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Author Topic: mysterious engine failure on the cb718  (Read 4787 times)

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Offline paulages

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mysterious engine failure on the cb718
« on: June 25, 2009, 07:31:57 PM »
I don't know what god I offended while in europe for two months, but my bike has officially paid for it.

I posted in another thread that I found a piece of the cam chain in the oil pan during an oil change and inspection. I pulled the engine, split the cases and changed the chain, no problem. I had caught the chain failure before it actually came apart.

After reassembly, it ticked a little louder on the top end than I remembered. Cam was timed properly, and I adjusted and readjusted the valves. Anyway, I was riding it up a long hill and the clutch suddenly went out. As I limped up the hill, the engine suddenly felt like I was running out of gas. I flipped the petcock to reserve and finding neutral brought the bike to a stop. Back at the shop, I found that my clutch pushrod was flattened on both ends and was toast. I replaced this with a spare and fired her back up.

...and it's only running on 3 cylinders. #1 is sputtering back through the carb. After a cold compression check, I find 120 PSI on 2,3,4, and ZERO on number 1.

Time to pull the head and find out what went wrong. I'm praying for valves and not a hole in my piston... :'(
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paul
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Offline OldSchool_IsCool

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Re: mysterious engine failure on the cb718
« Reply #1 on: June 25, 2009, 07:39:09 PM »
Man!  When I ignore my bike, she simply refuses to start!  I think your bike is TOTALLY over reacting!  She didn't smell another bike on you, did she?

Serious tho, I hope it turns out to be a faulty compression meter.
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #2 on: June 25, 2009, 07:43:03 PM »
Quote from: OldSchool_IsCool on June 25, 2009, 07:39:09 PM
Man!  When I ignore my bike, she simply refuses to start!  I think your bike is TOTALLY over reacting!  She didn't smell another bike on you, did she?

Serious tho, I hope it turns out to be a faulty compression meter.

If only I HAD cheated! Two and a half months without riding sucks. I can't imagine a faulty meter that only fails on one cylinder... unfortunately.
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paul
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Offline OldSchool_IsCool

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Re: mysterious engine failure on the cb718
« Reply #3 on: June 25, 2009, 07:47:54 PM »
Did anyone have access to the keys while you were gone?  Maybe SHE is the one with the guilty heart!
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Offline Pinhead

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Re: mysterious engine failure on the cb718
« Reply #4 on: June 25, 2009, 09:49:06 PM »
Oh no, say it ain't so!

Twisted cam possibly?
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #5 on: June 26, 2009, 01:22:55 AM »
Quote from: Pinhead on June 25, 2009, 09:49:06 PM
Oh no, say it ain't so!

Twisted cam possibly?

Logically, it has to be something I did upon reassembly, but I don't know what yet. The cam looks fine.. valves are opening and closing fine. If it hadn't ran fine for a bit before failing, i'd think I might have bent a valve installing the cam cover, like the 550s like to do. But... it was fine at first. I hope it was a valve, but going out up a hill sounds more like a holed piston from detonation. Anyway, I'll pull it apart ASAP and find out.
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paul
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Offline mrblasty

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Re: mysterious engine failure on the cb718
« Reply #6 on: June 26, 2009, 02:06:16 AM »
Thats enough to give a guy the #$%*s, good luck.  My bike likes to drop two cylinders when I'm 80 miles from home and its raining ice cold buckets. ::)
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Offline Hush

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Re: mysterious engine failure on the cb718
« Reply #7 on: June 26, 2009, 02:16:20 AM »
You didn't look lovingly at an Aprilia while you were in Europe did you?  :o
I figured pretty early on that my Honda is female, hard to start-hard to stop-needs constant attention or she sulks-takes ages to warm up-new shoes cost $400 a pair (Metzlers)-looks better with bling (more chrome added)-looked like a cheap date at $700 initial cost but has blown out to $4000 plus. :D
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Offline turboguzzi

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Re: mysterious engine failure on the cb718
« Reply #8 on: June 26, 2009, 06:04:52 AM »
Hei Paul

maybe you forgot to bolt back the number 1 cyl's rod?  ;)

wondering what you'll find, zero comp is strange indeed.

the clutch rod melting sounds like a lack of slack in the cable.

A holed pisotn would have released a HUGE smoke cloud, so find it hard to believe.

Shaise indeed,

TG

 
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #9 on: June 26, 2009, 12:42:33 PM »
Quote from: turboguzzi on June 26, 2009, 06:04:52 AM
Hei Paul

maybe you forgot to bolt back the number 1 cyl's rod?  ;)

wondering what you'll find, zero comp is strange indeed.

the clutch rod melting sounds like a lack of slack in the cable.

A holed pisotn would have released a HUGE smoke cloud, so find it hard to believe.

Shaise indeed,

TG

 

it could have released a mushroom cloud and i wouldn't have known, as i was simply trying to make it up the hill with no clutch! i'm betting on bent intake valve. it would explain the puffs backwards through the carb when running, and if stuck would obviously kill compression. i have no idea how though, unless i caught the valve tip at an angle while installing the cam cover.
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paul
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Offline mlinder

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Re: mysterious engine failure on the cb718
« Reply #10 on: June 26, 2009, 01:04:02 PM »
Man I so hope it's just a bent valve, Paul.
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Offline turboguzzi

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Re: mysterious engine failure on the cb718
« Reply #11 on: June 26, 2009, 02:48:30 PM »
If it would be a holed piston you'd smell it too!

i always unscrew all the valve adjuster screws before mounting the cover so they are flush with the rocker arm, so less chances of them catching on a valve. also put a rubber band between rocker pairs to hold them up and cut them after installation.

TG
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #12 on: June 26, 2009, 05:20:40 PM »
Quote from: turboguzzi on June 26, 2009, 02:48:30 PM
If it would be a holed piston you'd smell it too!

i always unscrew all the valve adjuster screws before mounting the cover so they are flush with the rocker arm, so less chances of them catching on a valve. also put a rubber band between rocker pairs to hold them up and cut them after installation.

TG

I usually do the same when assembling a 500 or 550, but I haven't had the problem before with a 650 head. I pulled everything but the head but had to leave the shop. #1 intake valve feels a little sticky when depressing it. I'm pretty sure that's the problem.
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paul
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #13 on: June 26, 2009, 08:01:15 PM »
Well, the verdict is in... bent intake valve, caused by improper cam timing. Before I pulled the cam to replace the chain, I had scribed the timing location on the cam and sprocket. Well, something was wrong. I retimed it properly before the actual failure, bit the damage must have been done already. There are indentations on the piston where they kissed. There are faint marks on 3 and 4 as well, but compression was 120 PSI (cold) on both, so I'm going to assume they are ok. Them's some close clearances, folks...
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Offline MRieck

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Re: mysterious engine failure on the cb718
« Reply #14 on: June 26, 2009, 08:10:22 PM »
Quote from: paulages on June 26, 2009, 08:01:15 PM
Well, the verdict is in... bent intake valve, caused by improper cam timing. Before I pulled the cam to replace the chain, I had scribed the timing location on the cam and sprocket. Well, something was wrong. I retimed it properly before the actual failure, bit the damage must have been done already. There are indentations on the piston where they kissed. There are faint marks on 3 and 4 as well, but compression was 120 PSI (cold) on both, so I'm going to assume they are ok. Them's some close clearances, folks...
You got bit in the ass Paul....it happens to everyone the goes out on a limb (including me more than 5 times). ;)
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Offline bill440cars

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Re: mysterious engine failure on the cb718
« Reply #15 on: June 26, 2009, 08:33:22 PM »


      Too Bad about the 718, Paul. Sure am glad it wasn't worse. Need to get that sucker going, so you can put some "Carefree" miles on it. ;)

                             Take care Paul,  Bill ;)
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Offline razor02097

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Re: mysterious engine failure on the cb718
« Reply #16 on: June 26, 2009, 08:42:08 PM »
so the pistons hit the valves are they screwed?  Sad to hear about your engine woes  :(
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #17 on: June 26, 2009, 09:53:10 PM »
Quote from: MRieck on June 26, 2009, 08:10:22 PM
Quote from: paulages on June 26, 2009, 08:01:15 PM
Well, the verdict is in... bent intake valve, caused by improper cam timing. Before I pulled the cam to replace the chain, I had scribed the timing location on the cam and sprocket. Well, something was wrong. I retimed it properly before the actual failure, bit the damage must have been done already. There are indentations on the piston where they kissed. There are faint marks on 3 and 4 as well, but compression was 120 PSI (cold) on both, so I'm going to assume they are ok. Them's some close clearances, folks...
You got bit in the ass Paul....it happens to everyone the goes out on a limb (including me more than 5 times). ;)

Coulda been worse! This is an easy fix at least. I have to order a new head gasket (it hurts, since it looks perfectly fine and has practically no miles on it) and a new valve. I have deeper valve pockets now! Man, at least it's not a piston.

I'm a little a confused about how this all happened though.. when I clayed the cylinders for the valve clearances, it was with the non-slotted sprocket. The slots allow the cam to retard, not advance. So...if I had it wrongly timed, it was at worst timed at the mockup point, which should have been ok. Hmmm. At least retarded to 105 lobe centers it should have a hair more room.
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paul
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #18 on: June 27, 2009, 10:52:23 AM »
I forgot to mention something... there is a small hole in the offending valve's seat. I don't know how the hell this happened unless some foreign object got in, and with thr valve touching the piston there was no room for it to go when the valve closed. Don't know what this means really. I have to clean the pit out and see how deep it is.

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Offline bwaller

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Re: mysterious engine failure on the cb718
« Reply #19 on: June 27, 2009, 11:14:36 AM »
Bummer. This is just another test of your resolve Paul, keep a stiff upper lip.
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Offline mlinder

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Re: mysterious engine failure on the cb718
« Reply #20 on: June 27, 2009, 11:19:12 AM »
We can prolly just JB Weld that pit on the seal, Paul....  8)

I'm kidding. Kidding.
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #21 on: June 27, 2009, 11:22:03 AM »
Quote from: bwaller on June 27, 2009, 11:14:36 AM
Bummer. This is just another test of your resolve Paul, keep a stiff upper lip.

Eh, it's not the end of the world. At least I got to take a look inside. The pistons have light carbon buildup, and the honing is still visible on the cylinder walls.

The real bummer is that I don't have a rider at all right now, and the weather is absolutely perfect. The BSA engine is mid-rebuild right now as well.
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Offline moham

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Re: mysterious engine failure on the cb718
« Reply #22 on: June 27, 2009, 12:08:45 PM »
Sorry to hear about your travails but I'm sure you'll get it sorted. Died this mean you'll have to weld some material and then cut a new seat? I'm totally clueless, just guessing...

As an aside, I got the 650 carbs today but haven't looked at them. I'm sure they're fine. Thanks again...

Mo.
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Offline paulages

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Re: mysterious engine failure on the cb718
« Reply #23 on: June 27, 2009, 01:19:54 PM »
Quote from: moham on June 27, 2009, 12:08:45 PM
Sorry to hear about your travails but I'm sure you'll get it sorted. Died this mean you'll have to weld some material and then cut a new seat? I'm totally clueless, just guessing...

As an aside, I got the 650 carbs today but haven't looked at them. I'm sure they're fine. Thanks again...

Mo.

Those seats are hard. I'm not sure how I'll need to repair it yet. I'll take it to the mahine shop Monday.
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paul
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