Engine Trouble Runs rough at low RPM?

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Tressie

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Hi,

Just got my 48cc Happy Times running and have a quickie question about how poorly it runs.

Regardless of throttle position, while at low RPM my engine seems to run very rough and might even be firing only on every 2nd revolution. As speed increases it reaches a point where power increases markedly, and it starts to sound like it's firing more consistently.

My guess is that at low RPM I'm only getting enough fresh charge in the combustion chamber to ignite on every 2nd or so rev. It's maybe taking a couple of turns to exhaust enough spent charge and replace it with fresh mixture.

So how do I address this? Enriching the fuel air mixture by changing needle valve setting might help, but I'm worried this might mess-up my fuel consumption. Lengthening the induction column length sound a better means to tackle the problem, so I would be very interested to hear others experiences with this mod.

Thanks for any help or suggestions.

T :sick:
 
I'd first wait till the engine gets thru the run-in period.
 
Sounds like the needle is set too rich. Raising the clip (lowering the needle) should fix it, and improve your fuel mileage.
 
Thanks Purple Haze and crassius,

Yeah, there are a couple of factors I hadn't considered then. The 'tightness' of a new motor may have some impact, as may the 16:1 run-in fuel mixture. Changing the plug to an NGK B6HS gave it a better idle, but it would only keep running at idle with the idle mixture screw all the way in.

Well I guess the best place to start then is with the needle valve setting. Okay, I'll give leaning-it-out a shot this afternoon, and get back with the result.

Thanks for the tips, Guy's. Muchly appreciated.

T. :)
 
Hey T man, First, If you have a stock carb, It doesn't have a fuel mixture screw. All it has is an idle ajustment screw. This has NOTHING to do with the fuel mixture, it only raises or lowers the idle. On a stock carb you will need to adjust the needle setting or change jet size to affect the fuel getting to the cylinder.
Using a NGK B6HS is only the first step. The next thing to do is replace the plug wire and boot. Just use a good auto grade wire for this.
Then, Make sure all wires are secure, not touching bare wire to metal, and soldered if possible. And never use the white wire for ANYTHING. Cut it off, cap it off or whatever, just don't use it. not even for the kill switch. For that simply take one kill switch wire to black, the other to blue. BUT, BEFORE you hook up the kill switch, hook up the ENGINE ONLY. Sometimes the kill switch itself is defective and will cause problems. So, Unhook the switch and try it.
And crassius is right about letting an engine break in some. They always seem to smooth out a little after being broke in.
Big Red.
And I also meant to say, Purple Haze is probably right about the needle setting. It's something to try anyway.
 
Sounds like the needle is set too rich. Raising the clip (lowering the needle) should fix it, and improve your fuel mileage.

Yep, leaning-it-out improved the situation quite a bit. The idle speed went right up, but settled back to a nice smooth 'putter' when the idle mixture was wound out from it's fully-in position. I didn't really get a chance to warm the motor up much this afternoon to see, but low RPM running appears now much smoother, though has become a little sensitive to throttle position in achieving this.

The spark I got with the standard CDI gear seemed pretty strong. I believe there are some issues with the stock ignition at higher RPM, but I'm aiming for a lower speed, de-tuned configuration of about 200W.

Standard 20mm (19.1mm OD) copper water pipe and fittings appear to be available quite cheaply in oz, and promise to make an attempt at the extended induction column length mod fairly manageable. I wonder how well soldered copper fittings would take an electro-deposited coating of tin? Guess we'll find out.

Thanks for the great input, fellas. Much appreciated.

T. :)
 
Okay, I'm slowly running-in this engine and it's running better with each slow 15-minute ride.

What's the deal with chain and idler noise, though? I've got the chain nicely lubricated, and the idler is positioned straight, level and central, yet the rattle is driving me nuts. What's the cure for that? Do I have the chain on inside-out, or something?

T. :confused:
 
the standard chain will stretch about 1/2 inch over the first 100 miles or so - adjust it as it gets looser and it will get quieter unless it's rubbing on something - I also like to grind the sprocket teeth narrower at the tips to help the teeth engage the center space of the chain more easily & quietly
 
Pain in the @&& chain

First, Start with making sure the drive sprocket is PERFECTLY straight and in line with the rear sprocket. You can use a straight edge, But I have a very simple way of doing this. Unhook the chain, make sure the tensioner is out of the way, and pull the chain straight back. You can move the chain left and right and will be able to see, on the drive sprocket, when the chain is coming STRAIGHT off the sprocket. The teeth of the drive sprocket will be centered in the chain. Then, very carefully, bring the chain STRAIGHT BACK from there. The chain should be hitting the rear sprocket dead center. If not, adjust the rear sprocket to do so. You may have to use thinner, (or thicker,) spacing to accomplish this.
After you're very sure the sprockets are lined up, next comes the tensioner. Install the tensioner and lay the chain across it from sprocket to sprocket. I think you'll notice that the chain is not running straight in the roller. It goes from one side to the other in the roller. This is because the tensioner is straight, but the frame always goes to the rear at an angle. So simply twist the tensioner arm to fit the angle of the chain, and that the chain rides in the center of the roller.
Then, (What!!! thought you were through?) Install the chain, raise the bike, pull in the clutch and spin the wheel. Because if you're using a rag joint the rear sprocket IS NOT perfect. I've been doing this for YEARS and hardly ever get this PERFECT. There is probably a little back/forth in the sprocket. This will leave you with a spot thats just a little tighter when the chain goes around. This tight spot is where you adjust the chain. If you adjust on a loose part, then when it hits the tight spot it will be TOO tight.
Besides getting the engine straight and tight, This is the most critical part of any build. A chain that's not in alignment, or very, very close, will give you nothing but headaches. Yeah, A PITA, but it's gotta be done.
Big Red.
 
the standard chain will stretch about 1/2 inch over the first 100 miles or so - adjust it as it gets looser and it will get quieter unless it's rubbing on something - I also like to grind the sprocket teeth narrower at the tips to help the teeth engage the center space of the chain more easily & quietly

Hey crassius, I agree a chain settles down with some break in, But, If the chain is installed properly there should be no need to grind on the teeth. If yer having a hard time getting the chain aligned then, yeah, grinding the tips will help. I just gotta get the alignment right if it kills me though. Then again, Like I've always said.
IT'S YOUR BIKE, BUILD IT YOUR WAY.
Big Red.
 
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