What does the spring and plunger under the cam, actually do? Thanks.

 

Just finishing a full case split rebuild and I believe it is the decompressor. Tom at TC told me it wouldn't run without it.

XC 406EX

 

I'm in the process of rebuilding mine...just got in for the night.The pin you saw under the cam is the decompression pin.I was told by a couple sources to remove it when you get your new cam.The reason is that the lobe for the decompression pin gets removed(atleast for some aftermarket cams)from the cam when they create the new grind.I was told it will run,and without the lobe for that pin the spring/pin would probably float around 'til it wrecks something.I should know by tomorrow night.

 

The motor definitley runs without the decompression pin.My cam had the lobe for pin contact removed.It fired right off.

 

I was wrong, yes it does run. definately check to see if you have a decompression lobe on your cam. The decompression lobe only lasts about 5 degrees on the cam, so it looks like a small quick hump. I changed cams today so I got a good look at it.

Tinner has got a good point, if you do not have a decompression lobe, this would allow the pin to come out and travel through your motor cutting your day short to say the least.

 

I put istalled a new piston(10.5),cam and port work.When I finished it fired right up.About two minutes into break-in the thing went"tink" and quit(it did this before the mods but not that loud).I was afraid to try and start it again...fearing that I dropped a valve or something.I pulled the valve cover off and everything was as it should be,it also turns over(by wrench)with compression.I may have been too cautious,but I don't know if the cam and piston will not work together because of reliefs and liftafter it heats up. There's no resistance turning it over with the wrench.

 

I guess this thing is just cold blooded.I need better gas too.

 

The decompressor relieves stress on the starter motor without is the engine will crank slower and wear out the starter faster. I think the pin stops the decompressor cam once it starts??? Not sure

 

Thanks for the help guys. I think it is to stop the decompression thingy, once it starts. If you have the decompression lobes on your cam, you definitely need the spring and plunger underneath it. Thanks.