Modern Vespa

I put a big bore on my '08 Lx 50.

After the scoot gets warm - 10/15 ride . . . . it will bog/surge coming off the line and won't idle. I'll ret-start after it dies (rev'ng while i'm starting) And get it to eventually go. This process will last for a couple of minutes.

I've replaced the CDI, the fuel actuator at the bottom of the tank, and I have spark when it's not starting. The valve clearances are spot on. I feel as if I'm running out of ideas.

I've also cleaned out the carb a million times. It's tip top in there. Accel pump squirts just fine.

Fuel line replaced - vacuum line replaced . . .

Any other ideas? Short trips it's fine, but as I wrote - longer 10/15 minute trips where it has a sustained 3/4 throttle, is when this starts.

Would an improper float level cause this? It's using more gas than it can refill? I'm tempted to buy a crappy $30 carb off of amazon to see if that's it.

thanks,
dave

You might want to check your intake. It can cause alot of issues when cracked.

(No. 4)

in what way? from the air filter to the carb? may i ask how a cracked intake would effect things - it's all just air coming into the carb. vacuum?

Start the scooter, let it idle a bit, and then spray the outside of the intake tube with carb cleaner. If it stalls there's a leak in there somewhere.
This is the method used by Robot on Vespa Motorsport.

Sorry to hear this - it sounds like you've done all the right things to get it going again. (Obviously without success yet.)

Was the bike running properly before you installed the big bore kit?

Have you tried a new spark plug (my LX ran like garbage for months because of a bad "brand new" plug... in the meantime, I spent hours and dollars replacing my carb and a bunch of other stuff )

You mentioned float height - did you check it's set correctly when you had the carb off for cleaning? What about the fuel petcock - does it stop fuel when bike is off, and good flow when bike is running (or sucking on vacuum hose)?

Finally - you probably want to post this is the "General" section to get more help. Your post won't get as much visibility in "Project Reports". I think a minor correction is needed - I'm pretty sure carb cleaner is highly flammable and will cause the engine speed to pick up.

what size carb jet, are you using a freeflow air filter of some sort or the OE sponge, are you running an aftermarket pipe, did you get the high perf camshaft along with the kit, etc

I have a similar issue on my BBK LX50. It's the fueling. If you're still on the 75 try a 78 or 80.

I too upgraded my LX50, and some of the problems I had along the way were a split o-ring on the electric choke that made the thing run weird at different times.

What is your current jet sizing?

I like the mention about your spark plug and that is a cheap and easy thing to swap in. I fouled one up pretty good when I tried to run a #90 main jet and the scooter was bogging from 1/2 throttle and up. I dropped down to a #82 and the scooter ran very bad until I got it up to high revs for a bit which cleaned off the plug! Italian Tune-Up!

If the bike isn't faster than before, check the cam timing. You can easily put these off a tooth.They will still run, but with the bigger displacement, perform about the same as it did before the BBK.
Also, you might have a dirty idle circuit, or the air/fuel screw isn't adjusted properly, so it's idling off the pilot jet, and once it warms up, it's getting too much fuel. Or the autochoke isn't working, so it's either adjusted so it starts without it, or it starts with it, but it doesn't change, so it's running rich once it warms up.

"After the scoot gets warm - 10/15 ride"...

Have you considered a soft seize?

Hi All! Thank you so much for all your input. Here's my attempt to answer all of your suggestions.

1. intake - i sprayed carb cleaner - nothing - it's sealed up nice and tight
2. I did get the upgraded cam - and the power is awesome overall - big step up from the normal 50. when it's running, it runs like a dream
3. the airfilter is the upgraded red one with the oil.
4. did this happen before? well, one time it did it, but only that one time. This started significantly just after installing big bore. It does it less so now, but still does it if i'm riding for a longer period of time. I can feel it happening as it starts to bog while on-gas.
5. sparkplug - been through a bunch . . .
6. jet. it was at 72 stock. then 78, then 80 - happens with them all. i just put in the 78 from the 80.
7. mixture. so if it screw it all the way in . . . clockwise, nothing seems to happen. If I unscrew it will eventually die. so i screw it back in until it idle's nicely. then i rev a bit. it would die upon returning to idle. i would screw it in until it ran nice and then returned back to normal idle.
8. float - I bent the little tab down a smidge so the bowl will hold a higher level of gas. will see what this does.
9. accel pump works just great
10. choke - when this happened the other day, i unplugged the choke and got no different result. took it off today and things seemed fine in there. i don't see any orings in there - nothing is split. the needle slides in and out just fine. i know this is part electic and part heat activated. once it warms up, the needle retracts or something.

thank you all! i kind of want to buy a cheap new carb on amazon and see what happens . . . they're like $30 . . .

cheap gamble but I heard they don't work very well

You do need that oring on the auto choke. It's on the choke part sitting in a groove in the black plastic.

IMO You also should be working with a known value on the air mix screw like 2 1/2 turns open or 3 turns open. Start with it closed and open it up to 2 1/2 to start. I am 2 3/4 turns open with an 82 jet and seems to be good for me but I am at sea level.

I keep one of those cheap carbs around to trouble shoot too. You wont want to keep it on the scooter for very long because they don't offer the performance that you need but they are awesome for extra parts IMO. You can swap all the gaskets and orings over to your good carb.

So awesome! There's a groove but no o ring - will post a pic.

Do you have a cheap troubleshoot carb you like?

My mistake. Robot said in the video "Any change in the idle speed". I assumed stall. Skip to 3:10, or watch the whole thing. Good info.

https://youtu.be/xoPVQCs919w

Keihin CVK PD18J
I bought three of these a few months ago from Amazon but does not seem to be available anymore! I scavenged two for parts and kept one unmolested.

Thanks everyone - will check that video out for sure. Since I changed the float level and adjusted the mixture . . . we went on a long ride that normally would cause the bog/stall.

It was pretty cool out, but it happened a week ago under similar circumstances.

Going to cross my fingers, but i still don't see how the idle mixture can cause this issue, when as soon as you give it gas, that idle jet is take out of the equation.

Everything says that too low float level causes lean running, not bogging/stalling . . . .

so who knows - going to order that cheap carb to see what happens and going to rewatch that scooterswest video. part of me doesn't want to touch the mixture screw. If/when it does happen again I'll film video for sure.

Another question - when i turn the mixture screw all the way in . . . nothing happens . . . i have to unscrew it quite a ways before it stalls.

thanks,
dave

I always adjust mine with the motor off but I'm not an expert. There is an o-ring at the end of that needle valve we are talking about too. It's small!

Are you familiar with this chart? It really helps sort through what kind of tweaks can be tried IMO. We are talking about the idle mixture screw (IMS) and the changes it can influence are between idle and "barely open throttle" so if your scooter is running ok there you need to look elsewhere.

When I play with this circuit I always end up adjusting the throttle position screw on the top of the carb by where the throttle cable connects to get the idle where I want it. Have you had to adjust this also?

Also if anyone knows of an adjustable needle jet for these carbs please share. I made my own by pressing on a small washer but I would have preferred to purchase a factory one.

Cool chart. Note that "I.M.S." stands for "idle mixture screw." The intended meaning of "straight dia"[meter?] isn't clear to me.

Is my question of a possible soft-seize completely off base? He said the first time it happened was after the big bore kit was installed. Then it happens after the scooter warms up 10-15 minutes ride.

Not at all off base. Increasing the displacement throws off the engine's designed balance between intake, jet size, and exhaust. Who knows what a DIY job did or didn't do to re-establish that balance. But I avoid those mods myself, so didn't comment. I guess you could get one of those thermostats with a sensor that mounts between the spark plug and the head and see what's going on there.

Soft seize - had to google that . . . .

As far as most recent developments . . . no problems since i futzed with the float height . . . .but it hasn't gone through the ringer yet.

as far as soft seize.. . . . i read it to mean that the piston gets stuck due to overheat/underlube. the engine never stops - it bogs . . . then fires . . . then bogs . . . also - you can always crank. I don't think it's a soft seize, but i'll keep looking.

love that chart showing he different stages of the throttle!

I did a great job of installing the kit if I do say so myself. proper torques . . . malossi, cam, variator, all that stuff . . . . it was legit. I've done a lot of work on cars . . . .and rebuild old BMW airheads . . . .

I upped the jet, just like scooterwest said to - to 80 . . . .I am at altitude - so if anything i'm over jetted. Maybe i should change the needle height - is that the same as the needle taper in the chart or the clip settings?

also - when i screw the mixture all the way in (lean) it doesn't falter. only when i unscrew (rich) will it falter. does that mean anything?

I thought it was the clip position. I couldn't find an adjustable needle for my carb so i made my own by pressing on a small stainless steel washer.

Seemed to make a difference but I know it's less of a step than a true adjustable needle jet.

That doesn't sound right.

I think it means your bike is running rich at idle. Opening the mixture screw should add fuel, which apparently your bike doesn't need.

doesn't make any sense? is my idle jet too big? it was stock?

i guess the only thing to do is to take it all off and inspect

I don't see how you can do much analysis until you get that auto choke O-ring installed. When it was missing in mine the scooter ran strange.

I had the exact same problem almost with my stock 2006 LX50 - got so frustrated with it it's been sitting in the garage for like 4 years now lol.

Mine would have trouble coming off of idle right from start up though - warm up didn't change anything. I cleaned the carb I don't know how many times. New plugs, new fuel petcock below the tank, new vacuum and fuel lines, fresh air filter. Has a 78? main jet I think.

It idled fine, but would bog down as soon as throttle was applied. When it was occasionally able to get to full throttle, it ran great as long as you kept it at full bore. I'm ready to tackle it one more time I think - would take it to a shop but it's such a pain to get it there. I'd like to get it running well again, and then install a big bore kit possibly. It is a great little runabout.

Wish me luck and any and all suggestions are appreciated!

By the way, my LX 50 has about 5k miles on her - could the valves need checking at this point? Maybe I should start a new thread...