Topic: 1973 cb750K top end slapping and rattling noise at idle and low rpm

[fritzer_108]

I just got my 1973 CB750- project up and running for the first time today and was met with this sound:

I tightened the valves and checked the clearances as well as tightened the cam chain. Any ideas as to what this could be? Thanks in advance.

[KAndrew]

Pull your clutch in. Does that make a difference in noise?

[fritzer_108]

Not that I could tell. It shifts wonderfully in all gears.

[robvangulik]

I think that with a bit more rpm and a synchronisation of the carbs it will sound a lot better, it is running very slow now.

[y2kc0wb0y]

Timing chain tensioner?


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[fritzer_108]

I am going to retighten the cam chain and see if that helps anything.

[KasperK/Denmark]

I would go for a carburettor syncronisation.

[fritzer_108]

I retightened the cam chain and that did nothing, so I took a look at the lubrication system. I have an oil pressure gauge and when I removed that and kick it over, I had decent flow coming out of the hole. I loosened the valve cover to get a decent look at the rocker arms and noticed that oil was only squirting out on cylinders 3 and 4 and that 1 and 2 were pretty dry. I did a google search and couldn't find a thread regarding something like this. Thanks for the help...I have a 1974 Honda CB360, but I am new to the 4 cylinder family.

[bryanj]

If there is no oil on one side the oil restrictor jet is blocked on that side and you will need a cam, cam tower and 4 rocker arms plus a top set and patience.
STOP RUNNING IT NOW and take the engine out

[fritzer_108]

I figured as much. I haven't run it since I took the video out of a suspicion that there is a blockage. Just kept kicking it. I really wish I had one of the frame kits on it right now.

[754]

This is usually  a clue..... you check the valves and one is quite loose.
 I will be starting  on a new batch of frame kits , this coming  week.
If you know you are getting  one, you can cut frame now to get the valve train off, and fix it. Then put kit in later.
 I have run with the kit and frame piece out, but not good for cornering  ha4d

[fritzer_108]

Well I found my issue. The PO used silicone RTV to seal the bottom on the cylinders to case, which resulted in a clogged oil line. It flow freely now out of the top now...maybe too freely. I suspect that I do not have the oil jets in the head. I took before and after pictures of the teardown and looking back at them, i can't find those jets there in the first place.

[754]

If there is evidence of silicone, where the case halves have joined.. be very suspicious.. or anywhere else it visible that it squuezed out... there may be more to come off,  and clog more up..  trust me , been thru this.

[bryanj]

You need those jets, Yamiya do them(both sorts)

[fritzer_108]

I found a pair of oil jets from a friend who has a massive 750 stash. I will get new o-rings and hopefully this solves the problems. Surprisingly, the rocker arms and shafts have no scorings or pitting from the lack of lubrication.

[754]

The shafts dont take the beating, it the cam lobe, rocker pad and the  cam bearing in the cradle.
 You may not he aware , but early motorcycles often had open valve gear, drag bikes too. The only lube rocker shafts on dragbikes got was from a grease gun.,
 How long did you run like that ? You probably lose pressure to the main bearings , if the oil finds an easier oath tban feeding the mains.

[HondaMan]

I have recently rebuilt a 750 that was ridden from Michigan to California (after the new owner bought it from a Michiganite, or maybe it was Minnesota) with a brief stop here (Colorado) to have a Transistor Ignition installed during his ride. He wanted to 100% resto the bike, so he brought it back here on a trailer a few months later. When I tore down the engine to find a perfectly-good top end (rockers and cam stuff) I discovered there were NO oil jets! Suspecting maybe the bottom end had suffered (it was, after all, a 2000+ mile ride), I found the crank and bearings in perfect condition, all right at 0.0012-0.0016" clearance range. It had around 9k miles on it, IIRC.

The holes in the cam bearings limit the oil flow to the rockers and cam bearings by their passage sizes, which are just barely larger than the oil jet metering hole size. The 7 lower holes in the oil jets are 0.033"-0.035" diameter and the metering hole is 0.0375" diameter: this is the same on the older, larger, domed-type oil jets in the K0/K1 heads, which had 13-14 strainer holes for their 0.0375" metering hole, all of which were 0.031" size. Those little strainer holes have become vital when these engines have gone thru the hands of those who like to use RTV sealants...

[PeWe]

Heavy duty cylinder studs will reduce the oil feed hole too.
8mm diameter, stock 6mm.
I had one hole in head that was between 8.5-9.0 mm.

[ekpent]

 We are Michiganders Mark 

[fritzer_108]

I finished putting the bike back together and took it for a quick spin after making sure the lubrication system was working as it should. It pulled nicely in all revs and it will probably work even better after I sync the carbs and check the points again.