Author Topic: Loss of Compression  (Read 577 times)

Offline Stevie1262

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Loss of Compression
« on: 12.08. 2018 00:26 »
Advice please after changing the Magneto and cleaning carb (tank full of rust) getting no compression ...compression easy ok when I picked it up so lifted the head and found this, head bolts were all new, and looks like bottom end has been rebuilt but that’s looks like unburnt oil ....pistons look fairly new and no wear on bores,  right cylinder at exhaust, only top half,  is coated with what feels like  carbonised oil, seems to be stuck to cylinder ...
my guess is leaking valves I will lift barrel off tomorrow is this likely to be rings? And why only one side ...no lip felt at top of bores ..my guess is some major engine work done sometime and was taken off road shortly after  can’t see it being anything other than leaking rings and valves ....

Offline RDfella

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Re: Loss of Compression
« Reply #1 on: 12.08. 2018 12:48 »
Always easier to diagnose before stripping down rather than after. First thing I'd have done was listen at the inlet and exhaust ports whilst turning it over (easier if someone else is kicking it over). Any hissing sound whould mean valve leakage, which could be as simple as tappet clearance needing to be adjusted. Rings? Unlikely. Some rings need to be installed right-way up, and are marked 'top'. However, if these are fitted the wrong way round, there will still be reasonable compression. A broken ring (caused during fitting barrels) will usually show a vertical scratch mark in the bore. Again, there would still be some compression, as the other two rings will still be working. But you suggest there is little or no compression. Very odd. Most likely a valve issue, but more info required.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline edboy

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Re: Loss of Compression
« Reply #2 on: 12.08. 2018 13:20 »
hello, certainly needs top end decoke and clean. i would also clean piston grooves and change rings. they are probably stuck in their grooves if you have poor compression

Offline Stevie1262

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Re: Loss of Compression
« Reply #3 on: 12.08. 2018 15:23 »
I should have added I bought this as a non runner needing minimal attention to get it running....should have known better broke all my rules ...no history how long it’s been off the road, compression wasn’t great and valves def not seating I was guessing carbon build up due to dampness, looking in plug holes confirmed that, but I was hoping to run it and see what happened valves will need replaced and ground in the springs and guides look new, the Pistons wiped up and look new as well the carbon build up on the bore certainly looks like stuck rings as LH bore is super clean the oil in the sump and tank was clean and a bit gloopy ...so going to lift barrels now and see what can be done. I did check everything before lifting the top end off ...fingers crossed I can get away with a hone, rings and new valves ..cheers for advice

Offline Stevie1262

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Re: Loss of Compression
« Reply #4 on: 12.08. 2018 15:25 »
....and yes there was a hissing at the valves when turned over but I hoped it would start and blow the carbon out the back  *eek*

Offline RDfella

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Re: Loss of Compression
« Reply #5 on: 12.08. 2018 15:54 »
Looks like you've got it under control. Good luck with the project.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline Stevie1262

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Re: Loss of Compression
« Reply #6 on: 12.08. 2018 16:38 »
Lifted Barrel and yep rings stuck other piston not so bad but lots of carbon and burnt oil

Offline Stevie1262

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Re: Loss of Compression
« Reply #7 on: 12.08. 2018 17:36 »
Ok last question I promise.... is it worth vapour blasting head to remove carbon and have a look at valves? It’s on standard bores will a hone remove the carbon

Online Greybeard

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Re: Loss of Compression
« Reply #8 on: 12.08. 2018 19:48 »
May I suggest a larger photo size. A width of 1200 pixels is OK.

Offline RDfella

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Re: Loss of Compression
« Reply #9 on: 13.08. 2018 20:43 »
Carbon can be hard and stubborn to remove, so I prefer more aggressive means - like a small wire brush in a cordless drill for CI heads. Alloy heads require gentler treatment. Pulling valves and doing seats is a must. As for honing bores - I'd be a little cautious, as it's easy to take out a thou or more. I'd prefer to clean up using 240 grit wet / dry by hand. Good luck.
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Online morris

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Re: Loss of Compression
« Reply #10 on: 14.08. 2018 08:23 »
Ok last question I promise.... is it worth vapour blasting head to remove carbon and have a look at valves? It’s on standard bores will a hone remove the carbon
A night’s soak in a bucket of water with 250 gr of caustic soda added to it will do wonders to get rid of the carbon
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