Topic: cb750 upgrades

[jscro73]

i've got a 73 cb750 that i'm looking to upgrade. I was thinking about workin the heads, beefin up the cam, and upgrading the carbs. i'm just looking to make a pretty solid vintage street bike, that will do good on long trips.
does anyone have any suggestions?

thanks,
Jason

[Duke McDukiedook]

How much money are you looking to spend?
For what you are proposing, to do it properly would cost you a few grand.
Once you start working on one thing, you'll want to do something else, etc.
Once you go doing that you might as well change all the oil seals and gaskets, then you would want to change the cam (& roller assembly) and primary chains, see how the costs add up?

What is the mileage currently? Is there anything wrong with your bike at this point?

I would recommend leaving well enough alone if the bike runs OK as far as the engine is concerned.

Spend your money on upgrading your steering head bearings to tapered and rebuilding your swingarm with bronze bushings. New drive chain and sprockets, new shocks and springs for the front would also help.

Or just buy a newer VFR or Goldwing if you want a dependable touring bike for that amount of money.

[Gordon]

I'm not saying you shouldn't do whatever you want to your bike, but if what you're looking for is a solid, vintage street bike that will do well on long trips, then you've already got one. 

[GammaFlat]

I'd upgrade the ignition.  I may get flamed for this but there are some good options.  You may want to put on a fairing as well if you're going on long trips. 

[jscro73]

i know that what i have is already great, i just want to make it faster. I bought the bike when the odometer says it has 64k on the clock. I took it to a guy and let him look it over as a second opinion and he told me that it felt way too strong to have 64k miles on the motor, so i'm guessing that someone rebuilt it. Going through the suspension is a good idea, i hadnt really thought that much about that.
What i'm really trying to do is do a top end build that will give the bike more bottom end with out sacrificing alot of top end. I ride mainly around the local area, but occasionally do take overnight trips and long rides. I guess what i'm trying to say is i want to make it a bit of a hot rod, but i dont want to bore the cylinders yet, i would just like to stay in the head/cam area. I'm not real familiar with these SOHC bikes, so i'm just looking for suggestions.
ALso,
whenever i nail the throttle open, my bike bogs a bit and then gets into it. Is this normal? i've heard that this was fairly normal for these bikes without accelerator pumps. is there a fix?


thanks,
Jason

[andy750]

Welcome to the forum Jason and you`ll find all your answers and more here. Plenty of us have upgraded or rebuilt these motors but as others have said its going to cost you. Unless you want to spend 1K+ on upgrading the top end (just dropping a cam in wont do much on its own so you would then consider porting, new valve springs, retainers, etc etc...soon adds up) Id consider this first:

1. DynaS ignition or one of Hondamans ignitions.
2. Steering head bearings - tapered.
3. Refurbish the swingarm
4. Upgrade the suspension (Progressive springs in front, progressive/hagons/YSS/works shocks (depending on budget) in the rear.
5. New spark plugs
6. Clean your carbs
7. Good tires.

Do all that first and see the difference it makes. In stock form these bikes can tour all day.

If you want more do a search and find out options/prices....

good luck
Andy

[jscro73]

andy,
thanks for the welcome and the information. i appreciate you laying it out the way you did. i was looking for some pointed guidance, i guess.
i'll keep you posted as i work through that list.

jason

[andy750]

Jason,

Here is something else to consider..first part is just upgrading the top end...second part is adding an 836 kit (in my case I already have an 812cc kit with cylinders to match - I got this list from trawling the forum and making it up to suit my needs:

cam
Valve springs
Stock retainers
stock valves
stock guides
rocker arms?
heavy duty studs
base gasket
head gasket
engine gasket
rings
valve seals
cam tower pucks
adjustable cam sprocket

This alone will cost around $650 (without labour).

Add 836cc pistons with bored cylinders and then your talking more $$s. Im going for the increased low end  -midrange power. This is on my K2.

On my K4 I kept it stock and just refreshed the top end with the first list. Makes a difference for sure. My K4 is at 70,000 miles and has been used for touring (1-up and 2-up).

good luck
Andy

[stickman]

First my observation after reading your post, sounds to me like you think a 750 bogging down on take off is normal.  Its not.  And since you are not that familiar with the 750, my suggestion is solve your bogging down issue before proceeding to spend the big bucks.  You Just might not need to spend those big bucks to beef it up some.  My suggestions are:

First and foremost, find you a local old skool mechanic(one who has some grey in his temple and has been a Honda mechanic for 30 yrs or so).  Get him to ride your bike and give you his thoughts on the bogging.  Depending on where you live, you could ask a forum member who is close to you for an opinion.  You could be looking at:

a tune up, clutch adjustment or carb cleaning and/or rejetting to solve your problem.   The Dyna ignition or equivalent is highly recommended.  Take a look at both the front and rear sprockets.  Count the teeth to make sure you are stock, should be 18 on the front and 48 on the rear.
If still stock gearing you may want to consider swapping out the front for a 17 tooth gear to give you that extra umpff you are looking for and when ready to tour, stick the 18 tooth gear back on. 

Some advice I learned long ago about approaching a mechanic:  Describe to him what the bike is doing not what he needs to do to fix it. Describe the symptoms only.  Let him determine the cure.  Frequently its quicker and cheaper.  Get on the same page with the mechanic and he can save you lots of $.

I echo advice given earlier, if you think you want a quicker, more powerful ride, you are almost always better off to go to a different bike to get it rather than trying to upgrade. 

You have a great bike that may just need tweeking.  Best of luck.  Ed

[Old Scrambler]

Ed just gave you my conservative opinion in spades!  My K3 is now returned to stock,but years ago when I wanted more grunt I changed the counter shaft sprocket, and rejetted with a 4-1 header and a K&N filter in the stock box. No bog here! Get that fixed. I still run standard ignition but considering the upgrade.

[Dave K]

Of course start off with what Ed and others have said. But, depending on the RPM you are attempting to give full throttle, it just may bog. A through though tune up is needed first. I have had my K3 since new and at real low RPM's and a handful of throttle will make it bog. I think though after 50,000 miles I don't bog it, since I don't flog it until I get above 2000 RPM. These bikes are so easy to tune with a dwell meter, timing light, feeler guages and carb stixs.

[jscro73]

i'm pretty sure it's tuned. i may not have been too clear about when the bogging begins. Usually when i'm cruising and then punch it is when the bog happens, not from idle.

i appreciate all those mod suggestions. those will come in handy.

i had a guy whose been working on hondas for a long time go over it, and he said it was really strong for the amount of miles that are showing on the odometer. i'm currently painting the tank right now, so when i get that finished, i'll be able to ride more and pay more attention to the motor.

thanks for the info about the sprockets. i'll take a look into that when i get home.

jason

[MCRider]

Of course start off with what Ed and others have said. But, depending on the RPM you are attempting to give full throttle, it just may bog. A through though tune up is needed first. I have had my K3 since new and at real low RPM's and a handful of throttle will make it bog. I think though after 50,000 miles I don't bog it, since I don't flog it until I get above 2000 RPM. These bikes are so easy to tune with a dwell meter, timing light, feeler guages and carb stixs.

Agreed.

"whenever I nail the throttle"?? needs to be discussed. You can't be rolling along at 2500 RPM in 3rd gear and nail the throttle and expect anything. You need to be approaching the torque peak in the right gear before nailing it if you expect it to perform.

Otherwise its like shooting a gun. You've got to squeeze it. Sometimes more than others. But nailing it is reserved for those special times.

[MCRider]

And generally the search for more bottom end comes from more displacement. Hard to get more bottom power with the same size. Possibly raising compression, but that means new pistons and might as well go bigger.

More likely you can make a small motor hyper. A big motor can be downtuned. All things being relative.

I'll add a stocker will respond well to cylinder head work. A little pricey but you can keep it stock that way.

PS: I reread your post and see you mention heads work. (only one head BTW)  I stick with a stock cam. Some 29mmMikunis carbs would be good on a stocker. That could be sweet.

[jscro73]

with the dyna S ignition, will i need to replace the coils and spark plug wires too? or will i just bolt on and go?

jason

[andy750]

with the dyna S ignition, will i need to replace the coils and spark plug wires too? or will i just bolt on and go?

jason

Bolt on and go works.

[jscro73]

i'm working on that dyna s, but until then, i checked my sprockets... 18 front, 45 back. i replaced my chain. the other one was pretty rusty. i had a good one laying around, so i'm cleaning the old one for a spare. I'm also in the process of painting my tank, so i'm waiting to get that done before i do more tuning. I cant wait to get this thing rolling! I think i'm going to focus on getting that new ignition, then doing the head bearings, swing arm, brakes (rear) and fixing little things like my seat cover, rear brake light switch, blinker relay. I'm looking forward to having a solid bike this year.

thanks for your help.

[ekpent]

Stock back sprocket should be 48 tooth  and 18 front. Some people drop to a 17 in front for a little quicker out of the hole.

[MCRider]

Stock back sprocket should be 48 tooth  and 18 front. Some people drop to a 17 in front for a little quicker out of the hole.

True seems it should be 48 on a 73. 45 was stock on CB750/CB750K0. Someone made the conscious effort to retrofit.

Shameless spam alert. I have a good used 48 that's been cryogenically treated for sale if OP is interested.

[scottly]

If you go to a 17 with the 45, it will be close to a 18 with a 48. Counter-shaft sprockets are cheaper, and easier to change.