GP200 (SIL) poor performance after ¾ throttle PLEASE HELP

[ericthered]

Hi all,
My names Eric and I’ve just come back to Lambrettas after a long break, I had about 15 during the late 70s and early 80s (mostly LI’s but a couple of TV’s too), I also had an SX200 in 1997 till 1999, a few of these I tuned myself for a bit more speed/power (nothing daft) and was always happy with my results.
Anyway, after looking at my old photos I scraped together some cash and went out about a month ago and bought a totally standard SIL GP200 with an AF clubman exhaust and imported by AF Rayspeed in 2005 (now has 3600 miles on the clock).

It had been off the road for a couple of years so gave it a new plug, air filter, cleaned out the tank and carb etc and started second kick. First time I rode it was to the MOT garage and was a bit disappointed by the top speed, best I could do was just to say 60 on the flat with head a bit low and keeping my legs tucked behind the legshields (I’m 6 foot and 14 ½ stone). It gets to about 55mph and just seemed to refuse to go any further, fuel felt weak. Checked out the fuel tap and didn’t look very good so fitted an Italian fast flow one, still no better so pulled jets out of 22mm Jetex (yes I know they’re not too good, lol). It had 123 main and a 45 pilot.
Called AF and they explained about ethanol etc in the modern crap they call petrol and said try a 128 main and if too rich drop to 126.
While I waieted for parts to come in the post I pulled the barrel off and checked piston etc, all good so while it was out I matched the inlet and exhaust to barrel and smoothed out the worst of the casting faults.
First run was with 128 main fitted, had nice pick up and good through the gears but still didn’t want to go past 55 (126 didn’t feel right).
Sorry I think I’m making a short story too long but just published my first book so got into a habit of writing, I’ll stop waffling and list what I’ve tried….
123 main, 45 pilot = too weak and struggles to get past 55 (if at all).
124 main 45 pilot = too weak and struggles to get past 55 (if at all).
126 main 45 pilot = too weak and struggles to get past 55 (if at all).
128 main 48 pilot = runs right but still struggles to get past 55 (if at all)
130 main and 50 pilot = far too rich all round and still struggles to get past 55.
Removed air scoop (a tiny bit better) but still struggles to get past 55 (if at all)
Drilled air box (5 x 12mm holes) no difference.
Fitted Casa air bellows (nice and stiff ones) no difference.
Drilled lots more holes in airbox, no difference.
Checked over carb, all ok.
Checked all connections and all ok.
Checked out the exhaust and its all clear (from what I can see).
With the 128 main and 48 pilot the scoot ticks over lovely and performs well until I hit that 55mph or last ¼ throttle. I’ve nailed it in 3rd and it seems to get to 50mph pretty well, drop into forth and gets to 55 then says “that’s far enough!”
Tried NGK plug with 0.5, 0.6, and 30th and no difference.
Tried champion with same gaps and no difference.
On the way to Whitby I did manage to get 60mph down a long hill, but it didn’t want to do it!
Am I missing something here!
Any suggestions or ideas?????
Regards,
Eric.

[johnny LIS150]

its probably not just about the fuel delivery its likely to be a combination of factors.

You have a completely unknown scoot as far as performance goes, so 55mph may be all it has ever been capable of.

I would suggest that you look at the problem as a whole and not just the fuel and air flow.

Check the LT & HT ignition wiring especially the stator . I removed my stator plate last week to find the green ignition wire had become trapped under the stator plate when the stator was tightened up and shorted out by the previous owner! and the stator wires exiting the housing were rubbing on both the flywheel and chafing on the stator housing exit hole.

What gearing does the scoot have ? you may have a set of gears that give you great acceleration but is limiting your top speed by 5+mph Did the previous owner change the primary sprocket size ?
Check that your brakes are not rubbing and the hub bearings are lubricated and not worn.

What type of tyre do you have and what tyre pressures are you currently running in your front and rear tyres ,do you know ?
Tyre pressures are the most overlooked factor in tuning (except in racing of course) and underinflated tyres at 55mph could account for 3+mph due to rolling friction (hysteresis) alone.
Good article on tyres and recommended tyre pressures on the MB site .

Every mph above 55-60mph requires an exponential increase in power to overcome friction losses Its not linear ! think about aerodynamics you and the scooter . Are you for instance wearing a jacket that inflates like a sail when you get going ? surprising how many riders do ,without giving it a second thought. Of no consequence at low speeds but crucial at cruising and top speeds on a bike or scoot.

You may think that all this is irrelevant but have a read on some of the racers blogs facebook etc they leave nothing to chance in their quest for a couple of mph

[mick1]

When you say 55mph is that using the lambretta speedo or a GPS ? If it's a genuine 55 mph with the GPS i think you're close to the top speed of a standard lambretta. The gearbox and exhaust may be an issue in that if it's too tall you may struggle to get the rev's in 4th. I'd check your timing as it may be too retarded, but like i say 55mph for a standard Lambretta is a bit below par but still in the ball park.

[ericthered]

Thanks for the replies chaps.
I’m only using the speedo, not got a GPS, but it seems to feel about right when it says 50 and 55.
The scoot just has a std GP gearbox and no sprocket upgrades.
I went through the brakes and bearings etc again but nothing amiss so tried upping the tyre pressures to 21 front 31 rear and it did make a difference but no noticeable difference to top speed and the dieing thing at 55.

Not got a flywheel puller yet so couldn’t pull that off to check things (can you alter the timing without taking the flywheel off with Indian electronic??). But what wires I could see looked fine.

Then I put my tightest (skin tight and not very becoming of a chap my size!) jacket on to cut down wind resistance and no improvement (to the scoot or my looks).

So I thought I’d go back to where I started.
I put the air scoop back on, taped up the twenty 10mm holes I’d drilled (four rows of 5 holes about central to the air filter) at the back of the air box then tried it with the 128 main and 48 pilot, seemed to go a little better but felt suffocated.
So I took the air scoop off, did not really notice anything there.
Then I took the tape off the top row of 5 10mm holes……….

WHAT A TRANSFORMATION!!!!!
IT FLEW!!!
But here’s the strange part of the story, I had already tried it with no airscoop and 5 holes drilled in the back of the air box as recommended somewhere I read (could be MB developments?). But the holes where drilled in the centre of the box (one in middle, one above, one below, and one either side so like a cross), and it made no difference at all??
BUT, with the holes in a row just above the top of where the filter is the bloody thing flys!!??
I guess it must be something to do with the way the air comes in (vortex or something? Air resistance in the air filter? Who knows!)
With the 128 main and 48 pilot I’m now getting to 60 mph without too much difficulty (on the flat) and have even got to 65 before I ran out of straight!

It felt slightly weak at WOT so I put the 130 in and it took it ok with no loss of top end etc. Going to try the 50 pilot tomorrow and that should richen the WOT a tiny bit and order a 131 and 132 just to see what they’re like.

So it seems that the position of drilling holes in the box has a critical effect on WOT, well at least it does on mine.
Just out of interest I tried the Champion and NGK (both gapped at 30th) and found no difference at WOT or any other throttle position.

Very happy with it now and it seems to be behaving as it should.
Strange about the hole positions though?
Might even try a sprocket upgrade!
Or whats the biggest tyres I could fit and where do I get them from?

Thanks and regards,
Eric.