Author Topic: 3.5HP timing  (Read 541 times)

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Offline kevinbush

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3.5HP timing
« on: 15 Nov 2021 at 09:32 »
Hi, can anybody help with timing my 1915 3.5HP. I have rebuilt my engine after seizure. I lined up the timing marks on all three gear wheels, but once I turn the flywheel a few revolutions the marks no longer line up.

Thanks in advance Kevin .


Offline cardan

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Re: 3.5HP timing
« Reply #1 on: 16 Nov 2021 at 01:35 »
Hi Kevin,

I no longer use timing marks to time an engine - if they line up once it's timed, good, but if not it doesn't matter. Lots of engines now contain parts they didn't leave the factory with.

Veteran engines are extremely boring. Almost all have zero overlap, and the exhaust valve closes as the inlet valve opens at TDC on the non-firing stroke. If there is a bit of overlap, just spread it evenly between the exhaust and inlet. Original timings of the era often had the exhaust closing about TDC, and the inlet opening a little (say 5-10 degrees) after TDC.

So it's a simple plan:

First understand which way the engine rotates. Anti-CW viewed from the flywheel side.

Next figure out which way the camshaft turns. I think on the 3 1/2 there's an intermediate gear between the mainshaft and the camshaft, so the cam rotates the same way as the engine?

Fix the half-time pinion on the crank, and the gear on the camshaft.

Set the engine to TDC on your favourite cylinder. Handy to have an outside flywheel! (Being a Douglas, both cylinder will be at TDC.)

Rotate the camshaft (in the 'forward' direction) so that, for the cylinder you have chosen, the exhaust valve is just closing and the inlet valve is just opening.

(In your dreams, at this point BOTH the timing mark on the half time pinion AND the timing mark on the cam gear are pointing towards the shaft for the intermediate gear.)

Drop the intermediate gear into place - maybe the timing marks will all line up, maybe not. Don't worry.

Now rotate the engine forwards one complete turn (360 degrees) to TDC again. On the OTHER cylinder, the exhaust valve should be just closing and the inlet just opening.

If so, you are done. Well, done enough that the engine will run.

Maybe report back on what goes wrong when you work through the steps.

Cheers

Leon


Offline graeme

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Re: 3.5HP timing
« Reply #2 on: 16 Nov 2021 at 10:01 »
Good simple explanation Leon
When my dad restored the 3 1/2 I now own, we initially set up the gears as per the marks, and couldn't get any sense out of it - particularly the keyed magneto gear. We did just as you suggested and went back to basics, and got the bike running well, then painted on new marks!
Cheers, Graeme

Offline kevinbush

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Re: 3.5HP timing
« Reply #3 on: 16 Nov 2021 at 16:31 »
Thank you Leon and Graeme, will give it it a go. Thanks Kevin

Offline kevinbush

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Re: 3.5HP timing
« Reply #4 on: 19 Nov 2021 at 19:36 »
So update . I have enclosed pic of my engine to prevent confusion. I removed gear wheel 3 ,then set bottom of engine at TDC as directed,then refitted middle section and top section of crankcases,then set valves as directed, I noted that exhaust valve closes a lot before inlet valve starts to be opened,so i set the valve position just as both valve springs under pressure.
I then found I could not fit the gear wheel three in position, so then removed top two sections of crankcase again ,refitted gear wheel and did same procedure best i could, note the timing  marks do not line up. I have found at this point the flywheel spline facing at 3 o'clock towards the front of engine,after 360 degrees rotation of flywheel, found  both valves in similar positions and the flywheel spline position.
What do you think, shall I refit engine and see if bike runs. I will check if points  opening just before TDC, I forgot to do this. Is there an easy way to find TDC as with cylinder heads fitted seems difficult?

Thanks again Kevin

Offline cardan

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Re: 3.5HP timing
« Reply #5 on: 19 Nov 2021 at 22:44 »
Hi Kevin,

The test of whether the valves are timed correctly is to turn the engine forwards - on the non-firing stroke the exhaust valve should just be shutting at TDC, and the inlet valve is just opening, or about to open in the next 10-15 degrees or so. When correct this is true for both cylinders.

If you don't have a degree disc, you can make do with a dress-maker's tape and a marker pen. Fit the flywheel. Mark TDC relative to a fixed point - vertical or a fixed wire pointer. Run the tape around the outside of the flywheel (360 degrees) and measure the circumference. Divide the circumference by 10, and mark this out out on the flywheel before and after TDC - this will be +/- 36 degrees. You can use the "36 degrees before TDC mark" to time the magneto (on full advance). You can use the 36 degrees after TDC mark to make sure that the inlets are not opening too late - they should be well opening by the time you get half way there - 18 degrees after TDC.)

OK - there's a problem with my instructions, which were to leave the half time pinion (gear) on the crankshaft (1), and to leave the gear on the camshaft (3). I had envisaged just taking taking out the intermediate gear (2), setting the engine to TDC, setting the cam so the valves are "rocking", then dropping the intermediate gear (2) into place. But I see the top of the oil pump shaft is in the way! Are you able to drop the oil pump out?

I assume in the factory the oil pump went in last!

If you can do that, my instructions should be OK.

Re the timing marks, when the engine is at TDC I assume one of the 180-degee marks on the half time pinion points directly towards the intermediate gear spindle, Mark A on the photo below lines up with it, Mark B then points directly at the camshaft, and the mark on the cam gear lines up with it. But, as in my opening comment, don't bet on it! Remember that the position of the intermediate gear is completely arbitrary - it will look good if the timing marks line up but the engine will run exactly the same regardless of where you throw it in. Even the number of teeth on it is unimportant! It is the position of the cam relative to the crank that is important, thus the importance of timing the engine.

Good luck. Be confident!

Cheers

Leon

Offline graeme

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Re: 3.5HP timing
« Reply #6 on: 20 Nov 2021 at 08:59 »
Hi Kevin
Finding TDC can be done by removing one of the valve caps and poking a wire in, a bit of a fiddle I know but it is possible. Another method that works on any engine is to get a spark plug, knock out the insulator so that it is hollow, then put a balloon, or better still a condom over the plug body, and see when it blows up the most before deflating again. This is really quite accurate.
Cheers, Graeme

Offline Hutch

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Re: 3.5HP timing
« Reply #7 on: 21 Nov 2021 at 03:31 »
Kevin,

All great advice from Leon and Graham. I just remembered I had a photo copy of a photo copy of a photo copy etc of the The Handbook of the Douglas Motorcycle 4HP 1920 and attached is the section on timing the valves and magneto - as explained by the factory ! Hope it helps and doesn't complicate anything further :-).

The marks on the timing gears in the picture are not very clear but you can still just make them out.

cheers

Hutch

Offline graeme

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Re: 3.5HP timing
« Reply #8 on: 21 Nov 2021 at 09:37 »
Thanks Ian
I never knew about the flywheel keyway method of finding TDC! Very useful information
Cheers, Graeme

Offline kevinbush

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Re: 3.5HP timing
« Reply #9 on: 26 Nov 2021 at 08:37 »
Thank you guys for the help, apologies for delay, busy as ever. I will advise how it goes. Its brilliant that the such a strong Douglas presence in Oz, take care and thanks again Kevin

 

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