Modern Vespa

Hey fellas,

I have an LT150 from 2003 with 11k miles on it (the last 1k were put on me with no trouble), and the engine has a severe issue of dying the moment I let it idle. I had this issue two weeks ago when it stalled out twice while idling on a 15 min drive, and I've since:
1) checked the vacuum tap (it provides fuel when vacuum is applied and cuts off fuel appropriately, so it seems fine.)
2) checked the fuel and vacuum lines; they are supple and uncracked.
3) replaced the spark plug; it was mildly blackened and sooty.
4) replaced the intake manifold; it was slightly cracked and has some grey rubber cement smeared on it, so I just got a new one.
5) cleaned the idle jet, main jet, and the carburetor diaphragm with carb cleaner.
6) cleaned the air filter.

When I did these steps last night, it started up perfectly and idled for 10~ minutes no problem while I adjusted the speed and mixture screws on the carb. However, I took it for a ride this morning, and after starting up/idling well , it once again stalled out just a couple miles into the ride.

It is now to the point where I NEED to give it PLENTY of throttle for the poor thing to start up at all, and it dies the moment I release the throttle and let it idle.

It seems I am back to square one with this issue except it is now somehow worse, my guess is something broke/shook loose on that first ride since putting everything back in place this morning, but I am reaching my wits end. Any wisdom would be sorely appreciated!

sounds like you have an air leak. or shook something loose.

go back thru the intake system and make sure everything is sealed up properly. then double check the evap lines if you've still got them connected.

if you hosed down the diaphragm with carb cleaner, you may have damaged it.

double check the auto choke is working and the o-ring is intact and sealing as well

so check all the exterior first and then jump back into the carb.

good luck!

Thanks for the response!

I tried the ol spray carb cleaner around the intake seal/fuel line/vacuum tap areas while turning the engine over, but no area I sprayed delivered any change to the sound or ability of it to start.

For what it's worth, I didn't touch the diaphragm rubber itself with carb cleaner; I only wiped down the plastic tube connected to the diaphragm that pushes in and out of the carb body, but I'm gonna take the carb apart and see if anything looks out of place.

It also sounds like you adjusted the air fuel and idle screws before the bike was warmed up. Run it for about 20 minutes, then do the adjustments. See if that helps. Black and sooty and the dying at idle once it's warm sounds like the electric choke might not be opening once the bike is warm.

I had this same problem on a Buddy 125. it ended up being a bad spark plug cap.

the tube that connects to the diaphragm chamber, that's a useless evap tube, remove it. that alone may be the issue with starting-- especially if you've topped up the tank.

2nd motovista's advice, recheck the A/F mix once warm it should be 2.5~3ish turns out. then double back on the choke to check operation.

I tested the spark plug by holding it by the cap to the metal frame and checking for a bluish spark while turning the engine over - I saw what looks like a healthy spark, so I think my plug and cap are still good.

So, the previous owner took out the evap systems and that tube is about 3 inches long with an open end - should I stick a filter in that tube or seal it off?

check the filter in the tank is not clogged ( see photo)

Welp, I took the carb back off and inspected it, and nothing about it appears clogged or damaged (I should note that the carb was previously cleaned and tuned about 1000 miles ago). Upon reinstalling I have the same issue though: when I try to start it, I can hear the engine turning over (and I've made sure that the sparkplug has a healthy looking spark to it), but the engine just refuses to start up beyond that.

I did take the spark plug back out to look at it after turning the engine over, and noticed slight dampness on the plug - from what I can tell from this link:

"Won't start" diagnosis for beginners

...it seems the fuel delivery isn't the problem, and now I'm wondering if the valve clearances need to be adjusted? Any advice?

I previously checked to make sure the fuel tap

a) kept fuel from leaking when no vacuum was applied, and
b) let fuel flow through the fuel line when vacuumwas applied

and the tap performed well on both fronts; I haven't looked at the filter part of it yet, but do you think that could be the issue if the vacuum diaphragm of it works fine?

you can stick a filter on there if you want. do not plug or seal it in any way.

check the valve clearances. they generally behave a little differently when they're out but stalling a difficult restart is a symptom.

This isn't enough to rule out a faulty cap. It's $5. Try it, might fix your probem.
My Buddy 125 was at 12,000 miles. Would run fine at speed. When I would stop at a light, it would die. It would start and keep running as long as I gave it gas. i went through the carb twice, checked the valves, replaced the petcock and was eyeing the stator. I had a donor bike with a spare coil and cap. I swapped the whole thing. Problem solved. I went to further isolate it to the spark plug cap. The connection between the cap and the plug had slightly worn, so it wasn't making full contact 100% of the time. At idle, the scooter shook just enough to interupt the connection. When revved, it would smooth out enough to not be a problem. In the early stages of this problem, I also tested for spark the same way you did. Saw it, and tore into the carb.

Huh, I think I might get a new fuel filter and spark plug cap and see if that improves the situation. I am close to sending it in to a mechanic but I can't decide how much longer I want to spend on fixing it myself ahaha

Remove the pilot jet and make sure you can see air through it. Just about anything will clog them. A guitar string will open it up, if you can't see air.
If it is clogged, the odds are that you've got the idle too high, and that will also make it very hard to start.

As long as you are getting reasonable responses here, stay with it. Any bloke with a credit card can wheel it into the shop. The kid working on it may or may not know more than you. Scooter shops near me get $125 and up per hour. Stay at it, fix it, learn.

Since you said it idled fine for 10 minutes and then died, that means it was working properly. Something changed. Vacuum leaks don't tend to change. Adjustments definitely don't change by themselves. There could be a tiny piece of trash floating around in the carb that you knocked loose when you cleaned it but did not get it out. I would suspect the vacuum petcock, because I've had a lot of problems with them on many bikes. Ethanol tends to melt the rubber diaphragm. But if it runs ok at higher speeds, then it has to be getting fuel. Maybe try disconnecting the fuel line from the carb, and feeding the carb from another fuel source, like is commonly used to run an engine with the fuel tank removed while synchronizing carbs. I use an unlit propane torch to search for vacuum leaks. If there is one, the engine will suck in the propane and the engine speed will increase noticeably. Propane won't damage anything like carb or brake cleaner can.

Thanks for all the replies guys, truly. I took the carb apart once more, this time with the most thorough cleaning and most careful reassembly yet. I was able to discuss my issue with a mechanic friend who thinks it's the choke based on what I explained to him, and I made sure to clean the choke bowl and needle on the carb, but to no avail. Because it is a cheap part I'm going to buy a replacement electric auto choke just to rule out the choke being the issue, and if that doesn't solve it I'll move onto replacing the fuel filter and spark plug cap.

I've resolved to keep at it myself come hell or high water - I've dissected the damn thing this much and learned invaluable knowledge as part of the journey, and I'm not giving up when I know the solution is within reach.

Woah, potential issue on the automatic choke staring me in the face: when I removed my choke from the bowl, the needle fell right out of the plastic choke box along with the spring. I'm looking at videos of carb disassembling/choke testing and so far I haven't seen a single instance like mine where the needle completely detaches from the choke box upon removal - they all seem to remain attached and wobbly. I wonder if this is a sign that the choke is definitely broken?

Also, alas, the choke is not as cheap as I thought - all out of country options within the $20-30 range have $30 shipping unless I wait 2+ weeks for it to arrive. Darn.

No, that's now right. We have the Keihin and Walbro auto chokes both.
https://scooterpartsco.com/index.php?main_page=product_info&cPath=801_3501&products_id=10406
https://scooterpartsco.com/index.php?main_page=product_info&cPath=801_4154&products_id=9288
But most people who buy the choke or replace the slide end up coming back for a new carburetor in the end.

The choke, or "bystarter" is at least in my opinion a stupid design. I would much prefer a manual choke. But if you are going to have an automatic electrical choke, it should be wired backwards to the way these things are. With these "bystarters" you have to have voltage going to them to turn them OFF. Meaning if you don't have voltage going to them, the engine won't run, because it is going to be on full rich. It would make a lot more sense if they were designed so that they only required voltage to turn them ON. Where I live, most engines will start without a choke, or enricher circuit. So if I lost voltage to the thing, the engine would still start, and continue to run. If you unplug the wire going to a bystarter with the engine running fine, it will quickly become very rich and stall.

This is a very interesting idea. Keihin has a customer service line and might be interested in hearing more about wiring their electric chokes backwards. But it's probably not a good idea for people working on their own scooters to wire the choke backwards, realize that didn't fix it, then drag the thing to the dealer without reversing the wires or telling the shop what they did. That could be a very expensive repair bill, because nobody who works on these things ever asks themselves, "did the customer reverse the wires on the electric choke?" They'll just keep replacing stuff, and writing down what it costs on the work order.

I am equally convinced that the melted-wax design of the autobystarter is brilliant and idiotic.

Failures like this are, though, pretty rare. At least they seem to be on Vespas and Buddies.

I think ScooterDude means by "backward" is that the failure mode of these things is with the enrichment "on" rather than "off."

I have never seen a carbureted motorcycle with anything like this. They either have a manual choke or a manual enricher circuit. Maybe scooter manufacturers think that people who ride scooters are not smart enough to know how to use a manual choke or enricher. I never understood why only scooters have these things.

Well, an update:

I replaced the broken choke with the new part, put the carb back in, but ran into the same issue still: it turns over but only catches when I give it plenty of throttle while starting, and dies the moment I let off the throttle. I'm gonna run a compression test to see if that's where my problem is, but sheesh.

I think to make the scooters as automatic as possible. When I talk to younger riders about setting the choke on a cold start with an older bike, they roll their eyes and remark that it is ridiculous in this day of FI machines. I have been riding and wrenching on scoots for over 50+ years. With the latest retro craze on restoring and riding older machines, I have had more than one person show up and ask why their bike won't start without a lot of throttle twisting/kicking, spinning. Then I give them a quick tutorial of the art of choking the carb. Some info gets lost over time, I guess.

. If the electric choke is working properly, you should not have to touch the throttle. Doing so will just flood the engine.

did you ever check the valve clearances?

I checked them a couple weeks ago but just checked them again now after running a compression test: engine came in at 90psi, and both valve clearances were fine, so I'm guessing it's something with the piston rings. Darn.

These are equipped with a compression release. It's designed into the camshaft. You'll need to disable it to get a true compression reading.

Hmm, this is something I missed when reading up on how to perform a compression test. Do you have recommendations for how to disable it?

you'll need to partially disassemble the timing assembly to remove it, then reassemble and test. it's kind of fiddly, but if you're in that far what's another few steps.

however, because it was running I doubt it to be rings/compression-- unless there's something wrong with the valves/cam.

I'd circle back to basics: intake and carb, spark and ign system