Questions about DOHC with new exhaust

[ETFerguson]

I really really really don't want to beat a dead horse here but I've searched and searched and even had another thread about this a long time ago but can't seem to find a clear answer. Maybe something definitive from TOOLS or Rerun will put my mind at ease

I just finished putting my Mac 4-1 exhaust on my 82 K and I love it! I'm planning on starting it this weekend but I'm thinking I may either need to A) re-jet the carbs or B) sync them again?

I've heard one or both "need, don't need, may need" to be done now that i've changed the way the bike breathes (see why I'm confused) and I just can't get a darned straight answer! Can someone please explain why either of these may need to be done and maybe a short description of the "science" involved. I don't want to put out money if I don't have to. I just cleaned, mostly rebuilt and synced the carbs in the fall before I decided on this exhaust and the bike was running well.

Thanks in advance for your infinite wisdom!

[TOOLS1]

An exhaust has nothing to do with carburetor sync. And it is very rare that an exhaust change will require a jet change. You will just have to run it, and do a plug chop to make sure it does not need any changes to the jetting though.
TOOLS

[Re-run]

I will echo what TOOLS said. I have never heard a pipe change needing a carb sync. It may be that the sound change has made the need for a sync more apparent, but it will never cause a need for it.

Jet changes are also very rare with just a pipe change. Maybe if you went to drag pipes, then maybe. Otherwise as already said, do a plug shop to be sure, but you will most likely be fine.

[ETFerguson]

Great thanks so much guys I feel much better now. All this worrying for nothing, and this is supposed to be fun! :D

I'm thinking I'll be fine because honestly this bike isn't running silly smooth anyways. I'm going to have to do some more engine work I think at the end of the riding season. There's still a little ticking noise which may be my cam chain and I'm sure that at some point it will need a top end rebuild with valves etc. I am scared what that may cost me in parts alone...

On Monday, April 6, 2015, Re-run [via Honda CB750'S] <[hidden email]> wrote:

I will echo what TOOLS said. I have never heard a pipe change needing a carb sync. It may be that the sound change has made the need for a sync more apparent, but it will never cause a need for it.

Jet changes are also very rare with just a pipe change. Maybe if you went to drag pipes, then maybe. Otherwise as already said, do a plug shop to be sure, but you will most likely be fine.

The ride IS the adventure. The destination is just to get gas!

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[ETFerguson]

One more thing though:  if I do need to play with the carbs because they are running rich or lean I can do that by adjusting them with the air mix screws yes? Like I said, I just had them done last fall and they were in good shape and the mixture was good as far as I know. Hopefully moving from a 4-2 to a 4-1 hasn't changed this too much.

[Re-run]

Well, in most cases, a simple screw adjustment will do it. If you have air screws, in is richer, out is leaner. If they are fuel screws, it is the opposite. I think you have fuel screws, but double check the manual to be sure.

Don't turn much, generally 1/8th turns, especially on fuel screws.

[LukeM]

A little ticking noise may be valvetrain related. Get the FSM from the home page, and check the following:

Cam Chain Adjustments
Valve Adjustments
Compression Check

All of these will help your motor run sillier (or did you mean silkier?). :-)

Then you can look at adjusting mixture, idle speed, air screws, and the like.  Before tweaking screws and stuff, make sure you have a baseline setting for where your motor is now.  That way, if you get it too out of whack, you can go back to those settings and start over.

Carb Sync is not that difficult, presuming you have the gauges and the proper screwdrivers to work on the carbs.  It does make a big difference when they are in sync and running well.  A piece of advice: Try to get the carbs sync'd at different RPM levels: idle, part throttle, mid throttle, and close to max.  You may find that the engine is a bit lumpier at idle, but sync's up nicely at other RPM levels.  A balance over the range will give you best performance for normal riding.

Another piece of advice: Get a box fan or equivalent blowing across the cylinders while working on sync.  If it's gonna take a while (and it may, as this is your first time), the heat in the cylinders may get too high.

I hope this helps.  The nice thing is you have a rider right now...you're just making it better, with your own two hands!  That's the cool part.

Luke M

[ETFerguson]

Great advice Luke, thanks! I had the compression worked on last year too and it's pretty good. All within 10% of each other which I though was good for an 82

On Monday, April 6, 2015, LukeM [via Honda CB750'S] <[hidden email]> wrote:

A little ticking noise may be valvetrain related. Get the FSM from the home page, and check the following:

Cam Chain Adjustments
Valve Adjustments
Compression Check

All of these will help your motor run sillier (or did you mean silkier?). :-)

Then you can look at adjusting mixture, idle speed, air screws, and the like.  Before tweaking screws and stuff, make sure you have a baseline setting for where your motor is now.  That way, if you get it too out of whack, you can go back to those settings and start over.

Carb Sync is not that difficult, presuming you have the gauges and the proper screwdrivers to work on the carbs.  It does make a big difference when they are in sync and running well.  A piece of advice: Try to get the carbs sync'd at different RPM levels: idle, part throttle, mid throttle, and close to max.  You may find that the engine is a bit lumpier at idle, but sync's up nicely at other RPM levels.  A balance over the range will give you best performance for normal riding.

Another piece of advice: Get a box fan or equivalent blowing across the cylinders while working on sync.  If it's gonna take a while (and it may, as this is your first time), the heat in the cylinders may get too high.

I hope this helps.  The nice thing is you have a rider right now...you're just making it better, with your own two hands!  That's the cool part.

Luke M

Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.

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