Author Topic: Confounded by Bad Compression  (Read 1493 times)

Offline RichardL

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Confounded by Bad Compression
« on: 26.05. 2014 19:06 »
Looking for wisdom or even wild ideas.

In reaching the end of my repair/rebuild, I adjusted my valves to 0.008 and 0.010, then ran a compression test. Both cylinders ran up to over 150 PSI on about four kicks. This seemed pretty good, being about the same or better than I was getting before the start of repairs last year. With everything back together I was feeling cocky and guessed it might start on first kick. Very wrong. After about 12 kicks there was obvious difference in how much compression I was getting. Tested again and both cylinder were around 75 PSI or less. Gave one cylinder an oil squirt to test the rings and no change.

So, I'm looking for ideas, opinions, guesses and, even, sage advice as to what is going on here. A few added bits of info: new head gasket, well annealed and coated in copper sealant; in the rebuild, honed bores to a looser fit (0.004"), because my pistons showed scoring; reused my old rings because they showed no signs of wear, even though the pistons were scored (I could see this as an arguable cause of the problem if I hadn't, one hour previous, had god compression); valve train seemed to be working fine with TDC found with dial indicator.

How about: Blew a leak in the head gasket between cylinders? Snapped the cam shaft? (Easy enough to check if I want to pull off one of the valve adjuster covers just put on.) What do you think? No wild guesses will be sniffed at. No offense will be taken if your suggestion starts with, "Richard, you dumb..."

Edit to the list: maybe i fell asleep when installing the idler gear and missed a tooth?

Thanks

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Online bsa-bill

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Re: Confounded by Bad Compression
« Reply #1 on: 26.05. 2014 19:13 »
Throttle wide open for both sets of compression test?
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline RichardL

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Re: Confounded by Bad Compression
« Reply #2 on: 26.05. 2014 19:30 »
Bill,

Interesting thought. I just now tried it both ways. Got about 100 closed and 75 open. Given the every-other-rotation  behavior for puffs from the open plug holes, I'm thinking it can't be a sheared camshaft.

Richard L.
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Offline RichardL

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Re: Confounded by Bad Compression
« Reply #3 on: 26.05. 2014 19:45 »
Something else occurs to me, it seems so unlikely, because the valve train appeared to be running so smoothly, but could I be out of the cups on the tappets? I thought I had resolved this in this build a while back when I noticed one adjuster too far out of the rocker arm. The reason this occurs to me now is because my 150 PSI test was with adjuster covers off.  Maybe adjusters are hitting the cover and I just imagined 12 kicks of good compression?

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline RichardL

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Re: Confounded by Bad Compression
« Reply #4 on: 26.05. 2014 20:07 »
Nope. Adjuster covers off and still seems to be perfect valve train movement but still sh*t compression.
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Offline muskrat

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Re: Confounded by Bad Compression
« Reply #5 on: 26.05. 2014 21:27 »
Bugga. That was my first thought too Richard.
 Did you check the ring gaps. Honed bore + old rings would mean their a bit large.
Did you have the head apart and lap the valves? Did you at any stage turn the motor over with the pinions out of sync?
Comp tests should always be done with a wide open throttle, and will take at least 10 kicks to reach max.
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Offline RichardL

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Re: Confounded by Bad Compression
« Reply #6 on: 26.05. 2014 21:51 »
Thanks, Muskrat. The ring gap issue occured to me about a half hour ago while driving to the hardware store. If I made the bores 0.0015 larger in D, the ring gap is going to be about 0.005 larger. I'm going to try the oil squirt method again, but with real oil instead of thin Marvel Mystery Oil (if that's known to you).

It's getting clear that some disassembly is on the way. The screwed up part is that I'm not exactly fast and it's hard to dedicate large chunks of time to rebuilding. Our ridiculously long, near-record cold winter offered extra building time, but today was a beautiful warm holiday and I really wanted to ride. Now it becomes questionable if I'm riding this season.

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline RichardL

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Re: Confounded by Bad Compression
« Reply #7 on: 26.05. 2014 21:59 »
Did you have the head apart and lap the valves? Did you at any stage turn the motor over with the pinions out of sync?

I didn't lap the valves because it was a crank shim that brought on the rebuild. Are you asking if it's possible that the valves got bent due to out of sync timing? (I sure as hell hope they didn't.)

Richard L.
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Re: Confounded by Bad Compression
« Reply #8 on: 26.05. 2014 22:13 »
I doubt the valves got bent, you'd have no compression at all.
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Offline muskrat

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Re: Confounded by Bad Compression
« Reply #9 on: 26.05. 2014 22:24 »
"I doubt the valves got bent, you'd have no compression at all."
Not necessarily, it only takes a tad to give low but some comp.
A common mistake is to turn the motor to line up the dots & dashes and touch a valve. It must be done without the pushrods in or turn the cam while the piston is down.
Cheers
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Offline RichardL

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Re: Confounded by Bad Compression
« Reply #10 on: 26.05. 2014 22:36 »
In my case, it was all done on the bench before barrels were attached. I kept thinking that the dot on the crank pinion was pretty small and obscure, but it seemed the only one that was there. The next question is: if I missed the timing mark, am I gauranteed  bent valves with 9:1 pistons?

Richard L.
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Re: Confounded by Bad Compression
« Reply #11 on: 27.05. 2014 10:12 »
Quote
I gauranteed  bent valves with 9:1 pistons?

pretty much so - DAMHOK - well ok I'll put my hands up, I reckon in a moment of focuslackabilty I did the dreaded deed that Muskrat alludes to, can't exactly recall but can offer no other explanation.

Very small bend in the valve in fact I never did SEE it but the give-away was a small mark just out of the pistons valve pocket, wasn't testing for compression then so didn't notice anything at the time but while adjusting the tappets noticed one valve just very momentarily hesitated when returning, wasn't much and could easily be missed.
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline muskrat

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Re: Confounded by Bad Compression
« Reply #12 on: 27.05. 2014 11:16 »
OK Richard here's a test to try. Buy a cheap compression tester. Cut the hose 1/2 way. Insert a T piece and then attach another hose with a fitting for your air compressor. Now there's two ways to go.
1st wind motor over to TDC on comp stroke insert comp gauge and pump in some air. Listen for a leak in the ex and inlet ports. Same on other side. Best if pipes and carb are off.
2nd same as 1 but fill the combustion chamber with diesel and look for it in the ports.
50 to 100 lb/sq" should show something up. Lay bike over to drain fluid from chamber before turning over to the other side.
Good luck.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline RichardL

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Re: Confounded by Bad Compression
« Reply #13 on: 28.05. 2014 03:59 »
Peace of mind on a couple of fronts. First, I can be pretty sure I don't have bent valves. Second, I may not be losing my mind, so much. Also, it didn't take much time to get to the answer (regarding gear timing). I didn't even need to turn the crank over 97 times. Had to remind myself during disassembly not to let the idler come out with the inner cover, otherwise, no peace.

Richard L.
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Offline muskrat

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Re: Confounded by Bad Compression
« Reply #14 on: 28.05. 2014 11:09 »
OK so it's back to the think tank! I'm outta ideas.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7