Modern Vespa

Howdy! I just picked up a 50 special this past weekend as my first small frame. It's the funky one with 9" wheels and it's got a V9A1M engine stuffed in it with a Dellorto 16/10 carburetor.

Problem is it's 4-stroking whenever I give it any throttle and doesn't clear up at any throttle position. Plug is black and oily. I've sorted through the usual stuff: cleaned the carburetor, flushed the fuel tank, new plug, filed points and set gap, reset timing and checked with a strobe. It has a nice blue spark and starts up second or third kick. It had a bad fuel tap and a worn float needle, both of those have been replaced. Carb has the stock 63 main and 50 idle jet in it.

My thoughts so far: 1. It runs well, almost as it should, with the air filter off. I noticed when rebuilding the carb that the gasket between the carb and filter doesn't QUITE match up with the holes on the carb, so maybe I need to make a new gasket? 2. It has the old school two-coil stator with one coil being the HT coil with the spark plug wire running from the it out the engine case. I thought I read somewhere that these are generally considered kind of crappy and should be replaced with something more modern, please correct me if I'm wrong. 3. And finally, a 16/10 carb for a 90cc engine seems small. The 19/19 came stock with the V9A type engine, did it not?

Apologies for the long post, just wanted to be thorough! And as always, thanks for any and all help!

The 19mm Dellorto is stock on the Primavera/ET3 motors. You should have a 16/16 Dellorto with your 90 engine. If you have a 50 idle jet, that is too large, the idle jet is located on the outside of the carb. Check that the choke is not stuck open.

What is your mixture screw set at? It should be around 1.5 - 2 turns out from fully seated. This screw will control fuel mostly at idle but does affect the rest of the carb's range at every throttle position. Points ignition should be no problem but yea CDI is nice since you don't have to deal with any mechanical adjustments. I have read that a failing condensor can lead to a boggy feeling when riding, I'm not sure if this symptom will foul the plug.

Look at the: V90 specs.

Good luck and congrats on the smallie!

Hey Mr. J! Thanks! Good to know on the stock carb size. 50 IS a little high for the idle, isn't it? =P That's because I got my jets mixed up. Idle jet is the one on the outside, correct? That one is a 38. The 50 must be the starter jet? Forgot to mention that I've got the mixture screw set to 1.5 turns right now. I've gone as far as 3 turns out and .5 a turn out with mostly the same results. I'm going to try and make a new gasket for the air box this weekend and see if that helps any.

Ha, yup 38 on the idle and 50 on the starter, check.

Are you using NGK B7HS spark plug? I edited my previous post and failed to mention about a failing condensor or even the HT coil.

condensers

How about a pic?

I'm running a Dellorto SHB 16.16 on my 50s. The dellorto uk website lists it as having 74 main and 42 idle, which is what mine has. I think Scooterwest sells it with those jets as well.

Interesting that the spec for the V90 SHB 16.16 is 63 main and 38 idle......

take some pics of the engine and around the top end for us to see condition of the engine...

do this:
- unhook the choke wire from the knob to make sure that the choke plunger is closed... go for a ride and get engine up to temp. did that help...??
- pull out the choke plunger and make sure it's rubber seal is not ridged which hinders its ability to close fuel flow = if not replaced recently then put that on your to do list
- replace spark plug cap since as these go bad you get plug fouling and inconsistent spark = fouled plugs and poor engine running

I had a plug can years ago that did what is happening to you. Spent 3.99 and 10 mins later I was back on the road and happy. The spark plug caps that keep the "top hat" on the spark plugs are the best to use.

http://www.motorcycle-superstore.com/34125/i/ngk-spark-plug-resistor-cover?device=c&matchtype=&network=g&gclid=CIjfgKmtxr0CFU1bfgod2xcAUA

reading:
http://website.lineone.net/~smallframes/carb.htm

Hey spiderwebb, funny you should mention the choke cable. The first couple of runs around the block it ran a lot worse than it does now. Turns out the choke cable nipple got caught on the seat where it goes into the carburetor which was holding the choke open. Fixed that and it ran better, but still 4-stroking. I'll check out the rubber seal and make sure it's in good condition. Also good call on the plug cap, I'll replace that and the wire this weekend. Here's some pics of the engine and the bike:

erratic sparking feels like 4 stroking
unhook the choke cable all the sway and then go for ride

Q
- does it 4 stroke all the way up the throttle range = i.e. from idle to WOT...??


one of my scoots has a issue like this that comes and goes depending on the day and how bumpy the roads are... the float gets stuck open and the carb runs rich due to too much fuel in the float bowl.

this could be what is happening to you:
- when you rebuilt the carb did you replace the float needle, the float and the little seals...??

and how are the points...??
new or old

that could be "it"

Are you riding that without a cylinder shroud and cooling fan shroud?

If you are, you need to remedy that because it might contribute to a seize.

Spiderwebb: It does 4 stroke from idle to WOT. I replaced the float needle and checked that the float actually floats in a bowl of gas. Not sure on the points. They look relatively new, I sanded them a little bit for good measure.

Astromags: I haven't ridden without the shrouds. I took them off to fiddle with timing and so I could pull the head off and check to make sure the piston wasn't in backwards.

sanding points only helps for a week
install new points asap
they are cheap

if you do it "right" you can reuse the wire that is between the points and the condenser... that way you don't have to solder the new wire to the condenser that comes with the new part.

if you do try to solder this wire onto the condenser do make sure you have a nicer soldering gun that get a bit hotter than the cheap stuff. If you do this quick like then the condenser won't get fried.

happened to me

ps - pop off the flywheel and see if this wire is frayed and shorting out on the inside of the flywheel. This also happened to me and causes a grounding issue.

good luck

Hey spider,

I replaced the spark plug wire and cap and got a super strong and bright spark going. Eliminated the four-stroking at idle to half-ish throttle. Still four-stroking at WOT. I think this may be due to the main jet being a 68 instead of a 63 as I thought I read the first time. I've ordered a 63 and 65 to try out and I'll add points to my list.

Thanks for all your help!

BGM makes a nice jet set for lambrettas and vespas

check Jet200.com
look in the vespa section
or call them and ask
you need to make sure you get the set that is the right mm

you get 10 jets for around 15 bucks
if you buy one jet at a time from most shops they are around 4.99 each =

do make sure you choke is not adjusted a bit off and is ON when it should be OFF

good job

ps - you can remove 1/23 of the filter metal stuff that is within the stock air flyer box and than could make it happy... or clean it well / drill a few small holes in the cover so a bit more air can get to carb (not too much...!!).

If no luck at Jet200, try treatland and ask for Benji (great service and quick shipping).

Dellorto Jets.

If I'm not mistaken, you need 5mm jets for your main on SH style carb.

Alright, an update.

New carb (splurged for the 19/19)
New condenser and points

Bike starts inconsistently. Gas is reaching cylinder, timing is correct (static), getting a good spark, although sometimes it stops sparking all together for short periods of time. When it does start it revs quickly and cleanly, no four-stroking. I disconnected the stator wires and this seemed to increase the chance of it starting. At idle, it will run for a few minutes with maybe a misfire or two and then stop dead unless I twist the throttle to keep it going.

My last attempt at starting lasted an hour with no luck. I swapped in ANOTHER set of points and a condenser. Cleaned the carburetor several times. I had spark and fuel, but no amount of kicking would start it.

At this point I suspect maybe a new stator is in order? Or is there something I'm missing?

setting the gap of points can be tricky
if not perfect then engine won't run right
keep adjusting them and hopefully the spark shall become consistent

when I had points I had backfires due to a short on the wire that goes from the points to the condenser = was wearing / touching the inside of he flywheel.

stator wires do just get old and a rebuild might be in order

if there is a short the vibration of the engine might be just enough to cause a wire to make contact with the case = no spark

where the stator wires exit the case is where a short can happen & on the back of the stator.

spark
compression
fuel

sounds like a short

pop off the flywheel one more time and check out the wires coming out of the back of the stator, make sure none of them look old and frayed anywhere along there length to the junction box.

keep at it