JP3 Decompressor set up

[Wrinkley]

Hi All

 

Trying to hand start my JP3 was difficult with the front decompressor not working! I don't know how to adjust this. The engine has "buttons" down the crankcase side and a rod/lever arrangement to take the "buttons" out. Any help on how to set this up would be gratefully received. One this is sorted I can then put it up for sale.

 

Regards

 

Graham

[martyn 1]

Hi Graham

 

I take it you meen you have the buttons on the door below the exhaust, these are not adjustable. if the valve clearances are correct then it will decompress. Does the button push in fully the back of the knob should touch the seal nut if fully in.

 

Martyn

[davey b]

Are you sue it's not working? If that valve is already open on the cam the lever will appear slack. My JP3 has the levers on the top covers so I can't help with the linkage, sorry.

 

I would like to get a hand crank for the top, though. Mine is a marinized industrial so would have used a handle on the crank shaft. If anyone knows of any parts available...

[by'eck]

This image shows the cam follower arrangement on a JP2, very similar to your JP3.

 

 

I believe the crankcase buttons you refer to when pressed in, lift the broad based cam follower, holding the exhaust valve open. I'm guessing there is a forked mechanism the other side of the button which forces & holds up the appropriate cam follower for each cylinder.

 

As this is a simple arrangement it should work assuming you are able to push all the buttons in. If not removal of crankcase door will reveal the problem.

 

Sorry I can't offer more advice, but I also have the cylinder head mounted decompressors & my parts diagram shows only that type as well.

Edited January 8, 2012 by richardhula

[Timleech]

I drove a railway loco, in my student days, with a JP2 which had that problem. We never got to the bottom of it, I was young & innocent then (!), so we just started it by swinging it over compression. From memory the button just pushes a peg under one side of the cam follower. As suggested, first check that tappet clearances are correct. My next step would be to slacken the fastening of the button onto the door, to establish whether there is any adjustment possible by moving it up or down on its mountings. I seem to remember it's held with three screws, could be wrong.

 

Tim

[Proper Job]

Have a look at the photo's on this site: link

 

There are some of the link mechanism that actuates the decompressors.

[Timleech]

Have a look at the photo's on this site: link

 

There are some of the link mechanism that actuates the decompressors.

 

So far as I can see that only shows the remote mechanism, for pulling out the knobs, as used on gensets etc, and not the actual internal workings which as I understand it is where the OPs problem lies.

 

Tim

[Proper Job]

Ok.......

 

I wasn't sure from the OP if the problem was on the mechanism or the buttons.

 

I would suggest removing the crank case door and checking the button units. Are they all the same? Are they all fitted the same? Is the one that is not working the same length as all of the others?

 

Another thing could actually be the link mechanism. The link can move on the shaft (IIRC it's only a small grub screw that holds it in place) and when the lever is set to 'decompress', the arm does not push the button in enough to lift the valve.

 

Mine are indivdual, so I tend to give them a 'tap' in with the palm of my hand to decompress

[martyn 1]

As previously mentioned, the buttons on the side are not adjustable, the button assembly is held in with three bolts but is located on a register so one location fit only. when you push the button in the single pin on the inside pushes under the tappet which in turn lifts the valve.

 

To check all the possibilities as to why it isnt working. firstly check that the button does fully push in (back of button should pretty much touch the nut behind it and with the rocker cover off you should also see the valve move (making sure the valve isnt already opened by the cam.....) . Secondly with the crankcase door removed check that you dont have a flat worn on the top of the pin as this will effectively stop it working. and thirdly check that there is not excessive wear on the rocker shaft or incorrect valve clearance ever of these will stop the decompressor from working properly.

 

If I can find some pics in our archive of the system dis-assembled I will post them later.

Edited January 9, 2012 by martyn 1

[Wrinkley]

First, thanks for all the above. I have now taken the decompressor door off to have a look as all the checks advised did not find the cure. Inside the cover were three sub castings for the decompressor button/rods. These sub castings had three sets of three holes with H, C, and L on the bottom three holes. The hole where the rod comes through was slightly off centre so I guessed it was a cam type off adjustment for the hight of the rod. All the sub castings were set up on the C hole so I moved the offending decommpressor to the H hole and re fitted the door. Turned engine over slowly in case of valve piston hitting but all clear. I can now turn the engine over! I am going to adjust the centre decompressor tomorrow as this needs a little more clearance so I can swing the engine over like the ones on youtube.

 

Thanks