1981 CB750 C custom "DOHC" Engine ISSUE

[Dwrecktum]

Recently I purchased a 81 CB 750 custom, aside from the tank it looked to be in excellent condition except a few minor dent in the tank but the owner said he never tried to get it running and had no idea what was wrong with it... I thought for 300 dollars I want it! Got it into the garage and got into tinker with it... cleaned and lubricated the carbs up real nice... gotta battery onto it and tried to turn it over with nothing but some clicking... Pulled the plugs to release the compression and see if it would turn over... Still nothing... Thus I have a seized engine and need a little guidance as to what my best options are gonna be... I'm willing to rebuild it... I will be pulling it apart soon to see whats actually seized whether its in the top end or bottom...
So: A couple questions
1: Is it common for these engines to seize? could there possibly be something that i'm missing?

2: If i get new pistons and rings how exactly do I know what to get the cylinders bored to??? (of all the fucking around i've done on quads and bikes i've never had to replace a piston) Lucky me i suppose...

3: I'd love to take in any pointers involving this engine I currently don't know a pile about it... Which as of tonight I realized i'm gonna get to know this engine better than I thought I would :) lol :(

[Re-run]

Well before assuming it is a locked engine, try to get the bike into 5th gear and push it then dump the clutch.
If the wheel will turn, then the engine should not be locked. It could just be that the solenoid is shot.
Another thing you could do is remove the starter motor and hit the starter button real quick and see if it spins. If it does not, then again, bad solenoid.

[LukeM]

You can also detect if the engine is locked by removing one of the side covers and trying to turn the engine with a large socket or wrench.  If it's locked up, the crankshaft you're wrenching on won't budge.
 
When the key is on, the run switch is on, and you hit the start button, the solenoid should click and close the high current contacts, putting the entire battery current across the solenoid toward the starter.

+1 on Re-Run's comments.  Might be a bad ground for the starter, or the solenoid is shot.

Luke M

[Dwrecktum]

More info on what i did last night... Checked voltage @ battery its good. Checked starter resistance to ground. GOOD Performed voltage drop checks between all connections including the solenoid... Checked voltage output on starter side of solenoid is good...
Removed spark plugs and starter would still not turn engine.
Removed timing cover... Here's my big mistake... The bolt i chose to turn on said 10 on it... But what i didn't realize that it was actually machined down to more of a 7 or 8mm shank... W/ a screwdriver in the #4 spark plug hole my partner felt the piston move upwards only slightly before i broke the bolt... :( used a pin style punch to remove portion of bolt from the end of the crank... (got incredible lucky)
Got a new bolt from a bike wreckers today...

Little info about me: I'm a H/D technician i work on on road trucks in alberta canada... I'm incredibly familiar with starting systems and issues because as you can imagine at -25 to -40 Deg Celcius during the winter trucks tend to not start... So I traced everything to my knowledge...
In my free time I fix old quads and the occasional dirt bike... Always only one cylinder though... This will be the first engine of this type i've played with... So i'm more than willing to take all advice that anyone here has as i'm sure you've all dealt with a lot of similar issues...

[Re-run]

Well still try to turn the starter before assuming engine. An engine tear down is a pain compared to pulling the starter.

[yellowdog]

If it's benn sitting for a while. Fill the cylinders with PB Blaster for a few days while you work on other things.

[Re-run]

Another good idea there. Pb blaster or some other penetrating compound.

[Dwrecktum]

Alright... story continues! reading back I realize my first post sounded like i'm an idiot... It was 3 am.. I was tired and frustrated... But that night i did spray a pile of fluid film into each cylinder...

This evening... First... Pulled the starter and confirmed that it spins like a dream... Tried turning the crank on the genny side... No movement at all what so ever... Pushed it down the road at all our might in 5th dropped the clutch and turns out the clutch slips... lol... likely adjustment... seems like its cranked up pretty tight... Rolled it back in the garage, and put it up on the stand...

Tore the engine out... when removing it was easy to see in the exhaust pipes that #3 had been running lean or hot... and #1 rings had been passing a bit of oil...

Engine on the bench:
Tore the valve cover off and unbolted the cams... "E" is the only damaged one showing excessive wear but cam appears fine with no real pitting and surface is in good condition... All other components are good ish...  cept 1 cap screw that broke when trying to remove it... i haven't figured out how i'm getting it out yet... Removed the cylinder head... Took a bit of mild tapping but it eventually came... Now the issue... RUST #3 and #4 being the worst... used a wood dowel to hammer on the pistons that were near TDC and eventually got them freed up... Some gradual cranking back and forth and eventually full rotations were possible with no real restriction... Removed the cylinder block... Which was a nightmare to get off!! and Voila...

#3 and #4 wrist pins don't move freely... #1 and #2 look like new ish... So ultimately I need pistons rings pins... Everything below looks good except some surface rust on the #4 connecting rod which seems to come off no problem... I've decided to refuse to split the case... There's no reason to do so... I'll need to either hone or bore the head... I'm thinking its gonna get bored a mill or two over...

**!!Now i really need the suggestions... What should i get... and from where and what else should i be looking for just in case... !!**

Sadly this bike needs a little more tlc for an oil change but i'll love it all that much more when its done...

[LukeM]

Wow...a lot of work for 3 in the morning!
I'm in the middle of a rebuild on my 750L as well.  Finally getting the parts to my mechanic today, and he'll work on it this week while I'm traveling.

Broken bolt for the cam caps: I have 2 of them on my original head.  Time for some extraction tools.

I'd recommend finding another engine (if one's available) to pull parts from.  New ones from the dealer will be pretty expensive, especially for what you've described so far.  EBay and Craigslist might be a good source for the used engine, and I'd check EBay for some of the parts as well.  There are several good Internet sites on this board as well.  Check with Mitchberry, as he's got some contacts on parts through his store.

Good luck, document the process, and let us know how it goes.  We all need the encouragement to keep these old bikes going.

Luke M

[Dwrecktum]

I hate doing extractions on any bolt... I think I'm gonna leave it sit in a plume of fluid film/wd40 for a week before i even try... Its only one bolt and one cam cap... I think the cam is gonna be good... the tensioners seem good everything was nice and tight before disassembly...

When we got the cylinder head off my brother for some reason decided to turn it upside down which was obviously catastrophic all the shims and the "caps" (i'm not sure what the other peices are called) fell onto the floor... I now have no idea what was where... They all appear to be in good condition... But now i'm left with a puzzle that I likely could have done with out... Whats the easiest way to solve it????

[jayme4038]

I have question. I just got a 1981 CB750 custom. It will start and ride but when it sits there at idle and gets hot, oil starts to come out of front of engine. I can't tell where its coming from because its inside the (i dont know what they are called exactly) slots on the engine. Does this sound like a cracked block, gasket or? As i said it only does it when i let it sit there and get hot... When i'm riding it and I crack on it, it sputters but if i just ease into it, it runs fine. Can anyone give me some pointers. I got the bike for $800, did I get ripped?    

[Rodgil]

It is possably a leak from the head gasket, at the join between the cylinder head, and the cylinder. You should be able to see where they meet. If this is where the leak is coming from, you may me able to stop it, or reduce it, by re-tensioning the head bolts. Remove the fuel tank, and valve cover. Using a tension wrench, loosen and re-tighten the head bolts to the correct torque (3.6 to 4 kg/m, of 26 to 29 ft/lb), one at a time. Do this twice. That proceedure cured my oil leak.

[Rodgil]

It is possably a leak from the head gasket, at the join between the cylinder head, and the cylinder. You should be able to see where they meet. If this is where the leak is coming from, you may me able to stop it, or reduce it, by re-tensioning the head bolts. Remove the fuel tank, and valve cover. Using a tension wrench, loosen and re-tighten the head bolts to the correct torque (3.6 to 4 kg/m, of 26 to 29 ft/lb), one at a time. Do this twice. That proceedure cured my oil leak.
If you are in the US, your carburettor has an accelerator pump, to give extra fuel when you quickly open the throttle. That is a possible cause of your hesitation when you crack the throttle, if yours isn't working properly. Check the workshop manual on this forum for more info. It also lists possible faults and causes.
Good luck!