Topic: 1978 Cb750 K Rebuild Questions

[denward17]

Good progress!

I would say check oil level after each ride (while warm) and no oil leak.

[M 750K6]

I checked plugs regularly too and did my first oil and filter change around 100 miles. I opened the oil filter up, just to be sure nothing was hiding in the pleats. Just cut the top and bottom metal rings off it, to open it out. I also went round all the engine mounts to check tightness and cover screws. I also rechecked tappet clearances at 100 miles.

[HondaMan]

 Is there anything ya'll think i should look out for/things you would keep an eye on for the first few hundred miles?

Watch for oil weeps: the 5 places most often in the later engines is around each of the 4 exhaust spigots (front of the engine) and the middle of the head, top of the cylinders at the joint, on the front of the engine. They come from different sources.

[jmelton]

Thanks all, I've added these to my notes and will keep an eye out. Fingers crossed its smooth sailing from here!

[jmelton]

The rain stopped long enough for me to get a 20 or so mile ride in (still staying close to home in case something happens). Overall things went well but i noticed towards the end of my ride once the bike was good and warm that when idling at stop lights my pressure gauge was reading 0 PSI and at 3-4k rpm pressure was probably around 40 PSI. Normally, when just idling from cold around 1100 RPM the gauge reads above 40 and rpms of 2k+ it reaches up to 70-80 PSI. Obviously, i expect pressure to drop slightly as the oil warms up but not that much.  My dummy light remains off even when the gauge isn't reading pressure. Does this seem excessive/out from norm?

I'm planning on doing an oil change now that i'm approaching 50 miles, fingers crossed i don't see too much glitter from the break in  .

[bryanj]

I suspect your gauge is not accurate

[MRieck]

I suspect your gauge is not accurate

Word. Those gauges are just gross indicators. If you want to really know oil pressure at low RPM's get a Mac, OTC, Snap On etc gauge. They really work.

[denward17]

What weight oil are you running?

Look at your gauge good to understand incremental values, mine is different between 0-10 psi than the upper ranges.

When I first started mine after top end rebuild, I ran 15w-40, when hot it seemed to be around 10 or just below when at idle.  I run 20w-50 now and it is a little higher.  At cruising speed I think it is around 60.

[jmelton]

Denward you're right about the increment values being different 0-10 PSI, but i'd still figure i should see 10 PSI at idle when hot. Its possible the angle i was looking at the gauge made it appear to be below 10PSI. The gauge is a marshall gauge, I don't know much about the brand and it was on the bike when i bought it a few years ago so its definitely possible the gauge isn't accurate. So long as it tells me i have oil pressure i guess i don't need to know exactly what it is. I'll just be sure to keep an eye and an ear out as i go, it would be a shame to seize up an engine after i put so much time rebuilding in to it  .


I'm currently running 15w-40 and plan to switch to some 20w-50 a little later once the bike is broken in. Before rebuilding i ran 10w-40 and the shifting felt really clunky when warm so i figured i'd try something new.

[HondaMan]

In my old notes I showed PSI for three different oil types:
10w40 at hot idle was 3-6 PSI, depending on brands (Pennzoil was lowest, and it foamed in the tank).
20w40 at hot idle was 5-11 PSI, depending on which of 2 brands I ran (neither is available today, though).
20w50 at hot idle was 9-19 PSI, the lower being Valvoline and the higher being Castrols' XLR (the closest to the Castrol today is the Bel-Ray EXL Mineral in 20w50).
It's useful to note that the PSI switch on these bikes may toggle ON briefly when hot and thinner oils are being used, like when coming off an Interstate to a stop sign.

I didn't have any 15w40 available back then (circa 1972).

[jmelton]

 On my drive home from work today the bike was feeling great when it died all of a sudden at the light. I probably should have anticipated this when i needed to kick start it to get it running when leaving the office. On the bright side the bike started first kick, on the down side i had to get a coworker to give me a jump twice (the initial jump and then right after it died at the next light XD). I ended up being that #$%* at the light revving their bike up and down so that it wouldn't die on me again. Once i got home the bike was very hot, the external oil tank was too hot to touch. Should i be concerned about how much i was reving at stop lights/how hot the bike got this soon after a rebuild?

I guess i need to check the charging system and see why my battery is dead 

[HondaMan]

A low/dead battery can make it act that way (no idle), for sure.
The oil tank at engine operating temperature can be pretty hot: once my 750K2 burned a (permanent, smile-shaped) image of the oil cap into the inside right thigh of my ex-wife during a 300-mile trip on a summer's day when I had 10w50 oil in it (too light for 95-degree riding): she wore her jeans so tight there was no feeling on her upper legs when sitting, and the dark mark on her pants was our first clue. Normally it is pretty uncomfortable to place your hand on the tank when in city traffic.

She never forgave me for that.
Oh, well...

[jmelton]

The advice i'm picking up from that story HM is to not let my wife ride with me and only the women i pick up from the bars... and maybe to run heavier oil in the NC summer heat 

[HondaMan]

The advice i'm picking up from that story HM is to not let my wife ride with me and only the women i pick up from the bars... and maybe to run heavier oil in the NC summer heat 

Well, if you find my ex in one of those (easy to ID: just ask to see the inside of her right thigh, about 8" down from its top, there's a dark 'smile' there), my only advice would be: run!
She's a looker, but so is the snapdragon!

[jmelton]

Finally had a break in the rain and was able to ride to/from work today. Battery charge is up to 12.8V before start and the charging system seems to work (12.8-13V at 2k RPM peaks at about 13.2-13.5 V at 4k RPM). Up to about a hundred miles so far, i had a few oil "weeps" appear but they seem to be from me not tightening my tappet covers well enough.

I did an oil/filter change and topped the engine off with 3Quarts last week but I have noticed my oil level when hot/warm is reading overfull by about 1/4" or so. Maybe i didn't get all the old oil out or the jug i poured from was over full and the increments were in accurate. I haven't seen any smoke or noticed any unwanted puddles of oil on the ground. Should i drain the tank slightly to reduce the oil level or let it ride until my next oil change?

I'm planning on resetting the cam chain tensioner, checking the tappets and possible doing an actual carb synch this weekend assuming i have time  .

[denward17]

Finally had a break in the rain and was able to ride to/from work today. Battery charge is up to 12.8V before start and the charging system seems to work (12.8-13V at 2k RPM peaks at about 13.2-13.5 V at 4k RPM). Up to about a hundred miles so far, i had a few oil "weeps" appear but they seem to be from me not tightening my tappet covers well enough.

I did an oil/filter change and topped the engine off with 3Quarts last week but I have noticed my oil level when hot/warm is reading overfull by about 1/4" or so. Maybe i didn't get all the old oil out or the jug i poured from was over full and the increments were in accurate. I haven't seen any smoke or noticed any unwanted puddles of oil on the ground. Should i drain the tank slightly to reduce the oil level or let it ride until my next oil change?

I'm planning on resetting the cam chain tensioner, checking the tappets and possible doing an actual carb synch this weekend assuming i have time  .

Sounds like good progress!

With the oil being over filled it probably won't hurt anything (except maybe it might blow some out), but if mine I would drain a little out so that the level is about 3/4 up the hash marks.

[scottly]

Battery charge is up to 12.8V before start and the charging system seems to work (12.8-13V at 2k RPM peaks at about 13.2-13.5 V at 4k RPM).

13.5V is a bit on the low side.

[HondaMan]

The oil does expand a bit when hot: in my 750 it rises about 1/4" on the dipstick (at gas stops) during interstate touring speeds. In the colder weather or high in the cooler mountains, it doesn't do that.

[M 750K6]

The oil does expand a bit when hot: in my 750 it rises about 1/4" on the dipstick (at gas stops) during interstate touring speeds. In the colder weather or high in the cooler mountains, it doesn't do that.

May also be because it holds a small volume of air directly after a run. Not cavitation, but it does get thrown around and churned up in areas like the gearbox.

[jmelton]

I ended up pulling a little oil out so now i'm at the max fill line when hot. I figure i'll probably dial that back at the next oil change so i'm sitting around 3/4 full or so but we'll see. I do see a little bit of "smoke" or more likely condensate coming out of the crankcase breather hose that runs along the swing arm. Its not an excessive amount but fairly consistent. Is this to be expected with these 80+% humidity days or do i need to keep an eye on it? The route i take to and from work is roughly 20 miles (40 min or so of driving) so the bike does get pretty warm by the time i shut it off so i figure there shouldn't be too much of a condensation issue.

Scottly, you are probably right I think i may need to tweak the voltage regulator (or whatever else is impacting the charging) a little bit. I put on a little over a hundred miles this week and the battery voltage has dropped to around 12.4V.

Overall i'm really happy with the performance of the bike, the long route i take home is on mostly back roads and i've been enjoying the short bursts of speed/shifting through the gears before slowing down for cars. I haven't really pushed past 6k RPM but i can tell there is a little more to be had once i open up the throttle.

[HondaMan]

Something to note: the oil tank caps of the early "K" bikes (up to K6) were breather types, characterized by a large cork seal on the underside the cap. The later ones have a plastic, threaded dipstick for the cap and the overpressure burps release at the breather pipe, usually exited near the back of the bike (if all the hoses are still there!). This was a "smog control" added to reduce crankcase emissions.

Sorta...