Modern Vespa

Ok, so I've combed the forum and the internet for information on this bogging issue, but have not found a post that seems to match what is going on with mine. First, the info:

P125X, completely stock (as far as I know)
Carb: Dellorto SI 20 20 D
Starter jet: 60
Idle jet: 9892. 48
Main jet: 160 / BE4 / 96
Spark plug: NGK B7HS
Stored for 28 years, cleaned the carb and the gas tank, filled it up with new fluids (including gear oil), it started, I bought it... I put new fuel/oil lines, new fuel cock and oil filter, greased the cables, new tires, and took it on the road. It was running fine, tho it seems that the top end of 3rd and 4th gear could never be reached (it would not wind out all the way). I was thinking about rejetting as I'm in Santa Fe at 7000 feet and had read that that might be the issue. But, really, it started fist kick (2-3 cold) and just take right off.

Then... I was cruising down the road in 3rd(?) midspeed, and it bogged and died. It sounded deeper and then petered out. There was gas in the tank, but I added some just in case. It started, but bogged with throttle. I did a quick carb tune (air/fuel mix) and it seemed good. Took off, but in ¼ of a mile, bogged and died again. It idles perfectly. The spark plug looks to be a pretty good chocolate brown and dry. Maybe a tad dark(?). I managed to get it home by having the idle sped cranked all the way up and then keeping the engine revved while playing the clutch in and out a bit when it started to bog, luckily I was not far from home.

So here is what I have tried:

Opened the gas tank cap to be sure it was not a vacuum thing
Re-did the carb tuning, I did this a few times with the idle speed turned all the way up and also at a reasonable idle to see if it made a difference either way.
Brand new spark plug which after getting it home looked a tad wet, but then, I was gunning the engine and working the throttle all the way home...
Rebuilt the carb, replaced the float and needle

Now it still does the same thing. Might be a little better, and not bogging as quickly, but it is still not rideable.

I am going to check the exhaust, maybe do the drano thing... It's got 3500 or so miles on the speedo, but the numbers aren't lined up, so I'm thinking its probably 13500, so the exhaust could be clogged. Would something like that cause this to happen suddenly? I will check the fuel line as I read something about a crimped line causing not enough fuel, but then again it would be running lean right? Could it be an air leak? Should I decoke the head?


So, I know that this topic comes up a lot, but if nay would do me the favor of revisiting it and giving me some opinions / advice / diagnostic techniques to try, I would greatly appreciate it. Thanks!

Another quick thing to check before doing anything too drastic is to ensure the jets are clear. You may think they are but split it in to 3 to check properly

I did that when I rebuilt the carb. Clean as a whistle. May have time today to do the Drano exhaust cleaning, if not I'll get to it tomorrow. Has anyone done this with good results? Should I bother?

Sounds like a fuel starvation issue. With the tap off, disconnect the fuel line from the carb, turn on to see if u have good flow. It should b pouring out, not trickling (have a bucket or pan to catch the gas). Your fuel tank tap has a filter in it which may be clogged. You may need to drain the gas, pull the tank and remove the tap (requires a special wrench) for cleaning or replacement.

So, today I pulled the tank and checked the fuel line as it was a bit tight when I installed the new one. I rerouted it so there was more slack in case there was a crimp or something (although there was no indication of that looking at the line). No dice, it is still doing the bog...

It idles like a dream, it revs in neutral just fine, and will go down the road for 1/4 block, then will bog and begin to stall, if I pull the clutch n and rev it it comes back to life a bit, then if I re-engage the the gears, it will go, but will be very slow and boggy and jerky, kind of starting and stopping. It's gotta be something with the fuel. Again, it has a new fuel tap, and line, and a newly rebuilt carb. I'm thinking that I may put the old fuel line back on just to check the slack in it. The new one was a different thickness, I think the interior diameter was the same, just the wall of the tube was thinner.

Please! Any Ideas?

Well, now that you say it again, fuel starvation would account for the cuttting out but not the bogging. Bogging is usually the result of a fuel or electrical. Are you sure your carb is adjusted correctly with the air/fuel adjustment screw 1 and 1/2 turns out for starters? Have you messed with the stator at all that would cause timing issues? since you just rebuilt the carb, that seems a likely culprit. maybe tear it down again or at least blow everything out with an air compressor to make sure nothing is caught up in there. Gas tank is clean?

Does the timing go out just like that, so quickly? It seems it would gradually go... I know nothing about timing...

No. But I wouldn't sweat the timing. I was just asking if you had monkeyed with the electrical at all.

My guess is your problem remains with the carb.

Timing is pretty bulletproof on a stock P. unless the stator came loose somehow, or you've sheared the Woodruff key, I'd scratch timing off the list.

Phew! Ok thanks. Tomorrow, I'll pull the carb. again, check it thoroughly and spend some real time with the tuning.

Hey, what about the exhaust? Anyone think it could be the exhaust? Should I try the Drano thing? http://vespamaintenance.com/engine/exhaust/index.html

Did u find out the problem as having the same issue on my vespa t5... Cheers

Im having a similar issue. Did you ever end up resolving it? Here is a video of my issue. View My Video

I have had the same and was tearing my hair out. I would drive around 10km and then the motor would die, Thought it was the plug as I had that problem before. Before I removed the plug I tried to kick start and it ran again. managed to get to work and home. Next day I could only get about 500 meters and the motor would bog and die. kick start ride a bit and then stop again. Had issues with crimped hose before as well so removed the tank and checked, no crimp. All filters cleaned including inside tank and carb. . Replaced hose with a clear hose and held the tank in the air and opened the fuel valve. By doing this I found that the fuel line was sucking air from the breather in the tank so I would get fuel, a big slug of air and then fuel again. put a piece of wood in the breather pipe to see if it would help and then still got air, but less. After that I blocked the breather pipe completely with a cork and got a good fuel supply. Replaced the tank and put a 2mm hole in the top of the fuel cap to stop a vacuum forming, out of pure frustration to see if I could get it to run. Drove the bike today for more than 30km and had no issues what a pleasure. The only thing that bothers me is why the breather was letting in so much air into the fuel line, and that I don't have a answer for yet.