Manual Decompression Buttons

[nicholastanguma]

A very experienced high performance thumper tuner of the old school sort has advised against manual decomp releases for high comp single cylinders, citing problems with apparent ingestion of dirt particles into the engine. As off road specific motos such as the Yamaha XT500 and Honda XR600 are both kickstart-only with manual decomp levers on the valve covers I'm assuming this advisement against manual decomp is leveled squarely at decomp buttons and not levers.

Is this correct? And if so, how do manual decompression button valves ingest dirt particles into the engine, seeing as how all such devices I've ever seen on Harley-Davidsons automatically close as soon as the engine roars into life?

 

[javajoe79]

Because that little port could also pull air in on the intake stroke. If that little port had a little filter on it, then you should be fine.

 

[verdesard0g]

I had one on my Kawasaki Green machine back in the day. I think it had a oneway valve so only let air out, not in. It had a lever on the handlebars.

 

[Zuber]

The old two-stroke decomps were to help slow down the bike, mostly when racing. They had no filters.
Modern four strokes have an automatic decomp that cracks open one exhaust valve below 200 rpm to help get it started with an electric starter. The exhaust pipe is relatively clean.

If you are kick starting a large single, the manual decomps that mount on the handlebars work best - if you have experience starting a single. These typically crack open the exhaust valve. No problems with dirt.

Those hardly ableson things are better installed on a WWII tractor.

 

[1stDeuce]

Yes, the adage applies to buttons only, not levers. If the decomp button is surrounded by dirt and dust, it COULD suck in a tiny little bit of grime if you happen to kick or pull the rope on an intake stroke right after you push the button. However, the chances of that happening, and then the dirt causing issues and not just blowing back out on the next exhaust stroke, or as soon as the engine fires is slim, IMO. If you see a bunch of crud around the button, just don't use it if you think there could be an issue.

We have decomp buttons on our fire dept chainsaws. It seems to me like they cause the saw to flood and refuse to start more often than not, but that's just my experience. I prefer not to use them, and I think the saws start better if I don't, but I understand that this is harder on the pull rope, and on me.

I believe the decomp on those older 4 stroke dirt bikes was to allow people who came from 2 strokes to just hop on and kick the crap out of them till they start. In reality, it's much easier to start them if you roll them over slowly to find the beginning of the compression stroke (resistance). Then roll it just past TDC, reset the kick lever to the top, and give it a good kick. I find this to be way less work than just kicking till it decides to start.

FWIW, my XR400 has an internal auto decomp system that holds an exhaust valve open slightly at very low rpm's. (Presumably so you can just kick it till it starts, like a 2 stroke...) It sticks quite often and makes the bike harder to keep running when it's cold. Lots of people remove the auto system and report better starting, and I'll soon be one of them. There is also a handlebar lever that does the same thing, operating a cam the pushes down an exhaust valve a tiny bit. This is used to make kicking it to clear a flooded condition easier, not for starting. Neither will ingest dirt, as they're both completely internal, releasing or sucking a tiny bit through the exhaust rather than right outside the engine.

BTW, you might get more responses in the lightweight cycle area, rather than the GVM thread, which tends to be for autos rather than bikes.