Topic: 78 CB750F ?

[billyfrank]

Hey all,

My 78 CB750F only oiled the top end on cylinders 1 and 2 pretty much frying the cam and towers on 3 and 4. I scrounged up some replacement parts and am going to tear into it this weekend and was curious as to what I should look for as the problem. The PO had the whole top end off before he gave it to me all burned up. I am assuming he plugged the oil holes somewhere. Should I just start at the top and work down or is there something more specific I should look for?

Any advice is welcome, thanks.

Billy

[SOHC Digger]

http://www.cmsnl.com/honda-cb750f-750-super-sport-1978-usa_model7240/partslist/E++02.html

If you go to this page and look at item #6, the oil oriface, I think that's your problem.  I am not positive, but I believe it's function is to force some oil to be directed up to the cam.  If your PO had the top end apart, it is very easy to overlook this little part.

[billyfrank]

Thanks, I will check there during the tear down.

Billy

[HondaMan]

Hey all,

My 78 CB750F only oiled the top end on cylinders 1 and 2 pretty much frying the cam and towers on 3 and 4. I scrounged up some replacement parts and am going to tear into it this weekend and was curious as to what I should look for as the problem. The PO had the whole top end off before he gave it to me all burned up. I am assuming he plugged the oil holes somewhere. Should I just start at the top and work down or is there something more specific I should look for?

Any advice is welcome, thanks.

Billy

The orifice has tiny holes that are easily plugged if the PO used gasket sealant on either the cylinder's base gasket or the head gasket. There should not be much (or any, usually) sealant on these gaskets, for this reason.

Another thing that can cause this destruction: missing O-rings. One goes around the aforementioned oil orifice, but ANOTHER one goes on the other end of the cam bearings, on both of them. If the second one(s) is missing, the oil pressure on that side is about 3 PSI, not nearly enough to lube a cam journal.

[excellrec]

I'm just waiting on gaskets to put together my rebuilt f2 head from the same problem. I've found it's happened a reasonable amount to people. I went ahead and soaked everything in degreaser and shot high pressure water and air through all the passageways to clear everything out and then cleaned until it was like new. Mine appeared to have been because of gasket material and sludge clogging up the passageways along the studs and in the towers. The plan I decided on was to just make sure not to use silicone gasket sealer and make sure there is no gasket overhang anywhere so basically nothing can break off and clog the jets. I will say that odds are best if you do a complete teardown (if you arent already) and thorough clean of everything in that motor because if something clogged up that part there is likely more of it circling around elsewhere waiting to do it again.

[Mandic]

There was a good point raised.  Check the studs for sludge build up.  Make sure you have good clean oil throughout the bike.  And remove the filter and the "oil cooler" to take a look at things.  Some have said the oil cooler setup causes aeration and restriction that doesn't help the oiling of the F top end.  And the F top end is already lacking as far as oiling is concerned. 

[HondaMan]

If you have an engine with the heavy-duty studs installed (like from R/C or APE or someone), then the oil feed passages are much narrower than OEM. While this does not impact the oil flow when they are clean, it does mean that it takes almost NO sealant to plug them. In any case, the sealant (or any debris) easily plugs the tiny holes in the oil jet(s) in the heads, which is mostly where this problem occurs. Make sure to pull those out and check the passage to the nearby stud and the holes in the jet whenever the head is opened.