Scooter Loses power after 2 miles - bogs down badly By bigndum (Read 807 times)

[bigndum]

Cant figure this one out yet....

49cc TaoTao 4 stroke scooter.  Less than 2000 miles on engine.

Scooter starts fine, runs fine -  for 2 miles.  Almost every time I go 2 miles it abruptly loses power...and or fades until it shuts off.  If I stop for a few minutes it starts again.  This has been going on for months.  Does not seem to have lost any power due to engine damage.  Up to the two mile mark I can run up to a GPS verified 40mph.  Same as two months ago.

Here's what I've tried so far....
New gas filter
New Coil and Spark Plug lead
New Spark Plug
Drained the tank and added fresh name brand Premium fuel.
Checked to be sure fuel could flow freely from the tank.
Checked the gas cap vent (not the problem)
New CDI
New Vacuum fuel controller valve
No fuel filter at all
Mixture adjustment
Checked ALL vacuum and fuel hoses and they are either almost new or in great shape. (no soft spots or kinks)
Larger main jet (started at 75...tried a 78, 80 & 85)  Did not help -but lost top end on the 85 main
Raised the float bowl level slightly (bent tab just a hair away from bowl)
Compression is 130
Valves are set at .003 Inches (verified a few times)
Tires are properly inflated

The consistency makes me think it's either a fuel delivery problem or an overheating issue.
But everything in the fuel system seems perfect.
If it was an overheating issue I would think I would have damaged the engine by now and lost some compression.

What's left?
I'm wondering if this could be a problem with a valve guide or valve seat.
How about a sticking needle on the float bowl?

Bag of tricks is empty....HELP !

[cyborg]

When it happens again see if there is a vacuum created in the tank,,,turn the cap,,,if you hear a suction you have a fuel tank venting problem,, if it is that just drill a tiny hole in the cap and problem solved

[cyborg]

That is a very common and maddening problem,,, also it may be a vacuum loss at full throttle and the vacuum fuel tap is closing ( I had that with a Vespa I had),,,,

[bigndum]

It's not the fuel cap. I tried stuffing a rag in it so it could breath...same thing.
But, the loss of vacuum at full throttle...now that is something to check. Thanks

[cyborg]

It's easy enough to check,,, get it running at idle and put a set of hemostats on the vac line,, then take her out for a run

[cyborg]

If you're anything like me,,, I'm used to big bore twins ( 900 cc + ducatis and bmws) and four cylinders ,, I'm always pinned and that's where my Vespa had the problem,,, very similar symptoms to yours ,,, runs great two miles down the road peters out and died,,, found the problem by switching to clear motocross bike hose ,,, I got it on idle clamped the line and the problem went away,,, that was a ball buster

[cyborg]

also i didn't see it in your posts,,,when you pulled your carb bowl and adjusted the float did it feel like it had fuel in it ? it may have a crack and is leaking and sinking,,,not doing it's job,,,it's a long shot but it has happened

[jct842]

yup.......sounds like it may be running out of gas. A fuel pump also can cause this. Warms up, something expands, now there is a vacuum leak and no fuel thru pump or petcock.

[bigndum]

Cyborg,
I think you hit the nail on the head.

I ran a line direct from the tank to the carb and it hasn't shut off since.

Owe ya a beer my man ! 

[cyborg]

Great news!!!!!!!! Glad you figured it out!!! Damn I hate those Fukin things,,, I'm all mechanical on my bikes

[cyborg]

I appreciate the beer,,, gimme a piston and I'm happy,,, thank you

[kyvitti84]

I have a tank urban racer 150cc and when it started getting warmer this year my bike would lose power and I could walk faster than it would go. I looked and looked on the internet for the cause. in the mornings it would ride fine 35-45 degress F. in the afternoon I would lose power after 5 mins riding 55-65 degree F. changed moter oil all my problems went away. it was something I should've done but was lazy. learned a lot from it. hope this helps somebody.