Topic: 650 into 550 Cam Swap Thread

[NastyNip]

I thought that I would make this thread as a joint project/question thread about the process of dropping a '79-82 CB650 SOHC Cam into a 550.

This is going to be my first cam swap, so I'm sure there will be a few questions along the way. There are a few scattered posts about the topic, but none that fully document the information and process in a single place, so hopefully this thread can be that 

The aim is to give a little more power to my 1978 CB550K with a drop-in, sub-$100 part. Because my bike uses the superlean PD 46C carbs, exhaust/intake mods are somewhat more laborious because I have to rejet (with hard to find/non-existant replacement jets  ) so I felt that a cam-swap would be much smoother sailing.

After ebay being dry of 650 cams for the past couple months, I checked out MrCycleParts and received multiple responses within a week.

The cam+tach drive gear is coming in a few days, I'll update w/ pics when I get it.


Quick Overview of facts thus far, let me know if anything is wrong:

The 1979-82 CB 650 Camshaft is nearly identical to the 550 cam, in-so-far that it was ground from the same blank, and the only difference in features are the slightly different lift and duration, and a differently toothed tach drive. Timing and clearances should be the same, however some have reported that a tiny bit of modification is sometimes necessary with the head. These reports are varying, so I have not found out exactly which heads need clearancing and which ones don't.

Using the 550 sprocket on the 650 cam allows you to drop it in, the timing should be identical and not require degreeing, and the firing order should be the same also. Truly, a 'drop-in' mod that serves as a mild upgrade for cheap.

Now for the Questions:

1. I know that the tach ratio for the 650 is different than the 550. That's fine, I was planning on getting new gauges anyways. My question is, what exact tach ratio am I looking for when buying a new tach?

2. Can you guys forsee any particular problems with this swap because of the super-lean nature of my '78 550? Those PD 46C carbs are just awful to work with (but that's for another thread!)

[Scott S]

1.) CB650 and CB750 tachs will work. Don't remember the ratio off the top of my head. Or, run an electronic tach.

2) I rejetted my PD carbs ('78 550K), but I also had a mildly ported head, Jardine 4-1 exhaust and a 1.00mm overbore.

 More details about the rest of the bike? Exhaust? Stock air box? Oh, I also had to clearance the head slightly in a few spots to clear the cam lobes. I have pics somewhere...

[cb650]

If its a latter 550 the 650 tach is a direct bolt on.

[NastyNip]

1.) CB650 and CB750 tachs will work. Don't remember the ratio off the top of my head. Or, run an electronic tach.

2) I rejetted my PD carbs ('78 550K), but I also had a mildly ported head, Jardine 4-1 exhaust and a 1.00mm overbore.

 More details about the rest of the bike? Exhaust? Stock air box? Oh, I also had to clearance the head slightly in a few spots to clear the cam lobes. I have pics somewhere...

I'll see if I can dig up the ratios for 650 and 750 tachs, and post the findings here. Thanks for the heads

I'm running everything completely stock for now, so I can get a solid baseline, then go from there. Stock airbox (with tool tray), 4-4 exhaust, etc.

A 4-1 exhaust might be down the road. Scott, might I ask where you got your new jets from, and what size?

[Scott S]

I set the float height at 13mm, raised the needle one clip, kept the 42 pilot and went from a 90 to a 105 on the main.

[Scott S]

 Some threads about my bike and doing what you're doing. Somewhere in the build thread I think you can see where we clearance the head.

http://forums.sohc4.net/index.php?topic=98867.0

http://forums.sohc4.net/index.php?topic=102369.0

[SAEmiles]

If its a latter 550 the 650 tach is a direct bolt on.

Where did you find this information?

I have a 77 550 and the stock 550 gear does not mesh on the 650 cam. So if this is true, they must have switched after 77.

BTW, for what its worth mine fit without any head modification.

[trebuchetxlt]

Tacho ratio info can be found at;

http://forums.sohc4.net/index.php?topic=41940.0

[cb650]

If its a latter 550 the 650 tach is a direct bolt on.

Where did you find this information?

I have a 77 550 and the stock 550 gear does not mesh on the 650 cam. So if this is true, they must have switched after 77.

BTW, for what its worth mine fit without any head modification.

Didnt say anything about the gear or drive.    A 650 TACH will bolt on in place of a 550 tach no problem.   The earlier 500 maybe some 550's had a smaller tach.
I know this cause we did it in my driveway in about 5 minutes. With 2 diffrent members.

[Scott S]

If its a latter 550 the 650 tach is a direct bolt on.

Where did you find this information?

I have a 77 550 and the stock 550 gear does not mesh on the 650 cam. So if this is true, they must have switched after 77.

BTW, for what its worth mine fit without any head modification.

 I think he means the 650 tach will fit the 550 gauge bracket/holder.

[NastyNip]

Tacho ratio info can be found at;

http://forums.sohc4.net/index.php?topic=41940.0

Thanks for the link!

So According to that, the smaller SOHCs (550 and under) run a 7:1 Ratio tach, while the larger SOHC (650+) run a 4:1 ratio tach. As long as I have the 650 tach drive gear, running a 4:1 tach should give accurate readings.

[NastyNip]

Some threads about my bike and doing what you're doing. Somewhere in the build thread I think you can see where we clearance the head.

http://forums.sohc4.net/index.php?topic=98867.0

http://forums.sohc4.net/index.php?topic=102369.0

Awesome thread! Very relevant to my project, thanks for the link!

[NastyNip]

Update:

I got my 650 cam and tach drive gear in the mail, and everything looks good to go for the swap.

Here are some pictures of it+tach drive for academic purposes (click for big)



The cam as it came.



The teeth on the cam, which is different than the 550.



and how the tach gear and cam mesh.

I'm having other problems with the bike right now (see my other thread  ) so this swap is taking a backseat to it at the moment. But hopefully I'll get to tackling it within the next few weekends. I'll update with pictures as I go along.

Sidenote, I hope everyone is stoked for the beginning of riding season!

[trebuchetxlt]

Good luck with the install, it's fairly straightforward just watch out for clearance issues between the lobes and head. When I did my swap I had to shave a little off the head at the 1 inlet and 4 exhaust positions.

[jtran000]

will be following this also.  is this possible with the 77 model?

[Scott S]

 Yes. Look at the links I posted earlier. It shows how we did it on my '78.