Symptoms of a near seize event or just fuel starvation?
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Enthusiast
Vespa PK50XL2
Joined: 30 Mar 2020
Posts: 61
Location: Norway
Mon Sep 14, 2020 11:22 am quote
While driving as fast as my 75cc PK can go it suddenly slowed (cut out) down as it was running out of fuel. It did not stop, just went down to around idle. There was no grabbing at the rear wheel. It just revved down for a couple of seconds and lost power/speed. Pulled the clutch just in case.
Drove to the side of the road on just about idle. Turned it off and checked for a kink in the fuel line (Fuel line is as short as I can get away with). All good. Started it up again on first kick and it behaved perfect for the rest of the ride. Even when running it hard.

What could be causing this?

(Tank has fresh gas. Plug looks perfect (more towards rich than lean) and throttle response is great. No hanging revs or high idle. Idle is nice and stable)

NEX_20200912_132745.jpg
This is the plug after 30 km. Does not look to lean to me.

Addicted
2010 S50 Modified "Punkin"; 2003 ET4 150 "ISO"
Joined: 18 Feb 2017
Posts: 534
Location: Deepinnaharta, Texas
Mon Sep 14, 2020 11:49 am quote
Greetings:

I have had a few siezes with restarts. Every time the rear tire locked up solid, leaving a looonng skidmark on the pavement and a short one in my underpants. I'm thinking random electronic malfunction or a leaky vacuum hose causing the petcock to close and fuel level in the carb bowl to go too low.
Enthusiast
Vespa PK50XL2
Joined: 30 Mar 2020
Posts: 61
Location: Norway
Mon Sep 14, 2020 12:16 pm quote
There is no vacuum hose on the PK, but it might be a fuel issue. As the plug looks fine and there was no lock up I also think it is a issue with fuel or electronics. I just want to try to get to the bottom of this problem so I can rectify it

I think my jetting is on the generous side for the 16.15F carb with a standard exhaust w/sip "performance manifold stub" and a Polini 75cc.
I'm currently running a 72 main. 73 four stroked and 71 was a bit to lean.
Addicted
2010 S50 Modified "Punkin"; 2003 ET4 150 "ISO"
Joined: 18 Feb 2017
Posts: 534
Location: Deepinnaharta, Texas
Mon Sep 14, 2020 1:21 pm quote
Greetings:
Quote:
There is no vacuum hose on the PK
Is it a manual fuel tap then? Hope it's fuel. Intermittent electrical problems are a PITA.
Enthusiast
Vespa PK50XL2
Joined: 30 Mar 2020
Posts: 61
Location: Norway
Mon Sep 14, 2020 10:13 pm quote
Yes the PK's have a manual fuel tap. It might get clogged by crud in the tank. The carb is brand new so everything should be good there. Same with the jets.
I'm anyway unsure of this symptom and would like to get some insight in what could be causing it.
Addicted
2010 S50 Modified "Punkin"; 2003 ET4 150 "ISO"
Joined: 18 Feb 2017
Posts: 534
Location: Deepinnaharta, Texas
Tue Sep 15, 2020 8:03 am quote
Greetings:

Just a Hunch: Check the air filter. I've had this happen with a 9 year-old foam Honda filter crumbling. A chunk wedged in a reed valve and held it open. It restarted soon after it stalled. Also if the fuel lines are old, detached rubber-crud could get in the float valve inlet and prevent fuel flow.

I'm about to go through a fuel system evaluation and fuel line replacement on my ET4. Not the same symptoms - a rough "Handoff" from Idle to Midrange, but I suspect 17 year-old tank screen and lines might be the issue.
Enthusiast
Vespa PK50XL2
Joined: 30 Mar 2020
Posts: 61
Location: Norway
Tue Sep 15, 2020 8:38 am quote
Air filter on PK is a steel mesh insert that is easy to clean and "never" wears out. The filter and airbox was replaced last year and have been cleaned when I put a new carb on it
If the fuel system is to blame I suspect it is sediments in the tank or fuel tap.

Good luck with the fuel system evaluation. Keeping these scooters running is half the fun
Ossessionato
73 & 74 Rally, 76 ET3, 80 P200, 06 PX150, 61 Ser 2, 65 Silver Special, 86 & 96 Elite 80s, 2015 HD Road Glide Special, 2011 Ural Tourist
Joined: 18 Jan 2012
Posts: 3750
Location: Oceanside, CA
Tue Sep 15, 2020 9:36 am quote
Is the top end new? You definitely want to baby it during break in.

Wonít know if you seized it until you pull the head/get a small camera down there to look.

Is the jetting stock? Stock jetting sadly isnít enough to run wide open for an extended period. Good enough for around town but no highways IMHO.
Enthusiast
Vespa PK50XL2
Joined: 30 Mar 2020
Posts: 61
Location: Norway
Tue Sep 15, 2020 9:55 am quote
Top end is a couple of years old. Broken in and have not given me any problems. Jetting is far from stock since it has a 75cc Polini cylinder on it. I was trying a 73 main and it four stroked like mad. Went down to a 71 and it was fine, but went up to 72 just to be safe. Polini suggest a main between 68 to 70, but I feel this is way too lean.
The only other unoriginal part on the scooter is a SIP "performance manifold stub". A steel cylinder to exhaust stub that is supposed to have better airflow. Bought mostly because it's better quality than the stock aluminum one. Was worried about exhaust leak at the manifold, but the area is nice and dry/clean.

I have checked for air leaks around the inlet and the carb. All seems good. It drops revs if I spray a lot of carb cleaner around the carb manifold, but I suspect that has to do with it getting sucked into the airfilter that is 1 cm from the manifold.

It still fourstrokes if it hits high revs going downhill. Also four strokes if I pull the clutch. Starts at first kick. Keeps a steady idle and revs down to idle when i let of the throttle. Things I hope suggests that the jetting is ok?

I'm a newbie when it comes to classic 2-stroke scooters and I try to learn as much as possible, which is also why I would like to hear all possible causes to this "problem"
Enthusiast
Vespa PK50XL2
Joined: 30 Mar 2020
Posts: 61
Location: Norway
Wed Sep 16, 2020 2:25 am quote
I think I might try to jet up on size just to be sure everything is cold enough. I rather have less RPM than risking some breakdown at WOT

I am also unsure what the "performance exhaust manifold" needs when it comes to jetting (if anything, my old one was not restricted but have a 90 degree bend and not a smooth curve like the new one).
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