Author Topic: How does my decompressor work?  (Read 6977 times)

0 Members and 1 Guest are viewing this topic.

jbdrury

  • Scooter
  • **
  • Posts: 18
  • Karma: 0
on: June 19, 2009, 07:55:52 pm
The general gist of this is a question as to how exactly a decompressor works, but is related to this story:

So, I can get my Enfield to start....sometimes.

Last season, it would kickstart with no problems, maybe one or two kicks and she would be good to go. My routine would go something like this:

1. turn gas on
2. ignition off, choke off, kick through two or three times
3. ignition on, choke on, kick through until I could pull the decompressor lever down
4. push through until I got a little resisitance
5. kick and she would start right up!

THIS season, I struggled to get her started, cleaned out the CV carb, was about to buy a new one, cleaned the carb again, and she finally started.
However, she doesn't start without some difficulties. Its odd; I leave it alone for awhile and come and do my normal routine and she'll start. I kill the engine after a few minutes of running. And then I try to start her again, this is where the problems start:

FIrst of all, she seems almost OVER-compressed? Its very difficult to kick through, and it almost seems its somehow connected to my decompressor. After some experimenting with the decompressor I get my bike to start again, I THINK its because I actually reach down with my hand (trying to keep from burning myself) and actually lift the decompressor switch myself as I kick through the most difficult part of the compression. This somehow seems to work.

So, I read the "envy for the one and two kickstarter" topic, and I've recognized that this might come down to my cleaning the carb and not having it adjusted properly. But could it also somehow be related to my decompressor switch? How does this switch work? Am I correct in assuming that I'm only supposed to be able to pull the lever down at certain times in the kick cycle? How should it be adjusted?
Or am I crazy and this all related to my carb?

Thanks, John


ace.cafe

  • Grand Gearhead
  • *****
  • Posts: 14,457
  • Karma: 1
  • World leaders in performance/racing Bullets
Reply #1 on: June 19, 2009, 09:17:02 pm
I don't have an AVL, but the mechanism seems to be cable-operated.
Perhaps your cable needs to be adjusted, so that it can operate the valve lifter properly.
Home of the Fireball 535 !


Vince

  • Grand Gearhead
  • *****
  • Posts: 1,693
  • Karma: 0
Reply #2 on: June 19, 2009, 09:35:07 pm
     The AVL  decompressor does not work very well. Don,t over adjust it. Just push to compression, then put some pressure on the kick start lever until it passes TDC, Then continue on with the start drill. I don't use it on the AVL motors.


plane95302

  • Scooter
  • **
  • Posts: 68
  • Karma: 0
Reply #3 on: June 19, 2009, 11:05:16 pm
 ;)


HRAB

  • I should really edit this
  • Grease Monkey
  • ****
  • Posts: 389
  • Karma: 1
  • Been there, doin' more...God willing
Reply #4 on: June 19, 2009, 11:57:13 pm
The AVL Decompressor works by holding the exhaust valve open "a smidgen". The cable operates a cam on the tappet cover plate to hold the valve open to make starting easier.

You can not lift the valve with the lever. The cam WILL hold the valve open in decompression mode if you hold the lever and slowly kick the engine over until the valve rises and the cam follows it.

Using the decompression ln this way was one of the suggestions for taking the strain off of the sprag clutch before the sprag problem was solved by the  factory. Use the decomp lever to alllow the engine to turn over once or twice with the electric start to gain momentum befor releasing the decomp and starting the motorcycle.
jim
BMWMOA www.bmwmoa.com
Iron Butt Association www.ironbutt.com/about/default.cfm
(Formerly) CRA# 118N www.cra-mn.com (I got smarter in old age)
74 Honda XL350 'Scoot'
81 Honda CB650C
86 Gold Wing with California Sidecar
00 Honda ST1100 'Large Marge'


Lane

  • Scooter
  • **
  • Posts: 40
  • Karma: 0
Reply #5 on: June 20, 2009, 03:49:34 am
Slight pressure (don't force it!) on the decomp lever and kick over (with key and switch off but gas on).  You'll then feel the lever lower some more.  Now its holding the valve open, and its easy to kick over.  I usually prime kick it 3, 4 times to get the oil pump going.  After that, let go and its a 1 kicker to start!

I originally thought my decomp was broken until I learned how it works!


Lane

  • Scooter
  • **
  • Posts: 40
  • Karma: 0
Reply #6 on: June 20, 2009, 03:52:59 am
     The AVL  decompressor does not work very well. Don,t over adjust it. Just push to compression, then put some pressure on the kick start lever until it passes TDC, Then continue on with the start drill. I don't use it on the AVL motors.

Work well for me.  Just have to learn how it works.


PhilJ

  • Grand Gearhead
  • *****
  • Posts: 2,246
  • Karma: 0
Reply #7 on: June 20, 2009, 10:41:21 am
JB,
I've reread your routine several times to try and get a fee for what your doing.

I think you've got to many steps in it. Kicking through with the decomp pulled in is ok in the first part. It doesn't need to be pulled in again. Just push through the compression, just as it eases up is where you should start from for the starting kick. I also think you may not need the choke on. The clue to that is walking away and coming back it will start sounds like it's a little flooded.

Try it without the choke and don't give it any throttle. I can't tell from your profile where you live, if it's pretty cold you may need the choke, if it's warm you probably don't. But if your idle is set to low you'll have a harder time getting it started the first thing in the morning. Just raise the idle screw a little ( 1/4 to 1/2 turn) it will probably start. If after warm up the idle is to fast lower it a little but the next morning you'll probably need to repeat the process. This is what I have to do because I like the idle running at about 1K rpm when warmed up. The mornings until it's warm it idles too slow.

Hope this helps.


Geirskogul

  • Grand Gearhead
  • *****
  • Posts: 729
  • Karma: 0
  • The world isn't beautiful, therefore it is.
Reply #8 on: June 20, 2009, 11:31:43 am
You put steady pressure on the decomp as you kick it though - it'll "drop" twice.  Once when you first put pressure, then a little more as you push the kick lever down.  Once it's down the farther amount, the "Second" time, you kick the bike to life.  Or that's when I prime it for five or so kicks, THEN kick it to life.
All hail Sir Lucas, Prince of Darkness.

When an idiot thinks it's the same as not thinking at all!


Jerry

  • Scooter
  • **
  • Posts: 82
  • Karma: 0
Reply #9 on: June 21, 2009, 12:36:52 pm
I don't use the decompressor.  What works for me is: 
1. Gas on.
2. Choke on.
3. Switch on.
4. With bike on centerstand press down on kickstart lever until peak compression.  Keeping steady pressure on the leaver wait until it eases past tdc.
5. Let the leaver return to the top of its stroke and kick it through. 

Mine usually starts on the first kick.
I couldn't figure out how to effectively use the decomp. either.  It seems the only use would be for "priming" kicks and I've never needed to do that.  The only difficulty I have is that when on the sidestand it's hard to get enough pressure on the lever to get over compression.  I've toyed with the idea of lengthening the lever on the tappet adjustment cover to get better leverage.  Anyone have any thoughts on this?

Jerry


Geirskogul

  • Grand Gearhead
  • *****
  • Posts: 729
  • Karma: 0
  • The world isn't beautiful, therefore it is.
Reply #10 on: June 21, 2009, 09:15:07 pm
Jerry, I read some old posts that Kevin from CMW did saying something to the effect of "use the decompressor, if only to save the kickstart pawl."  Think of how much pressure you're putting on that little piece of metal.  I use mine, if only to get it past TDC without having to stand on the lever.  Something will break using the "force it" method.  My motto used to be "if it jams, force it.  If it breaks, it needed replacing anyway," but then I started watching my spending and realized that a little care goes a long way.
All hail Sir Lucas, Prince of Darkness.

When an idiot thinks it's the same as not thinking at all!


Jerry

  • Scooter
  • **
  • Posts: 82
  • Karma: 0
Reply #11 on: June 22, 2009, 02:04:13 am
Saving the pawl is one reason I would like the decompressor to be more effective.  I don't bear down too hard when getting over tdc.  Just steady pressure...it takes a while to ease it past.  That's why I really can't do it on the sidestand.  It seems to me that lengthening the leaver would allow for lifting the valve against the pressure of the valve spring which would make the leaver more usable.

Jerry


KB8ANY

  • Scooter
  • **
  • Posts: 91
  • Karma: 0
Reply #12 on: June 27, 2009, 01:14:46 am
Yo:

The decompressor is a sleeve on the exhaust tappet that holds the exhaust valve open when you pull down on the lever mounted at  the left controls.  You can see the little half-wheel that does the lifting if you take off the tappet cover.  It presses against the underside of the lip of the sleeve when you move the lever, forcing the sleeve up against the circlip that holds it captive on the tappet.

It uses the same principal as the decompressor on the older bullets.  You let some of the mixture out of the chamber to make the engine easier to turn.  The only problem is that it is "harder" to use, because it works directly against the exhaust valve spring.  On the older designs, it was simply a valve - it looked like a smaller version of the intake or exhaust valves - in the top of the head, and when you decompressed it only worked against the pressure in the chamber.

At one time I took the decompressor off because it is so hard to use and it makes a lot of noise, fading in and out of clatter as the engine rpm changes.  But I put it back in and finally learned how to use it.

Get ready to go, and pull down (i.e., in the decompress direction) on the decompress lever.  Kick the bike a couple of times.  Let up on the lever, and kick very slowly.  The INSTANT ( ! ) you start to feel the piston starting to push back because of the pressure building up in the chamber, stop pushing on the kickstarter, and pull on the decompress lever to let the pressure out of the chamber.  Then kickstart it.  It will go easily.

You have to stop pushing on the kickstarter the instant you feel the piston starting to push back because the pressure builds so fast with very little kickstarter movement  that you won't be able to force the sleeve up with the handlebar lever.  With practice you will get faster and faster, dispensing with the couple of extra early kicks (which really only tell you where the piston is) so that somebody watching who does not know about the new Electra will think you are doing it the old way.

I'm working on doing it by hand, just like the Indian guy on YouTube.  If I ever get to that state of grace, I'll post it on YouTube myself and tell you all to go look at me.

Good luck.

Paul