Topic: Which determines jet size - engine or carbies?

[jawntybull]

I am working on a K7 CB750 that I bought recently, and have found that it has the F model carbies, not the K model; PD41B instead of PD41A. The bike is stock everywhere else from what I can gather; 4 into 4 exhausts etc. I've read that the only difference between the A and B carbies is the jetting, but I don't know this for sure.

The K7 has a main jet spec of 105, whilst the F model has a spec of 110. When I recently bought carbie overhaul kits for the "CB750 K7" they came with 115 jets. The carbies themselves have 110s in them (ie standard for this carbie). So my question is - should I jet for the engine (105) or the carbie (110) or use the new jets (115)? At this point I've put the new jets in just because they look so shiny...

[gnarlycharlie4u]

I am working on a K7 CB750 that I bought recently, and have found that it has the F model carbies, not the K model; PD41B instead of PD41A. The bike is stock everywhere else from what I can gather; 4 into 4 exhausts etc. I've read that the only difference between the A and B carbies is the jetting, but I don't know this for sure.

The K7 has a main jet spec of 105, whilst the F model has a spec of 110. When I recently bought carbie overhaul kits for the "CB750 K7" they came with 115 jets. The carbies themselves have 110s in them (ie standard for this carbie). So my question is - should I jet for the engine (105) or the carbie (110) or use the new jets (115)? At this point I've put the new jets in just because they look so shiny...

neither the engine or the carb size alone determines the jet size. both together do.
actually it's the demand for a good fuel/air ratio that determines it.

I can't say for sure what your bike should be jetted to, it depends on a multitude of factors, including not just engine aspects but, elevation, atmosphere, etc... You'll need to determine the jetting for yourself by doing a plug chop.

#110 main jet and #35 is a good place to start if you're at sea level. with the stock airbox.
pods and a different exhaust will certainly require rejetting though, probably 115-120.

you need to read this page:
http://www.hondachopper.com/garage/carb_info/carb_info.html

trust me.

[TwoTired]

I've read that the only difference between the A and B carbies is the jetting, but I don't know this for sure.

It's true

The K7 has a main jet spec of 105, whilst the F model has a spec of 110. When I recently bought carbie overhaul kits for the "CB750 K7" they came with 115 jets. The carbies themselves have 110s in them (ie standard for this carbie). So my question is - should I jet for the engine (105) or the carbie (110) or use the new jets (115)? At this point I've put the new jets in just because they look so shiny...

Depends on if the induction and exhaust is still stock.  If so, use the K7 jet/set up parameters.

If the exhaust and/or induction has been changed, the jetting/set up won't be in any Honda book or reference.  You will have to find out what works best for your particular set up.

Cheers,

[BobbyR]

As TT said it is dependent on how your bike is set up. I have a K8 and the previous owner jetted it to 120. I don't know why, he was a mechanical Engineer and Porsche road racer. The bike only had 5K miles on it as he passed suddenly. I find notes in the shop manual about possible mods, I have no idea what mods if any he did other than rejet.  I have great response all through the powerband and the plugs will show some richness after a year. I would go the 115s and see how she responds.

[splitt]

Carb size is determined by both the displacement and the maximum rpm's that the engine will be turning.

The correct jet size is determined by a comination of things, as stated previously. I was taught to think of an engine as an over glorified air pump. The jetting is determined by how much air is being pumped through the engine. Each engine and each situation will be different.

Before changing jets, start from the beginning and make sure that the engine is tuned properly. (valve lash, point gap, timing, cam chain adjustment, spark advance, new spark plugs, fresh gas, clean carbs) Then synchronize the carbs before doing the plug chops. I wouldn't want you to waste a bunch of of your time and money by trying to rejet an improperly tuned engine.