Decompressor eliminator

[sanddune51]

Whilst playing around building a cylinder head, I came to realize that there is a far simpler method to disable the decompression mechanism. Bill Jurgenson has never mentioned it as a method (and I've trawled through each and every one of his posts )

Just for fun I'll let you fellas have a go at working it out before I tell you what I did with my motor? I'll post up photos in 2 or 3 days in any case.

Mark.

[dandywarhol]

Does it involve a welding kit?

[edfmaniac]

A tap, some red thread locker and a hex head grub screw is all you need.

[sanddune51]

Nope. Keep guessing

Mark.

[sanddune51]

O.k. I was contemplating the doing away with the decompressor mechanism, but at the same time I was thinking about lightening the rocker arms. It became obvious to me that I could achieve both goals at a stroke.

I removed the rocker arm cam follower pad that is actuated by the decompression plunger. If the pad does not exist, the rocker arm can't be actuated and thus the left hand exhaust valve will not be held continuously open while the engine is being started.

This is the rocker arm in it's unmodified original form. A line can be seen where the decompression plunger has been acting on the pad

This is the rocker arm after modification. The pad is much narrower. The rocker arm is consequentially lighter (nearly 7%), and most of this is a reciprocating mass so will have some (small) benefit. As I was feeling quite inspired I went ahead and lightened and polished the rest of the rockers. I can report that the engine is running sweetly

Mark.

[edfmaniac]

That does nothing to address the reason for removing the decomp system in the first place. The only reason to remove it is because the weights fly off of the timing gear and tear a big hole in your engine case. And if you plan on running the engine hard enough to test the stock decomp system, you also need to replace the stock balancer drive gear with the solid one from the Raptor 660.

[dandywarhol]

Interesting that if you google 660 decompressor failure, by far the majority of failures are on quads. Could there be a significance that quads are more liable to over rev in low gear on the overrun - no limiter will do anything about that

[edfmaniac]

Interesting that if you google 660 decompressor failure, by far the majority of failures are on quads. Could there be a significance that quads are more liable to over rev in low gear on the overrun - no limiter will do anything about that

I would assume that has more to do with the number of units in circulation and the fact that ATV guys tend to abuse the hell out of their quads. Not sure if the last part of your statement is a question or not. Removing the projectiles that fly through the motor case at high rpm will prevent said projectiles from doing so.

[sanddune51]

I really don't see the fly weights coming apart from the cam sprocket as an issue. Bill Jurgenson, the accepted guru for all things MZ Skorpion has not mentioned it. He states that the primary reason for its removal is to be able to carry out a cranking speed compression test. As far as I'm concerned, if it has never been an issue for him, I'm sure as hell not going fret over it. I believe the likelihood is minimal. Saying that all things are possible and a ham fisted home bodger could soon wreck any part of any engine if they were let near it.

I can't find a single post on this forum where this has been definitively documented as having happened. Of course the balancer shaft drive gear is a completely different matter and is well documented on this forum and elsewhere, but this was not intended as a matter for my initial post. All I was trying to do was show an alternative method to prevent the engine decompressing at cranking speed. The method I showed achieved that goal, with the bonus of lighter rockers too. Now all I have to do is find my 40 year old compression tester

Mark.

[dandywarhol]

Interesting that if you google 660 decompressor failure, by far the majority of failures are on quads. Could there be a significance that quads are more liable to over rev in low gear on the overrun - no limiter will do anything about that

I would assume that has more to do with the number of units in circulation and the fact that ATV guys tend to abuse the hell out of their quads. Not sure if the last part of your statement is a question or not. Removing the projectiles that fly through the motor case at high rpm will prevent said projectiles from doing so.

It was a statement edfmaniac, I tend to punctuate pretty well

What I was stating was that a rev limiter is of no use if the drive wheel in controlling the engine - as in downhill and low gear. The rev limiter on a 660 should protect the engine from flying apart assuming the motor is regularly serviced. Even extending the rev limit 1000 rpm with an Ignitech box should still offer reasonable protection

[edfmaniac]

It's a known issue. If you've ever taken the cam gear apart, you'd know why. The pins are a press fit and not a very tight one. There are multiple solutions on the market, but the easiest one is to get rid of the whole mechanism. I'm pretty sure none of Bill's bikes have these weights in place. Here's a quote from one of his posts, "The deco unit is known to be of questionable stability at continuous high RPM so it is almost always removed from racing engines." Later in the same post he refers to high RPM as any sustained running above 7000rpms. So if you are going to be bouncing your engine off of the rev limiter, remove the weighty bits. If you are going to be putting around and occasionally going above 7000, you probably shouldn't have to worry about it. I found it easier to just get rid of the bits and spend 5 minutes with a tap to hold the parts in place that are needed to maintain oil pressure in the cam bushings. Learned about the mod on one of the Raptor forums where there are plenty of photos of what happens when you don't remove the weights.

I just think there is a lot more to gain from removing the parts than leaving them in there if the only reason they are in there is to maintain oil pressure.

download.jpg (8.78 KiB) Viewed 8162 times

[dandywarhol]

Ok