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Aftermarket decomps........making them work.

traffic903

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I thought I would post about how I have fixed my issues with aftermarket decompression buttons, and the process of modifying them. I welcome all feedback and other ideas. This is just what I have found to work. Some might say not to bother and just buy genuine, but here in Aus that equates to almost $40 a pop!!!
 

traffic903

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Ok so the theory. These things don't really work that well on the bigger saws. Most times they close up as you are pulling and not when the engine fires. My 660 project was an example of this. I wasn't all that concerned at first because I never use decomps. But after having now had problems with starter components, coupled with the Highway popup piston increasing compression, I wanted to use the decomp more regularly.

But it was just garbage. It would close as soon as I pulled on the starter, and invariably that would cause the handle to be ripped from my hand. I'm sure the timing advance was also contributing!

What this mod does is to give the internal bearings a groove to sit in when the button is open. That groove is just enough to add some resistance to the button closing, of which there was basically none beforehand. So far it has worked very well for me. I did try drilling the holes larger, but this just increased the leakage from the button when closed. In fact I have now started plugging one of the holes (the cheapies have 2) with a pop rivet which gets filed flat. This limits the leakage even more.

I know it seems like a lot of work to go to for a decomp but they are expensive here, AND I just like to find ways of making things work better.

Let me know what solutions you have found to also work.
 

traffic903

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I think some guys use a oem husky decomp since they supposedly take more pressure to release. I just plug the decomp in my smaller saws.
Yeah the smaller saws don't need em. The Husky decomps are even more expensive than Stihl here, if you can believe it!!
 

stihl_head1982

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Not to mention the extra large hole in the side(?)
Could you picture an OEM beside this aftermarket one to compare the holes.
The ones I've seen comparatively the hole is huge compared to the OEM.
 

traffic903

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Not to mention the extra large hole in the side(?)
Could you picture an OEM beside this aftermarket one to compare the holes.
The ones I've seen comparatively the hole is huge compared to the OEM.
Yes and the larger the holes, the more they leak I have found. Blocking one off totally still leaves plenty of vent to bleed off compression. The OEM hole would be about one-fifth the size I reckon. So that all helps, but it's definitely the groove which makes it work properly now. Feels just like the OEM one.
 

traffic903

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Last stihl decomp I bought was from cannings stihl melbourne over a year back, I'm sure it was around $100

I remember it being visciously overpriced

Useful thread, thanks
Just ridiculous isn't it!! $40 for an 1128 numbered one locally. That's a bargain!
 

traffic903

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I can confirm it is possible. If you do the mod and then distort the end of the shaft in a vise before gluing the button it stops that chance. I did that on one of them to test the theory, but so far the glue is holding up well. Certainly something to be aware of though.
 

Larry B

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Had the same prob with OEM Hyway and farmertec decomp closing too soon and ripping handle out if my hand. Drilled the hole in the side slightly larger and fixed the problem. All 3 would pop the cap off when it kicked. Big bore 660 180 psi with advanced timing.
 
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