Modern Vespa

Would hugely appreciate help.

2004 Piaggio LT 150 - its bogging down on quick acceleration (aka applying twist of wrist quickly) at all speeds (from a stop or while at high speed, mostly on hills). If I take it up to speed slowly it seems to be mostly fine.

Actions I have taken:

1) pulled carb and cleaned it TWICE both jets and inside of float bowl
2) changed fuel line
3) checked fuel filter, looks good - I pulled petcock to do this
4) air filter is fine
5) no vacuum leaks, tested with carb cleaner
6) removed full EVAP system (just in case it wasn't working properly)

Seemed to be better but then I put 87 grade unleaded in the tank and my problems immediately started again as I left the gas station. So I drained the fuel from tank and float bowl of carb. I put in 90 grade unleaded (that's the highest I have in my town).

But I am still getting the bogging down on quick wrist acceleration at all speeds? I am at my wits end here. Any help would be much appreciated.

Did you happen to clean out the accelerator pump circuit? It draws fuel from the float bowl and uses the plunger (highlighted in red) to pump a shot of fuel to a jet nozzle in the carburetor's inlet.


With the engine OFF and the air boot removed from the back of the carburetor, look into the carb and press down on the silver tab on top of the accelerator pump; you should see a spritz of fuel get squirted in towards the throttle butterfly from the accelerator pump nozzle.

Appreciate the reply! I didn't clean it specifically. But Yes I am getting a big shot of fuel when that is depressed. I cleaned around it and also just took the carb off the bike to double check re the fuel shot.

Could it be some form of detonation from using sub 95 RON fuel... Ive read 95 RON (OEM recommended) translates to 91 rated fuel here in the USA... so could it some form of detonation from using 90 fuel.

I'm curious why filling up the gas tank with 87 (and accidentally overfilling at this time) made the problem so pronounced?

I was thinking of going to 93 rated unleaded if I can find some AND maybe changing the petcock?

I wouldn't think 87 would necessarily cause sluggish acceleration. I'd worry more about getting a bum tank of fuel and needing to clean the system again.

I highlighted part of your reply, because I wonder if your LT has the evaporative emissions control (EEC) system installed? It's possible that fuel has gotten in there and it 's playing havoc with your fuel/air ratio?

Overdue Valve lash maybe?

No, it doesnt have the EVAP system. I removed it as part of the process of trying to fix this issue. Removing it didn't seem to help.

I dont think its bad fuel, as I have tried changing the fuel a couple times. My only concern re fuel was if it was not high enough octane. But from your opinion, you dont think that would be the issue.

Appreciate the help!

Hmm, Ill have to check that this weekend and get set of gauges to measure the valve spacing.

How many miles are on the LT 150?
When did it last run well?

7,700 miles... It ran fine about 3 months ago. It first gave me issues when I had a passenger on the scooter and was going uphill. It ended up bogging down and giving me some minor backfires and then stalled. I live in a town with a large number of very long steep hills. I have taken no passengers since.

It has continued to now bog when climbing hills with quick throttle application even with no passenger.

fuel is unlikely the problem unless it's been contaminated with water-- highly likely given the recent weather we've had.

how far apart did you take the carb? remove the air fuel mix screw and clean that passage way out?

air filter, new? oiled correctly and airbox lid correctly sealed?

no cracks in the intake manifold or the intake hose?

spark plug? valve adjustment? how's the exhaust look not rotting away, sealed and not leaking, nothing took up residence in there?

you say it started when 2-up and that leads me to think maybe something in the suspension was contacting when compressed.

bogging can also be a byproduct of the transmission playing up. how's all the wear items in there look?

That is a great list - I will start running through those this weekend;

- Air filter looks good but i didnt put all the screw back when I reattached the airbox for ease of future work... I only put about 20% of the screw that held in the air box cover back on, there shouldnt be too much extra air BUT worth a shot

- i didnt take apart the air mix / fuel screw, i just cleaned the bowl and both jets (BTW the jets were 82 main and 34 idle)

- no water in the gas as the bike was stored in a garage and I changed the gas to 90 octane today

- i havent changed the spark plug, will do on this advice and also check the valves (will save the valves as last resort as its harder to work on)

- no idea on the wear in the transmission - thats above my ability to wrench, id have to take it in somewhere

Thanks all, I will update after the weekend.

I got to check some of the things you all suggested - thank you!

It seems to have been the air filter (it was soaking wet with oil in spot near the intake, i squeezed it out). I also put all the screws back on the air box instead of just the top and bottom (I had not put all the screw back so that I could more quickly work on the bike, but now they are all back).

It seems to be working fine now. I suspect that it was the air filter being clogged with too much oil. This must have been making the bike run rich as not enough air would have been getting through.

Appreciate the help everyone!

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ALSO, just in case someone is looking for how to access the carb on the 2004 LT 150 / LT150 - it isn't as obvious on this model:

1. Remove the seat (two bolts at the front)
2. Remove rear rack (3 bolts)
3. Remove the inner part of tail light and turn signals.
4. Remove rear cowling (two screws behind the front access panel and two large screws either side about 4 inches back from access panel)
5. Remove battery
6. Remove gas cap and then battery tray (the battery tray has two screws, one is under the battery)

Not long after it started happening again … about a week.

I changed the spark plug and it now has been running fine for months. Sorry to have taken so long to update.

Bizarre but whenever I hit bumps and was full throttle going up hills in SF it would bog… I can now confirm it was 100% the spark plug and maybe a bit the air filter.