high-quality synthetic oil, so I guess it'll do fine. If it really comes down to it, I'll pull the carb apart...I'd like to avoid that if possible.

Thanks for the advice =D
 
B

bike916

New Member
Jun 28, 2009
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Sacramento, Ca
Hello all. First post. I just did a conversion with a pk80. Mine has the same issues as the OP. It coughs or the way I put it...it seems like it has some cams in the engine with a lopey low end, but top end opens up nicely. I'm using chevron 2 cycle oil with a 16:1 ratio as well. What is the needle everyone talks about? Which spark plug should I upgrade to? Thanks guys and I'm glad to be here.
 
Bikeguy Joe

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
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Go to 24:1 for one thing, 16:1 is just too much oil. You can't tune an engine that is running on the wrong mix. (I know what the "instructions" say).

The needle we refer to is the slide needle located in the slide, inside the carb.
 
Norco John

Norco John

New Member
May 26, 2009
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Brookston, IN
I was running 16:1 for the first 2 gallons. Now that I have almost 200 miles on this thing I've reduced that to 24:1 and I have much smoother low-end (I was having the surging/jerking thing, too) and more power at the top (27MPH top speed v. 24MPH)

Another thing I noticed with the heavier oil was that my fuel filter would never fill up, indeed, I'd have to take the cap off the tank and blow on it to get the fuel flowing. Now the filter is full and I don't have to pressurize the tank.

I wouldn't play around with the carb needle/jet until a final oil/fuel ratio is settled on. The jet passes only so much liquid through it at any given time, and if there's more oil, there's less gasoline, so I think there's a good chance of running the engine too lean. I tried that with 16:1 and the engine wouldn't run over 15 MPH. I raised the needle back up (by the roadside, no less) because it didn't feel right.
 
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