Topic: A straight-forward CB550 to a CB650 engine swap

[turkey2143]

After reading some ancient posts in the sohc archives I learned that putting a CB650 engine years 79-82 is a bolt on thing to put on a 77' CB550K frame, and all I need after that is the 650 carbs and an appropriate exhaust, but what is the actual cut off year for swapping?

How far into the future can I go before my bolt-on-age won't work anymore?

[Frankenkit]

well, 79-82 650 engines are all basically the same in the bottom end... they differ up top with CV carbs or mechanical slides.  That's all well and good, but the 650 is also electric start only, so unless your 550 engine is dead-dead, you might be setting yourself up for more work than you really want.  Don't get me wrong- I love my 650- it's powerful and sounds great- but the carbs are a pain in the rear, the electrics are, too, and I do believe I've replaced almost everything that can be, in the pursuit of a truly reliable machine.  If you use a 650 motor, you may be in for some "fun" finding carbs that will work well, and/or pods/airbox.

What it comes down to is: What is your motivation here?
And.
There are people here who will know more than I do for sure...
I want to say 82 nighthawks might be the limit because at some point the DOHC motors started going belt-drive, I think.

[turkey2143]

Hmmm... well my 77' 550 with stock carbs & airbox, and a cheapie MAC exhaust has about 21k miles on it, and is starting to seep more oil out of the head gasket than I'm used to, but has otherwise been very reliable.

My motivation is high, but I've been seeing some salvage bikes in the 79-82 era with smashed front ends, but otherwise pristine engines and carbs (~5000 miles!!) that can be had for peanuts.

How many miles does your bike have? and what specifically has had to have been replaced?

[Frankenkit]

Mine has 30,000 miles on it.
The ignitors melted out, but that's relatively common.  Pulsers weren't working right, either- 1-4 wasn't functional.
Both coils were cracked.

Whether or not these things were the product of or in any way related to my rotor shorting internally when it got warm is anyone's guess, but I doubt it. Rotor got rewound, and because I had the promise of greater output, I got the stator rewound, but that probably wasn't entirely necessary...

Plug wires and caps got replaced, but that's just maintenance.

Starter solenoid got replaced, though in retrospect it may have been alright.  The cable from solenoid to starter motor was bad, but wasn't easily diagnosed because I didn't know to look at it and no one else suspected it, either.  

Regulator/rectifier got replaced as well as they tested on the very outer limits of acceptability.

I went through a couple starter motors but that was because of the cable... though the first WAS grounding internally- the commutator rubbed against the housing.

...and that's my electrical experience.

I strongly suggest you get a new head gasket because it would be WAY cheaper, if only because you know what you're getting if you have your ol' familiar motor as opposed to a new unknown.

[paulages]

you have to splice the 650 charging system into the 550 harness- no big deal. the CV carbs can be made to work with the stock airbox (haven't done a slide carb engine), you just have to glass shut the stock opening from the airbox to plenum and recut it to fit the 650 plenum. CV carb models are '81-82.

[ieism]

Is this a stock 550 motor? 21.000 Miles is nothing, a lot of guys here have 4 times that milage and no problems. My 550's both have over 70.000 on the clock without leaking oil.

A 650 motor in your 550 frame would be really cool, but what are you trying to achieve? If it's just an oiltight engine you want, changing a few gaskets is a lot less work than putting in a new engine.
I think an old 650 will not make more than 60 HP now. A properly rebuild 550 with a few mods will make 55 HP, so that's a small difference.

[turboguzzi]

Is this a stock 550 motor? 21.000 Miles is nothing, a lot of guys here have 4 times that milage and no problems. My 550's both have over 70.000 on the clock without leaking oil.

A 650 motor in your 550 frame would be really cool, but what are you trying to achieve? If it's just an oiltight engine you want, changing a few gaskets is a lot less work than putting in a new engine.
I think an old 650 will not make more than 60 HP now. A properly rebuild 550 with a few mods will make 55 HP, so that's a small difference.

  Check your figures, unless you are talking about meaningless mfg. claims, stock 500-550's gave 36-38 hp @ rear wheel and 650 mid 40's. getting 55 rwhp from a 550 is not just a few mods, oh no, ....

that said, i agree that a weeping head gasket is no reason for a 650 swap, that's really easy to rectify.

if you want more ooomp, then yes, worth the while, but you could have an easier time boring to  608 with a big nore kit, retain the classic looks of the 550 head and have a fresh top end. hard to beat in my book.

TG

[ieism]

Sorry, I didn't mean rearwheel HP, I never had a 650 so I don't even know how much hp that makes on a dyno. I was just trying to compare the two bikes.

My 550 is 45 Hp on a dyno now, with just a ported head and different exhausts and K&N filters. I'm pretty sure a tired stock 650 will struggle to make 45 HP on a dyno.

Slightly overboring and mild porting would be a simple, cheap and good solution IMO.

[manjisann]

Upsides to the 650 engine, adjusting valves is TONS easier than on the 550's.

Personally I like the mechanical choke on the 500/550 carbs, the 650 has a cable choke that connects to a plastic butterfly thingy that is brittle and made of unobtainium (just as Kit).

I have a 650 currently and I had a 500, loved them both, but I agree with what everyone is saying. If the only reason you are thinking of changing engines is due to a weepy headgasket, just fix the gasket. Besides, if the donor bikes you are looking at have front end damage, they likely were layed over hard and there is no guarantee that they're any more oil tight than your 550.

In the end though, it's your choice.

HTH

Brandon

[turboguzzi]

Sorry, I didn't mean rearwheel HP, I never had a 650 so I don't even know how much hp that makes on a dyno. I was just trying to compare the two bikes.

My 550 is 45 Hp on a dyno now, with just a ported head and different exhausts and K&N filters. I'm pretty sure a tired stock 650 will struggle to make 45 HP on a dyno.

Slightly overboring and mild porting would be a simple, cheap and good solution IMO.

yep, 45 rwhp is pretty nice for your light mods, good work. did you bump compression a bit?

TG

[MaaseyRacer]

Hmmm... well my 77' 550 with stock carbs & airbox, and a cheapie MAC exhaust has about 21k miles on it, and is starting to seep more oil out of the head gasket than I'm used to, but has otherwise been very reliable.

That is most likely the little rubber pucks in the head, very common with the CB500/550.  If you replace those that will probably fix your oil leak.

Also, is it possible to just swap the cylinder jug, pistons, rods, crank, and head off of a CB650 and swap them into a CB550? Thus retaining the CB550 charging system, and kick start operation.

[paulages]

Hmmm... well my 77' 550 with stock carbs & airbox, and a cheapie MAC exhaust has about 21k miles on it, and is starting to seep more oil out of the head gasket than I'm used to, but has otherwise been very reliable.

That is most likely the little rubber pucks in the head, very common with the CB500/550.  If you replace those that will probably fix your oil leak.

Also, is it possible to just swap the cylinder jug, pistons, rods, crank, and head off of a CB650 and swap them into a CB550? Thus retaining the CB550 charging system, and kick start operation.

http://forums.sohc4.net/index.php?topic=40414.0

[ieism]

yep, 45 rwhp is pretty nice for your light mods, good work. did you bump compression a bit?

TG

Well they shaved a little bit of the base of the head, but more to straighten it than to raise compression. I never measured compression before rebuild, so i'm not sure how much that helped. I think most of the gain came from the porting, and running exhaust without restriction.

Sorry to steal the topic.....