Power Valve Adjustments

MikeS · Mar 24, 2004

I have read here and there about changes to the spring / preload.

Am I correct in these assumptions?

Adding Preload will delay the opening point.

A lighter spring will quicken the "time" to full open.

Adding more preload and a light spring will produce a later opening point but the PV will be wide open sooner.

thanks

Just trying to sort this out to understand before I start playing.

marcusgunby · Mar 24, 2004

You are right but dont concern yourself with how long it takes to get full open-they are almost like a switch when you watch them.As long as the timing is right for when they open it will work better.Also when you change the engine by a reasonable amount it may need a different opening rpm.

People say dynos arnt needed but i dont see how you can optimise it without.You can see so clearly when its right or wrong.I now feel this area is as important as jetting.

DEANSFASTWAY · Mar 24, 2004

Marcus what jetting were you running in your 03 KX125 ?

marcusgunby · Mar 24, 2004

From memory
main 400
needle middle clip
pilot i went as low as 30 with AS at 1 turn, the pilot is the furthest away from optimum when std.

Did you get my email? im not sure which address you have nowdays?

MikeS · Mar 24, 2004

Thanks Marcus

I don't have dyno access right now so we will have to give it the seat of the pants test.

We are trying to get that snap going into the turns. It is a 58mm EG big bore. I am going to map the port timing this weekend and double check it. We advanced the timing alot per EG and it has the snap but the top end drops of flat...real flat.
Jetting is nice. I figure give the power valve a go.

Again thanks for the confirmation

Mike

marcusgunby · Mar 24, 2004

Steve is the resident 58mm piston expert maybe i can get him to chime in with ideas, whats your exact engine setup mike?

elf · Mar 24, 2004

On my ktm I set the pv to open later than stock. This gave me more bottom with a wider spread of useable power. It also gave it less hit. Of course on the ktm its as easy as turning a screw to ajust it. Just out of curiosity how much did you advance your timing?

jaguar · Mar 25, 2004

You have crisp bottom end power when your PV is closed, and it don't rev far enough when the PV is open. So when it changes from closed to open is not the problem. Its porting most likely. Being a 125 you can't have strong low end and also have it rev to the moon. If the track is tight then stick with the low end. If it is open with long sections then raise the ports for more top end.

MikeS · Mar 26, 2004

Marcus

My combo is not the norm. 03 TM125E.

Alot of the basics will apply anyway. The 04 TM uses a light spring with more preload. Claim is a smoother transistion.

We have limits in pipes/silencers. Using your not so popular HGS pipe. It is the best for mid/top power so far. The Doma spreads everything out...maybe too much. A Pro Circuit was a good mid range pipe that could be run hard but they are no longer in the market.

Jetting has been sorted out with a DDJ (3/4-full, taper added) 182/185 main.
Bike pulls clean and quick.

I added some preload last night and Jr said he can't tell. Well I may leave it for now as it is alot of work to change it (2 hours)

Jag

As for the comment on you can't have both top/bottom. Well we have gotten very close. Better bottom with almost the stock "killer" top.

The top goes flat for us and I am leaning towards the port timing. The transfers and exhaust port of the 58mm kit do not uncover at BDC of the stroke.

So I was curious about the PV effect to add to my learning curve.

Thanks Guys.

bclapham · Mar 26, 2004

one thing to remember, quite often, the base of the cylinder is skimmed off to go back to stock port timing with the 58mm piston. On some bikes (my RM, LOL) the pv governer linkage is just a rod sticking up from the bottom of the cases and since the cylinder is dropped lower, on one cylinder i had, the pv was running slightly open all the time- just check this before sending yourself in circles- i little fettle is all that is needed.

Mike: what carb does that TM use??? 38 or 39.5???? ive got the 38pwk on my RM- 1470 needle, i was running advance on the ignition but it would ping- ive gone back to stock with a slightly milled head, same bottom and more top- it was pinging yesterday @ wide open with a 168 main, all i had was a 172 and no more ping and it still kept revving out! nice!

MikeS · Mar 26, 2004

38 PWK

PV rod is adjustable so I can get it close just nice.

Testing tomorrow, I am going to play with timing again. The HGS pipe really revved way out on the stock motor so we are going to give it a shot.

I cannot run that lean, it falls on it's face real fast.

The 97 TM125 Eric did pulled like a tractor and revved slower but longer. This 03 TM125(150) revs real quick. Low end is improved, mid is real strong but the top signs off to fast. Junior needs to shift more.

If this doesn't work then off to the lathe we go. :clue: The TM uses a step on the cylinder head, I can use thick base gaskets to get the ports .020" higher then adjust the head to correct the squish without affecting the volume or shape of the combustion chamber. I will map the ports first and consult the master. :think:

bclapham · Apr 9, 2004

i just had a moment of inspiration, LOL!!!

can we calculate the best RPM range when the PV should open?

if so, on most bikes, you can access the PV cover- simply get a tachometer and rev the bike and watch for the PV to open and then adjust it accordingly to the RPM range when you want it open!

anyone got a spare tacho strobe, LOL!!!

DEANSFASTWAY · Apr 9, 2004

I like to use a timing light with a tach on it .

bclapham · Apr 9, 2004

Dean, is there a magic RPM when the PV should open???

DEANSFASTWAY · Apr 9, 2004

I dont know as a rule but i heard tell of like 125s being like 8k and 250s being 6k . Kaw used to have some team green articles that had different gov ramps and springs with the different rpm levels .I think the new honda elect P/V has some sort of check or spec I read somewhere but I have to go back and look at a book. ??

bclapham · Apr 9, 2004

this should do the trick!

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=41727

marcusgunby · Apr 9, 2004

Bruce i think you can be close with 8K on a 125 but it may vary a few hundred rpm on each bike and also depending on mods, the 04 rm125 didnt show the pv dip until it got a better pipe on it.The HGS showed the dip more than the other so would need the pv opening time adjusting earlier than the 2c pipe.

Power Valve Adjustments

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