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decompressor

Posted by Expat Stu <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Expat+Stu' width='330' height='300' border=0 style='border:none;'> on March 25, 2026, 6:44 pm

I am not sure I spelt that correctly!

Online research has shown some OSSA bikes had decompressors fitted, but it says it was to improve engine braking,and I do not understand that.

If I fitted one,could it work to make starting easier?

250cc piston port 2 stroke.

Re: decompressor

Posted by Greeves Rider <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Greeves+Rider' width='330' height='300' border=0 style='border:none;'> on March 25, 2026, 8:53 pm, in reply to "decompressor"

Fitted to improve engine breaking,but not for engine starting, they were also fitted to Villiers engines

Re: decompressor

Posted by JERRY <iframe src='https://members5.boardhost.com/classicbikemart/info/user=JERRY' width='330' height='300' border=0 style='border:none;'> on March 25, 2026, 11:32 pm, in reply to "Re: decompressor"

I never new that thanks GR.

The oldest rocker in town!

Re: decompressor

Posted by Will <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Will' width='330' height='300' border=0 style='border:none;'> on March 25, 2026, 11:54 pm, in reply to "Re: decompressor"

How do they work to improve engine braking?
It sounds completely counterintuitive to me, but then I'm a numpty as far as engine design, and especially 2 strokes, is concerned.
I'd expect a decompressor to reduce compression which is a source of the engine braking (albeit together with the inertia of the flywheel).
So how and/or why does it work?

Re: decompressor

Posted by tigerville <iframe src='https://members5.boardhost.com/classicbikemart/info/user=tigerville' width='330' height='300' border=0 style='border:none;'> on March 26, 2026, 11:02 am, in reply to "Re: decompressor"

From Google AI: How does a decompressor help with engine braking on a two stroke motorcycle?

(It's very plausible except it mentioned an exhaust valve!)

A decompressor (or compression release valve) on a two-stroke motorcycle enhances engine braking
by opening the exhaust valve, allowing the engine to function as a high-resistance air pump rather than compressing air. By releasing pressure during the compression stroke, it creates a constant braking drag effect similar to a four-stroke, which was vital for slowing down in racing or on steep hills, particularly with older drum brake systems.

How it Works: By releasing pressure right before top dead center, it stops the engine from using stored compressed air to push the piston back down, removing the engine’s typical power impulse and replacing it with drag.
Benefits: It helps slow the bike down during corner entry or on steep, downhill descents without relying solely on the braking system, reducing brake fade.
Usage: Often operated by a lever, it is typically used on vintage two-strokes or special racing applications (like flat trackers) to provide intense, instant deceleration.
Other Uses: In addition to braking, this mechanism, especially when automated, helps alleviate high cylinder pressure to allow easy starting.

AndyB

Re: decompressor

Posted by Andy C <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Andy+C' width='330' height='300' border=0 style='border:none;'> on March 26, 2026, 1:09 pm, in reply to "Re: decompressor"

"by opening the exhaust valve"

Since when have 2 strokes had an exhaust valve - think that destroys the credibility of the article.

On a 2 stroke opening the decomp with the engine running is not going to improve engine braking - quite the reverse as there will be no compression.

Who needs a signature anyway......

Re: decompressor

In the interests of finding the truth -

Digging deeper with Google AI -
(Please note the last few words!

In a two-stroke motorcycle, a decompressor (or compression release) helps with engine braking by
turning the engine into an air pump that consumes energy from the rear wheel.
While two-stroke engines naturally have very little engine braking compared to four-strokes, adding a decompressor creates resistance through the following mechanics:
1. Energy Dissipation through Pumping

The Problem: In a standard two-stroke, as the piston moves up, it compresses air (consuming energy). However, as it moves back down, that compressed air acts like a spring, pushing the piston back and returning most of that energy to the crankshaft. This "spring effect" is why two-strokes normally freewheel so easily.
The Decompressor's Role: When you open the decompressor valve, the compressed air is vented out of the cylinder near the top of the stroke. This means the "spring" is removed; the energy used to compress the air is wasted rather than being returned to help the piston go down.

2. Creating Vacuum Resistance

On the downward stroke after venting, the piston must fight against a partial vacuum or work to suck air back in through the small decompressor orifice. This "heavy breathing" requires significant physical work, which is powered by the momentum of the rear wheel, effectively slowing the bike down.

3. Practical Usage and History

Off-Road Control: This was a popular feature on large-bore two-stroke dirt bikes in the late 1960s and 70s. Riders used it like a "third brake" to help slow the bike down quickly on steep descents or entering corners.
Modern Context: Today, decompressors on two-strokes are primarily used to make starting easier by reducing the force needed to kick the engine over.

Warning for Premix Engines: Excessive engine braking on two-strokes can be risky. Because the engine is lubricated by oil mixed with fuel, closing the throttle while the engine is spinning fast (engine braking) can lead to oil starvation, as the engine is moving rapidly but receiving very little lubricant.
Would you like to know more about the risks of oil starvation during engine braking, or are you looking for a decompressor kit for a specific bike?
AI responses may include mistakes.

AndyB

Re: decompressor

I've been out for a 60 mile ride for lunch at Claverdon on the Tigerville since my earlier post.

AndyB

Tigerville?

Quick question. Tigerville - T140 Bonneville converted to Tiger spec by any chance ?

Who needs a signature anyway......

Re: Tigerville?

Correct.

AndyB

Re: Tigerville?

Posted by Andy C <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Andy+C' width='330' height='300' border=0 style='border:none;'> on March 27, 2026, 7:53 am, in reply to "Re: Tigerville?"

Had a T140 Tiger in my teens, the best air cooled Triumph twin I have ever had.

Sometimes tempted to get another one.

Who needs a signature anyway......

Re: Tigerville?

I've had this over 25 years, but don't ride it much, as although I'm well over 6 feet tall I find my '66 Triumph 350 more comfortable.

AndyB

Re: decompressor

Posted by Will <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Will' width='330' height='300' border=0 style='border:none;'> on March 26, 2026, 7:23 pm, in reply to "Re: decompressor"

Thank you for that explanation.

Re: decompressor

Posted by 6TDen <iframe src='https://members5.boardhost.com/classicbikemart/info/user=6TDen' width='330' height='300' border=0 style='border:none;'> on March 26, 2026, 9:04 am, in reply to "Re: decompressor"

Strange, I always thought it was to assist starting. I never have experienced one but I would swear my mate had one many years ago on his BSA B31 and used it for that purpose. Can’t say I paid much attention back in the day though so could be wrong.
Den

Re: decompressor

Posted by Tonupdave <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Tonupdave' width='330' height='300' border=0 style='border:none;'> on March 26, 2026, 9:45 am, in reply to "Re: decompressor"

On all the twostrokes I have had it was used to stop the engine there is also one on my Ariel used to get the piston just over compression for starting and used to stop engine as it has a magneto .

Tonupdave..T.U.D.

Re: decompressor

Posted by geoff the bonnie <iframe src='https://members5.boardhost.com/classicbikemart/info/user=geoff+the+bonnie' width='330' height='300' border=0 style='border:none;'> on March 26, 2026, 10:25 am, in reply to "Re: decompressor"

On 4 strokes, it used to be an aid to starting and worked very well on my old Matchless G5 single but not seen one on a 2 stroke

Re: decompressor

Posted by the old one <iframe src='https://members5.boardhost.com/classicbikemart/info/user=the+old+one' width='330' height='300' border=0 style='border:none;'> on March 26, 2026, 10:59 am, in reply to "Re: decompressor"

fitted as standard on puch maxi mopeds (50cc 2 stroke) never improved braking tho , you pulled lever in , then either pedelled or turned pedel whilst on stand , once engine was turning over , you release cable , and hang on for grim death , 1.5 bhp can be terifying
the bullets also had them (350-500 cc 4t) and once you understood them and the ammeter "flicker" less days were taken off work

Re: decompressor

Tonupdave..T.U.D.

Re: decompressor

Posted by ChazzyB <iframe src='https://members5.boardhost.com/classicbikemart/info/user=ChazzyB' width='330' height='300' border=0 style='border:none;'> on March 26, 2026, 2:47 pm, in reply to "Re: decompressor"

My Mobylette had a decompressor to stop the engine. Probably used when starting, too. Pedal to get engine spinning with decompressor open, then close it. Ring-ding-ding.

To advertise here, please call...

Re: decompressor

according to that AI slop the things are fantastic for breaking on a 2t , is pulling on the decomp on a bend or at a junction the same as having a disk brake conversion fitted

other search engines without AI slop are available , and are recommended

Re: decompressor

Its strange suggesting using a 2 stroke engine to brake. On my 2 stroke 3 cylinder SAAB, they fitted a device operated inside the car to disengage the engine for downhill so it actually speeded up. The reason being, and this will apply to bikes as well.
Going downhill there is no petroil burning or being drawn into the cylinder so the rings are staved of oil and the engine will seize if going down a long hill. Thats what was noted in the 1961 manual for the car. It also stated to blip the throttle often to keep a good oil supply to rings and bores.
Be aware any old 2 stroke coasting down a hill will not be getting much oil. When i ran 2 stroke bikes, i used to pull in the clutch on long descents

Re: decompressor

De compressors were not fitted so much on road bikes more on trials bikes which gave engine braking without locking up the rear wheel. They were also common on pedal started autocycles and mopeds to get the engine turning over for starting.

Re: decompressor

"De compressors were not fitted so much on road bikes more on trials bikes which gave engine braking without locking up the rear wheel"

So that is how it helps with engine braking, I get it now, never considered that TBH.

Who needs a signature anyway......

Re: decompressor

They were also fitted to Siba Dynastart equiped Villiers 31A engine as fitted to the Bond Minicar. It released compression so that the starter motor could turn the engine. Once engine was spinning over you released the de compressor and the engine fired into life (hopefully)

Re: decompressor

Posted by Andy C <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Andy+C' width='330' height='300' border=0 style='border:none;'> on March 28, 2026, 9:52 am, in reply to "Re: decompressor"

Was that the same motor as used in the blue "Inavlid" cars many years ago, they were Villiers 210cc if memort serves.

Only bike I ever had with a decomp was a Bullet 500, came in handy for getting it over TDC before giving it the big kick.

Once you got used to the ammeter "twitch" for want of a better word it would more often than not fire up first kick.

Who needs a signature anyway......

Re: decompressor

the invacar used a sach 500cc engine , quite fast as well

Re: decompressor

Posted by Expat Stu <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Expat+Stu' width='330' height='300' border=0 style='border:none;'> on March 28, 2026, 7:38 pm, in reply to "Re: decompressor"

Errr, lot of stuff to look into there! Thanks everyone!