93 4BL, powervalve servo cycling but not closing at low rpm

[HOTSHOT III]

Hi all and thanks for the add. I had no idea this forum existed but hopefully someone can shed some light on this problem.

93 4BL DTR with 3 wire servo, cycles open-closed-open OK when turning on ignition but doesn't close when engine is started (I believe it should do this and then open as the rpm increases as in the YouTube vid).

https://www.youtube.com/watch?v=Wo_bI40UyWE

Black/yellow signal wire was disconnected and isolated when I got the bike this week, I've plugged it back in and it makes no difference. Bike sounds good at 6k rpm+ tbh, revs out OK but at low revs it sounds weak and reminds me of the one I owned 25+ years ago with the powervalve fixed open (which is effectively what this is atm).

Has anyone ever measured the voltage through the black/yellow signal wire from the CDI unit? At cranking speed I'm getting up to 19v which would suggest the CDI works OK and it's maybe just a goosed electrical component inside the servo but it's odd that it cycles like it should. Hope someone can help!

[declan]

@HOTSHOT-III from what I understand that's usually a bad cdi although I don't know for sure

[NINJA]

@HOTSHOT-III Hi and welcome aboard.

[Calum]

Hello and welcome!

I would definitely try and look at the simple stuff first.

Are you sure the valve is connected correctly. Sometimes people get the servo cables crossed over and thus this would simulate the problem you are describing.

[HOTSHOT III]

Thanks for the welcome and all your replies guys, will update this if and when I make some progress!

[SpookDog]

Is it possible to remove the pulley cover and watch what is actually happening? Might be a dumb question but I don’t have a servo powered valve...

[SpookDog]

Oh yeah! Could it be needing a decoke? PV sticking in the bushes maybe?...