1UA & 1WT and 31K power valves

[Deleted]

Hi guys

are there any differences in the power valves between the above models

will the 1UA /1WT fit a 31K

or do they cause any issues

thank you

Howard

[jon]

I’m interested in the outcome of this too.

I’ve measured OEM valves from both bikes and can’t see a difference. That’s not to say they haven’t been replaced wrongly before I measured.

However, Fowler’s show a 31k part number for the 1UA so I imagine there the same.

Did hear of aftermarket billet valves being bike specific though.

Jon

[Deleted]

Exactly where i want to go with this Jon

The aftermarket billets state 1UA and 1WT,

I did buy a set for my 31K 1983 LC2 but unsure if they are the same as above as I want them for a 1985 F1 which is also 31K derived

[4l04ever]

From what I have seen, the OEM power valves match the cylinders of one model (31k??) better than the other model. The aftermarket ones have been optimised to be a better fit so are specific to each cylinder variant.

[Deleted]

So the 1UA and 1WT aren't suitable for an F1 or 31K, Correct?

[4l04ever]

Jul 23, 2019 9:20:04 GMT 1 @hblair said:

So the 1UA and 1WT aren't suitable for an F1 or 31K, Correct?

They may not be a perfect fit so not optimum. For billet ones, you need to ideally get ones that match your cylinders.

[tony1964]

I know Martin at Mad Biker Designs sells billet ones, they are marked up 31k and 1ua, the 1ua ones are 1mm different on the exhaust port, i believe the original Yamaha ones going by the part number fit all models.

[midlifecrisisrd]

All bikes had 31k pv's fitted

All pv's of the after market type will work in any cylinder but I'd set them up by looking up the exhaust port, not the pulley mark

The 1ua exhaust port height is slightly higher but not much

Just another anomaly on the 1ua/1wt. The bike makes max power at 8500 but pv's don't open fully till 10200.

Never going to work optimally

Steve

[Deleted]

I don't even get to 10200 on my 83, I usually pull up at about 9-9500

Longevity of my engine is the culprit here lol

I'll check out MBD, the last lot were from FFTP

Thank you for putting in your ideas

[bare]

I've found 31k PVs don't even come close to matching the ex port on my 1ua cyls.
From memory.. about 3 or so mm too small on both; Top of port AND Bottom of port.
Test them?
Rotate them, in a cyl, on the bench then look and measure.
Maybe yours fit perfectly.. maybe they don't ?
IF they don't ? A carbide burr fairly easily reprofiles the things for a clean unobstructed ex gas transition.
Do spread the transition slope to at least the centre of the PV tho.
No cheating by just nibbling away the offending edges

[Deleted]

hi Bare

Not sure i'm that good a craftsman apart from that i've no idea what you are trying to explain, i'm not a mechanic by, i stumble on to things and somehow stumble out, in other words I know enough to get me out of trouble and into it at the same time

TBH I never properly looked at a PV in its cylinder to check what it does

A few pictures would assist if you have them

Howard

[ianboom]

We had a play with the powervalve timings on the dyno, that was a few years ago!

From memory, we set the valves to open much earlier as the bike revved faster than the valves opened and they were also fully open before the bike revved out.

I used Zeeltronic in the end, but Ironically have the powervalve control unit with the DIL switches soldered in that I originally used in my desk!!

I also remember setting the powervalve cables feeling the valve against the cylinder bore, rather than using the marks.