Decompresion mechanism repair.

Message

wistech · #1 Post by **wistech** » Wed Jan 12, 2005 2:12 am

So how does this little device work and why does it fail? Lets have a look see. First off when the engine is not running a spring pulls the flywieght towards the center of the assembly.This in turn rotates a small cam pin so it interferes with one exhaust valve bucket. When the cam rotates around while cranking the pin hits the bucket halfway up the compression stroke thus lowering compresion and helping the starter get the motor spinning.

wistech · #2 Post by **wistech** » Wed Jan 12, 2005 2:14 am

Once speed increases the flywieght moves out ,rotating the cam pin so it no longer hits the bucket and hopefully the engine has fired and running.

wistech · #3 Post by **wistech** » Wed Jan 12, 2005 2:16 am

Here is what the front of the cam pin looks like. The small shiny pin is what the fingers on the flywieght hit. The yellow pointer is showing where you have to remove material from the end of the cam to be able to press the assembly off. This is why some have said the device is nonrepairable. Ha

wistech · #4 Post by **wistech** » Wed Jan 12, 2005 2:21 am

I believe the main reason for all the failures leading to destruction is that there was way to much play in the overall fitting of the parts. Everything is running at high speeds extremely close to each other.
Look at how close the pivt pin is to the cam bearing cap and the egde of the head. I talked to Cord today and he said he has had to remove material from the cam caps to keep them from touching.

wistech · #5 Post by **wistech** » Wed Jan 12, 2005 2:28 am

One of the things I do is hand fit the pivot by shaving off metal at the end of the pin to get around .001" of side play. This keeps the wieght from moving sideways and wearing an edge on the fingers of the flyweight.

wistech · #6 Post by **wistech** » Wed Jan 12, 2005 2:31 am

The pointer is showing the clearance that has to be kept to a minimum. If it gets to be to sloppy the fingers can actually slip off of the cam pin and force the weight into the bearing cap .

wistech · #7 Post by **wistech** » Wed Jan 12, 2005 2:34 am

Here is a pic of a flywieght Happyboy just sent me. One finger has an edge worn off it and the other has broken completly off. The piece is apparently nowhere to be found.

wistech · #8 Post by **wistech** » Wed Jan 12, 2005 2:40 am

Here is a pic of a newer style powdered metal flywieght (left) and a prototype I made out of stainless steel. (right). The early cannondale weights were fully machined .
The top center is the newer pivot pin which is held in with a screw instead of a soft rivet that is staked on the inside. I use locktite on the screws and also stake the back side of the screw so it is impossible to ever come out or come loose.

I dont believe the powdered metal weights are bad but they dont have as tight a tolerance as the old billet wieghts and seem to have more play in them. I think if they are tightened up on the pivot they should work fine.
Unfortunatly about half of all the cams I work on have to much wear on the weights and need to be replaced.

We are working to get a suply of billet weights so they can be replaced as needed. I built a couple by hand and I can say its way to much work to do manually. The screw type pivots ,screws and cam pins are available from ATK . I will post part numbers when I figure out what I did with them. LOL

comander420 · #9 Post by **comander420** » Wed Jan 12, 2005 3:42 am

great work wistech, hopefully one less bug to worry about. Are you going to take up some work on the side to fix these for cash or can you write an instruction page about fileing or grinding the rivit down a bit to get the tighter rolerance that is required so they don't wobble so much, and how to stake the pin back in afterwards? I'd be happy to have it done either way, you or me, as long as it gets done right. I just don't have the time, knowledge or cams to experiment on.

Happyboy · #10 Post by **Happyboy** » Wed Jan 12, 2005 4:03 am

Got the cam back and it looks great. Thanks John!