15 mph top speed with an 80cc engine

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SomeGuy

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Nov 22, 2015
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So I bought an 80cc bike engine kit a couple months ago, put it together on a mountain bike, and it started off weak. I figured it just needed some adjustments and time to break in. I made sure things were tight and that the seal on the intake was fine, but top speed didn't change. 100+ miles and 2.5 gallons of gas later (thing sucks it up fast!), still no improvement in top speed of approx 13-15 mph. Yet it starts fine, idles strong, and revs really high when the clutch is disengaged, but when I'm riding it has no power.

Is it possible that I just have a dud engine and I wasted my money? I have no idea what steps to take. I was led to believe these engines are capable of 20+ MPH on their own and had fuel efficiency of 100-150 mpg (not 40 mpg). Any help is really appreciated, let me know if you need more details.

Some Guy
 
2door

2door

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Have you checked the torque on your cylinder head? 120 to 140 inch pounds is recommended but I'd check the head gasket too. A compression loss will kill performance.

Spark plug gap? .024 to .028 is a good starting point. And yes, 16:1 is too much oil if that's what you're running. It might help to look at carb tuning but even with the stock main jet in the carburetor you should be getting better performance than 15mph.

What size rear sprocket do you have? Tooth count? Rider weight? Hilly terrain? Wheel bearings greased and adjusted? Brakes dragging? If you leave the fuel petcock on when the bike is parked does it leak fuel from the carburetor?
Just some ideas and questions to ponder.

Tom
 
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crassius

crassius

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I would drop the needle a lot before even looking at the plug - it must be way rich.
 
Bikeguy Joe

Bikeguy Joe

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NEVER make a carb adjustment- especially one to make the engine leaner without checking the plug first.
Common sense.
 
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SomeGuy

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Nov 22, 2015
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Thanks for the fast replies! I decided to take up some of your suggestions and check up on some things real quick.

So the details: I am a 190 lb. rider using a 44 tooth rear sprocket, mountain bike wheels roll without drag, and I am generally riding on flat roads or sidewalks. Lately I have been using a fuel mix of about 13:1 (10 oz oil to a gallon). Previous to that I had been using 16:1 and 32:1 mixes to see the differences but I hadn't noticed any.

Since you mentioned the torque for the cylinder head, I got a torque wrench and tightened each a bit to about 120 inch pounds. I have not checked the head gasket yet but I'll look into that. As for the spark plug, I made sure that the gap was to manufacturer specification. I also checked the plug and it looks wet/oily with some carbon build up on it but the very tip of the plug is dry.

After adjusting the torque on the torque heads, I did not actually notice anything. I was just riding for a mile in about 25 degree weather so it wasn't ideal conditions anyway (I'll do more testing throughout the week.) Anyway, there are a few things that do bother me with the bike:

When the fuel petcock is left on when my bike is put away, I do notice fuel leaking from the carburetor.

When I am riding with the clutch disengaged and the engine off, squeaking noises come from the engine.

Other than that, it just takes a couple minutes to warm up but I am not able to go fast at all. When I'm going uphill or on rough terrain, it starts to lose RPMs until it dies. I have tried pedaling really fast to speed up the engine (my phone app said I hit about 29 MPH) and it almost seems like its pushing itself maintaining speed for a little while but then it quickly drops back down to about 15 or less MPH.


Thanks a lot for the suggestions. All help really appreciated!

SomeGuy
 
2door

2door

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Keep you oil/fuel mix at no more than 24:1. 32:1 is even better. Too much oil will not help anything and will impact performance.

The fact that your carburetor leaks when the bike is parked and the wet plug is good indication that the float needle valve isn't closing, probably due to crud in the valve. If you didn't clean and flush the fuel tank before installing it the crud came from the tank. Install an in-line fuel filter, remove the float bowl and clean the carburetor. A little air through the fuel inlet nipple will usually clean the needle valve. If that valve is being held open by dirt it will cause a very rich condition and low performance. It will also allow the carb to overfill and leak.
Start there and let us know how things progress.

We're assuming the choke is fully open. It should only be used when first starting the engine then opened after a few seconds of running. Make sure the choke is open.

Tom
 
Kioshk

Kioshk

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Oct 21, 2012
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Tom makes a good point about the choke. I'm assuming you have an NT carb. If so, the lever-up position is choke, and down is no choke. Be sure that your choke-plate isn't loose. I had a similar issue long ago, and it turned out to be a defective choke-plate nut. The plate wasn't secure to the lever, and the assembly was also loose. The engine vibration caused the plate to drop into about 30% choke-on position, and my performance sucked.
 
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Brassneck

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I'm new to these engines as well...but could the exhaust be plugged? Sounds like youve been running a rich mixture, so perhaps the restriction is due to exhaust...? I'm sure others with more info could chime in on this idea.
 
2door

2door

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Brassneck said:
I'm new to these engines as well...but could the exhaust be plugged? Sounds like youve been running a rich mixture, so perhaps the restriction is due to exhaust...? I'm sure others with more info could chime in on this idea.
Click to expand...
This is also a possibility. The OP said he had used a 13:1 fuel/oil mix. That's a lot of unburned oil that could plug up things. I've seen exhaust ports more that half blocked by oil and unburned fuel deposits when an engine was run rich with an excess of oil.

Tom
 
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crassius

crassius

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for them as thinks dropping needle right away is unwise, I believe that a radical change is most likely to show whether one is looking in right place or not as quickly as possible - easy to change it back if nothing good happens

few people have good luck with plug reading, and for many, it just confuses them

OP's later peek at plug says 'too oily, too rich, or both' - at this point, even glazed cylinder might be considered
 
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SomeGuy

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Nov 22, 2015
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Michigan
Yes I am using an NT carburetor and I always make sure to flip-off the choke when its ready and while I'm riding. Since you mentioned that about the carb leaking, I decided to open it up. The jet pin is bent perhaps from tightening the idle adjuster screw too much in the past, I am unsure what that might cause. There was some sealant gunk in the intake connection area so I cleaned that and dirt outside the carb, but the valve and the float area were clean. I also ended up putting more gas into the mix (maybe around 20:1-24:1).

I put it back together and it still runs the same. I am also noticing that the squeaking from the engine while the clutch is disengaged turns into a crunching or scraping sound inside the gearbox area as I pedal faster. So there is quite a bit of drag while I'm pedaling as you might imagine.

Not sure what may be happening there, perhaps the engine is defective? Does it just need lube? I find it odd that, although it revs really high when the clutch is disengaged (sounding like it has power), when I engage it with the rear tire suspended in the air, it doesn't spin all that fast (probably equivalent to the speed it runs with me on it). So what should I do next?

SomeGuy
 
crassius

crassius

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the squeaking may be from the drive sprocket fittings - open the cover, take out the bucking bar, put a lot of grease inside & push the bar back in (don't lose the ball bearing if it falls out when removing the bar) - fill the recess where the clutch actuator cam is in the cover with grease too - now, look for rub marks from the chain and if any post where they are & we'll see what can be done

now open clutch cover and first thing, look for rub marks inside cover - if there are post again, if none, put a peanut sized blob of grease on small bevel gear & rotate gears to spread it

not sure what is happening to your carb, but from now on only adjust idle higher while holding throttle open by hand as you don't want to push anything out of round - BTW, are you sure the slide is right side up? I've seen slow bikes due to slide upside down
 
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SomeGuy

New Member
Nov 22, 2015
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Michigan
Thanks for the suggestions crassius! I'll try greasing those areas of the engine soon. And no, I did not put the slide in upside down, I have it put together the way it should.

As for status with my NT carb after cleaning and re assembling it, it hasn't leaked fuel after leaving it with the petcock open for over a day. But last I rode the bike, performance didn't seem any different. If it means anything though, I am using an offset intake so it can fit the frame but the intakes fit to the carb doesn't seem like it would be as airtight as it would with the stock one.

Also, I have seen "Racin Carburetors" selling on amazon and such, and they sound like they would be better in every way. Would one of those be worth the while? Like here is an example: http://amzn.to/1T5TYaN

SomeGuy
 
2door

2door

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Don't fall for that advertising hype. Especially from a seller that can't even spell.

Racing, speed, high performance, are all examples of marketing 'buzz words'. Your NT carburetor will give you all the performance you can expect from a stock Chinese 2 stroke. The carburetor you linked to is available from several other sources. It is sometimes referred to as the RT. Not a bad carb but it isn't going to fix your problem. No matter how they spell 'racing'.


If the carb doesn't fit the intake manifold tight enough to provide an air-tight seal then you have a problem. Any air (vacuum) leak downstream of the carb will cause issues.


Tom
 
crassius

crassius

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Sep 30, 2012
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I doubt a new carb will fix anything, I'm hoping it is just something rubbing somewhere & you'll spot it, you might want to look up 4-stroking to understand it too & maybe lean out your carb if it needs it.
 
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