HT bicycle engine goes fast, slow, fast, .....

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kallsop

Member
May 2, 2009
106
1
18
CT US
My sons 66cc/80cc HT bike was running fine. Today, at or near to full throttle, it runs for a second or two, the engine loses almost all power for a second or two, power comes back for a second or two, rinse and repeat. I replaced the cheesy stock plug cap with a nice NGK cap, thinking it might be loose, but that didn't help. I moved the carb needle down a notch from middle where it had been, up a notch from middle, no difference. Maybe a CDI about to go BP on me?
 

Fuji Moto

New Member
Jan 17, 2010
59
0
0
Canada
Re: HT goes fast, slow, fast, .....

mine was doing that once i switched to the cns carb and extended intake, so i put together a stock carb from someones old mangled parts, and used a windex bottle trigger spring for a slide spring, put that contraption on the extended intake made sure it was airtight now it runs fairly nice, but unfortunately I got a bad engine somehow with a ridiculous amount of play in the main crankshaft and the magnet is almost out of alignment with the mag, and on the other side the drive gear is riding on almost less than half of the clutch gear, but i have a nice bottom end that is tight for free and just needs a top end as for the cns carb I guess the only thing I have to do is somehow get smaller jets, or fill them. Im working on making a short ported custom intake, if that doesnt work, does anyone know how to get jets for the cns?
 

kallsop

Member
May 2, 2009
106
1
18
CT US
Re: HT goes fast, slow, fast, .....

I have a newer stock carb (the one with the black plastic filter housing with downward facing intake) and it's been running fine until today, so I think we are chasing different problems. At lower rpm's it runs normally. At WOT, on the flat, it does the power cycle thing. I didn't try WOT up a hill to put the engine under load and drop the rpms a bit, but I suspect that will run clean.
 

Mind_Reader7

New Member
May 1, 2010
392
1
0
Northam, Western Australia.
Re: HT goes fast, slow, fast, .....

Mines does this when there's not enough fuel going through the lines. Press the primer button 3-4 times and try. Or get it started, and then pull the clutch in and just rev it up and down a bit for about 20 secs, then pedal and release the clutch again.
 

city of angels

New Member
May 24, 2009
470
0
0
tucson az
Re: HT goes fast, slow, fast, .....

it sounds like you have a bad carb how did you install the slide after you took it out did it went down all the way very smooth it could also be your trotle cable spray some wd 40 and see if that loosens the cable a llittle
 

GearNut

Active Member
Aug 19, 2009
5,104
11
38
San Diego, Kaliforgnia
Re: HT goes fast, slow, fast, .....

To me that sounds like the classic symptoms of fuel flow starvation.
It could be one of the following:
1. Fuel valve screen inside the fuel tank is clogged.
2. Fuel valve is clogged or has too small of passages manufactured inside it. (cheap Chinese junk)
3. Fuel filter is clogged, if you installed an in line one. (It's a good idea to have one anyways.)
4. Pinched or clogged fuel line.
5. Crud stuck inside the fuel inlet on the carburetor. (above the float valve, not jamming the float valve open which would cause flooding which you are not experiencing)
6. Poorly venting gas cap on fuel tank.
7. The fuel tank is almost out of fuel.
 

Kevlarr

New Member
Jul 22, 2009
1,628
4
0
Mi
Re: HT goes fast, slow, fast, .....

Mines does this when there's not enough fuel going through the lines. Press the primer button 3-4 times and try. Or get it started, and then pull the clutch in and just rev it up and down a bit for about 20 secs, then pedal and release the clutch again.
That's not a primer button, all it does is push down on the float to over fill the bowl. Just holding it down for 3-5 seconds is enough. When I do that I don't even need to use my choke.
 

Venlaw

New Member
Apr 7, 2010
30
0
0
Canada
Re: HT goes fast, slow, fast, .....

I had the same problem. Ran perfect one night, next day was bogging at mid-high rpm for a bit and had perfect idle. Messed around with the clip settings, clean filter, everything. Replaced the carb, problem gone.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,837
252
63
up north now
Re: HT goes fast, slow, fast, .....

To me that sounds like the classic symptoms of fuel flow starvation.
It could be one of the following:
1. Fuel valve screen inside the fuel tank is clogged.
2. Fuel valve is clogged or has too small of passages manufactured inside it. (cheap Chinese junk)
3. Fuel filter is clogged, if you installed an in line one. (It's a good idea to have one anyways.)
4. Pinched or clogged fuel line.
5. Crud stuck inside the fuel inlet on the carburetor. (above the float valve, not jamming the float valve open which would cause flooding which you are not experiencing)
6. Poorly venting gas cap on fuel tank.
7. The fuel tank is almost out of fuel.
Good advice. Also, see if your main jet unscrewed.