Jump to content

S3 Low Compression Head Replacement


Robinhobbin
 Share

Recommended Posts

Has anyone installed an S3 Low Compression Head Replacement? My 2023 Gasgas 250 TXT Racing is too lively for me and it feels like the throttle is either at idle or full on with very difficult low rpm control. The thing just jumps out from under me with the slightest twist of the throttle. It’s also hard to start and almost impossible without leaning it against a tree or a rock to stand up and kick the crap out of it. My old Yamaha TY250 was much easier to control and start. I’ve installed a slow throttle that proved somewhat helpful but now I’m considering an S3 low compression head. Does anyone have experience with them and what can I expect? Thank you so much for your help in advance. This issue sent me to the hospital with a punctured lung and 4 broken bones. I need to fix it.

Link to comment
Share on other sites

 

Welcome to the board.

I don't know anything about your bike specifically, but decreasing the compression ratio will both help starting and reduce the motor's torque.

Other things to try would be retarding the ignition timing and making the low-speed jetting richer.  Both of those things will likely make it easier to start too.

Also, try riding in a higher gear (or gear the bike taller).

A flywheel weight will decrease the motor's rate of acceleration, but only if you have excellent clutch skills.  Otherwise, that mod may just get you into more trouble.

Edited by konrad
Link to comment
Share on other sites

 
 
 

I put an S3 low compression head and a flywheel weight on a 2017 Sherco 250.  It didn't really make much difference!  I also fitted a smaller engine sprocket.  That slowed it down a bit, although I later went back to the stock size. 

On reflection it was probably just getting used to riding that particular bike that made the biggest difference, rather than the modifications.

Link to comment
Share on other sites

 
 

My 2015 GasGas 300 came with a flywheel weight from the previous owner.  I noticed immediately that it did not have that "pop" at small throttle inputs, it had to be revved more to get that response.

Made the bike feel sluggish to me.

I took the flywheel weight off within a couple days. 

A flywheel weight may provide the result you're looking for.

  • Thanks 1
Link to comment
Share on other sites

 
4 hours ago, Wishiwastwentyfive said:

It’d cost me about $250 to make the change and I don’t want to wast money on something that won’t work.

If you are looking for an inexpensive option to a low-compression head, consider installing a thicker base gasket.  This decreases the compression ratio by increasing the clearance volume, however it also changes the port timing.  But in your case that may help achieve your goal by moving the torque up the rpm range slightly.  

Increasing the thickness of the base gasket by 0.5mm will have a very noticeable effect.  Ideally, you would measure your current base gasket first, but a lazy person might just order a 1mm gasket and test the result.  See: https://www.thehellteam.com/products/gas-gas-genuine-parts/gaskets/

I've written some basic information about heads here: https://www.ossa-efi.com/home/engine/heads   If you have access to a lathe, a zero-cost option would be to increase the combustion chamber's blend radius.  This will slow the rate of combustion and thereby make a less aggressive power delivery.

 

  • Like 2
Link to comment
Share on other sites

 

A couple of years ago, I bought an 01 Rev3 270 with almost zero wear on the original tires.  Power was very sharp right off the bottom with the initial crack of the throttle.  It was much much sharper than my 2014 Evo 300 2t Factory off the bottom.  (The 300 Factory sort of took off mid range.)  I added a full millimeter of base gasket for a total of 1.5 mm.  Then I installed the heaviest flywheel weight I could find.  It was something from a Techno marked Ducati.  Power delivery is much smoother and it is easier to kick.  I would think anyone who likes vintage bikes like a TY250 would like flywheel weights on modern bikes. 

I also find that going down a tooth on the front sprocket helps.  The clutch out speed is slower so it makes clutch use easier in slow tight turns.  (I'm no clutch master.)  But, based on your injury description, you may have been going pretty fast.  A smaller front sprocket also reduces the tendency to build speed with a series of obstacles to clear like log-log-log.  If the throttle does get away from you, the bike will also run out of RPMs quicker with a smaller front sprocket so you can't get in as much trouble.  I have tried 2 teeth lower but find it reduces the ability to get up and over things in first gear.  A smaller front sprocket does reduce top speed if that is important. 

    

Link to comment
Share on other sites

 

Hmmm, I think mcman makes a good point about gearing the bike shorter that I had not considered.  I'd say gear the bike taller so it has less driving force, but gearing it shorter means a slower ground speed before running out of revs.  Use whatever works for you.

My opinion is that learning to use the clutch proficiently is always your best protection against a "too much" mishap.  The best advice I got when starting out in trials was, "One finger, and one finger only, always, always, always on the clutch lever."

 

  • Like 1
Link to comment
Share on other sites

 

Wow! So many great ideas I never would have known about. Thanks to all of you for your help. So glad I joined this forum! As far as when I crashed, I was only going about 5mph on a narrow rocky trail. I hit a rock and it bounced me backwards just a bit pulling on the throttle. The bike shot out from under me and I fell to the right on to hard pack and that was it. Broke 3 ribs (very painful), a clavicle, punctured and collapsed a lung and split my knee open a full 6” wide full depth. I was alone and about 2 hours from home but managed to get the bike back to my car and loaded. I then drove home. The doctor couldn’t believe how I did it. I’m pretty damned lucky to still be around. That’s why I’m squeamish on my bike now. Thanks again so much!

Link to comment
Share on other sites

 

"bounced me backwards just a bit pulling on the throttle."

That is exactly what I would describe as the throttle getting away from you.  In that situation, a smaller front sprocket makes the bike run out of RPMs quicker so it can sort of choke off or minimize the "getting away from you".  It is cheap and easy to try.

Link to comment
Share on other sites

 
 

I'm not sure if they are still available but there used to be cylinder head spacers. flywheel weights are good at slowing the acceleration and great on muddy off cambers but also slows the decreasing revs (i.e. keeps going). I softened the rear shock so the back would dip and take up a little bit of the power before take off, so add that to your list of things that cost nowt.

Broken ribs I know what they feel like, I felt like I couldn't even blink without squealing! and collar bone I used to hear it grind when trying get to sleep😮. Hope you recover soon.:thumbup:

Edited by richt
Link to comment
Share on other sites

 
 

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
  • Create New...