Sq4 Timing sprockets

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adrian.hannam
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Sq4 Timing sprockets

Postby adrian.hannam » Sat Mar 14, 2020 5:25 am

1951 Mk1 Simplex. Turning the engine by hand so that the timing chain travels clockwise, the chain regularly becomes slack between the camshaft and halftime sprockets. Continuing, it snatches up tight again. I have an old chain, it does the same. Looks like I am up for a new chain, but do I need to replace the sprockets too?
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Re: Sq4 Timing sprockets

Postby roger.fellows » Sat Mar 14, 2020 6:46 am

Do you have the 2119-49 'snubber' which supports the spring steel tensioner?

Is the dynamo shaft bent?

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Re: Sq4 Timing sprockets

Postby Paul_Linden » Sat Mar 14, 2020 7:50 am

That slack part of the chain in the second photo is very odd. When you turn the engine for this test are you using the nut on the end of the rear crankshaft (the one in the photo). The “physics” of the chain running clockwise is that the sprocket on the rear crankshaft pulls the chain down from the camshaft sprocket and so that part of the chain should always be in tension.
If you turn the engine using the camshaft sprocket or the dynamo sprocket then you are “pushing” the chain and you will get the slack bit you see.
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Re: Sq4 Timing sprockets

Postby adrian.hannam » Sat Mar 14, 2020 8:00 am

Roger; I have the snubber, not yet in place. The dynamo is a new Longman alternator.
Paul; I am using a socket and breaker bar on the front crank, turning anticlockwise.
I will fit the snubber and try turning the engine using the rear crank.
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Re: Sq4 Timing sprockets

Postby Mick D » Sat Mar 14, 2020 8:13 am

Hi

Something is causing the chain tensioner to compress, (see the position in the second photo compared to the first), resulting in a slack chain which then tightens again when the tensioner springs back.

Does the chain begin to ride up over one of the sprockets at some point? (worn chain).

Does the tensioner compress without seeing any significant riding up of the chain? eccentric sprocket / bent spindle.

Are all of the links in the chain free to move around their pivots?

You need to identify the cause before you can fix it.

Regards Mick

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Re: Sq4 Timing sprockets

Postby roger.fellows » Sat Mar 14, 2020 9:10 am

The alternator probably 'wants to rest' in fixed places. Those might do strange things to where chain tension arises??

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Re: Sq4 Timing sprockets

Postby Pete.Silson » Sat Mar 14, 2020 11:57 am

The camshaft might be culprit. As the follower goes over the cam peak it is pushed down the "off" ramp by the valve spring pressure and turns the camshaft ahead of the drive chain. The effect is less pronounced when the engine is running at speed but I suspect it is why the snubber was added to the Mk1 and the fibre rubbing strip was fitted to the iron 4G.

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Re: Sq4 Timing sprockets

Postby paul.jameson » Sat Mar 14, 2020 8:16 pm

Like Peter, I think that it is simply a matter of the position of the camshaft and the effects of the valve springs. The Longman alternator has no magnetic effects until switched on so it is most unlikely to be that.
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Re: Sq4 Timing sprockets

Postby adrian.hannam » Sun Mar 15, 2020 7:01 am

Installed the snubber, much improved. I need to develop a bit more mechanical nous, should have been able to solve that one myself. Later when I have lube set up I will turn it over with a power drill and see if it still snatches. Now I am plotting valve lift, get set for more stupid Q's.
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Re: Sq4 Timing sprockets

Postby simon.holyfield » Sun Mar 15, 2020 9:50 am

adrian.hannam wrote:Now I am plotting valve lift, get set for more stupid Q's.


Good idea. Setting the valve timings up per Waller showed me that the timing mark on the crankshaft sprocket was not in the correct position.
cheers

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