Author Topic: Compression on a 1916 Douglas  (Read 5110 times)

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Offline Ron

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Compression on a 1916 Douglas
« on: 12 Sep 2014 at 13:58 »
Hi folks, I just finished my 1916 despatch riders bike. I have full history of this bike. Can anyone tell me how the compression is on these bikes? I just revised the engine but only now just closed it up (my fault) and there doesn't seem to be a lot of compression. I will check the valves and valve covers or have to take the engine off the bike again? Can one start these bikes by turning the flywheel or do I have to run? Bummer. Thanks for all comments

Offline Chris

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Re: Compression on a 1916 Douglas
« Reply #1 on: 12 Sep 2014 at 16:27 »
Hi Ron
The compression should be quite high on both cylinders. Starting is normally by sitting astride the machine and with it in gear paddling away with the feet with the valve lifter lever raised and releasing the lever when the flywheel is rotating. By experiment you will need to have set the air and gas levers to the correct initial settings. Another way of starting the machine while on the stand is, again with the machine in gear, is to pull the rear wheel round. The compression is normally high enough that this is quite difficult to do without the belt slipping unless you have someone else to raise the valve lifter lever and to let it go when the flywheel is rotating. Chris.

Offline Ron

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Re: Compression on a 1916 Douglas
« Reply #2 on: 12 Sep 2014 at 17:39 »
Thanks Chris, the compression is next to nothing so I will check the valves first, they are new and perhaps not grinded in enough on the seats. To be continued. I will post when I find the cause. I was thinking piston rings also but even with worn piston rings the cilinders should have at least some compression. My guess is valves but will check that out. Thanks again :-)

Offline MRD

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Re: Compression on a 1916 Douglas
« Reply #3 on: 12 Sep 2014 at 20:35 »
Hi Ron
Be very careful if you try to start the bike on its stand. You should put the gear lever into “HIGH”, this will make it easier. As Chris said pull the wheel round but be careful not to hit your hand on the stand clip or edge of the rear mudguard. It might be a good idea if you are trying this method for the first few attempts to use a leather glove.
Once the engine fires it will rev up, so you will need to get to the handle bars and adjust the throttle/air control to bring the revs down and at the same time taking it out of gear in to the "FREE" position.
Hope this helps.
Regards        Martin

Offline Ron

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Re: Compression on a 1916 Douglas
« Reply #4 on: 12 Sep 2014 at 21:02 »
Thanks for your help Martin. Will do !

Offline Ron

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Re: Compression on a 1916 Douglas
« Reply #5 on: 13 Sep 2014 at 18:15 »
Anyone knows where I can get piston rings in case of I would need them? Thank you!

Offline Black Sheep

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Re: Compression on a 1916 Douglas
« Reply #6 on: 17 Sep 2014 at 05:50 »
Starting on the stand is a bit perilous - so easy to mangle your fingers. When I first started my 1914 2 3/4 I used a socket spanner on the flywheel nut to flick it over. When it finally started, the socket and handle spun round with the flywheel before shooting off into the far recesses of the shed. Normally I paddle to start. Compression on mine isn't great but then the engine hasn't been apart since the 1920s. 

Offline Ron

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Re: Compression on a 1916 Douglas
« Reply #7 on: 17 Sep 2014 at 06:24 »
Thanks for the info, but having a bit expérience with old bikes, I wouldn't dare to try this  :) I value my health lol. Nope I will dismantle the barrels now and order new piston rings. I 'll let you guys know how I will get the bike started. I hope the magneto still does the job. Its been six years since I had it rewound. Thanks for all the info, I needed it ;-)

Offline graeme

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Re: Compression on a 1916 Douglas
« Reply #8 on: 17 Sep 2014 at 10:20 »
Hi Ron
If you have no luck sourcing piston rings locally, JP Engineering in Adelaide (the piston makers) have an enormous range of piston rings.
Cheers,
Graeme

Offline Ron

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Re: Compression on a 1916 Douglas
« Reply #9 on: 17 Sep 2014 at 10:27 »
Thanks very much Greame  :mrgreen:

Offline Chris

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Re: Compression on a 1916 Douglas
« Reply #10 on: 17 Sep 2014 at 11:18 »
Hi Ron
Unless the valve seats were nicely cut and ground in properly so that they make a good seal this can be a significant source of loss of compression, assuming of course that you do have tappet clearances permitting the valves to reach their seats. Unless broken or missing, piston rings in themselves even if worn are unlikely to be so bad that compression will be as low as that you complain of. What is more likely to cause serious loss of compression is for the piston rings to be completely stuck in their grooves.This can and does arise from more than one cause. Under oiling and overheating can cause rings to seize in their grooves but also if carbon is permitted to build up extensively in the combustion chamber and on top of the piston it is not unknown for the top of the piston to become peened over trapping the top piston ring in its groove.  If you have the original cast iron pistons with wide piston rings then it is possible to use two narrow rings in the groove. Another cause of loss of compression would be if the bores are seriously scored and this unfortunately is a very common problem with the 2.3/4 hp models . The gudgeon pin in the early pistons was located in the piston by a small copper disc each side. With a high mileage and bore wear these discs can wear so thin and with increased piston clearance slip out of position permitting the steel gudgeon pin to move sideways creating the common tram lines down the bore. The only cure for this is to re-bore and fit new pistons. The later alloy pistons from JP have their gudgeon pins secured with circular wire circlips and even these have to treated with caution as they have not been shown to be foolproof. Chris.

Offline Ron

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Re: Compression on a 1916 Douglas
« Reply #11 on: 17 Sep 2014 at 11:50 »
Hi Chris,
Just dismantled the engine and you made me check again. I've blown in the sparkplug hole sealing the barrel with my hand and the air goes right through the valves without resistance I probably haven't grinded the valves in enough on their seats. I will do this again. They are new and I ordered them at the time at the London Douglas club. Another option would be to install the old valves which I'd rather not do. So I guess a bit more elbow grease won't hurt me. Thanks!

 

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