Topic: CB550F Engine Swap

[jackudig]

Since my engine needs work, can I replace it with a used K model engine?  Will it bolt right in?

Thanks.

[Duanob]

Yep same engine, different carbs and exhaust is about it. What kind of work does your F engine need? And how do you know the K engine will be any better?

[Bankerdanny]

Even the carbs are basically the same as long as the K donor is a '74-'76.

The 77-78 K's used completely different carbs (PD series), but any 550 carbs will work on a 77-78K motor as long as you have the intake manifolds that match the carbs you plan to use. You may need to play with the jetting though, so I would recommend using the carbs that match the engine.

[Tews19]

I always thought the F meant FAST! K meant Kupcake...... hmmmmm

[jackudig]

Compression is low on some cylinders.  138 160 138 148.  Pressure went up with oil in the cylinders.  I read the exhaust pipe temps ( cheap infared gun ) after shutdown and #1 cyl was about 100 degress cooler than the rest.  41000 miles and some oil leaks.  It seems to run alright and starts easily. 

Thought if I found a decent engine K or F I could go through it and swap it out while still riding until that point in time.

[Jimsun]

Compression is low on some cylinders.  138 160 138 148.  Pressure went up with oil in the cylinders.  I read the exhaust pipe temps ( cheap infared gun ) after shutdown and #1 cyl was about 100 degress cooler than the rest.  41000 miles and some oil leaks.  It seems to run alright and starts easily. 

Thought if I found a decent engine K or F I could go through it and swap it out while still riding until that point in time.

If those readings were before you put in oil, then those are some very nice numbers. My readings were 135/130/140/130 or something along that range from cyl1 to 4 respectively. The lower temp can be fixed by cleaning or re building the carbs.

[Tews19]

Those numbers are pretty nice if the bike sat for a while.

[jackudig]

Thanks for the input.  Maybe I worry too much.  You know how parents are with there own kids.

[calj737]

Often, the range of the numbers is far more important than the achieved number. You want a very even spread of compression across all cylinders to provide the best performance. Obviously, if the numbers are very low in all cylinders, you need to work the jugs.

As mentioned, for a bike that's sat a while, those numbers seem quite fine. You could drizzle some Marvel in the cylinders thru the spark plug hole, maybe even mix in some Acetone, to help "clean" the bores and rings, free up any contamination, then run the engine. It will smoke, but that's okay. Heat it up plenty, then drain and change the oil and filter. Refill with quality new oil. Run the bike some. Then re-test your compression. Should come up and even out. Unless you have problems (unlikely from what you've posted). But the cleansing step is still a good preventative measure.

[lucky]

They will not interchange!

Completely different design.

[Duanob]

They will not interchange!

Completely different design.

The CB550 SOHC4 K and F model engines won't interchange? That's a load of BS a mile high Lucky.  We put up with a lot of your quirkyness and that's fine, the board has an entertainment value as much as an informative value. But when you're wrong you are just plain wrong. And spreading bad information is just bad news for the board and it's members. All part numbers are the same for both engines, except carburetors and exhaust systems. I have both.

[Bankerdanny]

They will not interchange!

Completely different design.

I'm not sure what you mean Lucky. The carbs on my '74K and '76F are externally identical, including the linkages and manifolds. I know the model numbers and jettings are slightly different, but physically they should interchange fine.

The PD's on the '77-'78k's are obviously completely different with different throttle cable and choke setups. But the engines and heads are the same, so as long as you bring the manifolds, airbox, and throttle and choke cable setup with you, they should bolt right up to an earlier engine.