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Garden Tractors Discussion Forum

Different kinds of decompression designs.

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Russ from MN

04-12-2018 09:44:03




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I have a couple of medium size Jonsered chain saws that have a push button on the cylinder head, if you push that in before frying to start them it makes them pull easier, saving on the shoulder. My wife's motorcycle is a Suzuki 650 single, and it has a mechanical decompression activated by a solenoid, when you push the starter button it engages. Works great, that allows a little MC battery to spin that big 650 cc.

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Texasmark1

04-15-2018 14:43:58




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 Re: Different kinds of decompression designs. in reply to Russ from MN, 04-12-2018 09:44:03  
I like the BS design. just a little roll on the cam shaft, cocking a valve slightly open during part of the compression stroke. Cranking speed is maybe 100 rpm (slow giving time to let off compression for easy starting) but even at idle the engine is spinning fast enough that very little energy is lost and at governor 3600 rpm, it's not worth mentioning.

The other thing is they increased the diameter of the crank rope sheave giving more ft so not so many pounds are needed to get the desired torque to spin it over. Course, once it moves, it's no longer just torque, it's now HP, since rotation is present.

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Brian G. NY

04-12-2018 19:13:44




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 Re: Different kinds of decompression designs. in reply to Russ from MN, 04-12-2018 09:44:03  
I have a bunch of the Kohler "K" series engines with the ACR and I've never had one give me any trouble at all.

I had a snowmobile back in the 60s that had one of those manual compression releases.....It might have been a single cylinder Hirth on a Mopto-Ski?
Seems as I recall seeing a chain saw with one of those also.

I like the idea of the automatic solenoid operated release on a motorcycle.

My Kawasaki Mach III had another nice feature...the gas shut off was vacuum operated so as soon as you started the engine it would open and then close again when the engine was shut off.

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Russ from MN

04-13-2018 05:23:18




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 Re: Different kinds of decompression designs. in reply to Brian G. NY, 04-12-2018 19:13:44  
Yes, I think a lot of older carburetor motorcycles had the vacuum operated valve, mine is fuel injected so it has a pump.



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jeffcat

04-12-2018 21:49:51




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 Re: Different kinds of decompression designs. in reply to Brian G. NY, 04-12-2018 19:13:44  
The chainsaws I have are Mc Culloch saws. The pro mac 650 has that little red button on the side of the cylinder. It is called a DSP. Makes it a lot easier to start these almost 4hp saws. When you push the button in it opens the way to a 1/8 hole into the upper 3rd of the cylinder. When the engine fires it pops the little button back out and closes.



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Tom Arnold

04-12-2018 15:17:44




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 Re: Different kinds of decompression designs. in reply to Russ from MN, 04-12-2018 09:44:03  
Growing up in the 50's, I was mostly exposed to English motorcycles such as Triumph, Norton, BSA, Ariel and AJS. Many of those bikes were single cylinder engines in the 359 and 500 CC sizes. Starting one of these was a challenge without a compression release. The common method was a small lever on the handlebar that could be engaged by the first finger of your hand. You would begin by using the kick starter to slowly rotate the engine until it reached top-dead-center on the compression stroke, where it would stop dead. Then you would allow the kick-starter to come all the way back to the top before putting light pressure on it so that you could just feel the compression. At that point, you would leap into the air, squeeze the de-comp lever and put everything you had into forcing the kick-starter lever downward. When the kick-starter lever hit bottom, you released the de-comp lever and hoped that the engine would come to life. On the opposite side of the handlebars was another lever that rotated and made the choke plate in the carb open and close. Experience taught you how much choke to apply. If the engine was properly tuned, starting was rarely a problem. I rode a Norton 500 and a BSA Gold Star 500 and both bikes used this type of de-comp.

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Tom Arnold

04-12-2018 15:27:51




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 Re: Different kinds of decompression designs. in reply to Tom Arnold, 04-12-2018 15:17:44  
A huge number of garden tractors were powered by one of the greatest engines ever designed.......the Kohler K. Many of those Kohler's came with ACR - a short form for Automatic Compression Release. It worked by slightly lifting the exhaust valve off its seat. When the engine was spun over to try and start it, the ACR made it easy for that to happen by slightly reducing the amount of compression but when the engine RPM hit a certain point, centrifugal force removed the ACR and restored full compression to allow ignited fuel mixture to take over and keep the engine running. A very simple and effective way to make these engines easy to start.

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jeffcat

04-12-2018 21:56:41




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 Re: Different kinds of decompression designs. in reply to Tom Arnold, 04-12-2018 15:27:51  
It is so sad that Kohler dumped the "K" series of engines. If you did your normal maintenance they were almost forever engines. A 3000 hour engine was the norm. I have several machines with them. I really like the Briggs Vtech twins. Again, do your maintenance. Have no idea how long they will last but I have two with some pretty heavy hours on them and they are still putting right along.



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Tom Arnold

04-12-2018 22:14:46




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 Re: Different kinds of decompression designs. in reply to jeffcat, 04-12-2018 21:56:41  
Kohler did not kill the K-Series or the M-Series engines. You can thank the EPA for that. That regulation also put an end to the Onan flat twins, another great engine.



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oldsarge

04-13-2018 07:55:11




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 Re: Different kinds of decompression designs. in reply to Tom Arnold, 04-12-2018 22:14:46  
I agree.You're a whole bunch smarter'n your namesake!!!!!!!!!!!!!!!!!!!!!



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Tom Arnold

04-13-2018 11:07:40




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 Re: Different kinds of decompression designs. in reply to oldsarge, 04-13-2018 07:55:11  
Very kind Sarge. However, I have been on this earth for 14 years longer than the wannabe funny man. LOL



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