Is this CB750F a Good Buy?

[motogrady]

Bummer.

Now I'll be thinking half the day on what could cause this thing to bind, not only one way, but half the way.

Lucky's right, but befor pulling the motor I'd look under the valve cover at the cam chains, maybe the side cover and look at the clutch assembly also.

Don't get overwhelmed with the carb thing, take what you can apart, clean 'em up and be done with it.

You have a bigger problem on yer hands with the binding thing.

Anyone have any ideas on why this thing is locking up?

[seestheday]

Tools, do you mean that you can remove the valve cover while it's still in the frame?  I can't figure out how you would remove the cylinder head with the engine still in the frame, let along the block.

The FSM and my Haynes manual both say that you need to pull the engine to remove the cylinder head & cylinder block.

Excessive - don't bother with the carbs until you figure out why the engine isn't turning over.  Once you are ready to tackle them we have a lot of great resources and step-by-step guidelines.  You can also send them away and get them cleaned by a pro for about $350 or so if you'd prefer.  Much cheaper than buying a new set.

[excessivedemon]

Thanks for the great advice guys, I'll try to look under the valve cover - big gulp for me I'm a bit worried since I'm not a mechanic and it's kinda scary for me but with your help and the service manuals I may hopefully find my way around

Question though - would it be worth dropping a few oil on the spark plug sockets and try turning it again? I was thinking maybe since bike has been sitting for years all oil are at the bottom and there's zero oil on top and when pistons reach that point without oil there's not enough lubrication to make it go through so maybe that's why it's getting stuck both ways I try to turn it? And if so, how much is "just enough"?

[excessivedemon]

By the way, along with my question above (trying to put some oil on the spark plug sockets) I realized I made a mistake with my description of the problem - it turns not just 180 degrees but about 360 degrees (or a little more) both clockwise and counter clockwise, but gets stuck / locks up after that - I did not try forcing it further since I was afraid it would scratch the pistons / rings if I force it - don't know if that (360 degrees vs 180 degrees) makes a difference of what you guys think the problem is but I thought I'd bring it up.. sorry for the wrong info.. again thanks for all the help guys, you rock

[TOOLS1]

Either a piston is hitting something like a valve, or there is something in the crankcase hitting the crank. The reason you can turn it is that what ever it is only obstructs it at that point. No amount of oil, or lubrication is going to help.
The DOHC engines top end can be disassembled while in the  frame. It is only the SOHC K and F models that require the engine be removed. The frames are made with clearance to do this.
TOOLS

[excessivedemon]

Thanks Tools, I'll try my best opening it up and see what's going on inside. Anybody have any write up on how to do this?  I do have a copy of the service manual, will read up on it. Don't know if it will help but previous owner threw in a big box full of parts / spare parts (head / pistons / valves / cam / clutch assembly / etc...) as part of the deal. Hopefully I have in the box whatever it is that's broken. Thanks again.

[cdaiscool]

Could also be something as simple as compression stopping you, if one's sealed up better than another.

Take all 4 plugs out and repeat the test. If it still doesn't spin at all (it won't be easy, but it'll spin easier) then definitely got something going on.

As for the carbs, McGregor's has a guide on cleaning them. I never did it before, grabbed that guide, and rebuilt mine for a decent price. Fun stuff if you've already got the tools!

Nice purchase, btw!

[excessivedemon]

Thanks cdaiscool - I will try removing all plugs and then repeat testing, if it still fails, I'll go ahead and remove the head as Tools suggested - I'm really hoping / wishing / praying I don't have to open the head (fingers crossed)

Thanks for all the wonderful help and advice you are giving me guys, I really appreciate it

[seestheday]

Tools, how do you get the head over the studs while it is still in the frame?  Do you need to pull the studs?

Sorry, it's just that I literally just pulled the head and put it back on and I can't think of a way to get it off/on while it was still in the frame.

[TOOLS1]

It will come off. It's tight but will come off.
TOOLS

[excessivedemon]

Hey guys, just wanted to update the thread - so, I was supposed to try removing the plugs and repeat testing as cdaischool suggested (before going that head removal route) but the guy at the hardware store gave me a 16mm spark plug remover tool telling me all motorcycles (including big bikes) use 16mm spark plugs and only cars / trucks use 21mm spark plugs - so I bought it, got home and tried using it but it did not fit - my spark plugs are 21mm!

I bought a can of carb cleaner and wd40 along with the spark plug remover tool, and out of desperation I thought I'd try spraying wd40 on those 4 holes beside the engine where the carbs are supposed to go. While waiting for the wd40 to have any effect at all, I though I'd try checking the clutch as Motogrady suggested - a lot of oil fell off while I was trying to remove the bolts but was not able to complete the task since I did not have the correct tools to remove those bolts in the tightest of spaces.

Anyway, it was a no go, so after oil stopped coming out I just replaced the bolts back and tried testing the clutch to make sure nothing got broken with me trying to remove the bolts. Things seemed ok - then I then tried turning the engine over again and after 360 degrees - same problem - stuck engine. I tried turning the engine the other way around then it went 360 degrees, then 540 degrees, then kept turning on and on and wow.. it's not stuck - I went ahead and turned the other direction again and kept turning and turning and yeah, it's not stuck anymore.. I'm very happy with the engine not stuck anymore but I could not explain what could have made it go free? Was it the wd40? Or was it the clutch (probably some dirt got stuck somewhere but fell off after the cover was removed?

I guess a bigger question now is - I'm not really sure what caused the thing to be stuck at 360 degrees before - and I'm not sure what set it free - do I need to be worried about pistons / piston rings being in a bad shape? Do I still need to do a topend rebuild or something?

[easyrider12]

Hey excessive, welcome aboard.   I have about as much mech. skill as you, but i'm sure you'll figure it out.   Just my .02 cents, but you may have a small tool kit hidden behind your left cover in a small plastic shaped triangle behind the airbox with a proper plug wrench in it.   first thing i replaced on my bike 6 years ago was plugs and wires.   Good luck- wrenching is half the fun.

[motogrady]

You fixed the starter thing, you got the motor free.  Very good.
Clean the carbs, charge the battery, put 2 gallons of fresh fuel in it.
Check the oil.

Put it back together and see what happens.

[seestheday]

The spark plug in your bike should be an 18mm. Ngk d8ea or the iridium equivalent.

[seestheday]

fyi - fantastic carb cleaning manual:  http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revF.pdf

If you want to just do a quick carb clean to just get the bike running before you do a full carb rebuild then you can follow this doc (and not have to split the carb bank): http://honda-cb750-s.456789.n3.nabble.com/file/n3481241/900_Carbs_Rebuild.doc

NOTE: do not use toothpaste to polish the seats

[excessivedemon]

Hey guys thanks for all the wonderful help - I tried searching for the bike's tools but there weren't any, I could not find any spark plug remover tool in my area other than 16mm or 21mm - I tried purchasing a set of socket wrench (8mm till 22mm) but realized only when I got home that the tool is no good in removing spark plugs since it would not go through with the spark plug's long tip (where you connect the high tension wires) any ideas what else I could try? The socket wrench set had a specific spark plug remover as well but only at 21mm

As to the starter being fixed, I also went ahead and had the solenoid fixed today - it's totally destroyed and there's no way to save it - electrician setup a relay system in place of the solenoid - the problem I have now is with the new relay system, if the bike is in Neutral and I put ignition to "ON", the "Neutral light" will NOT turn on and I cannot press the start button - Only when I press on the clutch lever does the neutral light turn green and I am able to press the start button and have the starter run (again, this is all while the bike is already in neutral) - I definitely know for sure that with the old solenoid, I did not have to press the clutch lever for the neutral light to turn on if the bike was already in neutral - I brought this up to the electrician but it is what it is. Darn them electricians - what are your thoughts on this?

As to the carb cleaning steps, thanks so much, I'll be cleaning up the carb tomorrow - right now, (except for having to press the clutch even if the bike is already in neutral) I have the starter turning pretty good when I press the electric start button, and I hear the engine going boom boom boom boom - not stuck anymore.. I'm hoping soon as I get the carbs cleaned and in place tomorrow I can have the bike running (hopefully spark is good - no way for me to test right now since I can't pull them)

[seestheday]

Where are you located?  I'm surprised to hear that you can't get an 18mm spark plug socket locally.

[motogrady]

yer gonna need a plug puller.

and hey, you get that thing running, be effin' carefull.
I'm not kidding.

Be carefull with it.

[excessivedemon]

Hey Guys, just thought I'd update the thread - so I was finally able to cleanup my carb with the help of the links seestheday provided (thanks man), I found a customized 18mm socket wrench deep enough to pull out my plugs and do the regular spark plug test (they seem fine although might need replacement soon since spark is not bluish in color), then I managed to get a hold of those Shorai Batteries (advertised as really good) and got the bike cranking and running but with some really foul smelling dark smoke

I tried doing this a number of times turning the bike off then running it and it's really cranking really good but the smell is just so terrible my neighbors started complaining and I had to shut the thing down - any thoughts on what could be causing this? I was thinking maybe I should try to keep revving the thing tomorrow to maybe clean out all those dirt in the engine maybe?

Also, had to bleed the brakes since they were practically not biting at all - I was able to successfully do this but on the final bleed, I broke the brake bleed screw - how can I fix this? There does not seem to be any way for me to remove the screw there now - it's stuck without a head.. it's working for the time being but what happens when I need to bleed again in the future? Will this require me to buy a whole new set of calipers? Thanks again in advance.

[motogrady]

Good to see you hanging in there guy.

The bleeder, maybe an "EZ Out".
It's kinda like a tap, but has spiral grooves going the opposite way threads usually go.
Drill a hole out in what's left of the bleeder the size the ez out calls for.
Hopefully it will twist out.
A machine shop might be able to help, I've seen them take a welding rod
and weld it to the damaged bleeder part, then turn it out that way.
The steel welding rod won't take to the alum, but will to the steel bleeder.

As far as the smoke, that could be a ton of things.
Oil may have leaked into the cylinders over time loading them with sludge,
the rings might be letting oil pass into combustion,
valve guides or seals ditto,
but I would think that would be a blue smoke.

Are all 4 plugs the same color?
I'd run that thing for a minute or 2, pull a 4 of them and look for one that's different.
If all the same, I'd go back to the carbs and have a look.
That feeds all 4.

As far as spark, keep that battery charged with a charger till you get it running.
Weak spark is white, it get's blue when it get's a strong shot from the charging system.
weak battery, white spark.
(The plug thing, I just clean them with a shot of starting fluid, sandpaper, check gap, reinstall.)

That thing might just need to have the crap run out of it, blow out 10 years of whatever.

You're doing good guy, keep it up.