Topic: just bought a '73 cb750 K3, and...idle and choke and owners manual

[Hondawggie]

I found the cb650 and cb550 Owners Manual links on the 'Download Manuals' post at the top of this tech forum, but not the cb750 owners manual.

Okay, I am brand new to my 1973 cb750 k3, I have owned it 2 days only.

THE CHOKE:  my observation is, if the choke arm level is pushed DOWN all the way, so that the arm is vertical (straight up and down), the choke is off.

To choke the bike by turning the choke on, the lever has to be pulled UP.  (It is so warm and sunny these days the bike starts right up, no choke needed.)

Is this correct? I need to know this before it starts getting cool outside.
(If I only had the cb750 Owners manual... )

THE IDLE:  where is the idle adjust knob?  This carb bank has a 'pulley' setup with push/pull throttle cables that actuate the pulley.  One set of throttle cables, all carbs are affected.  So I'm expecting to find one idle adjust screw.  Maybe it broke off?

Where is the idle adjust screw on my all-stock '73 cb750 K3?

[Logan80]

You are correct about the choke.

The idle screw/knob is located just above the choke lever and about 3 inches in. It's like a set screw that raises the entire throttle assembly.

[Hondawggie]

Thanks man, I found the idle screw.  


This '73 cb750 has about 50,000 miles.  This is my 2nd day of owning it, naturally with that mileage I want to know something about the state of the engine.  

So I warmed it up, drove it around for 10 minutes, then did a compression check.  I'm really surprised about what I found:

#1: 162psi
#2: 160psi
#3:  161psi
#4:  164psi

So I looked for signs that the motor had been apart.  I mean, *surely* it's been rebuilt, having such good compression at 50,000 miles.

NOPE.  There are screws holding the valve cover on.  I compared those screws to the screws holding on the clutch cover.  The clutch cover has been off at some point, I can see the wear from a screwdriver in the heads of the clutch cover screws.  

Neither the valve cover screws or any other fasteners on this engine show signs of a wrench or screwdriver being taken to them.

The prior owner had a lot of relatively steep hills in his daily riding for the years he owned it, so to me that explains the clutch cover screws having signs they have been removed.

Is it just pretty freaky that this engine has that high of compression?  I know the guage is good, it's an expensive one and I use it on other bikes I own and compare it with my cheaper compression gauge, and assess the strength of the motor and it's good.

What's up with the crazy-good compression at 50,000 miles?   Just good preventive maintenance or something?

[Logan80]

These are 100k+ mile engines. You're only half way there at most...

[Logan80]

Mine has about 45,000 miles, sat for 15 years and after a carb rebuild and oil/filter change it fired right up. Actually it ran before the carb rebuild, just poorly.

[Old Scrambler]

My K3 had about 65,000 on it and was weeping a lot of oil at high speeds on the highway. The compression was still above 160 but I rebuilt it to replace all of the oil seals and gaskets. Take a look at the front of the head. The early K3 models have a noticable curve on the fins above #2 & #3 that was changed to more of an angular line on later models. Most K3s came from the factory with 105 mians. They work fine at altitude but 115 or 120 mains will give more power and top end...........even with stock pipes.

[Rio_CB750]

Shop Manual CB 750 http://www.honda4fun.com/man_officina/man_officina.html

Shop Manual CB 750 http://www.mediafire.com/?wqzmehqvkzl/

Owner's Manual https://docs.google.com/fileview?id=0B0XYNbk73smCOTZkMjhhNzQtN2UzYy00NDg5LWJmOWMtNzA4Y2Y0NDdjY2M1&hl=en

I have a K2, Briar Brown, got it as a basket case, motor locked (not seized, just rusted), every piece of rubber was rotten, I think the only thing that saved it was the fact that it was on it's center stand.  Went through the carbs three times, twice off the bike, once from the bottom of the bowls, which you can reach if you take your time.

Good luck brother!

[Hondawggie]

Shop Manual CB 750 http://www.honda4fun.com/man_officina/man_officina.html

Shop Manual CB 750 http://www.mediafire.com/?wqzmehqvkzl/

Owner's Manual https://docs.google.com/fileview?id=0B0XYNbk73smCOTZkMjhhNzQtN2UzYy00NDg5LWJmOWMtNzA4Y2Y0NDdjY2M1&hl=en

I have a K2, Briar Brown, got it as a basket case, motor locked (not seized, just rusted), every piece of rubber was rotten, I think the only thing that saved it was the fact that it was on it's center stand.  Went through the carbs three times, twice off the bike, once from the bottom of the bowls, which you can reach if you take your time.

Good luck brother!

Thanks all, and thanks fp for the manuals!  From the owners manual I see that my bike did not come with a 4-1 exhaust apparently.  It has a rumbling-sounding 4-into-1 exhaust on it right now.  But the owners manual, which has '1973' on the front cover, has photos inside of 4 mufflers for this bike.

If I want to find a used exhaust, the stock one, what should I pay for a good (not perfect, but good/presentable) set of pipes? 

Also -- do I have to find an exhaust only for the 1973 model year, or which other model year stock pipes will be okay for my '73 engine? 

[Johnie]

Your bike came with the HM341 exhaust. You can still get them from Honda. If you want a louder bike you could go with the HM300's which were on the 1969-1971 CB750's. The 300's you will find cost more. I paid $215 per pipe from my dealer,  but he gives me a discount as I buy a lot from him. Check out the for sale section of this board. Someone just posted selling some 341's here. Below is a pic of the 341's on my K4. You can click the pic to enlarge it.

[CBNJ74]

Searching high and low here so please excuse the question. Does anyone know what the OEM fork tube length is a for a 73 CB750? Just picked up and installed a standard set of CycleX tubes (23 13/16) and I'm hearing that they're 1" too long? please say it aint so.