Decompressor?

[jaan]

Is it useful to mount a Decompressor to my Montesa Cota? In your opinion, what kind of advantages I'll get? Any problems?

FRANCE TRIAL has Decompressor kits (49,80e).

[feetupfun]

They are good for letting dusty air into the cylinder

[swooshdave]

They are good for letting dusty air into the cylinder

I've used my compression release plenty of times, but for the life of me I can't seem to get anyone to tell me how exactly they work.

[tony283]

Can't answer for the Montesa 348 but for any twinshock with crap brakes they are good for steep descents where you need engine braking to be controlled, a bit like a Jake Brake on a 18 wheeler truck. They were almost essential on the Ossa! Also very good on bikes that were prone to "gassing up" after a long slow downhill, followed by the uphill where you would traditionally end up with the wwwwweeerrrrrh, wwwwweerrrh until all the crankcase compression had equalized and it fired cleanly..... early Bultacos were very bad on this aspect.......... and finally a very useful overtaking tool if you ever ride ISDT, ISDE or enduro events....... rocket up behind someone and give him a couple of squirts.... he will instantaneously slow down and look to see what has gone wrong with his engine and making that God awful noise..... at which point you zoom past!

Tony

[swooshdave]

I do know that I let a good "phraaaaaaaaaaaappppppppppp" out on a downhill at Bodner Ranch last year and it put a smile on Dick Mann's face (he was checking).

I've tried to read up on how it's suppose to work, and it still doesn't make sense.

Here's what I'm thinking. If engine braking uses the compression to slow down a vehicle, then a compression release would allow the engine to turn more freely and thus not actually slow down anything.

Now with diesels apparently the Jake Brake works because there is no throttle butterfly, and something about vacuum.

But now it's just making my head hurt.

[biffsgasgas]

As i recall the compression release in the cota was a second hole in the head correct?

Generally these were used to reduce compression for ease of starting. Most people put a second plug in place of the release as a backup.

On my yamaha enduro bike it puts compression gasses from above the exhaust port via a small hole into the exhaust. Running down the road the only thing i got from it was lack of power at higher rpms. If you leave it open too long there is the posibility of running lean and sucking in dirt with the head plug version.

--Biff

[bullfrog]

In normal operation down a hill with the throttle off, the piston compresses a charge . . . then that charge fires and drives the piston down. We are talking idle speed charge . . . but a firing charge, none-the-less.

With the compression release open, enough raw air is drawn in to the cylinder directly (not through the carb) that combustion does not happen. So the piston is not driven down by a "fired" charge. There is no power being produced by "fire" and the rear wheel rotation must "power" the piston down, resulting in braking effect. (Ever push start a balky machine? )

Ed

[swooshdave]

In normal operation down a hill with the throttle off, the piston compresses a charge . . . then that charge fires and drives the piston down. We are talking idle speed charge . . . but a firing charge, none-the-less.

With the compression release open, enough raw air is drawn in to the cylinder directly (not through the carb) that combustion does not happen. So the piston is not driven down by a "fired" charge. There is no power being produced by "fire" and the rear wheel rotation must "power" the piston down, resulting in braking effect. (Ever push start a balky machine? )

Ed

Ah, it finally makes sense. Thank you.

So back to the original question, whether to use one. For starting, it depends on the bike. If you have a hard time kicking it over, then yes.

For riding? I've found it most useful in downhills with very loose or sandy soil. We ride a lot in very dry conditions (what's know as "high desert") so this kind of terrain is common. We also ride "no-stop" so slowing is preferable to hard braking. Given that, I think down that I've moved from the "sheer rookie" stage, I could probably ride just fine without using the compression release. I'm the only rider I've ran into that even has one.

[tony283]

Dave,

They were an absolute godsend on the Ossa which had awful brakes, and in many cases on a steep slippery downhill were much smoother in their application that applying full BRAKE and CLUTCH. Also if you were unlucky enough to stall the motor then by deft use of the decompressor you could easily get the motor to smoothly refire. On the big bike like my Enfield it is the only way to stop the bike when in neutral for all the reasons Bullfog gave.

Tony

[swooshdave]

Dave,

They were an absolute godsend on the Ossa which had awful brakes, and in many cases on a steep slippery downhill were much smoother in their application that applying full BRAKE and CLUTCH. Also if you were unlucky enough to stall the motor then by deft use of the decompressor you could easily get the motor to smoothly refire. On the big bike like my Enfield it is the only way to stop the bike when in neutral for all the reasons Bullfog gave.

Tony

To the original poster's question, a Cota 348. Having never ridden one of those, I'll surmise with that size of engine, in top shape, could be a challenge kicking over. Don't know what the brakes are like, I assume better than an OSSA.

14mm Compression Release kits are US$25-30, so 49 Euros sounds a bit steep (about US$78). You may want to check out some of the pre-65 British dealers?

[dgraf]

yeah mon, compression release,good weapon on a 2 stroke for nasty muddy downhills and literally scaring the sh**t out of your buddies

I remember there were two different styles though, just make sure you get one with a one way valve set up. The two-way valve set up

was for short track/flatrack and will slow the bike down way faster than a one way valve......of course it will suck in mud,water,dirt and

low flying birds! it will also sometimes save you from a spark plug trying to foul out when flooded.

[reallymadmonty]

Jaan,

I fitted a decomp to my 348 some time ago, which I got from In-Motion (Dave Renham). It comes as a kit; with valve, cable and lever - it's easy to fit and I have had no problems with it. It can make starting easier - helps to get the piston over TDC, but it is a 'godsend' on long steep downhills, when wet brakes create wet pants! I believe it was about

[stevecarter1000]

Ref the 348 electronic ignition, what real difference does it make to the bike please?

Ta

Steve

[beamish owners club]

Is it useful to mount a Decompressor to my Montesa Cota? In your opinion, what kind of advantages I'll get? Any problems?

FRANCE TRIAL has Decompressor kits (49,80e).

The primary purpose of the decompressor is to aid starting and reduce shock stress on the kickstat mechanism. If you have owned a 348 or 349 where some genius has removed the decompressor and added another plug, you will may have been snapping kickstart shafts thinking it was a weakness of the bike whereas if fact it's a weakenss only if you remove teh decompressor. Honda TLR's suffer the same way but snap kickstart levers if the auto-decompression mechanism is removed.