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Knocking at low RPM that goes away

tdskip

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Hi guys - hope everyone had a good week.

So the good news is I got the 1980 CB750F (speedometer indicates 31,000 miles +/-) back on the road after rebuilding the carbs and brakes and absolutely love it. Electrical system came up and it rides surprisingly well despite still needing everything. The bad news is that I'm skeptical that she is going to stay on the road for long.

When I changed the oil prior to firing her up what came out when I drained the oil was super thick, which set off alarm bells. The oil, while dark in color, was free of any debris or metal. When I started her at low RPM, like idle to 2,000 RPM, when cold there is what I can only describe as a knocking sound that goes away once over 2,000 RPM. Once warmed the knocking is usually gone even at idle.

Bike runs very smoothly and pulls strong.

I have not done a valve adjustment yet, I have not tightened the flywheel bolts.

I have my suspicions but don't want to lead the conversation in any particular direction - what do you all think is going on here.

Thanks!
 
The chain between the crank and the transmission in these bikes has a hydraulic tensioner, activated off of oil pressure. So when the engine is cold or a low idle it tends to rattle. That may be it. If so, nothing to worry about.

 
X2 on that reason. Normal can rattle just when you are about to kill the engine, say when you let clutch out a wee bit fast to try to die, you hear it rattle almost like a hard pinging or knocking. Higher rpm makes it go away when the engine revs up to bump the oil pressure up. A brand new one can even do it.

The more the carb syncing is off the more it will rattle, part of the noise is from uneven cylinder loading from carbs not set the same to snatch erratically at the primary chain, even the carbs up and the more even rockover at dead idle quietens the motor up a whole lot.

The oil pump on these is relatively small in relation to the great numbers of engine parts that allow even normal oil leaks past so many tight parts to have an effect on oil pressure. Put a good pressure gauge on and they will run at 50 psi all day long at freeway speed but at hot idle the pressure will drop to as low as 5 psi (here in Texas at 100+ degree heat) and sometimes the oil light will flicker a bit at that. Rev the engine even a small amount and the light goes out as the pressure climbs instantly. The rattle occurs in that low pressure window.

Use no lighter than 10-40 oil. Many now use 20-50 in them to help with that among other things.
 
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