Modern Vespa

I haven't posted much lately, but I've been plagued by a problem with my 2001 ET4 150 for months; namely when I'm riding at WOT for several minutes (or sometimes sooner) it begins to loose power, eventually backfiring when I let off, then if I jockey the throttle I can get it back up to 50mph. Anything over that and it starts to choke again. And lately it's been doing this on fast accelerations as well.

What I've done:

* Replaced the vacuum line
* Cleaned the carb jets
* Replaced the intake manifold gasket. It was shredding, so I thought for sure this was it. Nope.
* Replaced the whole intake manifold. Same problem.
* Completely removed the evap crap (previously I had just disconnected the hose from the carb.)

It definitely ran much better, but still the same basic problem persists.

Now here's where I screwed up: I went to adjust the valve lashings. Had a bastard of a time getting the cover off, but finally got it done. I then adjusted the gap. However I apparently neglected to tighten one of the set nuts enough. As I was riding the bike suddenly made a bad noise and lost almost all power. When I opened up the valve cover again, I found the nut down by the timing chain. It was a bit worse for the wear but still managed to put it back on and readjust both (because I realised I had used the wrong gauge the first time). *sigh*

Started her up again and heard a horrible racket. Opened it up and found the other nut had come off It was in bad shape and couldn't be put on at all.

So I went ahead and ordered the adjustment screws and nuts from scooterwest and am waiting for them to come in.

In the meantime, how badly did I screw up the engine? Should I take it to the dealer to fix or try it (yet) again? What are the possible consequences of the nuts coming off?

Well, if you're lucky you didn't do any damage at all other than chewing up the lock nuts. As long as you found all the loose parts and nothing made it's way down the cam chain tunnel to the bottom of the motor you don't have too much to worry about. The adjusting screws coming loose just means that the tops of the valve stems took a bit of pounding (part of the noise you were hearing) and the valves weren't opening all the way. Not likely to cause damage since the motor wasn't run for a long time this way.

-Craig

That's hopeful I did get the nuts out, so there shouldn't have been anything else to get down there. Now to decide if I trust myself to open it up again.

Once you finish adjusting the valves, change your oil to get any crap out.
This should be done after any valve adjustment as a rule.

What is the rationale behind this? Unless you are working with filthy hands and/or tools (which you shouldn't be), you aren't introducing any crud into the engine.

Enquiring minds want to know.

How do you know nothing got in when you took the valve cover off, or bumped some dirt with the back of your hand while they are in that area? you have a huge open area for dirt to get in while the valve cover is off.

This is standard practice on anything you have to open the valve cover on to adjust the valves or doing anything where dirt can get in the motor.

Just makes sense to me

You must have a filthy scooter and a vastly different one from any of mine, then. I keep my engine clean -- as anyone with an air-cooled engine should for good cooling -- and I've always done all my own wrenching... including valve adjustments. There is no "huge open area" on a Vespa when you take the valve cover off... maybe 4 x 5 inches tops. And it's oriented nearly vertically, making it very difficult for something to fall inside. Never had a bit of loose dirt introduced, either. I'm often doing general maintenance when I do a valve adjustment and would be doing an oil change anyway, but I see no reason at all to do one just because you did a valve adjustment.

Don't ride on dirt roads then do you?

Also, every maintenance that requires valve adjustment, also requires an oil change. Why would you do your oil change on a cold motor, since motor should be cold to do valve adjustment and Hot to do oil change? That sets the order for doing the work.

Not often, and I suspect not many Vespa riders do either.

If I did, I'd sure as hell clean my bike before I did any maintenance.

You are one of the few that do that I bet.
I have never washed my motor.

Thinking of your original problem, have you tried changing out the spark plug wire? Could have an intermittent short heating up causing the problem.

Actually, I forgot that bit in the list of things I did. Replaced the cap and wire and the blue bit they come from

If you'd check, the subject of how to best do it comes up all the time on this forum.

I cleaned my engine when I did my 30,000mi maintenance (of which the valves are a wee bit delayed part of). I need to do under the carb, though. I missed that bit