On 76 Hondamatic CB750A at low idle there's a knock,once i speed the idle up the hammering leaves ?

Question

On 76 Hondamatic CB750A at low idle there's a knock,once i speed the idle up the hammering leaves ?

Answer 1

may be whats called "piston slap". your piston and bore may have some wear and their making noise until the rpms go up to move the piston into a position to stop slaping. the oil pump thing sounds possible too. you might try lucas oil to stop it.

Answer 2

Carbs out of sync.... If you adjust your cam chain tensioner as "bikeinkawboy" says you do have to rotate the crankshaft to a certain position... It has to be rotated clockwise until it is 15 degrees ATDC for the No. 1 cylinder. This position puts the chain slack at the rear of the engine and duplicates running conditions. You kinda need to see a manual drawing to see where 15 degrees ATDC is, it will be just past the F and T marks and lined up with the back edge of the advance mechanism's spring peg... Also I wouldn't recommend using any oil that is thicker than 10w40 such as 20w50 because motorcycle engines are smaller and built with tighter tolerances certain areas in the engine may get deprived of oil that is too thick to get though such as your main crank and rod bearings which on this engine are plain type and not roller bearings like later models. In short too thick of oil is a good way to "spin" a rod bearing especially when it is cold...You will also see a noticeable loss in power when using oil such as 20w50 because it is so thick it drags your engine down...

Answer 3

I agree with "barfoeng" on this one.

With the carbs out of sync, uneven impulses are sent through the engine, causing the clutch basket to rattle.

When the revs increase, each cylinder doesn't have a chance to run at a different speed than the others.

The constant load on the clutch basket, prevents it from rattling.

Answer 4

If you haven't changed oil in a long while, that's certainly a possibility.  However, the first thing that came to my mind is the cam chain tensioner.  Your bike was built in the days before automatic cam chain tensioners and you must do it manually.  Look in your manual and if someone else here remembers differently than me, please correct me if I'm wrong.  The last Honda chain I tightened was over 25 years ago.  

The tensioner is a small thing that protrudes from the back side of the cylinder block.  With the engine not running, loosen the locknut and lockbolt, let the internal spring tighten the chain and then tighten the bolt and lock it in place by tightening the locknut.  I don't think it's necessary to turn the crankshaft to any certain position.  

Piston slap occurs when the engine is cold and goes away as it gets hot, causing the piston to expand and better fit the cylinder.

Oh, cool bike you have!  My friend had one and it was a really smooth running machine.  A much better touring bike than the 750F I had.

Answer 5

your oil pump is old and it may not be providing the same oil pressure at adle as it does when the engine is reved.

You can probably  ''tighten'' up the pump by using a higher  viscosity motor oil.

For instance if you currently have 30 weight in the engine you can switch it to 40 weight or even 50 weight.

Also i suggest you use a straight # instead of a multiviscosity such as 10-30 or 10-40.