Topic: Rejetting required???

[88BuckMeister]

My '75 CB550 I'm assembling has 3 major mods to it... the first is a set of Dynoman 61mm big bore pistons. The second is a 650 camshaft mod. The third is yoshi replica racing exhaust.

With that setup, assuming I use the stock airbox and avoid pods, will rejetting be required and/or beneficial?

[newday777]

Most likely you will need to rejet.
You will have have to do spark plug chops to test the burn color of the plugs.

[Mark1976]

   Oh joy, what fun you have ahead of you. Keep the airbox (its just plain quieter), just my opinion, that and its moderately easier to get the jetting squared away. When I installed the 650 cam in my 550f, std bore, mototec gp pipe, w/stock airbox, I believe I ran a 40 pilot and a 105 main. You've got a bit more displacement so you're definitely going to be above that, how much is the 64 thousand dollar question.

[calj737]

Depending upon the cam profile and the rev range you ride it in, you may also need to move the needle clip one slot in addition to the 105 mains.

[scottly]

My '75 CB550 I'm assembling has 3 major mods to it... the first is a set of Dynoman 61mm big bore pistons. The second is a 650 camshaft mod. The third is yoshi replica racing exhaust.

With that setup, assuming I use the stock airbox and avoid pods, will rejetting be required and/or beneficial?

Usually a big bore and cam will not require re-jetting, as the carbs meter fuel based on the amount of air flowing through them. The exhaust may benefit from an increase in the main jet size, due to the scavenging effect of a tuned exhaust. Then again, using the stock airbox may negate that effect?

[88BuckMeister]

   Oh joy, what fun you have ahead of you. Keep the airbox (its just plain quieter), just my opinion, that and its moderately easier to get the jetting squared away. When I installed the 650 cam in my 550f, std bore, mototec gp pipe, w/stock airbox, I believe I ran a 40 pilot and a 105 main. You've got a bit more displacement so you're definitely going to be above that, how much is the 64 thousand dollar question.

Do you have any suggestions for a "rejetting kit" that comes with a bunch of options?

[calj737]

Start with plug chops before modifying your carbs. Plugs at idle, plugs at WOT, plugs at 6,000 RPMs. It’s cheaper, easier, and more accurate.

[Mark1976]

Start with plug chops before modifying your carbs. Plugs at idle, plugs at WOT, plugs at 6,000 RPMs. It’s cheaper, easier, and more accurate.

   As stated above, start with your stock jetting, time it, sync it and see what ya get. Its a known setup to start with. The only thing I might add is take note as well about how it behaves cold vs hot. You don't have any extensive modifications so it should be pretty straightforward. You'll need to determine if you have the right main size 1st, my opinion...

[bryanj]

No "kits" are worth it as they are all aftermarket jets

[Don R]

 Like Scottly said, some mods pull more air which also pulls more fuel. If changes are needed, start small.

[jonda500]

Like everyone is saying try the stock jets  - rejetting may not be required - get the main jet size sorted first!
John

[CycleRanger]

No "kits" are worth it as they are all aftermarket jets

Yes, make sure to use genuine Keihin jets.
https://www.jetsrus.com/

[HondaMan]

Using the 650 cam in the 550 will make the low end go rich (idle jet RPMs). The intake valve just opens much sooner than the old one did (like 5-7 degrees sooner, depending on which 550 you started with). This causes the airflow over the idle jet to be higher than it was before, which will draw more fuel-air mix. When doing similar mods in the 750 (which is to use a Megacycle 125-00 cam, almost identical to the CB650 cam) I usually have to go down a step from #40 idle jet to #35 (or #37.5 if I can find them) to get the low-end and midrange  transition to be smooth, so it doesn't fall on its face when rolling into the throttle and blacken the sparkplugs. In the 750, using the #37.5 puts the idle mix screws back to their normal setting, while using the leaner #35 requires going out a bit more, like 1-1/8 to 1-1/4 turn in stead of 1.0 turn.

[88BuckMeister]

Using the 650 cam in the 550 will make the low end go rich (idle jet RPMs). The intake valve just opens much sooner than the old one did (like 5-7 degrees sooner, depending on which 550 you started with). This causes the airflow over the idle jet to be higher than it was before, which will draw more fuel-air mix. When doing similar mods in the 750 (which is to use a Megacycle 125-00 cam, almost identical to the CB650 cam) I usually have to go down a step from #40 idle jet to #35 (or #37.5 if I can find them) to get the low-end and midrange  transition to be smooth, so it doesn't fall on its face when rolling into the throttle and blacken the sparkplugs. In the 750, using the #37.5 puts the idle mix screws back to their normal setting, while using the leaner #35 requires going out a bit more, like 1-1/8 to 1-1/4 turn in stead of 1.0 turn.

Hondaman when you walk in the room, everyone shuts up and listens lol. Gotcha. Ok duly noted, and thank you. Hey did you ever wind up finishing the mods book for the 550?

[88BuckMeister]

Understood, all. Thank you all for your advise. This is a bangin thread