Modern Vespa

Hi Guys

After searching and reading hundreds of replies to threads I have run out of things to check and now need your advise. My engine does not idle and stalls. It starts up fine and responds to throttle blips well. When I take the air cleaner off it seems to idle fine. I put the air cleaner back on, the engine bogs down and dies.

The engine is a stock Rally 200 engine. Only thing that varies is that the main jet is a 120 for some reason. I have ordered a 118 which will be arriving tomorrow.

I have tried the following

I have stripped the carb, soaked in carb cleaner overnight and re-assembled

New B8ES plug

New air filter

Idle speed screw is 2.5 out

Idle mix screw is 1.5 out. I have actually tried the idle screw at all positions, I do not see much change when adjusting this screw. When the carb was stripped I checked the screw and everything seamed fine. Can this simple mechanism stop working.

Replaced gaskets between engine / carb box and carb box / carb

I have done the carb cleaner spray check. No splutter when I spray between carb box / engine. Engine dies when I spray between carb / carb box. I have however read threads that this does not necessarily mean an air leak as the carb cleaner vapour is probably going down the open carb and killing the engine.

The engine is not installed yet, I am doing these tests on a 'test frame'. I would think that this shouldn't make a difference whether it was installed into the frame or not.

This problem is driving me mad!

Any suggestions??

If you are not seeing a change when you adjust the screw then there is a problem with the screw/hole.

On my first carb, the hole for the mix screw was cross threaded and therefore the screw wouldn't seat properly. I never could get the idle right.
I didn't notice it until i really inspected it. I could see from inside the venturi that the screw was crooked and enlarging the tiny mix hole.

You might have a similar problem.

1) air cleaner is plugged - unlikely.
2) Plug not gapped at .6mm
3) try a 7 plug.
4) running too rich. If you cut the air by adding the air filter and it stalls, then it isn't getting enough air for the fuel so that means it is going rich.
5) remove the idle mixture screw and blow so filtered high pressure air in there.
6) make sure the mixture screw has somewhat of a point on it, isn't bent, etc. someone on here had one where the point was flapping.
7) you are just placing the filter on and it is dieing? sounds like it is really on the borderline to begin with.
8) could be weak spark. I am starting to lean that way after typing the above.

tell us about that new air filter you bought. is it blue?

i had this problem with a p2. my prob was someone had fitted a race crank which has a longer induction as crank is cut, if this is the case you need a smaller pilot jet. cant remember what i went down too, but as you have said a 120 main jet in there i suspect there has been some tinkering, but if the crank is standard you can still lower the pilot jet size. a standard pilot jet is 55/160. check whats in it now

Compression?

inspected the screw and all seemed straight and fine, doubt that screw is cross threaded, I will make double sure tomorrow, thanks

1) agree, new filter
2) checked gap, sitting around 0.7mm
3) i have a bosch W5CC plug lying around and same problem. I think the W5CC is hotter than a 7 plug
4) agree, main jet is slightly bigger than stock and plug is wet
5)will try this tomorrow, thanks
6) mixture screw seems fine, previous reply also brought this up, I will post photos to confirm
7) yes
8) possible, I remember reading a thread and either Vader or SFvsr mentioned that the LML stators cause intermittent sparks. The stator I have is a Swiss stator. Remember reading that Swiss produces for LML, right?

thanks for you input, will keep you updated

no, air filter material is beige in colour...thanks for the reply

i did a compression test and reading is just under 100 psi, expected for used piston rings and cylinder

i think i am running rich...wet plug and 120 main jet instead of the stock 118 main jet

When I said earlier a stock Rally 200 I was only thinking that the engine has not been kitted. In actual fact I have converted from Femsa to 12V Ducati and therefore have installed a P200 crank

My current jets are as follows
Main = 120
Slow Running = 50
Air = 160
Mixer tube = BE3
Starter = 60

For a P200 the stock set up is
Main = 112 - 116
Slow Running = 55
Air = 160
Mixer tube = BE3
Starter = 60

I only learnt about carburettor jetting a few days ago. When you refer to the pilot jet, is this the same as slow running? Should I change to a 55 slow running for the Ducati / 12 v / P200 crank conversion?

another check I have performed is, removed the olio plug used to put oil into gearbox side of engine. started up engine and let it idle (without air filter ) using an compressor forced air into the casings through the removed olio plug, no change in idle, therefore seals are fine

no i would go down to 40 - 45 as this is what does your tick over, main jet is only last 1/4-1/3 of throtle, the jet is the small one next to the main jet and as said should be in one piece 55/160. also the timeing can effect running, what degree of retardation have you used with new ellectrics

I could be wrong, but I thought that under 100psi was getting kind of low?

ok will try a smaller pilot jet

Timing for stock Rally is 24 deg and for P200 23 deg.I never knew which one to choose but figured that they are so close it would not matter. My engine is set at 24 deg

Vespigi - have you tried just turning the throttle slide screw all the way in to turn up the idle rev speed?
Simple question i know.
The 120 main jet is not a problem with the idling.
The 1deg difference in timing will make no difference either.

Throttle slide screw is all the way in when I put the air filter on

Compression of 100 PSI is almost too low to start cold. If it were a Rally 180 (which had lower compression) I would say maybe okay, but not a Rally 200.

Threads on this site state the compression reading should be at around 130psi. The compression reading I get for a cold engine is 95psi. I therefore sent the cylinder and piston to the local vespa mechanic to measure tolerances and he said all is fine.

The mechanic also said that if the compression is around 100psi it is fine. I think the difference in pressure requirements is because here in Johannesburg we are about 5750 feet above sea level.

A search on the web came up with this
http://www.gregsengine.com/cylinder-compression-testing.html

Using the correction factor for 6000ft, would my compression be 95psi/0.8359=114psi if I tested my engine at sea level?

When rebuilding the engine I did not replace the rings, I assuming a new set of rings will improve pressure if the tolerances are correct

Coming back to the idle problem, my understanding is that a low compression will affect performance and make the engine difficult to start.

Once the engine is started will a compression reading of 95psi have an effect on idle?

Would the altitude have an effect on idling?

Maybe less oxygen so running rich. As I have said. Smaller slow running jet

Please post a picture of the top of you carburetor. I'm curious if you have the air correcter hole for the idle circuit built into the body of the carburetor or if you have the later type with it plugged.

I would recommend a change of your idle to something slightly smaller, like, 45 or 48.

I can only imagine that your engine starts fairly well when the mouth of carb ist being restricted, so let's not get too focused on the compression. Even though it IS low.

Please verify your piston has been installed in the correct orientation.

Ok gonna go and buy a 40 & 45 jet and see if this solves the problem.

Thanks for the advice

I will buy a 48, 45 and 40 jet

Without the air filter the engine starts after five or so kicks with choke pulled out.

Piston was installed with arrow pointing towards exhaust outlet port. Would the engine run if the piston is installed the other way round?

That is the earlier carburetor. Can you get the solid body idle jets where you are? You can just plug that hole and get later idle jets.

It will start up, but when you give it throttle it will stall out.

I am getting a little confused with the different terminology used. The idle jet is the one under the float cover right, referred to at the 'Starter' jet in the reference guide below. Right?

i am searching on SIP trying to find how the solid body idle jet you mention looks like. i will phone my mechanic to see if he has.

The hole I need to plug is the brass orifice on top of the choke mechanism?

Piston must be installed right, when I give throttle it does not stall

Or is the idle jet the slow running jet, this actually makes more sense

I am talking shit here, the hole you are talking about must be next to the idle (slow running) jet. Position 2 o'clock on the carb opening.

The jet inside the float bowl is the starter jet. It is only in effect when the choke is pulled out. (It's not really a "choke", it's a valve that opens the starting circuit. Think of the starting circuit as a miniature carb inside the carb.) The brass hole on top of the choke is the venturi for the starting circuit. If this valve isn't closing all the way once it's started it will run too way too rich and not for long.

Everyone here has been talking about the idle jet, aka the pilot jet, aka the slow running jet, which is located under the brass screw head just above the main jet stack in your photo. If it's too big it will also run too rich at idle and some at higher speeds.

Early style carbs had the idle air corrector separate from the idle fuel jet. All later carbs have the idle air corrector and idle fuel jet integrated into a single unit. Most early carbs were retrofitted by plugging the idle corrector hole and switching to a one-piece jet instead. The retrofit supposed solved some early flooding issues when the scoot was parked.

Your photo indicates you have an early style that has not been retrofitted and still has the 2 piece idle jet. (The air corrector is the small brass hole at about the 2 o'clock position relative to the carb throat). Later style integrated idle jets are much easier to get than the old 2 piece style one that you have. You can either try to find the old style jet, or plug the hole and switch to the newer jet. This is what SFvsr was referring to. I don't have a recommended procedure for doing this.

I hope this helps explain a few things.

Thank-you to SFvsr, Bazm60, Cncjerry and all the rest who have offered me guidance in this thread.

Thank-you to pdxjim for taking the time in giving me a great explanation on the terminology used. I have actually been thinking for months now how does the choke work and what does the choke actually restrict........"the choke is a small carb within a carb" is a brilliant explanation.

i am thoroughly enjoying the steep gradient of the learning curve I am on and do not want it to flatten out.

I have bought a 48/160 integrated one piece idle jet and will be trying it out early tomorrow morning, hold thumbs

That integrated jet already has a 160 air corrector built in and it will be way too lean if you don't plug the old one first.

Somewhere in some manual I saw instructions for hammering in some kind of lead shot plug to close it off. Hopefully someone else on this forum has some guidance about that. I'd hate to see you mess up the carb.

Have you made sure the choke is completely closing? It will give a similar symptom if it isn't.

Got up bright and early at 6am on Saturday morning and tried the 48-160 idle jet replacing the stock 50 solid jet. As SFvsr predicted the original jets for my Rally (solid jet with separate air corrector) are not available here in SA. I plugged up the air corrector hole with a tooth pick, started the engine without the air filter and engine idled OK as usual. I replaced the air filter and engine died.

As I had ran out of options to try I became desperate and started trying things that I knew would have little to no chance of fixing the problem. As I have a low 95psi compression I bought new piston rings, installed them and tested the compression. Again I read 95psi, my mechanic actually predicted this as he measured up the cylinder and piston tolerances and all were within spec. I will address this issue later when my idle problem is sorted. I then replaced the 120 main jet with a stock 118 knowing that the main jet does not come into play while idling. As I thought these 2 changes did not sort out the idle problem.
This is where I decided to throw in the towel and I removed the tooth pick so that the carb box cover would fit. In removing the toothpick the tooth pick snapped leaving the hole plugged. While trying to get the tooth pick stubby out I actually landed up pushing in right into the carb body. I removed the carb and gently poked and prodded with a thin electricl copper wire and eventually got tooth pick stubby out. Replaced the carb and for shits and giggles kicked engine over. To my amazement the engine idled with the carb filter on. I assumed that while poking with the wire I unblocked the idle passages.

All these tests were done on a test frame and I now had to install the engine. As I had fixed the idle problem I decided it was the perfect time to install my new SIP road pipe that arrived that day. After installing the engine and new pipe I took the scoot out for a test run at 8pm, the engine ran very rough and. I assumed that the scoot was now running lean because of the SIP pipe. I called it a day as I had solved the idle problem. This morning I installed the 120 main jet that originally came with the scoot and struggled to get the scoot started. I free wheeled down a steep hill popped the clutch in 2nd and the scoot reluctantly started. The engine was still very rough and I felt a sort of jerking motion from the engine. The scoot died shortly afterwards and refused to start and I had to push the scoot up the steep hill back home.
Thinking back to the replies to this thread I remember mention of a weak spark. I connected a spare plug in series to the plug in the engine. I kicked the engine over and saw no spark. I actually had to get got someone else to kick the engine while I had my nose right up against the plug and could barUsing the ely make out the tiny spark. Using the same spare plug I did the same test on my sprint and it now seemed as if there was a LED at the end of the plug.

Looking through my replies Cnjerry was the second person to reply to my post and actually predicted this. Just don't know why I decided to test this last.

How do I know if it is a faulty stator plate or CDI unit. I am leaning towards the stator plate as I my lighting circuit went on the blink for a while and then started working again. Also not knowing I bought a LML (Swiss) and remember reading that they sometimes produce a bad spark.

Where to now? Do I take the chance and fork out the cash for a new stator and then maybe a CDI unit?

the choke is closing fine

I find with electrical problems I usually pull off the flywheel and look around for a bare wire or some other problem. then ill take out the multimeter. that usually does the trick.

what, you leave your multimeter in the bike?

doesn't everybody? no just kidding, i keep it in the garage toolbox. i pull off the cover for the junction box and start checking the wires and stuff. you know, goths and malls.

ditch the 'swiss' indian stator........... theyre notoriously shite, the pick ups usually fail and all off and rub the flywheel. Get a genuine piaggio an carb wise have you got a spare to try? even a friends spare?