Adding a decompression valve

[BiodieselNC]

Wow, you want to add a decomp to a 40 cc motor? I have a 91 cc saw (660) that doesn't even have a decomp. (I've had nothing but problems with the decomp on my 361, so the hole on the 660 is plugged...and life is good.) Good luck, nonetheless...

It’s not an issue for me to pull a 41cc engine with 175psi compression. I have an 066 and 084 as well. The problem with the FS250 is that the recoil assembly, size of string I can fit is small, so I was constantly breaking starter rope and/or handles. Thought about adding an elastostart handle but never did it.

By the way, to report back after almost 10 years since I did this work, the trimmer still fires up on 2nd pull (1st when hot) and runs like a scalded dog. Have not had a single issue with the decomp valve or porting that I did. Porting and high compression actually reduced the fuel consumption on this little machine, and I definitely use this one piece of equipment more than all my saws, blower, other stuff. It sees at least as many hours / season as my lawnmower.

 

[Okie]

I've added them to old Briggs and Stratton that did not have a comp release.
I got the valve in hand, then just went at it. No problem once I had the chainsaw de-comp in hand for measurements, etc.

They worked great for me.

 

[Had Robinson]

I successfully added a decomp valve to a 130cc engine without a boss. Drilled and tapped a hole between the spark plug and and the squish edge of the cylinder. The fins had to be machined down to do this. Works great.

 

[Wood Doctor]

I just built an FT660 with a decomp valve for a friend who had bought the parts but gave up on the assembly. It started and ran well for me, but when cold, I could hardly pull it without using the decomp. He was grateful and cold started it immediately without bothering with the decomp. He said, "I never use them on any saw that I have and that includes the big 660 logger saws." I guess he was showing off, but maybe I'm just a weakling.

 

[Had Robinson]

Some guys are really strong or enjoy the punishment...

 

[BiodieselNC]

Yeah and I think you strengthen the muscles when you have to pull start a lot. I remember the hardest thing I ever had to pull was my old Grizzly 600 ATV, battery had gone bad and pulling with both hands and squatting down and pulling up with all my strength to turn it over fast enough to fire.

The main reason I appreciate and retain decompression valves is the reduced wear on the starter mechanism, rope, handle. I’ve broken starter pawls, lots of rope, and even a couple of handles.

 

[Had Robinson]

I forgot to give you kudos on noting that a decomp lowers the wear and tear on starters. This is a big reason to use them. I'm getting ready to put one on a 185cc engine. I'll be adjusting the bleed hole on the decomp. I think the rule is enough compression to be sure the engine fires. The China-clone decomps are cheap enough to waste a few....