Topping out at 20 mph...Too much fuel ?

Airbuzz

New Member
Aug 16, 2008
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Hi everybody. For the past couple of weeks my 47cc has been bogging down at 20mph. It will accelerate well to 19mph and then at 20mph it bogs down, dropping to about 18. This occurs whether you give it full throttle or partial throttle (just enough twist to accelerate to 20) so as to not flood it out.

The engine used to run fine. One of its sweet spots was at about 25-26mph. Then all of a sudden it started bogging at 20.

You may ask, has anything changed? I had to replace the float a few weeks ago. The original leaked. But I swear the bike ran fine for a while after it was replaced.

My saga continues and here is where it gets interesting. I was getting accustomed to the 20 mph top speed, so I just figured that I’d allow my 10yr old son to ride this while I build a new one for myself. 20mph is fast enough for him.

Well yesterday he coasted into our garage on his ride and said he figured out the problem, stating that he was again able to hit the 25mph sweet spot. His method was to starve it of fuel by mostly closing-off the fuel shut-off valve on the fuel line.

The whole time I thought that it was not getting enough fuel. Now it appears that it was getting too much. Any Ideas of what I can do to fix it so I won’t have to use the fuel shut-off valve method?
 

TexasDav

New Member
Aug 19, 2008
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Houston
The two things I would check, and are common, Is an air leak(spray carb cleaner around the intake and see if rpm changes) and be sure your motor mounts have not loosen any(causing your fuel to foam at high rpm) Good luck
 

Easy Rider

Santa Cruz Scooter Works
Jan 15, 2008
2,144
4
38
Nor*Cal
You might want to clean out your carb. Remove the needle and blow it out with carb cleaner (maybe it's not closing) and also check your jet. I had the same problem and I found out the jet was semi clogged.
Or it could be a common dreaded air leak.
 

Airbuzz

New Member
Aug 16, 2008
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But Why Would Starving It Of Fuel By Nearly Closing Off The Fuel Pet-cock Valve Make The Engine Run Fine ?
 

Easy Rider

Santa Cruz Scooter Works
Jan 15, 2008
2,144
4
38
Nor*Cal
I think because you need to close the petcock, its getting too much fuel. That's why I think your needle is not closing and its getting too much fuel. I recommend checking your carb again to make sure its closing. What you can do is take the bowl off and push the float up. If fuel keeps coming out, the needle isn't seating.
 

TexasDav

New Member
Aug 19, 2008
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Houston
Well if it was foaming of the gas then keeping the bowl empty would have no gas to foam, but that is just a guess. I just know mine lost power at was once there and I fixed the air leak then when my motor mount got just bearly loose I lost power again, thighten it and fixed the problem and my logic was all wrong with my thoughts of the carb being the problem. If your jets are the same and your float is working back the way it was before you replaced it, then it must be somewhere else then the carb it's self, I would think. These little motors are tricky even if they are simple. Best of luck and let us know the fix.
 

Airbuzz

New Member
Aug 16, 2008
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I'll perform all of your suggestions tonight and report back.

I'm intrigued most by how the fuel level in the bowl would cause my top speed bogging. I can't understand why this would have an effect (admittedly I don't know very much about carbs) but I did notice some fuel showing up on top of the engine possibly spilling out thru the airfilter housing. Maybe at full throttle when the bowl fuel level is lowered to a certain point, the release of fuel from the float fuel metering pin goes from a little stream to a flood, then the bowl is over full, and it spills out the air filter housing, the fuel filter being saturated increases the fuel-air mixture and also restricts airflow...hence the bogging. Huh...maybe this is it. I'll check it out.
 

Finfan

New Member
Aug 29, 2008
871
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Tucson, AZ USA
You might also check your air filter. I had a similar experience once when I forgot to close the choke and the engine wasn't getting enough air.
 

jasonh

New Member
Jun 23, 2008
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Longmont, CO
Once the motor is warmed up, turn the choke fully on. If the motor doesn't die, you probably have an air leak.

Just thought I'd throw my $0.02 as a side note...these are not for kids. A kid that's good at riding a dirt bike, maybe, but not for most. Especially where there are cars. Most states won't let kids ride these anyway.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
235
63
up north now
Alrighty then....you say it's too rich.

Clean your air filter, check the float level, and also make sure the float is not sinking- like if the new one cracked or developed a hole.

I would also change the plug, not clean it, change it.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
235
63
up north now
One more thing- double check the float shut off, it may have some crud in there and it's not shutting off all the way and causing the rich condition.

If you have to shut off the fuel to get it to run right, it should be one of those things.
 

Airbuzz

New Member
Aug 16, 2008
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Looks like I solved the problem, and of course thank you for all of your suggestions which were indispensable in helping me figure this out.

The carb was simply flooding the motor. The solution was to bend the arms of the needle valve assembly in the downward direction. I bent the arms down a significant amount. It was a bit of trial and error, allowing the arms to drop down and watching when fuel would release. I test ran the bike a few times with the float bowl re-installed, finally getting it to run well. It does 30 mph + without hesitation. See the first and second pics. Also of note is that the bike has probably been running very rich from the get-go. After every run, an oily fuel would build up in the muffler. Then it would leak out on the floor. This was probably un-burnt fuel that had been washing the cylinder wall down. Bad for wear!

All gone now! And thanks again ! Merry Christmas.

Jasonh thanks about your concern for my Boy. I'm watching him.

Also, see my next build, Pic #3. It'll be black, antique white, and orange. The frame is a little stretched.
 

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Cabinfever1977

New Member
Mar 23, 2009
2,290
1
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Upstate,NY
no bogging down here,my bike is idleing too high,
48cc dual start starfire II grubee kit with 48t
26"mountain bike with knobby tires
i think i need better brakes
 
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ibcnya

New Member
Oct 29, 2008
220
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0
Concord NC
But Why Would Starving It Of Fuel By Nearly Closing Off The Fuel Pet-cock Valve Make The Engine Run Fine ?
Same issue im having, was bad before, but worse now that ive got a SBP fuel valve. When I close the valve the engine runs fine until the bowl empties, then i have to open it back up.

I think because you need to close the petcock, its getting too much fuel. That's why I think your needle is not closing and its getting too much fuel. I recommend checking your carb again to make sure its closing. What you can do is take the bowl off and push the float up. If fuel keeps coming out, the needle isn't seating.
I was told this early today....

One more thing- double check the float shut off, it may have some crud in there and it's not shutting off all the way and causing the rich condition.

If you have to shut off the fuel to get it to run right, it should be one of those things.
My bike has this studder around 30mph, at that point it will begin to slow down as well....to much fuel in the bowl, huh? So like the OP said this was fixed by controlling the amount of fuel allowed into the bowl. By bendind the tabs on the float control this causes the needle to stay seated longer, and the float to go lower before opening the needle up to let fuel into the bowl. This makes alot of sense, seein how the tabs are basically straight from the manufactuer(well mine are anyways)...this allows to much fuel into the bowl because the needle opens as soon as the fuel starts to drop. If I bend the tabs where the needle will stay closed longer then this may help me out. Good thread....hope this works. Ive done so much to remedy this annoyance.

My engine definatley runs better with the fuel valve off though, so maybe slowing the fuel flow to the bowl is what i need to do.
 

Ghost0

New Member
Mar 7, 2008
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Bellingham, WA
If it still runs better when closing the valve and your float is properly adjusted then in my opinion you need a smaller jet. You can first try to lower the needle to its lowest setting by raising the clip to the top notch on the needle. Most say this only effects low to mid but at least on mine when the needle is all the way down it never comes completely out of the jet tube so it will lean it out a bit on top end.