Modern Vespa

Hi Guys

Need your help in trying to find the problem to the following;

Just re-built my Rally 200 engine and driving around the suburbs on short 10 min trips the engine runs great and accelerates hard. I therefore decided it was time for a test run on the open road.

1) As soon as I reached 80Km/h the bike started to jerk as if it was running out of fuel. I swung the tap over to reserve but the jerking continued.
2) The jerking lasts for a few seconds and then the engine pulls for say 5 to 10 seconds and then jerks again.
3) Engine sometimes died, bike coasted for a say 5 to 10 min and then engine starts up again
4) Initially, keeping the speed under 80km/hr the jerking disappeared. However towards the end of the ride the jerking got worse and started at around 50km/hr
5) On 2 occasions during the ride when the bike starting jerking I pulled the clutch and the engine revved extremely high with the throttle closed
6) When I got back home I drained the gearbox oil to check for fuel in the oil. I could not smell any fuel but the oil was extremely black. Scoot has only done about 50km since last oil change.
7) Popped headlight low beam bulb and brake light on ride
8.) Recently converted to 12V Ducati ignition
9) Stock cylinder
10) Carb 24/24E, running a 118 main jet. Did a plug chop in the suburbs and think I am running slightly rich, dark brown color on plug and oil splats from exhaust on rear wheel
11) Fuel tap is a SIP fast flow tap, I was planning on kitting the bike and had to replace fuel tap recently and therefore went for the fast flow.
12) On the short trips round the suburbs the bike really ran great; I am thinking that the problem surfaces when the engine gets hot. I will take the scoot out in the morning with a cold engine. Almost certain that I did 80km/hr in suburbs with no jerking.

Thanks

check your main jet, and make sure your choke is actually closing all the way.

Be sure no water in the fuel as well.

In addition to the above... Have you checked your timing with a gun to be spot on? and have you checked for any air leaks? (you can do a quick and easy check around your cylinder base gasket and head w/ some WD40 or starter fluid... give a spray directly at the seal area and see if it revs or dies. (same would go for carb to case connection, but don't spray into the carb)(use the little straw to spray)

I've had oil get dark pretty quickly.. I haven't changed it after just 50k, but I have after about 100 miles and it was dark.

I recently changed from a 120 to 118 as my plug was black from a plug chop. I think I should try with the 120 to see if the jerking goes away

I am almost certain that the choke is closing all the way

timing is spot on, checked with a gun I have checked for leaks between carb box and engine. If I have a leak I would suspect the oil seal on the clutch side. so maybe the clutch side oil seal if fine then

As a basic guide. Jerking/missing with a backfire is timing and or air leak.

Jerking/missing without a backfire is fuel shortage.

So i would check fuel filter in carb top, and a flow test of petrol.

Which Ducati conversion did you do, the crank/stator/CDI or the electrical mods to use the Femsatronic stator? If you did #1 try exchanging a known good CDI. If #2 try the Femsatronic CDI instead of the Ducati.

Another remote possibility could be a plugged vent on the fuel tank, but that's easy to check. If you have a stock cylinder and exhaust the 118 main should be pretty close with a BE3 mixer for jetting.

Recently had a scooter in my shop where the owner thought it was seizing up repeatedly as he drove, but turned out it was popping in and out of gear.

im gonna put all my chips on the table and say condenser. you can wire in a new one pretty easy and they are cheap too. keep us updated as you try different ideas. good luck.

dang. stupid E.I.

so I dont win any cash or prizes? how about a green ribbon for participating?

Ok took the scoot out this morning and same problem with a cold engine. I am moving away from the idea of a damaged seal that starts leaking bad when engine gets hot and I am gonna concentrate and fuel starvation.

If it's the same with a FULL tank & your cap vent is clear(AND no loose crap or sludge in the bottom of gas tank), I'm still on the same page as JJ, but thinking possible dicey CDI unit. Is it new, good brand? Do you have access to another one to swap in for testing? You could try this if not:


Gap real tite & leave your cowl home. You'll know if you still have spark when it dies.

To eliminate the possibility of an air lock, this morning I ran with the fuel cap wedge opened and same thing happened. Tank is nice and clean and there is no sludge Ducati CDI is new, it says 'Made in Italy' so it has to be a good brand (My name is Luigi) Dont have another CDI but will try the V oodoo external plug trick to see if spark is dying on me, good suggestion thanks

fuel line too long or routed funny? kinks?

fuel line length is right, i can just manage to turn tank 90 degrees and rest it on frame when tank is out

doubt whether i have a kink as i have installed one of these

What type of fuel line are you using?

If it is a soft rubber with a small ID than you might need to swap to proper harder type with an ID that does not restrict.

If your motor is kitted, or thirsty like a rally, at open throttle than you want to be sure to have the proper fuel line with no restrictions.

It does not matter if you installed a fast flow or not, if the hose ID gets smaller than the connections you will have some restriction.

Just an opinion.

I would completely replace the fuel with fuel from a different gas station as clean-slate starting point. Most of the hair-pulling issues like this one I've had on two-strokes have been from bad gas (water in fuel sits at the bottom of a carb and will collect and get in the way of the jet with engine vibrations)

If that doesn't do it, then I'd try what you mentioned above (go to a larger jet size) then work backwards. Though you mentioned going to a 120 from a 118, but I'd even go a little bit higher if you really want to get to the bottom of this - maybe a 122-125.

As someone else mentioned, check your fuel filters (carb and tank) and all the other spots other have mentioned.

Good luck.

hi guys

a quick update on this problem I had a few weeks back

turned out to be a twisted fuel hose

the 90 degree steel elbow is obviously not fool proof

Thanks for the update!

Rally's are notorious for pinched fuel lines between the oil tank and frame...

Where did you get the barbed angle hose? I like that!

Fitting is from SIP

They advertise by saying no more pinched fuel hoses......whatever

I had this problem and found out it was fuel starvation at high revs,Try changing the mixer tube,Changed the CDI to a good quality ducati one and upjetted to a 120 main jet and it was all good after that.
I've had cheap cdi's and they are shite.I've even had one melt before,
Hope this helps.