BGM PWK 28 idle behaviour

[kejj]

I have a question on idle behaviour.
When I set the carb up I noticed that it has 2atages of idle level. From full throttle it drops down to fast idle and stays there for a few seconds, maybe around 10. Then it drops down to a lower idle.

Could not figure out if this was normal behaviour, faulty carb or wrong settings.

Anyone noticed this?
I'm sure it's not airleak as it is stable in both idle levels. No Erratic behaviour or such. Stable but in 2subsequent stages.


In iron Lambretta cylinders we trust!

[molibdimum]

Have you had the carb apart and blown all the jets through?

[kejj]

Totally clean throughout


In iron Lambretta cylinders we trust!

[olliewtf]

i had this after my rebuild with the carb i had used before, a 28mm delly. i eventually put it down to an air leak, in my case from the base gasket. worth checking that out, either way the problem will get worse and worse if it is that. i convinced myself it was to do with running in, ie tight piston bore clearance blah blah but no just a simple air leak.
although plug colour was fine, it ticked over without racing, it was still that, the tiniest of leaks which got worse and worse til i sussed it.
could also just be slide sticking, cable routing causing slide to stick etc. thought id just give you the bad news first aha

[ukpeckerwood]

And is the slide closed when returning from full throttle? only asking because I had the same symptoms and only cured it by putting in a stiffer throttle slide spring

[Chris in Margate]

Have had this too. Stronger spring and also found the circlip on the needle was moving.

[kejj]

Interesting with the slide, will check tomorrow. Difficult to see while running. And I guess it wont be evident on tick-over stand still revving.

I was our driving for 2 hours today, both town and open road (No high way).

If I rev the scooter high, upon changing gear the revs stick for a short while with the clutch pulled in. Not much and didn't affect drivability, I just noticed it,
While out I set the tick over and air screw to idle better. Good instant throttle response and OK fall down of revs back to idle.

When driving around town with stoplights and low revving. the 2nd lower tick over showed itself more evidently. I noticed the this lower revs idle sounded awful lot like bog down overflow, Upon trying to hit throttle wide open it got this long vrooooooaaaaaaauuuuuwwww before hitting to rev. It's at this moment where, if I shut the scoot of, I would have trouble starting it. If it starts it would be with full throttle and still no revs, just idle chucking until it clears up. Evident says idle jet to rich. To much fuel in the idle position.

More investigation tomorrow.

If, when idling normally, I block the pipe it will die. Both exhaust and inlet is well secured and no leaks from there or cylinder head och base. super dry!
So for now I wouldn't believe its the air leak route.

Also, PWK is new to me, used to JETEX and PHBH. I have a PHBL I might swap with just to try the alternative out but it would mean I go down to 25mm.

[netweasel]

Here is a PWK how-to I posted on the LCUSA forum:

I recently picked up a GP200 that had a OKO/PWK 30mm fitted with an old PM Style ScootRS pipe. Although it ran, it was jetted poorly, not to mention that whoever last rebuilt the carb stripped most of the housing and jet threads. So I picked up a few 30mm powerjet OKO/KOSO/PWK carbs cheap off of ebay and decided to try various setups until I had it jetted perfectly.

These carbs seem to come from various manufacturing facilities. There are 5 modifications that you must perform.
1). Check and drill the float needle area to 5mm. (The large area of the brass insert. Be careful, and go slow. If you mess up you can pull out the brass insert, or drill through the bottom of the brass insert.)
2). Check and drill the float needle center fuel inlet to 2.5mm. (The small area of the brass insert.)
3). Check and drill the float needle area horizontal feeder holes to 2mm.
4). Check and drill the idle/slow jet outlet in the venturi to 1.2mm. To do this you will need to first unscrew the jet holder from the bottom of the carb. Start at .5mm or .6mm and work your way up to 1.2mm
5). If the OKO/KOSO/PWK carb you have uses round jets, swap out the atomizer/diffuser for a Keihin style hex jet one.

One of the main problems people encounter with these carbs is that they can't get them to idle without turning the idle adjuster all the way in. This issue is because of two reasons:
1). The first is that the idle circuit needs to be drilled out.
2). The second is that people are running way too lean idle jets. People always think that if it is lean it will cause the engine to race, which is true for main jets and slightly smaller than required idle/slow jets, but if you don't give it enough fuel to begin with it will not run at all and will mimic a too-rich condition at idle. To compensate for the too lean idle jet, people turn the idle adjuster all the way in which uses the slide cutaway to add more fuel allowing it to somewhat idle at a higher rev. Another side effect of a way too small idle/slow jet is that it will idle with the choke on.

The jetting I used was:

slow/idle jet: 48
slide: stock 3.5
needle: JJH
needle position: 3
main jet: 125
air mixture screw: 1.5 turns out
filter: stock air bellow w/ oiled foam UNI pod filter.

I used a set of Dormer metric drill bits for the 2, 2.5, and 5mm holes, and the Harbor Freight micro drill bit set for the 1.2 mm hole.

At WOT I can see the Powerjet being utilized. There are no flat spots, and the throttle response is crisp. The plug is a nice chocolate brown. I did these mods to three individual carbs, and they all respond the same on the scooter.

[Minority]

Found this site very interesting, although not directly scooter related.

http://www.mid-atlantictrials.com/Sales.html

[kejj]

It amazes me the amount of modifications we need to do to a PWK type carburetor for it to work on a Lambretta.
The PWKs are very popular in much larger bikes used in racing, dirt bikes etc. Why they work for them unmodified and not for Lambrettas must be a mystery!

I'll see if I dare to modify mine or just switch to a phbl instead. Thanks for all the info!