Modern Vespa

New owner of an ET4 150 that has been sitting for several years.

To date, Here's what I've done.
1. Drained old gas
2. Replace the fuel tank tap
3. replace fuel lines
4. inspected/replaced vacuum lines. Broke the tee from carb to Evap line, so upgraded to a larger diameter tee
5. Oil change
6. new plug
7. new air filter
8. cleaned the carb w/ carb cleaner and compressed air
9. ultrasonic cleaning of carb - jets look good
10. accel pump is working
11. tip over valve isn't plugged up and the ball is moving around easily
12. diaghram on needle valve appears good
13. drive belt and pulleys appear to be working correctly


here's the issue.
1.
With the vacuum line off at the carb, idles great, terrible acceleration and wot is about 20mph with me on it ~215lbs. On the center stand it will rev up but it accelerates very slowly.

Hook the vacuum line up and it sputters and dies within a few seconds. There is a slight vacuum at idle when the line is unhooked from the carb when you put you finger over the hose end.

Is the vacuum pulling the needle valve out of the main flooding it out?

Why is the acceleration so poor when the vacuum line is unhooked? Is it not lifting the needle?

there should be no vacuum line to the carb. are you talking about the evap line that hooks up on the left side just under the diaphragm cap?

if so disconnect it. you don't need it.

it sounds like the diaphragm isn't fully seated/sealed or the spring is binding. pull the lid off and check that is all squared away.

Thanks for the reply. The evap line that goes to the canister and the rollover valve has a Tee in it. Where does the smaller line off the tee go? When that is exposed to ambient pressure the thing idles like a dream but the acceleration is poor on the center stand and really poor w/ me on it. 20mph max

I'm 99% sure the diaghram is seated well and the spring is good to go. The needle valve isn't bent etc.

I'm at a loss.

A picture is worth 1000 words here.

Here are a couple pics. 1 pic shows the vacuum line in question. You can see the Tee in the distance of the picture connected to the Evap line.

the 2nd pic appears to be a vacuum port to me. I am trying to find where to plug in the other side of the small vacuum line from that tee.

Again, w/ the small line open to atmosphere. It jumps to life and idles perfectly. The acceleration is terrible and WOT gets to 20 mph.

If I plug that line into the suspect pressure port, it dies within seconds.

it is not a vacuum line. stop calling it a vacuum line. that is an evap line. see how it goes to the T, and from there one line runs to the charcoal canister. the other goes toward the left of the bike. we'll get back to that momentarily. that is not a vacuum port. it allows for ambient air pressure in the diaphragm chamber. do not plug it, do not attach anything to it. something is wacky in your evap system, or in your carb (diaphragm not seated, missing a part-- you did put the needle guide back in, and in the right way around, correct? or the spring is binding) or both!

so back to the evap crap. the small line runs to the T, then one of the larger lines runs to the charcoal canister, from the charcoal canister to the one way valve, from that valve to the barb attachment at the manifold. the other line jets off to the left side of the bike and up under the cowl to the roll over valve and then back up to the tank top off.

so, check all those lines for cracks, abraded holes, rot, etc.

or you could just pull it at the manifold barb and cap it with a plug and be done with it. this would also eliminate any guess work in that department, and make a quick diagnostic.

if the problem persists, then it's in the carb, intake, airbox, bellow or filter.

hth

In the second pic it appears that the carb isn't fully seated in the intake manifold. That metal point should be all the way into the intake rubber notch. You might have a leak there.

Validated no leaks in the intake side with carb cleaner during ruining. The carb was not seated in the intake as well as it should have been so did correct that. Removed and capped the intake pretty for it. It helped but it's still not right. The acceleration on the stand is certainly better but it won't go over 20 while I'm riding it.

District possibility the needle guide is in backwards, once it cools off I'll drop the bowl and have a look. Kill keep you posted. Thanks for the suggestions.

when you have the carb off, a quick check of the needle guide is to look down the Venturi and see if you can spot it. it should be flat not domed. the domed side sits against the diffuser that threads into the body.



a quick check on the diaphragm is to blow some compressed air into the orifice and see if it lifts the needle. and check the slide with your finger to see if it moves smoothly and it's not bound up.

I'll check it the guide in the morning. I did pull it out seems to only go in one way. I'll get a pic similar to yours in the morning.

On the diagram, is the orifice to put the air to the vent in my first pic?

no, into the Venturi, specifically the kind of oblong opening in the mouth of the carb that you can kind of see in the pic I posted.

Fwiw, a pic of the diagram showing it being seated correctly.

Fwiw, a pic of the diagram showing it being seated correctly. And another pic of the guide installed. Looks good to me. I also found the orifice to validate the slide operation. Worked perfectly.

Maybe the clutch is jacked up and the clutch is letting the belt slide down too far.

needle guide looks correct. double check the operation of the slide and diaphragm with compressed air. it may *look* sealed, but unless verified that still remains an unknown.

it starts and idles so you can eliminate those circuits. now you're into the diffuser, main, and needle.

did you remember to install the needle retainer?

what you're describing sounds like the diaphragm/slide/slide isn't operating properly, possibly a fuel flow problem.

of course it could be spark related as well, but considering that it starts and idles easily I'd look at that last.

I blew air into the orifice and the slide moved up easily. I'll do it again and pay better attention to the needle but if the slide moves, the needle has to.

The needle retainer clip is locked in. I'm beginning to think the clutch/belt may be the issue. On the center stand, it seems to rev reasonably well. riding it not so much....

I have not checked the valves.

might be, pop off the trans cover and you'll probably know straight away.

Variator and clutch are good. Belt is good. Still won't rev up and 20mph Max. Idles like a dream. Pretty weird. I'm open to suggestions. I guess I could start putting bigger main jets in it to see if that helps

man, I'm at a loss.

either the carb isn't completely clean and something is blocking the main jet circuit or it's not getting enough fuel, possibly double check the air filter and box are seated properly.

then it's onto spark. plug, cap, coil are cheap. CDI is not.

just for S&G's I'd pull the valve cover and have a quick look at the cam and rockers. figure as long as I'm WAG might as well take an extra min to investigate.

I'm going to run it without the exhaust for a run and see if that has any effects. I'll probably ultrasonic the carb again. After that it may be a Keihin carb upgrade.

Thanks for the help. I'm about out of ideas.

run it thru the ultra one more time and see if that helps.

quick check on the muffler is to kick it and see if you can hear it rattle.

if you decide to go for another carb PM me. I have several in very good condition that I'm looking to sell. I go thru them and run them on a motor to verify operation before selling and sending out.

Hey OP,
Greasy knows what he's doing.

One thing I noticed, in the pics of your slide and diaphragm, it looks filthy.
All the carb parts and orifi need to be completely clean.
A spec of grime will stop the main jet from passing enough fuel to run.

If it's just a camera angle or something, I apologize.

I just had a similar problem with a small 50cc motor.
I had everything clean and up to spec .(or so I thought)

There was the slightest glaze of stuff on the diaphragm pin.....and while it appeared to work properly, it was not moving freely while the engine was on.
Be real careful not to get carb cleaner on the diaphragm as it will swell and possibly not seal. (Don't ask why I know that)
If it does, let it air out for a few hours before you reassemble.

Good luck

Good thoughts. It doesn't take much air to get it to lift when it's off the bike. I think it's working pretty well. I just Sonic cleaned it again, blew everything out and reassembled. Will have another go in 30 min

No joy. At a loss. Thing will not rev up for anything. Even on the center stand it takes a minute.

Anyone in San Antonio have a working carb I could try?

If you have indeed taken care of the fuel side.....Check the spark side...

Verified the slide is coming up under throttle movement. Baffling. I'll check the spark side is doing it's thing. Sure feels like a carb issue though.

Rep placed the carb with new. Same issue. Starts and idles fine. Very low power. Tops out at 20mph.

I don't think it's a fuel problem. I get lots of fuel out of the line if I disconnect it. I ran it without the air filter to see if it was choked out.

Next step, I'll check valve clearances and replace the coil. Any other suggestions? CDI?

Is the engine running fine on the center stand?
(Throttling up OK?)

Variator/belt?

"I get lots of fuel out of the line if I disconnect it"

Shouldn't be any fuel dumping out of fuel line if the engine isn't running.
Check that the fuel tap is working correctly....
When you pull the fuel line off, no gas should flow.
When you apply suction to the little vacuum hose, fuel should flow.
If it's always flowing, or you get gas out of the vacuum hose, you need a new fuel tap.
(Mine failed last week)

Thanks for the reply. On the stand it accelerates very slowly. It will come up in rpm but it's far from instantaneous. On other quads, bikes etc, no load acceleration is almost instantaneous. Holding it wide open would be a bad idea. I can open this up and it takes 5-10 second to top out and it feels like it should be more.

The fuel tap is new. I should have mentioned it's running when I check the flow. It's running the fuel out of the bowl.

I changed the rollers and belt on the variator and clutch. Everything seems to be functioning properly.

The final drive could use a gear oil change but I don't think it's causing any extra resistance.

Main jet blocked!

Well...
If you are getting good fuel flow and you have tried 2 carbs, (after you adjust the valves) start thinking about ignition coil.

Start with valves so there are no variables.

Thanks for the reply. On the stand it accelerates very slowly. It will come up in rpm but it's far from instantaneous. On other quads, bikes etc, no load acceleration is almost instantaneous. Holding it wide open would be a bad idea. I can open this up and it takes 5-10 second to top out and it feels like it should be more.

The fuel tap is new. I should have mentioned it's running when I check the flow. It's running the fuel out of the bowl.

I changed the rollers and belt on the variator and clutch. Everything seems to be functioning properly.

The final drive could use a gear oil change but I don't think it's causing any extra resistance.

I cleaned the original carb 3 times, 2 of which were ultrasonic soaks. I bought a brand new OEM carb without any change.

I don't think it's a carb issue anymore

I'd next check valve clearances, compression, and plug wire for integrity and connectivity (both ends). Then suspect the coil.

Does anything different happen if you rev it (once) with the air filter off?

Starting the valves now. I took the air filter off yesterday. It would idle but almost die with throttle. The plug wire looked pretty good but I'll look again compression is a good idea. Let me check it. I hope i can find my compression tester.

Valves were good. Spark plug wire is good. Here's a pic of the spark plug. New with about an hour of run time.

Video of the acceleration of it will load.

That plug looks horrible for 1-hour of runtime. Looks like your running super rich. As black/sooty as that plug is, I'd expect to see black smoke and a real rich smell from the exhaust.

Yeah,
Plug should be brownish....especially with that little time on it.
I usually suspect the fuel tap not working right first, but if you have a weak spark, that could cause the fouled plug.
You're going to have to test the wire and coil.

time to dive into the ignition system. test the coil first and check the connections, that's easy. confirm that you have big bright blue spark and not some weak yellow-orange spark.

from there, ohm out the stator components. you may have a bad LT coil or pickup that's going away. from there, it's a CDI

the testing processes are all in the FSM
https://manuals.wotmeworry.org.uk/Vespa/LX150/LX150Workshop.pdf

good luck!

Cool looking suspect. Not blue. Resistance checks high.

If this checks out, it will be the first coil I've ever had go bad. I'll follow up when I get the new one installed.

Put in the new coil and a new spark plug. No change. It feels like it's choking. I pulled the airport of and tried to clock out of with my hand. Seemed to help a little. It's pretty rough to modulate the air at the right spot.

Checked the carb to be sure. The new one is installed. Clean as a whistle. Validated fuel flow out of the tank.

No idea what to do next. I have not checked compression as I left my tester in another state. I'll have to pick up another one I guess.