That auto-decomp chestnut

Ady · Oct 31, 2007

Having thought i had this thing wrapped up - i'm asking for ideas before i remove my cylinder head to take a look.
It was time to adjust the valves as i noticed the noise and the fact that the auto-decomp was struggling to work and therefore allow the engine to turn.
Having done my valves, the auto-decomp wasn't activating at all.
So, i set the lash on exhaust to zero and checked for its correct functioning and it was fine. It is afterall the modified device and has only been in for about 20 hours.
Therefore, it aint no good if the device is only functioning with zero lash - that's too tight...
Any suggestions, before i remove the head.

BTW, Top end serviced when cam chain was renewed about 20 hours ago.

Regards
Ady

ned37 · Oct 31, 2007

if you're talking about the cam operated decomp, you prolly will have to pull the cover to see how much lift you're getting and whether it spins freely. also check to make sure there's no vertical play in the rocker bearings/ shaft.
or by "auto decomp" do you mean the cable that activates when the kicker goes thru its motion? do you still have the handlebar cable connected? is either one binding? is it different with the tank off? is it different with only the auto decomp connected? is it different with both cables disconnected? if all of these are "no", then you prolly have a notch worn in the decomp shaft where it contacts the rocker. you will need to remove the cam cover to check it out.

Simon · Nov 1, 2007

Ady said:
Having done my valves, the auto-decomp wasn't activating at all.
So, i set the lash on exhaust to zero and checked for its correct functioning and it was fine.

Ady,

Most probably you were out from true TDC when you originally adjusted valve lash.

We all know how bleedin' sensitive these things can be :roll:

Simon

Taffy · Nov 1, 2007

RE: Re: That auto-decomp chestnut

do have a very careful look ady at the cam follower bearings as i was suprised at the 'instant' din that your top end produced!

to find tdc i pull the plug and the ignition cover as well as the inspection caps.

using a T-bar fixed with a socket, turn the engine watching the 4-stroke cycle of the valves. as the inlets close you want to see the ignition marks (both or either will do) come level with that allen screw hole over the top of the cover.

or stick to being a sparky! (LOL!!!).

regards

Taffy

Ady · Nov 1, 2007

Having been a 'Sparky' all day - I found half an hour to try a little experiment.
This time, i set the valve lash on the exhaust to .003 as opposed to .005 (or a tad tighter - if you use the 1/4 turn method).
And it seems better !
Can't ride her tonight but will have a test tommorow.... oops tonight, it's gone 12.00

Thanks
Ady

Taffy · Nov 2, 2007

ady

the pitch of the thread is 0.75mm and the tappet clearance should be 0.12mm so a 1/6th or 1/7th will do. i go for the 1/6th method because all i have to do then is say to myself that it's 10 to 4 or that it's 20 to 2 and then go back two numbers on an imaginary clock face.

the reason i think using feeler gausges doesn't work is that the pads that push the valves become cupped and therefore the feeler gauge doesn't get a clean run at it - it's like taking a scaffolding board potholing!

regards

Taffy

Ady · Nov 3, 2007

Worked all day today Taffy thanks.
Definately wearing somewhere though
Will wait till it goes completely before i investigate further.

BTW. Rang Suzanne - no chance 08, ok for 09, but i may be in Oz for that month.

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