Author Topic: Scraping sound when trying to start  (Read 683 times)

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Offline TheHulk

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Scraping sound when trying to start
« on: September 28, 2020, 07:34:09 AM »
Hey everyone. I just wrapped up a long and meticulous rebuild of a 77 CB550F. Rebuilt the top end, which included new pistons, rings, soda blasted engine castings, honed cylinders, lapped valves, new gaskets etc. Followed various instruction videos to a tee. Reassembled everything with a fair amount of assembly lube before closing it up.

Finally got it up and running and inspected, and rode Friday and all day Saturday without any major issues. Tried to start it up Sunday, and got it fired up but puttered out while warming up. Not too unusual, as I still need to properly tune the carbs and idle mixture. But when I went to try starting it up again, it almost seemed to make a rough, possibly scraping sound. hard to describe, but I could tell it was struggling and stopped what I was doing. I checked the oil level, and it still seemed to be at or slightly above the oil line. I did accidentally fill it a touch over the line.

I have a few theories, but I am new to this so I thought I would ask for some advice and guidance. I pulled the Cylinder 1 plug, and it was definitely fouled, meaning I was running too rich. Being so black in such a short amount of time, I am wondering if hydrolock could be a potential cause? I did forget to turn the petcock off after riding, could it be possible that the cylinders are flooded? I will take the plugs out and check tonight to test this theory.

I wondered if it could be that the oil pump failed, but my oil indicator light never came on. Also, I noticed a slight oil leak coming from the cylinder head side covers, which would lead me to believe that oil was indeed circulating, if it's leaking out of the very top of the engine (just a little bit).

Could it be the starter motor? 

Apologies for the lengthy post. I will take any advice to make sure my rebuilt engine doesn't seize up on me. Although, I may be overly paranoid.

Offline seanbarney41

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Re: Scraping sound when trying to start
« Reply #1 on: September 28, 2020, 08:39:06 AM »
If the battery is weak, Honda starter clutches can make some very weird sounds...or does the engine turn hard with the kickstarter?
If it works good, it looks good...

Offline TheHulk

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Re: Scraping sound when trying to start
« Reply #2 on: September 28, 2020, 08:48:10 AM »
If the battery is weak, Honda starter clutches can make some very weird sounds...or does the engine turn hard with the kickstarter?

It seemed to turn hard with the kickstarter too. Voltmeter on the bike put the battery at 12.2V when turning the key. I put it on the battery tender over night just in case and will test again tonight.

Offline seanbarney41

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Re: Scraping sound when trying to start
« Reply #3 on: September 28, 2020, 10:41:03 AM »
If the engine turns hard, scraping sound, on a fresh rebuild...the obvious choice is unfortunately a broken piston ring.
If it works good, it looks good...

Offline seanbarney41

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Re: Scraping sound when trying to start
« Reply #4 on: September 28, 2020, 10:43:22 AM »
Dont start crying or tear it down until you do more digging...nothing very conclusive yet
If it works good, it looks good...

Offline TheHulk

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Re: Scraping sound when trying to start
« Reply #5 on: September 28, 2020, 11:39:32 AM »
Dont start crying or tear it down until you do more digging...nothing very conclusive yet

Ha. I certainly hope this is untrue. I'll start digging to see if I can determine the cause. Say this were the case... would I be able to take one of those small automotive cameras (endoscope), peek inside, and see visible debris from a broken ring?

Offline PeWe

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Re: Scraping sound when trying to start
« Reply #6 on: September 28, 2020, 12:21:59 PM »
You can see scratches in the bores after ring issues, like too tight or no end gap.
Scraping sounds more like an oil rail that is outside its groove in piston.
Oil rails can be very thin.
But that should have scraped when the cylinder was mounted, crank rotated direct holding cylinder down before head went on.
High friction to rotate engine too.

Or primary chain issue, worn too long chain scraping.
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline TheHulk

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Re: Scraping sound when trying to start
« Reply #7 on: September 29, 2020, 05:10:25 AM »
If the battery is weak, Honda starter clutches can make some very weird sounds...or does the engine turn hard with the kickstarter?

Good news! It seems that the issue was in fact just a low battery. Definitely made some weird sounds when trying to start, and as a noobie my head went immediately to the worst case scenario. Nonetheless I did rent a borescope and took a peak inside. Definitely was running rich and have some carb tuning to do, but no hydrolock, no stuck bits, no major issues. Once she fired up I took off the tappet covers to make sure oil is circulating and it is.

Thanks everyone for the advice!

Offline PeWe

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Re: Scraping sound when trying to start
« Reply #8 on: September 29, 2020, 06:47:27 AM »
That is really good news!!
Even if money should not be a problem (if having deep pockets) all work to fix the worse scenario.
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Scraping sound when trying to start
« Reply #9 on: September 29, 2020, 11:59:24 AM »
Now get a battery MAINTAINER and keep her plugged in.
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)