Hi everyone, hope to soon have the dt125 back to life. Does anyone know how a pwk28mm flatslide would perform on this bike along with some Boysen reeds ? Thanks in advance. 

The PWK will work very well with the correct needle and jets. Look very carefully at the intake boot and airbox though, as they are often very restrictive and will reduce the gains from the PWK.

All Yamaha engines run much better with V-Force reed blocks fitted, and in most cases if they are bought direct from China, will cost less than Boyeson reeds.

They perform very well with stock reeds and stock carb. Unless you are doing other fairly extensive engine modifications, a large carb and aftermarket reed block are not going to improve performance on your 125 and will likely actually hurt it. Specifically, they do not respond well to larger carbs in my experience.

I'm not a fan of Boyesen reeds... they do give a neat off-idle "throttle response feel" boost in most cases. They do not increase power at full throttle and generally make the bike slower under full throttle acceleration in real life testing and experience.

Best "bang for your buck" on your model would be swapping to a complete 175 top end (and 175 carb if you have a 1980 3J0 model.)

Any DT engine which has the small DT100 type reed block fitted, will run better at all engine speeds, if the intake duct is modified and the larger RD250 type block fitted.

The V-Force reed blocks in conjunction with an intake spacer, and bigger carb help even more.

Any intake alterations are compromised by the very restrictive stock air cleaner and intake boot though, so these things need to be looked at, for best possible performance.

All 4 bolt 100, 125, and 175 motors work better with increased transfer flow, and there is a post on the TY part of this forum, showing how an expert 2T tuner goes about getting this done.

Yes any Enduro would benefit from professional porting and some "modern" upgrades.  Everyone should probably send their engine to Rich at Rich's Taylor'd Porting or someplace to have it modified to accept a V-force, larger carb, and porting to go with it.  No disagreement there.

When I was in high school, a friend got a used 1974 Camaro.  Two barrel carb on a 350 V-8 with the horrible emissions equipment they had those days.  Maybe 140 HP. 

He wanted to modify it like most of us would.  His Uncle gave him a four barrel Holley 850 CFM "double pumper".  Great performance carb.  He installed it with a two barrel to four barrel adapter.  It ran worse than stock. 

There was a guy in town that was well-known as an experienced hot rod tuner.  We took it over to him.  He laughed and said he was surprised it ran at all.  They he patiently explained that carb need to be used with the right camshaft, better cylinder heads, headers, higher compression, etc, etc. 

My friend asked for and got a new four barrel intake manifold for his birthday.  Still ran worse than stock and got about 3 mpg.  His uncle told him the Holley needed to be rejetted as it came off of a big block race boat.  So back to the tuning guy we went for jets and/or advice. 

The tuning guy was pretty cool and told us it was still WAY to much carb for that engine.  He said he had one of the then-new Holley 390 four barrels and would trade my friend for his used 850 double pumper straight across AND he would install the new carb and get it running good.  My friend accepted and it was the best decision he could have made.  That Camaro ran great!  It wasn't as fast as a Z28 or anything but it was fun to drive and even got decent mileage. 

Putting a 28 and Vforce reeds on a 125 is not quite the same as the above...  but it's still a lot of work and expense for "maybe" stock performance and a good chance it will perform worse than stock...  unless more extensive mods are done.  And the best "bang for the buck" will still be converting to a 175..."no replacement for displacement"...  And then if you can afford it, upgrading the 175 with porting, new reed block, etc. 

(But to take full advantage of the engine mods, you'll need at least a better exhaust.)

On all DT motors using the very small DT100 reed block, the reed block does tend to plug the motor up.

On any of the DT motors, fitting a cheap Chinese V-Force reed block, works really well, but on cylinders which use the DT100 type reed block, the intake duct must be modified.

Nibbi PE clone carbs are available in 24, 26, and 28mm, are very easy to tune, and take stock Keihin parts.

1980 DT125 does not use the smaller DT100 reed block. (the bolt pattern is the same, not the reed area and opening)

1980 DT125 has the same reeds and block as the earlier YZ125 20+ HP high RPM screamer..  You'd need to do significant modifications before the stock reeds on this model would need to be upgraded. 

I would suggest you try a V-Force reed block for yourself. A really effective modification, and if you buy them direct from China, low cost.

Both the Boyesen and V-Force might be geared for "low speed".  They kill HP at WOT compared to stock in my experience and testing. 

I have the V-Force 4 and my modified 175 loses 5 to 10 mph top speed over the stock reeds.  Boyesen's are similar. 

Some people love them...  the Boyesen's especially improve throttle response at lower RPM.  Not what I hoped they would be in terms of actual improved performance though.  (I am not alone in this conclusion)

The issue with ALL four bolt DT motors, is that the area of the transfer ducts is such that the flow is very restricted.

Later four stud (monoshock) motors, have significantly increased transfer duct area, and will respond much better to a less restrictive intake.

Not ever tried to intake mods on a motor, with stock transfer ducts, but would think It's very likely they don't make much difference?

A DT175 is essentially half of an RD350, and while the porting is not the same, mods to improve transfer flow on the RD cylinders work well on the DT ones.

Not ever tried any mods on a bike with stock exhaust and airbox, as I found very early on that changing those parts, makes quite a difference.