Topic: Project bike - CB750F

[CB750 SS]

Hey Guys, I'm in the process of rebuilding my CB.  Had everything together and back on the bike.  I went for a spin around the block and BAM! My #1 bank popped.   As background I just "freshened" the motor and it has about 48K on the clock.  On Sunday it will come out of the frame for a tear down.  Based on what I'm seeing I may have dropped a valve or worse.  There was metal in the carb and in the oil pump screen. It's bad!

Couple of question. 

1. Are all CB750 SOHC top ends and Heads inter changeable? ebay seems like a cheap option.

2. At this point I'm assuming the combustion chamber is damaged.  When I pulled the spark plug the electrode was gone! The end was beaten to a pulp.  Should I buy a used top end rather than machine it (new pistons etc)

3. New head? or repair and add new valve springs.

4. Cheap easy performance upgrades?

5. Insure the entire mess and light it on fire?

Thanks for the help. 

[dragracer]

If you can post some pictures of the damage it might help us to give you some direction. What year is the bike?? The 77/78F models used a different head and pistons than the other 750's so you if you go with another head, be mindful of this distinction.

I'm sure others will jump in with more useful information but thats the basic thing to look for when replacing the head. Either way, you would want to rebuild whatever head you decide to use.

[Terry in Australia]

Do the EBay/Craigslist thing for a complete running engine mate, it'll be cheaper in the long run. You say you've recently "freshened" the engine, what did that entail? Cheers, Terry.

[CB750 SS]

The engine is out and attached are a few pix of my motor.  It looks like the valve stuck and then got smacked by the piston.  It punched thru the piston and the valve end bounced around bending the other valve.

The piston sleeve looks surprisingly good! After a cleaning I can't tell the difference between #1 and the other 3 sleeves. 

My plan is to buy two new valves (intake and exhaust), all new springs (currently has original 1976 springs), a new piston (rings, C-clips and dowel) gaskets and have the top end professionally cleaned and sorted.  cheap parts supplier?

Couple of questions.  What caused this to happen?  My brother thinks the timing chain was too loose and therefore it went off timing.  I think the crank vent tube got pinched and restricted the air causing too little air movement, which restricted the oil and the valve then stuck.

Any insight is great appreciated!!!

[ekpent]

Crank vent tube had nothing to do with that in my opinion. Maybe was just over revved,did you wind it up hard,,way out of adjustment or had a mechanical breakdown of a weak part etc. You may have some extra metal in the lower end that may have to be addressed also. What did you do to freshen it up?  What year is the machine,1976 I think ?
  That is one wrecked piston,suprised your cylinder wall is good    Sorry about your misfortune.

[CB750 SS]

Interesting insight.  When I freshened the motor it was all new seals and piston rings.  I didn't replace any "metal" parts such as valves or valve strings. I also did NOT "polish" or clean the valve guides.  When I had it a part before I did notice that some of the valves were sticky.  But that was sticky when I used my hands and I figured that was laughable when compared to the forces of the motor. BTW - prior to breaking it didn't burn any oil (1 thru 4) and had good power. 

[CB750 SS]

BTW - I didn't rev it very hard at all.  Never past redline or for long.  I typically lug a motor than over rev.

[Terry in Australia]

Erk! What does your cam and rockers look like mate? I see you've used some gasket sealant on your head gasket, (normally unnecessary) so I wonder if you've blocked off an oilway in your head? This would cause the top end of your engine to overheat, and quite possibly the valve guide "grabbed" the valve.

That engine will need to be completely stripped and cleaned out, as there'll be bits of piston thru everything. Make sure you clean out your oil pump and make sure it works as well. (could be another reason for the oil starvation scenario) The valve seat looks pretty bad, you might be able to get it re-cut, or you might have to junk that head. looking at the work involved, I'd still opt for a good used engine, it'll be way cheaper than what you'll need to do to fix that one. Cheers, Terry.

[CB750 SS]

looking at ebay all the motors are 600 bucks and used.  If I buy all the parts and get a new valve train, have it professional sorted ($200), I'm still a little cheaper and have a "rebuilt" motor.  Seems the "off ebay" is simpler and gets the bike back on the street faster.  However, I also have a GS1000 which is near perfect and an empty garage.  So space and time are not a huge issue.

[Terry in Australia]

No worries mate, if you've got another bike and plenty of time and money, then go for it. What model GS1000 have you got? Here's a pic of mine, I really must start riding it again! Cheers, Terry.

[CB750 SS]

wow, yours looks great.  Here is a pix of mine during a 3,000 mile trip last year.  It performed perfectly.  BTW - it is a 1979 GS1000e

[Terry in Australia]

Well yours looks pretty sweet too mate, I like that pipe, is it a V&H? Are you a member of "The GS Resources"? Mine's a 1981 "E" that I built from parts. Cheers, Terry.

[lucky]

Hey Guys, I'm in the process of rebuilding my CB.  Had everything together and back on the bike.  I went for a spin around the block and BAM! My #1 bank popped.   As background I just "freshened" the motor and it has about 48K on the clock.  On Sunday it will come out of the frame for a tear down.  Based on what I'm seeing I may have dropped a valve or worse.  There was metal in the carb and in the oil pump screen. It's bad!

Couple of question. 

1. Are all CB750 SOHC top ends and Heads inter changeable? ebay seems like a cheap option.

2. At this point I'm assuming the combustion chamber is damaged.  When I pulled the spark plug the electrode was gone! The end was beaten to a pulp.  Should I buy a used top end rather than machine it (new pistons etc)

3. New head? or repair and add new valve springs.

4. Cheap easy performance upgrades?

5. Insure the entire mess and light it on fire?

Thanks for the help.

My #1 bank popped???

Why do you say "bank"?

[CB750 SS]

There is a new wrinkle to this madness. It seems upon closer inspection (with glasses) my #1 Cylinder has some damage.  So I need to either get a new Cylinder bank or have it bored out and get all new pistons, rings and such. All of which is $$$ 

As I plan to keep the bike forever and it's been a good month (@ work) I'm trying to decide what I should do.   

Do I keep it stock and go with a used cylinder bank? Cost is ok - 1 new piston (+rings etc), valve guides, two new Valves (In & Ex) and gaskets.

Or do I go up to an 836 and all new pistons.  If so, does that mean anything for the valve train? or do I just need to worry about Carb Jets and timing?  BTW - ebay has a big bore piston set (836) for 110.00 bucks.

What should I do?  Is 836 much of a gain?  I'm not a racer but like to take rides and traffic in the south (US) averages 80mph.

help!!!!!   

[cmonSTART]

Ha.  I'm going through a bit of the same right now.  The $110 836 kits on eBay fit K models which have a different piston dome, so installing that kit in your F bike will change the compression ratio a bit.  There's a bunch of info on this site about it. 

[lucky]

Hey Guys, I'm in the process of rebuilding my CB.  Had everything together and back on the bike.  I went for a spin around the block and BAM! My #1 bank popped.   As background I just "freshened" the motor and it has about 48K on the clock.  On Sunday it will come out of the frame for a tear down.  Based on what I'm seeing I may have dropped a valve or worse.  There was metal in the carb and in the oil pump screen. It's bad!

Couple of question. 

1. Are all CB750 SOHC top ends and Heads inter changeable? ebay seems like a cheap option.

2. At this point I'm assuming the combustion chamber is damaged.  When I pulled the spark plug the electrode was gone! The end was beaten to a pulp.  Should I buy a used top end rather than machine it (new pistons etc)

3. New head? or repair and add new valve springs.

4. Cheap easy performance upgrades?

5. Insure the entire mess and light it on fire?

Thanks for the help.

Do not make assumptions.
Just wait until the engine is all taken apart THEN decide what actions need to be taken.
Just because a spark plug is beat up, does NOT mean the cylinder head is not serviceable.