Modern Vespa

Having just returned to scootering after nearly 15 years away I have bought an 02 PX200E Disc (with Elec Start).
This issue is that when riding along at any speed I throttle off to brake or slow down for another vehicle and the engine seems to start to cut out.
Then when you roll on the throttle again it has a massive flat spot then after a split second the engine will pick up again.

I don't remember any of my old scooter behaving in this way and wondered if anyone had any ideas where to start looking?

many thanks in advance
Russ

Hi Russ and welcome.

sounds like incorrect air screw setting if it's at the very bottom if it's around quarter it's the slide. I would start by taking the carb and carb box off check all jets are correct and set right and not worn(slide) put it all back together with new gaskets. If it's still the same after that check the piston rings they may need replacing, poor compression can cause hesitation on throttle response.

Moved to Not So Modern.

Hey thanks for the advice I'll give it a try.

Also thinking about it I also noticed that at part throttle on the motorway last nighty it seemed to be 'pinking' like a car engine would when the mixture is to lean.

Could it be its running to lean on the mixture and that could be the cause?

Russ ;o)

oh oh.... you soft seized. yea that pinging is something you hear just before your engine 'spodes! boom!

I don't see much evidence for soft seizing, sounds more like a carburation issue. Try richening your idle circuit a bit (1 to 2 turns out on air mixture screw at rear of carb box) and see if it helps with the flat spot and partial throttle pinging. This is not uncommon, and is the easiest thing to check.

Russ is it a standard P200? pinking can also be caused by flywheel timing. I once melted the piston into a barrel of a pinasco 215 kit because I forgot to alter the timing. Although I think your problem sounds more like carb or piston rings.

As far as I know its a standard PX200E engine. I'm going to have a look at it today and ignition timing is something I had thought about.

I'll start off with the basics on the fuel side.....

I did find this post that sound exactly like the issue I'm having:

PX200 PROBLEM - FLAT SPOT - HELP !!!!!!!

Cheers
Russ ;o)

I like the idea of the plastic tray in the frame under the tank I've had loads of scooters without that in(in fact every second hand scooter I've had) and not had a problem.

let us know how you get on

Hello

long shot here check for fuel starvation and the air hole in the fuel cap, the idle screw is further out on the later px models, 1 3/4 to 2 turns from fully in.

Grumpy

Hey all, checked the mixture and its is running perfect, the spark plug is the correct colour that I'm told it should be.

Checked the fuel filler cap and it appears to have a breather valve in it but I can not test that.

any other ideas to try?

Russ

Hello

Test by blowing through the hole, you would be surpised how often it is blocked or run with it off


Grumpy

Hi Russ

Do you know how long the piston rings have been in? how many miles on the clock?

As far as I can tell its never had any piston rings and its done 22000Ks... probably needs them changing. Are you thinking that this could be the fault?

I spoke to a dealer today and he seemed to think it may be a stater plate issue?

Hello

My px 200 did 55,000 miles with the same rings and little wear, if your plug is the correct colour you have very little blowby.

Grumpy

yes was thinking that it could be. My last T5 I had I put rings in when I bought it, 4000 miles later they needed replacing(t5 ring wear quicker than p200 ones) and I had the exact same problem as you. So I changed the rings and the problem cleared.

Can you elaborate on how you checked the mixture? Sounds like you just checked the plug color or something?

It's not clear to me if you tried turning the air mixture screw out a few turns to test its effect on the flatspot. I'd just feel better knowing you tried this easy and common fix before moving on to more exotic remedies.

I just had this same symptom on my PX. I actually solved it by installing a different idle mixture jet, but you can get some idea if that's the issue by adjusting the screw and noting its effect on the flat spot.

Hi, I checked to plus colour to see if it was running lean or rich.

If the scooter ticks over fine when warm and when you rev it stood on the stand the revs should come down nice and even not take too long or be erratic your something like right.

Plug colour is not really a reliable indicator of partial throttle behavior, which is why I keep recommending you back off the air mixture screw a couple turns to richen up your idle circuit and get it right using the seat of your pants.

I'm not just making this up. Try the easy stuff first, then get all sexy with piston rings and whatnot.

Forgive my ignorance but am I turning it clockwise or anticlockwise to richen the mixture?

Counterclockwise (out) richens the mixture.

So I have tried turning the mixture screw to richen it up at the bottom end but it has made no difference.

Next steps anyone?

Russ

If you are running lean due to an air leak then richening with the mixture screw might not make enough difference to notice.

You said you checked the plug color, but never actually said you did a plug chop (http://www.incriminators.com/plugchop.html) or what color it is. You may be fine at idle but lean at speed.

Hi all thought I'd post an update as to what I have done and where I have gotten with the flat spot problem.

So after much reading on the forum I decided to do the following changes one at a time to see if any would cure the flat spot.

1/ Adjust the mixture screw. Outcome = No change
2/ change the rings. Outcome = No change
3/ Skim the head. Outcome = No change
4/ Change the exhaust. Outcome = No change
5/ Strip and Rebuild Carb. Outcome = No change
6/ Change Spark Plug. Outcome = No change

to say I am at a loss as to the problem is an understatement......... any other ideas?

Russ

Christ on a bike Russ!

sorry to hear all the things you've done had no result.

The only other things I can think of is if it's a cast barrel it could be worn, I think you said it had done 22,000 km or something like that. You could either re-bore your existing barrel and put and over size piston in(I think beedspeed are doing a range of 200 oversize pistons), or kit it up. But if you want to keep it standard I think you can still get hold of standard barrels.

Or as I thing someone else has said check your timing.

other than that I'm at a loss mate.

did u check ur cdi (electronic)? a bad cdi can sometime affect pinging

Well after a long a troubled search I have finally sussed out the cause of the problem..........

Crank oil seal!!!!! causing a very slight air leak and all my troubles.
Just about to pull the engine apart and fix it so I'll update ASAP.

Hi all and my last update on my flat spot issue.

I had the engine stripped down and rebuilt professionally. Ran the engine in and It still had the flat spot!

The last resort before I set fire to it was to take my scooter to my nearest Dyno. After an hour of changing jets only, no timing adjustments it runs properly.

So I spent all that money on a rebuild when I should have taken it to the dyno in the first place.

But now the problem is cured!!!!!

and I'm a happy man

if you read your whole thread you'll notice member HP touched on the carb as the typical

that guy has never been wrong when it comes to carb/jetting discusions

Thank you very much for reminding me ;o) and thank you HP.

I did try and set the carb up but I guess you can see a whole lot more going on with a computer screen on a dyno. And I suppose it always helps when you know what your doing.

Even tho it was a pain in the ass I have enjoyed every minuet of messing about.

sorry if i was all "i told you so", i just know from my own over thinking and how pleasent and mellow hiram (hp) would try and direct me on simple fuel/carb stuff

now a days i just try to remember the 3 rules of thumb

fuel
spark
compression

on a well maintained bike it is rarely the bottom two and more likely than anything the top one.......fuel

good gas
proper flow
clean jets
good enough mixture settings(they're never perfect )