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Discussion Starter #1
Hi guys...........
I recently had an interesting discussion with a guy who was chewing my ass about my 75 CB550. He was trying to explain how to get power from the motor. The idea was a S.O.H.C. CB650 head ( 1979 to 1982 ), larger valves and such. It is supposed to be a bolt on swap. Besides a change of carbs and exhaust, what else would be needed ? Would the end product be worth the time and trouble ?. Oh, I also did a repaint and some other changes since I posted a link to my pics. As soon as I get the new pics up, I'll post the link to them ( wish I was better with good old Bondo ). Thanks for your time

John
 

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I know the whole 650 motor is a swap into the 500/550 frame but not sure about the head. You may have to run a search over at the SOHC4.net board as it has been discussed there several times.
 

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I've heard the same motor enhancement with CB750's (using F2 heads with bigger valves). Just because you have bigger valves does not mean your motor will develop more power. You also have to have the motor built with supporting features such as the "right" exhaust and carbs to make power with the bigger valves. Without specialized equipment, setting up a motor to take advantage of these enhancements is beyond the average back yard tuner.

If you want serious power the formula is simple - more cc's equals more power. Other than that, look into adding a turbo or a JADO (sp) jet pack to your bike. (jet packs look cool at night...)

Your 550 is what it is - your better off doing handling mods to make it handle vs trying to add more power to the engine. No matter what you do engine wise - your not going to make your 550 into a bike that will blow away a 1000cc size bike. But you can make it handle just as well or better as the 1000cc bike.

Save yourself a few bucks and spend it in the right direction.
 

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bmartin, who ever told you you can use an F2 head with a K engine is full of shit.

FYI 1977-78 cb750F big valve heads are NOT interchangible with earlier K motors. You need to change the cylinder and the pistons if you want to run the bigger valve heads on an earlier motor. If you put an F2 head on a K cylinder the valves with hit the pistons, If you put a K head on an F2 you will see a noticable drop in compression. The reason is that the F2 pistons are dished to clear the bigger valves.

However the stock cb750K head is a fairly restrictive head and you can see large gains in power by doing port work and changing the carbs to compensate for the new flow. The cb750F2 big vavle motors do put out more hp than the previous motors and a large portion of that is due to the head.
 

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to answer your question by the way:

yes, the 650 pistons and head will bolt straight onto the 550 cylinders. the 650 and 550 pistons have the same pin height, but the 650 piston is way too domed to fit into the 550 head. you'll have to use the 550 cam sprocket, which will also bolt right onto the 550 cam. while you're at it, you could just use the stock 650 cam for a pretty big increase in lift, or try running one of megacycle's mild street cams for the 550. the blanks are the same for the 550 and 650, so don't listen to them if they tell you they can't cut one from a 650 core. mike reick suggested the 126-31 for my project. yes, you'd want 650 carbs.

from here on the SOHC4 board:

http://forums.sohc4.net/index.php?topic=27827.0

you have to use the 650 pistons with the 650 head.

sounds like it is easier to me to get a cb650 whole motor complete and just drop that in instead. No machine work, and makes decent power over the stock cb550 (most cb550s have about 45 hp, the 650 makes a jump to 50hp).
 

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Geeto - thanks for the input, I already knew that, I was just trying to get the point across that for the average joe, changing heads from on breed to another was not for novice and the kid was better off putting money into handling. BTW, I have one of my heads that I was going to port myself, I have some information on the subject, however; if you have additional info - I'd love to get a copy of it..

thanks
 

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quote:Originally posted by bmartin

Geeto - thanks for the input, I already knew that, I was just trying to get the point across that for the average joe, changing heads from on breed to another was not for novice and the kid was better off putting money into handling. BTW, I have one of my heads that I was going to port myself, I have some information on the subject, however; if you have additional info - I'd love to get a copy of it..

thanks
I have a few notes on the subject...I will see if I can dig it out...

The person to ask about porting is Mike Reick in Boston...the man knows tons about it. I'll dig up his contact info.

There is a salvage yard in New Orleans that I used to frequent that had a stockpile of cb750 f2 heads....most of them with bent valves because of miss info. considering your expirence with cb750s I kinda figured you know better but I felt like saying someting about it anyway so no noobs get the wrong idea.
 

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Geeto - I have access to a savage yard close to my home. The word on the street is cb750/450/350 parts are hot - so these parts are going to be harder to source in the future. As far as experience on CB750 - only a fool thinks he's got all the answers, I'm always open for information. Besides, the info may have just saved someone else from a costly mistake.

BTW - I have access to a 650 head if anyone is interested...
 

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typing this stuff on the internet is like having a conversation in grand central station....you may be only addressing a few people but pretty much everybody has the chance to listen in.
 

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Discussion Starter #10
Hi guys,
After reviewing the talk on the sohc4.net forums, I have pretty much given up on this idea. I think I have located a complete motor to install instead. Unfortunately, California puts the engine numbers on the title and insist on very very precise documentation as to where the motor came from. I think maybe I'll start looking for another bike, with more getty-up for a start or more of a play toy ( dual purpose ?? ). Thank You for all the imput and suggestions. Anybody wanna buy a 1975 CB550K ?.

John
 

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i remember trying to "improve" one of my bikes. developed it over a number of years until it became my race bike. years later, i put another (stock) bike on the road and only then did i realize that the factory had it right from the beginning all along. it idled, pulled well throughout the rev range, was smooth, and quiet. it provided excellent usability/enjoyment on the street. modifying the original form of a street bike virtually always yields a shitty street bike. if it was good, you'll probabley wreck it. if it sucked, you'll make terrible. the factories have tons of money and smart engineers working on coming up with good bikes. and they miss once in a while. most of us will miss every time.
-parks
 

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in 99% of the cases I will agree with you parks on this, but the cb650 is the natural evolution from the cb500-550 engine - basically the factory kept refining the design until they had a 650 motor that was really close to the SOHC cb750 that had gone out of production when the DOHCs came in. The 650 engine is literally plug and play in to the older chassis 500/550 but internally shares very little with the 500/550. too bad the bike they elected it to power was a mini chopper looking thing.
 
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