Honda 125 - HPP setup TIPS & TRICKS & TUNING

I see lots of questions regarding bikes not running right, fouling plugs, having bad low-end or bad high-end etcetc...

 

And every so often it's just the setup of the HPP exhaust system that seems the problem. So i thought why not post some basic info on how to set it up properly and alongside some tuning tips for those who want to get a little more out of their HPP engine.

 

 

SETTING UP THE HPP ON HONDA CR 125

 

First of all it's obvious that the parts need to be cleaned and the valves&guides need to be free of any carbon buildup.

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So let's start with just setting it up correctly. Many times guys seem to just bolt things together but forget to adjust the pre-load on the valves which is mandatory to have them shot closed and stay closed for good before they need to open up at certain RPMs actuated by the governor which sits lower in the cases. By running the HPP without this preload the valves have some freeplay in them and won't close good which makes you loose low-end and maybe even some mid-power as the slightly moving parts in the exhaust stream are definitely no good. There's a simple way to detect if you have any freeplay: just try to move the valves by hand. If they have a snug fit and are closed thight left&right then it's ok. But if they can be moved in/out just slightly you have to follow the steps below:

 

So before thightening that allen bolt above, just put some pre-load on that tiny bar just above the valve as shown in the picture below. This way you apply some pre-load on the tiny spring. Just push it all the way in so the spring gets opened up to the max. While doing so (!) close the allen bolt. Note the end tip of the tiny spring near my thumb which gets "bent" open. It really needs some force to hold it bent open and with the other hand thighten down that allen bolt above.

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So in the end it should look like this:

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Do this on both sides and before putting on the covers and re-check if you really have that small gap between spring and arm on both sides! If not re-do it on that particular side!

 

 

REBUILD OF GOVERNOR: 

 

Now here's some tips to get things back to performing as they once did. After all these years of use these governors develop some play. No wonder on engines that are at least 17 years old...This play can translate in sloppy/unprecise actuation of the exhaust system. Just recently i found the solution to get rid of any play in them:

 

1st of all you need to open the governor by using a extractor:

30356828556_877d895ba1_c.jpg

 

BUT - beware of holding on the outside of that shim! It might break off when applying pressure. Just get a grip farther inside where it's thicker so there is no risk at all:

30392978365_8f45c116f0_z.jpg

 

This is how everything looks once disassembled. Note the bearing inside that part i'm holding in my hands in the picture. That's the bearing which wears out and develops the play.

30356823576_0b4f771331_c.jpg

 

Here's the reassembled governor again and the needed bearing to get rid of that play:

30392986835_b0fe67ff9c_c.jpg

 

 

CHANGE OF EXHAUST VALVE TIMING:

 

I found that you can really change the engines charecteristic by working on the HPP timing. I personally run bigger 38mm carburetors which alter mid-top power quite significantly however they make for a slight hesitation, a slight bog before you enter the powerband. By keeping the exhaust system closed a little longer i was able to get rid of this bog and have a much more linear erogation. 

 

It's the tension of that main spring on the governor which determines the moment of actuation of the HPP exhaust system. On our production bikes we have no option to change this opening timing. It is set by the factory. Only HRC cases allow you to work on the governor springs as they have a seperate cover just for the governor so it could be taken out and replaced with different spring rates to open sooner or later.

 

Here's a look at such HRC cases. Check that black cover which allows you to take the governor out without the need to open all the right side of the engine case:

19355200592_a8a1cbd7dc_z.jpg

 

On a stock engine however you need to take off all the right side case to have access to the governor:

8525336080_50ebefb308_c.jpg

 

So when you want to change the timing you obviously have to work on the spring tension. A softer spring would make for sooner opening and a harder spring for retarded opening. I never had the need to have it open sooner but i know from a tuner that on earlier ATAC engines (1987-1989 which have the exact same governor &mechanism in the cases) they wanted it to open sooner and shaved the outer of the spring some..this obviously softens it.

 

To have it open later you need to add some pre-load. For my Mugen equipped engines, running a 38mm carburetor, i found that adding 3mm of preload is just perfect. As mentioned it almost completely eliminates that slight bog before entering the powerband.

 

How to:

Once again you need to open the governor. Just like shown above it's done with an extracor...

 

You then need to have some precise fitting shim. I inserted two 1,5mm thick shims. but obviously one 3mm spacer would do as well.

The exact measurements are:

Outer diameter: 26,7mm

Inner diameter: 22,4mm

Thickness: 2x 1,5mm / 3mm

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The reassembled governor will then look like this:

19173469080_ef8ae5e6f0_z.jpg

 

 

BEST HPP VALVES & GUIDES

 

Now lets turn our focus back onto the HPP valves & guides themselves:

The HPP was introduced in 1990 and up until 1997 the valves have seen only some minor changes. A big step forward though was in 98/99 where they added a L-shape to the valves and accordingly a groove in the guides where this L-shape fits into. This makes for a much better seal when the valves are closed which results in better low-end !

 

These 1998/1999 HPP valves are actually the best performing on ALL HPP engines (90-99)

 

Check the difference in the picture below with the "older" HPP parts behind (1990-1997) and the latest generation (1998-1999) in front. You can clearly see that L-bend on the left and the groove in the guide on the right. The same parts from 90-97 behind don't have this.

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These parts make for a good seal when closed. Just where the red arrow points to the older HPP parts would blow through. You could actually see through there:

8890149352_a6ae093341_c.jpg

 

These valves&guides aren't available anymore from Honda so you have to get them used on Ebay etc...

Here's the parts numbers:

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Following this setup guide you should have your HPP run to it's full potential and have a flawless running engine. If you did all this you can definitely rest assured any possible problem on your engine is not exhaust-valve related anymore ;)

Edited by nino

Awesome post

great write up nino

I just got some pictures that show the massive blow-bye of the stock 90-97 HPP valves&guides in closed condition when they are supposed to seal good for better low-end:

 

View from the inside:

32117878820_a09d2c094b_c.jpg

 

From the exhaust side:

32117878640_8f5abe5a5c_c.jpg

 

Compare this with the 98/99 HPP where the red arrow points to! No blow-bye at all. That's where these later HPP valves are superior and offer better low-mid power.

8890149352_a6ae093341_z.jpg

Edited by nino

Great write up, new to the forum and to the hpp valves, but I have a question for you. I recently picked up a 99 125, when I got it the tie rod and pinion shaft were in this position with the valves completely closed, with pre load and no play in them. If I remove the the tie rod from the pinion shaft the arm will raise up above the top gasket surface as if the govenor Is in the correct position but the pinion shaft is not, I tried to loosen the bolt and move it up with the valves in the closed position, ( was just guessing) but once you put the arm on both side will not open or close in the same position. My question is do you know what effect this would have on performance?  Would it open sooner? And is there any reason why the previous owner would have it like this or is it just wrong?  Havent rode the bike much since u bought it but it didn't seem to run bad although I've never spent much time on a 125. Sorry for the long post but this is probably the best info I've found on it yet so I had to ask. Thanks!

20170121_195143.jpg

Sorry for responding only this late...

 

This position is just wrong !! Obviously they had opened up the engine and when putting back the cases failed to put that rod in the correct position. You would need to open up the right-side crank to fix that:

-let the water out

-remove the kickstarter

-remove the exhaust

-remove the right side HPP cover

-remove the clip of that rod which engages to the valves

-lay the bike on the left side. This allows you to work on it without having a mess with gearbox oil all over the place...

-remove the bolts of the right side case

-lift the case up about 1 cm (1/2 inch). This should allow you to pull on that rod and have it move 1 dent/notch upwards.

-close all up again. Now your rod should sit in the correct position. If not repeat until it is about level to the main axle of the HPP exhaust system where you engage it with that clip.

 

This is done in about 15-20 minutes easily!

 

 

 

 

Edited by nino

My son and I are rebuilding a '92 and most of this stuff is new to me.  If I'm reading this right the valve should have enough preload to close its self, without the tie rod hooked to it?  Do ya have any leads on where to get a tie rod for a '92. I read some where different year rods are different. Do ya know the best year to go on a '92?

5 hours ago, tucan said:

My son and I are rebuilding a '92 and most of this stuff is new to me.  If I'm reading this right the valve should have enough preload to close its self, without the tie rod hooked to it?  Do ya have any leads on where to get a tie rod for a '92. I read some where different year rods are different. Do ya know the best year to go on a '92?

No-you need to have the rod coming up from the case engaged. Otherwise the valves won't be closed. So to adjust the preload on those valves you have to have everything attached and engaged.

I just checked on Partzilla and they state 93-99 are the same. Anyway - i'd say 90-99 are the same as all they do is connect the governor with the mechanism above...no difference in actuation or power.

BUT - in my article i was talking about freeplay and slop in the mechanism. That's where those rods also come into play! The joint there in the middle wears out and can have massive play which again makes for unprecise action of the exhaust valves which then can hurt performance. In order to have things working properly you obviously want everything to have a snug fit and work precise.

Part No. 20 is the rod i'm talking about:

36812494351_8ab6a88303_z.jpg

Edited by nino

Thanks, that thing is starting to get a little easier to under stand.  It sort of remind me of a watch or something All the gears and spring.

Can this preload shim just be a copper crush washer? These are availabe in that size.

And are there differences in spring tension throughout the years of build?

 

 

Edited by Henkies
11 hours ago, Henkies said:

Can this preload shim just be a copper crush washer? These are availabe in that size.

And are there differences in spring tension throughout the years of build?

 

 

The Material doesn't have too much importance. For my first governor mod i also used copper shims.

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The governors are basically identical and interchangeable between 87-99 BUT obviously they changed springs many times during those years . I just had a look at Partzilla and there you see that several models share the same governor. There's obviously minor differences in spring tension (exhaust valve timing) but i couldn't tell you exactly what the different opening timing is for these different governors. You would have to put your bike on a dyno and check the opening for the different governors...

87-88-89

90-91-92

93-94-95-96-97

98-99

Edited by nino

How interchangeable are parts on the early years of the HPP, say from '86 to '89?

On 14.9.2017 at 10:55 PM, ASP1227 said:

How interchangeable are parts on the early years of the HPP, say from '86 to '89?

86 has nothing to do with 87-89 (ATAC) which have nothing to do with 90-99 (HPP)

87 up actually have the same cases up until 97

98/99 have slightly wider cases up to the transfer ports. Otherwise they're still the same.

I thought HPP started in '86. My covers on my '86 say HPP. From what I've seen '86 and '87 have the same design with a few minor differences.

Ok, to answer my own question for others future reference, parts off an '86 cr250 powervalve are ABSOLUTELY NOT interchangeable with any other year except the '87, not even the bolts that hold in the valve guides. If your looking for parts for your bike you should get the right year. But the '86 is an HPP, just a very archaic version of the HPP. The newer versions are simpler and less maintanece prone. The manual says the '86 HPP needs service every 2.5 hours, they're not joking.

On 18.9.2017 at 5:01 AM, ASP1227 said:

Ok, to answer my own question for others future reference, parts off an '86 cr250 powervalve are ABSOLUTELY NOT interchangeable with any other year except the '87, not even the bolts that hold in the valve guides. If your looking for parts for your bike you should get the right year. But the '86 is an HPP, just a very archaic version of the HPP. The newer versions are simpler and less maintanece prone. The manual says the '86 HPP needs service every 2.5 hours, they're not joking.

Hey this thread is about 125cc ! ;)

Lol oops. It's the same principle I think, just everything is slightly smaller right??? ;)

About the hrc cases... I couldn't find any online, any idea how much $ they run? Or where a guy can find one? And my bike is a 97, interchangeable years?

And this might be a stupid question, but you use a puller to take apart the governor, I'm guessing it just presses back on no problem? My manual does not have any info on that. 

Great thread by the way. Thanks!

HRC is factory stuff...very rare & very expensive IF you can ever find one ;) Mugen kits same story.

 

Cases are really the same from 89-97. All that changes is the color afterwards until '97.

87-88 are the same as well but out of magnesium which corrodes ---> no good!

89-94 black

95-97 silver

98-99 are similar but have wider side ports at the cases ---> interchangeable as well BUT cylinders need to be 98/99 as well or you need to enlarge your cylinder to make it fit.

 

Yes - that governor gets just pressed back together. It's nothing that Honda wants you to service ;) That's why it is in no manual and also much overlooked. It has a big influence on how these engines run and especially on older ATAC engine they need to be free of any play to get the best out of that dated/basic design. The older ATAC exhaust system does't have these tiny springs that eliminate the freeplay. If you have a worn governor your ATAC valve won't open/close the way it is supposed to do and just hurt performance. Same with the levers that come up from the governor. Most have enormous play in their joints which is really bad. (the governor might move for actuation but because of the play that initial moverment gets delayed some and it might even not be able to do the full range of motion needed!). Everything needs to have a snug fit and free of play to ensure precise actuation of the exhaust valves.

Edited by nino

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