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no power and very low top speed HELP?

new exhaust, new carb, bigger jet, malossi 70cc kit,

all fitted with no problems.

It starts up and idles fine, rev up ok on stand. when i take it out on road there is no power and at top revs in goes peaks and drops, with a weeeeee urrrrrrrr, weeeeee urrrrrr.

this happens at 3/4 to full throttle.

i am going to check the head is on tight but can anyone throw info at me help me figure what is wrong.

no change if remove air filter

Piaggio Vespa ET2
Malossi 70cc cylinder kit (cast iron)
Stage6 pro exhaust
Malossi Carb kit with 19mm Dellorto Carburetor
98 Main Jet

what causes loss of power and no top speed?

Your engine is a 2 stroke,,and as such it operates according to VERY strict,,Generic Characteristics.

If it Starts and Idles OK,,,that absolutely eliminates any possibility broken rings,reed valves,,,and a bunch of other Mechanical stuff.

It is NOT missing,popping,sputtering at Mid/Hi speed.
Scratch Ignition Probs off the list.

It's SURGING.
The "weeeeeeURrrrrrrWeeeeeeURRRrrrrrr" is NOT LEAN as in Too Small a Main Jet.
And a 98 Jet May NOT ultimately be the Correct Jet,,,
But it should be Very Close.

Such an extreme surge is Fuel Delivery deficiency.
Float set too low,,,crap in Fuel Line,,or Filter.

It's Fuel Starvation,,literally Running Out Of Gas.

Because it continues to Run,,,,
That indicates restricted Fuel Flow.
Under hi consumption Rate,,,it uses all the available fuel.

That's the URRrrrrrrr.

Then More Trickles in as engine slows down during Urrrrr phase.
Enough to Re-Supply the carb,,,and initiate the Weeeeee art of the cycle.
Engine "re-starts" on the suddenly available fuel and Revs Up.

Only to burn off the small amount of available fuel which trickled in,,,,,,and begin Next Round of the Urrrrrrrr>Dead Motor,,Outa Gas.

.......................................................
To Fix it:

First thing to do is try putting it On CHOKE at High Speed,,,while it's Surging.

If it RUNS,,,,it'll run like crap from Surely being way too Rich on Choke.
It will sputter/stutter/burble.

A)
If it Continues to RUN like that and the WeeeUrrrrr is GONE,,,HOWEVER BAD it Runs.....
Then That tells You Main Jet is EXTREMELY lean/Small.
Reason>>Engine's Fuel Mixture shortcoming is being augmented by additional fuel from Choke Circuit.

Thus it CANNOT be a Fuel Supply Prob,,,,,Choking it would make That worse cuz it sucks LOTS more Fuel,,Lots faster.

B)
If Choking it at Hi RPM makes the Weeee Urrrr WORSE...it will show itself Worse by Shorter time between Wee & Urrr.
Cuz it's sucking the Fuel Much Faster.
It'll Rev a Bit More,,,maybe.
But the URRR will most likely get Very Bad,,,into a Flat Dead BLAH.

..............................................................
Fuel Delivery has 2 aspects to verify.

#1) Flow Rate
#2) Fuel Level in Float Bowl

Each of those symptoms can masquerade as the Other.

FLOW RATE:
There should be a Drain Screw on the bottom of the Carb.
But I'm not certain about that,,,maybe there's none??

IF there is,,,open it and catch the fuel in some kind of convenient container.
Engine Burns about 1oz per minute at 60~70 mpg at 30MPH
FreeFlow Fuel should be TWICE that approx.

Measure the TIME,,,20~30 seconds should flow at least 1 oz of fuel.
If it dont Drain Out at least 1 oz in 30 Seconds,,,there's a restriction somewhere.

REPEAT the test using end of Fuel Line.
Remove Fuel Line from carb,,,and TIME/Measure how much comes outa fuel line.

It should be MORE than from the carb Drain.
But at LEAST 1 ounce per 20~30 seconds.

*** I know most folks dont have a container to accurately meausure 1 Oz.
Use anything Clean,,,so ya can pour the gas back into your tank.
And measure a 1/4 Cup of Gas,,,that's 2oz
or a 1/2 Cup=4 Oz
Full Cup =8 oz

As Long as you're gettin at least 1 Oz per 20~30 seconds
1/4 Cup= Less than 60 seconds
1/2 Cup= less than 2 minutes
and so on

*Fuel Filter
*Tank Vent
*Fuel Valve clogged
*Fuel Line Kinked,Clogged
*Carb Bowl Vent
*Trash in Carb Needle/Seat

#2) Float Level...
Just Cuz it's a New Carb you cannot assume Float Bowl's Fuel Level is set Correctly,,,or even assembled correctly.

Common Suspect on Wrong Fuel on Del's and Most Euro Carbs is the way the Float's Lever bears on the Fuel Needle.

Most Jap Fuel Needles are just a Staight Pin.
Their float Lever simply Presses on it to Close it as fuel Rises.
When fuel & Float Drop,,,only GRAVITY drops the Fuel Valve Needle to allow more fuel in.

On Dellorto's,,,,MOST have a "NAIL HEAD" Float Needle.
And the float LEVER is a FORK which slips Under that Nail Head.
So the Float's action does both,,Push Valve Up & Closed,,,
AND PULL DOWN Physically to Open the valve.

LOTS of those type carbs get assembled "Jap Style"
Meaning,,,rather than being 'clipped" UNDER the NailHeaded valve needle Correctly,,,
They are installed ON TOP of the Nail Head.

The effect of that is that the Needle is held Almost CLOSED even when Fuel Bowl in EMPTY.
That does 2 Bad Things to Fuel Delivery.

1)> Fuel Only TRICKLES into Carb because Needle is being held MUCH Higher,,almost Closed
2)> Any slow,small Flow which Does enter the bowl to raise the Fuel Level,,, Gets Shut Off as soon as the Float starts to Rise much at all.
So the BOWL is never really Filled before being shut off.

The Engine runs on little more than FUMES at Hi Speed.
No Reserve Capacity in the Bowl.
And the Refill Rate of the tiny puddle is Extremely SLOW,,,,cuz the needle vavle is Nearly Closed even when it's "wide open"

If the tab on the Float is Slotted,,it Goes Under the nailheaded Float Needle.

I think(?) even most of those needles are Spring Loaded.
Make sure the thing is free and Not sticky.
Wouldnt cause fuel starvation though even if it was "stuck".

............................
Actual Fuel Level setting,,,,
I do NOT Know specifically.
I dont know exactly which carb you have either.
PHBG?
Whatever....
A GENERIC "close enough" setting thats almost universal is "LEVEL".
The Bracket on the Float,,aka FLOAT ARM should be about Parallel with Gasket Surface on Carb Body,,,when holding carb upside down and Needle supporting Float's weight.

Almost anything will at least RUN OK set like that.

Another Rough Guestimate---NOT RECOMMENDED.....
( Cuz of Hazards of fooling with gasoline)
Hold Bowl in place on carb,,,but not screwed on.
Let Carb fill with fuel,
Carefully move bowl away from carb and see how much Fuel is in bowl.
It Should be about 1/2 to 3/4 Full,
If it's less than Half Full,,,thats PROBABLY NOT enough.

But thats very subjective.
It's best to find and use Specifics ,,especially when trouble shooting.

the rules-of-thumb about Fuel Flow & Level will at least give you a clue that it may be Way outa Spec.

................................
A New carb is RARE to have any fault,,,
but I've seen enough with packing trash in them,,
Bits of metal from machining,etc.
Almost Never,,but it happens.
Always worth a quick check.

Check your Float Installation/Fuel Level,,and fuel Flow rate into carb.

........................
An Air Leak,,,,like Cyl Base gasket,,or Intake Manifold Leak,,,etc
is gonna act TONS worse at Idle while under Closed Throttle>Hi Vacuum condition.
The effect is Usually MINIMIZED at High Speed.

A Bad Air Leak can and DOES lean out fuel mix enough to cause Seizures,Holed Pistons,etcs at Hi Speed.

But they'll 99% NEVER run Right at LOW SPEEDS.
So only the High Speed half of your symptoms would possibly be related to an air leak.
While the Good Low Speed running all but eliminates the possibility.

Fuel Starvation is what you're dealing with.
It Could be something else,,,but 85% chance or better there's a Fuel Feed restriction somewhere.

According to the sypmtoms you've described,,,
that's where I'd start looking.

Good luck with it,,,oughta be a blast when you get it sorted out

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