Topic: 1976 Cb 750 Performance carbs

[jimmygmartin]

Hi, I'm new to the Forum, a long time bike enthutheust and auto Hot Rodder.  I've been bikeless for about 10 yrs and just picked up a 76 Cb 750, in a barn for about 5 yrs with no carbs. I want to build it for drag racing.  I was reading a write up on head porting and it said the stock carbs are restictive and recommended 30mm Mikunis for the street and 32mm for racing.  Can someone steer me in the right direction on budget carbs, like carbs from another used bike which will allow me some good flow.
Thanks
Jim

[gerhed]

Hi, I'm new to the Forum, a long time bike enthutheust and auto Hot Rodder.  I've been bikeless for about 10 yrs and just picked up a 76 Cb 750, in a barn for about 5 yrs with no carbs. I want to build it for drag racing.  I was reading a write up on head porting and it said the stock carbs are restictive and recommended 30mm Mikunis for the street and 32mm for racing.  Can someone steer me in the right direction on budget carbs, like carbs from another used bike which will allow me some good flow.
Thanks
Jim

Jim,
I have a two carb setup (2 34mm Mikunis) that I got from the Cycle Exchange in Wisconsin.
Breaths well!
Not exactly a budget setup though.
Phil

[sparty]

Jim,

First ,welcome to the forum.  Second, a set up of racing carbs for the track or strip does not equal BUDGET in any way.  Sorry for the bad news.  My bike is set up for the drag strip and street.  I am using CR Specials.  Not for the budget minded, but perfect in every-way.

Sparty

[Geeto67]

bone stock, I think the head is more restrictive than the carbs, when you port the heads, the carbs become obsolete quick.

however, why racing class are you going to run in? If you think you are going to run heads up against KZs and GS's then be prepared to spend cubic dollars to be last in your pack. The SOHC cb is really a bracket bike at this point and really it is about consistency more than anything else. That being the case, put the carbs and the heads aside and spend all your money on setting up the chassis instead as that will give the best bang for the buck.

Now since your bike came without carbs in general, I would suggest that you go with either stock 1977-78 cb750 carbs with the accelerator pump, or the 1977-79 GS750 carbs. Since you will have to buy manifolds for your carbs anyway, dynoman sells the gs carb upgrade and the GS carbs are mukuni VMs which I think are better carbs all around. I have bought Gs carbs for as cheap as $50 off ebay needing only minor cleaning but the manifolds will run you $100.

[754]

If you say Hot Rod, I say Webers..

if you say roasdrace, I may say something else...

[City Boy]

754 nails the ultimate carb choice,also for road racing ifn you ask me.Availability may be iffy!!!

[754]

There are always used ones around, you just have to know baseline jetting..

 I was surprised when I checked new price a few months back.. around 800 w/o manifold.. of couse you have to hunt up a manifold, but they are cheaper than I thought they would be..

 I got 4 Dellorto Duals sitting here......

[jimmygmartin]

Thank You all for the help. Since I have no carbs, and since I like to do things "AL LA CART" I want somthing cheap to get started.  The Gs carb recomendation is somthing to start with, if things go to my liking I can spend the big bucks later down the road.  I was just wondering if a Kaw Or Suz or Yam model would get me close, or somthing from a DOHC crotch rocket.
Again  THANKS!
Jim

[low-side]

I found something neat the other day that surprised me that it worked so well.  A set of carbs off of an '84-'86 Honda CB700 have exactly the same spacing as the stock carbs for the 750.  It took a lot of force, but I mounted them by getting the stock boots up to temp. by riding the bike, removing the stock carbs, lubing up the CB700 carbs, and just pushing them on.  The stock pull cable hooks right up, the push cable jacket needs shortened about 1 1/4", and you need to make a bracket for your new choke cable.  It was simple, cheap, and the carbs are 33 or 34mm so they flow great. 

[bert96]

I found something neat the other day that surprised me that it worked so well.  A set of carbs off of an '84-'86 Honda CB700 have exactly the same spacing as the stock carbs for the 750.  It took a lot of force, but I mounted them by getting the stock boots up to temp. by riding the bike, removing the stock carbs, lubing up the CB700 carbs, and just pushing them on.  The stock pull cable hooks right up, the push cable jacket needs shortened about 1 1/4", and you need to make a bracket for your new choke cable.  It was simple, cheap, and the carbs are 33 or 34mm so they flow great. 

Did you see any performance improvement?

[Gittfiddler]

I know this wouldn't fall under the budget catagory but Monty the X-pipe guy also offers a carb boring service as well as adding a 5th circuit.  Anyone have this done?  scroll to the bottom of the page.

http://www.x-pipe.com/motorcycle.html

[bert96]

I found something neat the other day that surprised me that it worked so well.  A set of carbs off of an '84-'86 Honda CB700 have exactly the same spacing as the stock carbs for the 750.  It took a lot of force, but I mounted them by getting the stock boots up to temp. by riding the bike, removing the stock carbs, lubing up the CB700 carbs, and just pushing them on.  The stock pull cable hooks right up, the push cable jacket needs shortened about 1 1/4", and you need to make a bracket for your new choke cable.  It was simple, cheap, and the carbs are 33 or 34mm so they flow great. 

Did you see any performance improvement?

Anyone else have done this? i would like to have more info about this swap.

Thanks

[dusterdude]

for a bracket bike and if you want to spend the money,the twin mikuni setup would be easiest to tune.

[low-side]

Very noticeable performance improvement, but my motor is not stock.  You can see my issues in "836 will do 110 in fourth, won't break 90 in fifth" under "Tricks and Tips".  I was at a point that the stock carbs weren't enough and I needed larger venturi carbs.  If you want to read up on flow, do a google search for "Porting the Honda 750".  It discusses the limitations of the stock carbs and makes size reccomendations, as well as how to port it to maximize performance.  Good luck.

[Geeto67]

correct me if I am wrong but aren't the cb700 carbs CV carbs? if so, there really isn't a performance gain to be had with a stock motor but there is probably improved rideability in partial throttle applications.

With a modded motor the larger venturi would be helpful, but CV carbs are not true performance carbs, typically they don't have good throttle response as compared with cable actuated carbs.

[low-side]

I was actually pretty surprised at the throttle response from them, but you know everything with a carb made in the last 30 yrs has had CV carbs.  I don't know how well you'll get off the line with them, but they can be had cheap and they will mount up much easier than most larger carb sets out there.  They make the grade budget-wise because these can be had for cheap while CRs and Smoothbores are pretty pricey. 

[bert96]

 Thanks that all i wanted to hear!

 The next thing on my bike.....cv carbs!

[Geeto67]

I was actually pretty surprised at the throttle response from them, but you know everything with a carb made in the last 30 yrs has had CV carbs.  I don't know how well you'll get off the line with them, but they can be had cheap and they will mount up much easier than most larger carb sets out there.  They make the grade budget-wise because these can be had for cheap while CRs and Smoothbores are pretty pricey. 

The reason CV carbs became the manufacturing standard for street bikes is because of rideability in partial throttle applications, they just make the bikes smoother all around. CV carbs adjust to the enviornment and they are regulated to the engine's needs via vacuum. Basically they adjust to their enviornment where as cable slide carbs the owner needs to make needle  and jet changes and other manual adjustments.

In the 1980s one of the big complaints about the honda CV carbs was throttle response particularly with the cb750/900s. Basically the vacuum passages were too big and the slides were slow to respond. It was a back in the day upgrade to put 750 carbs on 900s to get the smaller passages so they would snap open when you mashed the throttle. By the 1990s the big four had them figured out.

Why they are not performance carbs is that you are always waiting for the slide to catch up to the engine's needs. You can whack that throttle open fast but the engine vacuum is what pulls the slide up and you have to wait for that to build, as compared to a cable slide where it is instanteous. You don't see a lot of dedicated drag bikes with CV carbs for this reason. If you run a properly setup set of cv carbs on the dyno against a properly set up set of CR kehins, the kehins will win out every time in overall power, but the curves will be smoother for the cv carbs throughout the rev range.

Your other carbs were probably incorrectly setup for your application, so to put these new ones on it must feel like a night and day performance gain. However because of the nature of cv carbs it is hard to tell if they are operating optimally or not. If I were you I would put the bike on a dyno and play with the carb settings (needle, jets, etc) at WOT to see if they are actually opening all the way. They are going to compensate for the engine's needs so really you are looking for how fast those slides go up and where the fuel mixture is when the slides hit the wide open position.