Author Topic: cb 750 motor life  (Read 2414 times)

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Offline neil10

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cb 750 motor life
« on: June 23, 2008, 03:15:30 PM »
Has anyone reached a 100,000 miles on motor without rebuild

Offline DJ_AX

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Re: cb 750 motor life
« Reply #1 on: June 23, 2008, 04:48:18 PM »
I 2nd the question...  ;D
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Offline MRieck

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Re: cb 750 motor life
« Reply #2 on: June 23, 2008, 04:56:20 PM »
 I'm sure the engine would still run but as for performance. Mine has about 75,000 but has been rebuilt a number of times in different configurations.
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Offline Terry in Australia

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Re: cb 750 motor life
« Reply #3 on: June 23, 2008, 05:30:51 PM »
Yep, my mate Davey Dobinson bought a shiny new CB750K2 in 1972 and commuted/toured/raced it during his university years and it was his sole source of transport when he was a teacher here in Melbourne, until he finally bought a car in the 1980's.

His family lives in country Victoria, so he did hundreds of weekend rides to see them in all kinds of weather, and he had the first set of "waxed cotton" wet weather gear I'd ever seen, at the time. He did lots of interstate trips too, like his yearly trek to Bathurst for the Easter TT races in the 1970's and 1980's.

By 1980, when I started hanging out with him, his bike had done 109,000 miles, and had never been apart. Davey did lots of miles, but he never rode more than 2000 miles without an oil and filter change, and that bike felt "gruntier" than my (then) recently rebuilt K1, or his brother Pete's mint low mileage K2. Incredibly, at 109000, he was only on his second set of chains and sprockets, but couldn't remember how many tires he'd gone thru. He'd installed a Martek ignition when they first came out because he was sick of messing with points, but apart from that and some aftermarket pipes, he'd kept the bike pretty much standard.

Pete fell off Dave's bike shortly afterwards, and the fins on the cylinder block and head were mashed, so were replaced as part of the insurance work, and Dave forked out for new pistons and rings, but up until then we were sure that bike would have done 200,000 without any engine trouble. It was truly a case of one of those bikes that nothing ever went wrong with, and it felt so good to ride.

He's still got the bike, (although I haven't seen it for many years since he moved to Cairns, in Far North Queensland) and has amassed a great collection of NOS "cosmetic" parts (original blue tank, sidecovers, seat, pipes etc) for when he eventually takes it off the road (it's been continually registered for the last 36 years) and does a "cosmetic" restoration. Cheers, Terry. ;D   
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Offline KB02

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Re: cb 750 motor life
« Reply #4 on: June 24, 2008, 05:02:49 AM »
I have a friend who inherited a '77 (I believe) 750, with just over 90,000 miles on it, from a friend who pasted away. It's not running currently,  but we think mostly it's an ignition problem. As soon as he has the time (he is a very busy man) he plans to get it running again and hit the 100,000 mark with his friend's wife on the back. I told him I'm ready to help get it running as soon as he is ready for it.  :)
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Offline JLeather

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Re: cb 750 motor life
« Reply #5 on: June 24, 2008, 05:09:31 AM »
Roy, owner/operator of the Rider's Roost motorcycle campground in NC used to ride a CB750.  I believe he said his had 130,000 when he sold it and hadn't even ahd the valve cover off.  Of course, he also squeezed like 90,000 miles out of a shovelhead without opening the engine so I guess there's something to be said for really good preventative maintenance.

Offline neil10

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Re: cb 750 motor life
« Reply #6 on: June 24, 2008, 05:19:01 AM »
regular oil changes and oil filters 
don't thrash them
Extra benefit now is the new generation oils
Motors should just keep on keeping on

Offline 754

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Re: cb 750 motor life
« Reply #7 on: June 24, 2008, 05:41:51 AM »
One of my riding buddies had ..76 K I think on his the year I met him..that was at the end of 74 !

He later put an 810 kit on it ride it a lot more years without splitting the cases. His brother went bigbore, the next year I think and hade 80 or 90 K on it..
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Lost quite a few CB 750's along the way

Offline goon 1492

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Re: cb 750 motor life
« Reply #8 on: June 24, 2008, 09:40:07 AM »
It was truly a case of one of those bikes that nothing ever went wrong with, and it felt so good to ride.

It must have been a bike that was built on a wednesday and not on monday or friday. ;D
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Offline markjenn

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Re: cb 750 motor life
« Reply #9 on: June 24, 2008, 10:51:31 AM »
Engine life on an old, generally infrequently used bike like a CB750 will be one of those things that will be wildly variable.  Keep the motor relatively stock, perform reasonable maintenance, and drive the bike a lot (5K/year or more), and I think 100K without engine work could probably be expected, although most air-cooled inline-fours will be have tired top-ends, especially valves.  There are reports of air-cooled inline-four engines going 200K+.

But... it is a very rare CB750 that gets more than 1K miles put on it per year, so most bikes succumb to calendar time and corrosion issues long before mileage wear is an issue.  In aircraft where engine life is more closely tracked because of the safety aspects, lightly used engines have to regularly overhauled well before reaching their recommended hours in use simply due to calendar time issues.  Gaskets dry out, seals go bad, o-rings disintegrate, wiring deteriorates, fuel systems rust, valves stick, bearings corrode, etc.  This is what gets our old bikes 99% of the time, rather than miles.

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Offline deacon

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Re: cb 750 motor life
« Reply #10 on: June 24, 2008, 10:56:19 AM »
I had a 1971 CB750 that was stock. I had over 50 drag runs with burnouts and 7 grand launches, power shifted at the redline. I thrashed it to the red line powershifting through at least the first 3 gears everyday for 28,000 miles. The engine was perfect the day I sold it. Never touched it execpt adj the valves and cam chain and changed the oil and filter every 1500 miles.
I did however have to put a new clutch (barnett).
I still can't believe the abuse that bike took and never let me down.
I have a Cherry 1974 K4 with 15K on it and wouldn't think of beating it like that. I guess being 57 years old now I couldn't stand the beating LOL!!!

Offline neil10

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Re: cb 750 motor life
« Reply #11 on: June 24, 2008, 03:45:22 PM »
Gees I'm doing  30,000 miles on my bike a year, can't help myself love to ride it.

Offline 754

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Re: cb 750 motor life
« Reply #12 on: June 24, 2008, 08:52:53 PM »
I must have gone to the same school as Deacon, but did at least 200 runs.. ;D

And that motor was together 25 years..

 they are very hard to break...
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline markjenn

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Re: cb 750 motor life
« Reply #13 on: June 25, 2008, 11:59:12 AM »
Gees I'm doing  30,000 miles on my bike a year, can't help myself love to ride it.

30K a year?  As in nearly 100 miles/day, almost every day of the year?  Wow.  You commute on it?

- Mark

Offline Bob Wessner

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Re: cb 750 motor life
« Reply #14 on: June 25, 2008, 12:01:52 PM »
Gees I'm doing  30,000 miles on my bike a year, can't help myself love to ride it.

30K a year?  As in nearly 100 miles/day, almost every day of the year?  Wow.  You commute on it?

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Offline Terry in Australia

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Re: cb 750 motor life
« Reply #15 on: June 29, 2008, 07:38:44 PM »
Geez, if he's doing 100 miles a day on it, imagine how many miles he'll have up when he finally turns it around to come home! ;D
I was feeling sorry for myself because I couldn't afford new bike boots, until I met a man with no legs.

So I said, "Hey mate, you haven't got any bike boots you don't need, do you?"

"Crazy is a very misunderstood term, it's a fine line that some of us can lean over and still keep our balance" (thanks RB550Four)

Offline 754

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Re: cb 750 motor life
« Reply #16 on: June 29, 2008, 08:57:39 PM »
7 1/2 monthths at 4K per month will do it.

 I had lots of 4K months..
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline ken736cc

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Re: cb 750 motor life
« Reply #17 on: June 30, 2008, 09:27:33 AM »
My sandcast has 89,000 miles. It was tired and unhappy.
It ran well and started every time. I knew it was worn when it tested 15%
cylinder leakage. It is apart now and getting the best of everything.
No performance parts, just a nice, stock rebuild.
K0 sandcast(in process of restoration), K0 diecast, K0 cafe racer
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Offline smccloud

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Re: cb 750 motor life
« Reply #18 on: June 30, 2008, 10:52:23 AM »
Engine life on an old, generally infrequently used bike like a CB750 will be one of those things that will be wildly variable.  Keep the motor relatively stock, perform reasonable maintenance, and drive the bike a lot (5K/year or more), and I think 100K without engine work could probably be expected, although most air-cooled inline-fours will be have tired top-ends, especially valves.  There are reports of air-cooled inline-four engines going 200K+.

But... it is a very rare CB750 that gets more than 1K miles put on it per year, so most bikes succumb to calendar time and corrosion issues long before mileage wear is an issue.  In aircraft where engine life is more closely tracked because of the safety aspects, lightly used engines have to regularly overhauled well before reaching their recommended hours in use simply due to calendar time issues.  Gaskets dry out, seals go bad, o-rings disintegrate, wiring deteriorates, fuel systems rust, valves stick, bearings corrode, etc.  This is what gets our old bikes 99% of the time, rather than miles.

- Mark

My '72 CB750K2 gets at least 1000 miles in two work weeks when its not raining.  That isn't counting weekend rides or going out for a ride around the lake with my dad (for some reason a '72 K and '79 F/K manage to get a lot of looks).  Speaking of that, its about time to change the oil on my baby again.................
CB750 K2

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