Engine noise

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hawkyzf
Posts: 4
Joined: Mon Jun 07, 2021 8:01 pm

Engine noise

Post by hawkyzf »

Hi all, new to the group, but knowledge needed already, picked up a 1999 model last week and it has what I can only describe as a fast scraping (knocking?) sound from the front cylinder or crank?. Sounds forward and low on the motor ...when the rev's drop to >1000rpm the sound disappears (sounds really quiet, but it maybe that the front cylinder is not firing at this point?) tickover is at about 1500rpm bug it does drop off to 1000rpm for a minute or two, then picks bak up along with the noise ..any idea's....don't want to run the bike atm incase it's the cct's (still on originals/new ones ordered)..thanks in advance
tony.mon
Posts: 15943
Joined: Wed Jul 25, 2007 10:46 pm
Location: Norf Kent

Re: Engine noise

Post by tony.mon »

Is it a single "thump" from under the tank, revs drop, then pick up again?
Or in time with the engine rotation?
It's not falling off, it's an upgrade opportunity.
hawkyzf
Posts: 4
Joined: Mon Jun 07, 2021 8:01 pm

Re: Engine noise

Post by hawkyzf »

it's in time with the engine rotation, only goes when rev.s drop off
tony.mon
Posts: 15943
Joined: Wed Jul 25, 2007 10:46 pm
Location: Norf Kent

Re: Engine noise

Post by tony.mon »

Does it go away when you pull the clutch in? If so, it's the usual clutch noise that lots of Honda's has.

I think you're going to have to start it and post a video with sound.
But before you do, remove the plugs, remove the larger of the two round plugs on the lh side, and rotate the engine using a socket. If it rotates easily, with no resistance during two complete revolutions, the rebuild and restart.

If you can feel resistance at specific points during those two revolutions, then your piston is already hitting valves due to a cct failure, even if, with a bit of effort, you can roll the engine past that point.

Only revolve the engine in the correct direction- anticlockwise.
It's not falling off, it's an upgrade opportunity.
hawkyzf
Posts: 4
Joined: Mon Jun 07, 2021 8:01 pm

Re: Engine noise

Post by hawkyzf »

No difference clutch in or out.... I turned the engine over a couple of times with the plugs out and - no resistance, but the rear cam chain was going slack (not to the point of jumping teeth) at tdc...changed the cct's and found the rear cct was the problem (stuck out about halfway through full travel). Rebuilt, changed oil & filter, started up and no noise...let it warm up and idle for a while, nothing....looks to be cured ... although I did find that the cct's were different "batches", the front had a pink dot and the rear was white???, and the carb velocity stacks were also fitted the wrong way around (the longer one to the front cyl). Seems to rev a lot better too now it's been corrected.
tony.mon
Posts: 15943
Joined: Wed Jul 25, 2007 10:46 pm
Location: Norf Kent

Re: Engine noise

Post by tony.mon »

The paint dots are a production mark, there is no difference between them.
Good you caught it before pistons met valves, most times owners aren't that lucky
It's not falling off, it's an upgrade opportunity.
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