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General Category >> Rubber Side Down! >> Decompression unit
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Message started by hotrod on 05/14/19 at 13:12:46

Title: Decompression unit
Post by hotrod on 05/14/19 at 13:12:46

As long as I have a decent battery, will there be any problem with removing the decompression unit ?

Title: Re: Decompression unit
Post by verslagen1 on 05/14/19 at 13:59:03

As long as you have a manual lever as backup, probably not.
There are instances where you'll need to be able to throw it.
Unless you position the engine on the exhaust stroke you will have trouble push starting.

Title: Re: Decompression unit
Post by Ruttly on 05/14/19 at 14:37:27

Push starting a thumper , stick it in first gear , pull the bike backwards till it stops , this gets piston just past TDC , pull clutch in and go for it and don't forget to turn the key on. Yup I own BSAs too !

Title: Re: Decompression unit
Post by batman on 05/14/19 at 15:11:45

I would not try rolling the bike backward if it's older than 1995 ,you could damage the starter gear train , much better in any case to squeeze in the clutch , roll the bike forward, in gear , pop the clutch , forcing the piston past TDC, giving the motor over 1.5 revolutions (time to gather speed) before the next compression stroke.
What will work for a BSA ,will not work for the Savage as our motor rotation is opposite.

Title: Re: Decompression unit
Post by norm92de on 05/14/19 at 17:25:25

Batman,
I think we need the judges ruling on that. BSA rotates backwards?

Title: Re: Decompression unit
Post by verslagen1 on 05/14/19 at 18:33:44

Isn't it Harleys that if you push in backwards the kickstart gear will engage?

Title: Re: Decompression unit
Post by Dave on 05/14/19 at 18:34:59

The first summer of riding my Cafe' bike - I discovered the handlebar mounted lever that I made using a Honda choke lever was not sufficient, as it did not have enough leverage to pull the exhaust valves open if they were closed.....so my decompression release was "non-functional".

The good news is that most of the time it didn't matter, as the engine would start easily without it. Every now and then the piston would stop on the compression stroke, and when I tried to start the bike the Wiseco piston would come up on the compression stroke and stop turning - I would instantly let go of the starter button, put the bike in first gear, gently rock it backward until it stopped moving (as Ruttly does also), then pull in the clutch and hit the starter button......and it would fire right up.

I realize that Verslagen and Batman don't like pulling the bike backward. However I believe pulling the back backward gently engages the starter clutch and rotates the starter with far less stress than the violent kickback that breaks the starter gears and the engine case. Rotating the engine backwards for 1.5 rotations until you feel the resistance does a few things: (1) It pulls on the backside of the cam chain - which happens every time the engine comes to stop with a valve on the back side of the cam lobe.....and it doesn't seem to hurt anything. (2) It runs the oil pump backwards for a few turns....that doesn't seem to cause any problems. (3) It turns the starter slowly and probably puts far less stress on any of the parts than the system experiences when you use the starter to spin the engine over.....so nothing breaks or gets out of wack.

Going a summer without a working compression release was OK - but I really don't recommend it...I built a functioning compression release the next winter and it has been working fine. When you ride without a compression release - it can be clumsy if you stall the bike at a traffic light - the process of getting the bike running again can take way too long for the drivers around you to accept gracefully......it might even put you in danger.

I think the compression release should be one that you can use with a few fingers on the clutch and one to operate the compression release - so you can start the bike in gear if you stall the bike in traffic you can get moving again in a hurry. You just pull in the clutch, pull the compression release, hit the starter button, release the compression release, motor starts....let go of the starter button and apply throttle and let out the clutch smoothly and get moving - it sounds complicated but only takes a few seconds.

Title: Re: Decompression unit
Post by hotrod on 05/14/19 at 20:09:12

Good info there. Thanks Dave, and everyone.

Title: Re: Decompression unit
Post by batman on 05/14/19 at 20:30:05

I may be wrong , if so I ASSURE you it is not the first time ,and will hardly be the last. Having the stock decomp unit I must admit having never experienced this problem.

Title: Re: Decompression unit
Post by norm92de on 05/15/19 at 11:55:00

I don't have the specifics but i remember the compression release/s being very easy to operate on British bikes. Just like the one on the Yamaha SR400?

Title: Re: Decompression unit
Post by Dave on 05/19/19 at 12:24:33

5E5F425D09025455300 wrote:

I don't have the specifics but i remember the compression release/s being very easy to operate on British bikes. Just like the one on the Yamaha SR400?

Some bikes had a small hole drilled into the cylinder wall part way up the cylinder.....and pulling the compression release opened a valve that allowed air to escape while the piston was on the way up. The spring was light and easy to open the valve.....it could have a very small lever. Some other bikes had a similar valve in the head - some have mechanical devices on the camshaft that work automatically and are disengaged once the cam is spinning.

The device on the Savage holds the exhaust valves open - if they are not in the open position when you pull the lever.....it takes a lot of force to lift them off the valve seats.