Author Topic: CB 750 K5 Questions. All replies welcome  (Read 1112 times)

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Cowboy48

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CB 750 K5 Questions. All replies welcome
« on: April 04, 2006, 05:05:34 PM »
I am the very happy new owner of a 1975 CB750 K5 that is in decidedly good condition for it's semi-advanced age.  It does however have a few "personality traits" that need a little attention (shocking I know).  I am fairly mechanically inclined and plan to do most or all of the work myself.  I have the Haynes, Clymer, and factory manuals to guide me as well as you guys :-).  Any and all responses to the following questions are appreciated.

1. When the headlight is on should the front orange lights be on but not the rear orange lights?  The blinkers work fine but the front 2 lights are always on with the headlight.  Also, the "Turn" gauge light is lit solid when the headlight is on and does flash when the turn signals are on.  Is this normal?

2. I found the main fuse wrapped in tinfoil and a 14a fuse in the 7a headlight fuse spot.  Amazingly everything works fine with the proper fuses in place but I noticed that the headlight works with or without a fuse in the middle holder.  Gotta think this is wrong.....

3. The bike starts and runs great but bogs down if the throttle is really openned up.  It eventually clears out and can get from 55-75mph but something is not right for sure.  I'm guessing carbs right?

4. Should I leave the 4-1 exhaust or go to a 4-4?  All the pics I have seen of this model have 4-4 pipes.  Has mine been changed?  Is there and advantage to either?  My muffler has a big dent in it so I'm not oppossed to changing the whole exhaust.

5.  Is there any kind of conversion for changing to a wider rear tire with disc brakes?  Am I being a non-purest by asking this question?

Lastly (for now) let me say that I truely appreciate any input that anyone has.  I was thrilled to get a great bike like this to enjoy riding but to find that there is also an entire community dedicated to maintaining and discussing these great motorcycles is amazing to me.  Thanks for welcoming me into the fold.

Offline Jonesy

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Re: CB 750 K5 Questions. All replies welcome
« Reply #1 on: April 04, 2006, 05:44:18 PM »
Here goes...

I am the very happy new owner of a 1975 CB750 K5 that is in decidedly good condition for it's semi-advanced age.  It does however have a few "personality traits" that need a little attention (shocking I know).  I am fairly mechanically inclined and plan to do most or all of the work myself.  I have the Haynes, Clymer, and factory manuals to guide me as well as you guys :-).  Any and all responses to the following questions are appreciated.

First off, congrats! You've done the right things by getting the manuals before you dive in.

1. When the headlight is on should the front orange lights be on but not the rear orange lights?  The blinkers work fine but the front 2 lights are always on with the headlight.  Also, the "Turn" gauge light is lit solid when the headlight is on and does flash when the turn signals are on.  Is this normal?

The front two turn signals have dual filament bulbs and the dimmer of the two are supposed to come on with the headlight and serve as running lights. The rear ones only flash. The turn indicator on the handlebars should only flash when the turn signals are switched on. If it's on when the headlight is on, something's not right....  (If everything was in order, there is a beeper that should sound when the signals are on. This was to remind you to shut them off. It's annoying and most folks disconnected them or tore them out.)

2. I found the main fuse wrapped in tinfoil and a 14a fuse in the 7a headlight fuse spot.  Amazingly everything works fine with the proper fuses in place but I noticed that the headlight works with or without a fuse in the middle holder.  Gotta think this is wrong.....

Get new (and correct) fuses before you do anything else. If memory serves, the main is 15A, headlight 7A and tail light 5A. You will likely have to look for shorter fuses as the standard automotive ones are a bit long. While you're at it, clean the fuse clips as corrosion can cause them to heat up and potentially melt the fusebox...

3. The bike starts and runs great but bogs down if the throttle is really openned up.  It eventually clears out and can get from 55-75mph but something is not right for sure.  I'm guessing carbs right?

Maybe, but how do the plugs look? Are the carbs sync'd? How does the airfilter look? Go over all the tune-up items before you start to tear into the carbs... If the carbs do indeed need tweaking, the jet needles likely need adjustment. What size jets are in there now? Is the rest of the induction system stock? (I know, lots of questions, but required info to sort things out)

4. Should I leave the 4-1 exhaust or go to a 4-4?  All the pics I have seen of this model have 4-4 pipes.  Has mine been changed?  Is there and advantage to either?  My muffler has a big dent in it so I'm not oppossed to changing the whole exhaust.

It's up to you. If you want the bike to be perfectly stock, go with the original HMC-341 4-into-4 pipes and be prepared to plunk down big bucks if you want new replacements. Some like the sound of the 4-to-1 better, and in some cases it can give slightly better performance, but as to how much improvement, I can't say....

5.  Is there any kind of conversion for changing to a wider rear tire with disc brakes?  Am I being a non-purest by asking this question?

Again, it's up to you. The rear drum on these bikes was very good and can easily lock the rear wheel. The later F-model 750's had a rear disc and you could probably lace up a Harley 16 inch wheel to accomodate a bigger tire. Another popular mod is to convert the front to a dual disc setup. Many have done this and there are outlines on how to perform the conversion. This might be an easier route to take...
« Last Edit: April 04, 2006, 05:53:35 PM by Jonesy »
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Offline TwoTired

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Re: CB 750 K5 Questions. All replies welcome
« Reply #2 on: April 04, 2006, 06:41:28 PM »
3. The bike starts and runs great but bogs down if the throttle is really openned up.  It eventually clears out and can get from 55-75mph but something is not right for sure.  I'm guessing carbs right?

Well, it IS carb related.  The K5 carbs do not have an accelerator pump and the slides are mechanically operated from the twist grip.  Suddenly opening the slides lowers the carb throat vacuum so the jets don't feed until the air velocity increases.  Later carbs have an accelerator pump to shoot raw gas into the carb throats when you twist the grip for much better response to large throttle changes.  Your carbs, however, rely on a rich slow speed mixture to overcome the "leanness" of the opening the carb throats at low engine RPMs when the carb's air velocity is low.  You will never be able to snap open the throttle from idle and expect the engine to pick up speed.  More likely it will die unless you return the throttle back nearer to idle setting.  However, you should be able to get good engine response to throttle changes of about 1/2 the total travel of the twist grip.  If not, and a complete and thorough tuneup and carb synch doesn't solve the problem, then the idle air bleed screws can be adjusted to improve the throttle response.  Note, also, that air filter type and cleanliness can effect throttle response, too.

Your problem description could also be attributed to mid throttle mixture setting issues where the slide needles dominate the mixture equation.
Since you have mechanical slides and separate distinct mixture delivery devices that operate depending on throttle position, you might consider temporarily marking your twist grip so you will know which metering device it causing issues.  Also learn to read your spark plug deposits for combustion conditions.   I've heard the description "Bogging" applied to engines running too rich as well as too lean.  The combustion deposits tell which one is existant.  A comparison chart toward plugs reading is here:  http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Assuming your carbs are jetted and adjusted as the factory provided, there is a very good chance that putting the stock 4 into 4 pipes on it will negate any necessity to fiddle with carb mixture devices.  Factory tuned carbs work best with factory intake and exhaust components.  You can make the carbs work with aftermarket components, but it is an acquired skill.  However, factory carb setting read from a book, work pretty darn well with the factory provided components.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
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Those that learn from history are doomed to repeat it by those that don't learn from history.

Cowboy48

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Re: CB 750 K5 Questions. All replies welcome
« Reply #3 on: April 07, 2006, 10:23:47 PM »
Thanks for the input guys.

Cowboy48

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Re: CB 750 K5 Questions. All replies welcome
« Reply #4 on: April 07, 2006, 10:30:00 PM »
What about changing the carbs to a newer, better responding model?

Offline mrbreeze

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Re: CB 750 K5 Questions. All replies welcome
« Reply #5 on: April 07, 2006, 11:46:36 PM »
Don't go newer!!! Newer is manure!! Your carbs will be great if only you will attend to them.Learn them.Clean them.Rebuild them.But first...do what everyone else said....make sure that your valves and ignition is good to go.You will be so damn happy when it all clicks....Enjoy!!!!!
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theunrulychef

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Re: CB 750 K5 Questions. All replies welcome
« Reply #6 on: April 08, 2006, 07:58:17 AM »
One of the first things to do when you pick up one of these old bikes is to recondition the electrical system - you'll be glad you did down the line.  Grab a few beers & spend some time to go over each & every electrical connector and clean it (emory or sandpaper) or replace it with a new connector (I used blade connectors).  Some people even solder many of the connections - I would recommend not doing that until you're pretty sure that most of your electrical woes have worked their way to the surface.  Also, make sure to put dialectic grease in each connector.

You'll probably catch a few iffy spots along the way: frayed wires, dirty or loose connections, rotten insulation.  You'll gain a much more intimate knowledge of your bike's wiring and hopefully a few precious volts in the process.  Also, when you start to troubleshoot other problems, such as ignition or the misbehaving turn signal flashers, you can be more confident that your wiring is not the culprit.

Jay in Philly