PWK 28mm- revs not dropping when throttle closed

[Pompeydave]

Hi all, I've got a Casa 198cc kitted lump with a BGM big bore box, and Kytronic advance/retard box and a PWK 28mm carb. The compression ratio is too high due to a replacement piston having a more domed crown than the original, making it really hard to kick over. The unfiltered carb is running with a JJM needle, a 40 idle and a 120 main. My problem is that once warmed up the revs won't die down immediately when changing up through the gears. I don't know if it's the compression ratio being too high at the moment or that it's running too weak. It isn't running hot, and a plug chop shows the correct chocolate brown colour. The throttle cable is well lubricated and the slide returns to the bottom of its travel correctly. The choke is disengaging properly too. Once the throttle is closed, therevs die down after about 3-4 seconds. Any ideas please?

[Meds]

2 things sprig to mind
1. slide not returning properly check that it's all free to move and return, look for cable kinks, try it when moving the headset as that might be causing the cable to tension, sitting on the scoot moves the cable slightly.
2 air leak, if you have a rubber carb mount check that first, unfortunatly that the symptoms I had when my rubber hose failed due to overnighting of the jubilee clip.

[Wack]

Try adjusting the air screw when the motor is up to temperature next time, check the throttle cable has some slack first.

[Pompeydave]

The slide is definitely not fouled or sticking. I'm guessing that the overly high compression ratio isn't helping although I have no pre ignition. I've played with the mixture screw to no avail. Do the needle and jet setting sound about right?

[CHRIS in MARGATE]

I'm a 115 and a 36 and JJK.
I put a stronger spring in mine.

[johnnyXS]

sounds like the piston crown might be the problem.
I would hazard a guess that its incorrect flame travel and squish that is causing excess heat and a lean condition during combustion .
A comression ratio thats too high is known to cause a lean and overtemperature combustion
I would check the timing and try retarding the ignition a degree or 2 to compensate, which should reduce the heat and reverse the lean condition .

[Warkton Tornado No.1]

Hmmm. There’s been a spate of a similar problem with Dell’Orto PH series carbs of late because the clip retaining plate has been fitted beneath the clip rather than on top.

Funnily enough, I have a PWK to hand in the kitchen (where else?) that has a clip on the needle that is too small on it’s outside diameter.

That allows it to go up inside the nylon moulding that is on the end of the slide spring.

I have it in mind to acquire some bigger outside diameter clips to stop this occurring but a washer of the right size (on top of the clip) will achieve the same thing.

I hope this helps....

[Pompeydave]

Yes, cheers everyone. I'm going to source a stiff spring to make absoultely sure it's not that, but I'm going to lower the comp ratio first to take any possible pre-ignition and overheating out of the equation.