Why a decompression lever on my 2 stroke?

[cleopatrisha]

I have a 1981 yz250 project bike that has a decompression lever is this necessary on these 2 strokes? ? do I need to use it to start the bike when I get it put back together. my yz465 does not even have a decompression lever. any information on 2 stroke decompression welcome.

 

[Ol'89r]

Way back when two strokes were used for flattrack, many people installed compression releases in their heads to use as a brake. The compression release will slow your bike down if you pull it while riding at speed. Kinda like a jake brake on a diesel truck.

The down side was that they sucked dirt in the engine along with the air.

You don't have to use it to start the bike, in fact you can take it off and throw it away and plug the hole with another spark plug.

Just my $ .02

 

[larosche]

2 types

There were/are 2 types of manual compression releases. The cheap ones were just an opening from the head to the outside world. The better ones were one way valves that only let air out of the cylinder.
Check which one you have. They are fun to use when you want to surprise someone with the unique sound.
RAD DAD

 

[VintageDirt]

Maico compression releases were built into the cylinder instead of the head. This allowed you to use the release while you kick started the bike. It was easy enough to start a 490 without using the compression release, but using it saved the weak starter gear dealio. I don't think any other manufacturers used this type of setup.

 

[Bultaco206]

I've always subscribed that the dirt-gets-in-the-head thoery is the stuff urban legends are made of. I've run the head-type on my Buls (MX/DT/Trials) for 30-some-odd years and have never had an issue with dirt ingestion. When it's open, there's a positive flow of compression - turning the motor into a one-way air pump.

By the time the pressure would equalize enough to let dirt in - the motor has stopped running. A dirty air filter, or one with a bad seal, would let in way more contaminants than a compression release would.

 

[CommandoRoadster]

You don't need it, and don't really want it on that bike, unless you're trail riding, and want some engine braking, for downhills and stuff. Just adds a little extra weight, and is something else to break. It does make a cool sound when you pull it in, though! :)