Topic: CB 750 for Performance Use

[TurboD]

I would like to add to cbr954's question. Also what CC's are the F1 heads??

[Old Scrambler]

Thank you CBR954. Good Info

The F0-F1 head is the same as K models. The pistons have a slight dome as used on the K7-K8 models.

The 31.8% increase to the chamber size allows for larger valves and should improve the flow across the piston during ignition.

[cbr954]

stock K heads vary a fair amount too.  If you compare earlier heads and later heads you will see a good difference in the combustion chamber shape.

[Jerry Rxman Griffin aka MuthaF'er]

Thank you CBR954. Good Info

The F0-F1 head is the same as K models. The pistons have a slight dome as used on the K7-K8 models.

The 31.8% increase to the chamber size allows for larger valves and should improve the flow across the piston during ignition.

Dennis, the F0/F1 head is the same one used on the K7/K8 not the earlier K's. They may cc out the same but I'd be surprised (wouldn't be the first time though ) if that is the case. It is much different than the F2/F3. Getting numbers on all the heads would be REAL interesting.

[Jerry Rxman Griffin aka MuthaF'er]

Does anyone know where I might find specs for the different Honda camshafts?

Open/close is available in the Honda brand shop manuals but not lift.

[MRieck]

The 1975F chamber is different from the 1976F chamber. The 76 resembles the later K chambers......the 75 chamber has more material in it...more like earlier K's. That is what I have found.

[Old Scrambler]

Thanks Jerry and Mike...............I'm doing some 'bench' building and trying to mate some 2nd-over 12.5-1 J&E pistons to an F2 head. Some smoothing of the chambers and pistons will be done, along with porting. Right now I'm considering the Cycle-X #7 cam with over-sized valves and trying to calculate how much (if any) I can skim and shave to maintain or increase compression with a 25-thou copper head gasket.  I'm trying to avoid a thicker base-gasket shim for proper piston clearance.

[TurboD]

I would also be interested to see a complete listing of the different heads.

After looking at the cam timings online in the manuals (here). I am vaguely guessing the lobe centers to be 102.5 in and 105 ex for the K model, and 105 in and ex for the F models. Does this sound right??

[TurboD]

I see That Cycle x offers a 915 kit with 10.25 compression for the K engine, If a F2 head is used with this kit how much would the compression be lowered?

[Old Scrambler]

I once had a running F2 in which the previous owner had installed the K-type 836 pistons. Compression was about 8 to 1.

Based on that experience, your 10.25 compression may drop to about 9.

[cbr954]

Dynoman offers  f2 specific 915 pistons that keep the compression.

[TurboD]

Thanks, good to know.

I am actually looking into options for lowering the compression, I have not seen any low compression forged pistons.

[BPellerine]

dynoman has je turbo pistons that are forged low compression

[TurboD]

Thanks for pointing those out, I'm still learning all the Honda perfomance suppliers.

I have read that the 83-85 CB650 carbs will fit onto a 750, does anyone know this to be true? Are they a direct fit?

[BPellerine]

I have seen them on one bike on these fourms but the owner could not give me info as he had just bought the bike and the only thing he said was that he had to enrich them for startup and increase idle so probably some linkage work needed off a nighthawk I think.bill

[TurboD]

Thanks. I found a set some what cheap that are due to arrive this week. I'm just trying to find out ahead of time if and what else might be needed to make them fit.

[BPellerine]

you could try a search I cannot remember the thread I saw them on.bill

[TurboD]

I searched, just never came up with a solid answer or the correct search.

[dragracer]

I searched, just never came up with a solid answer or the correct search.

Send a private message to Bill Benton or Jon Weeks on the type carbs they used on the SamAuto bike. Bill can likely give you details on the jetting as well.

[jweeks]

CV carbs are what I'm relatively versed in. Nighthawk 750 Canadian CV carbs (34 mm) or Yamaha Seca CV carbs (32?mm) bolt on. Both are a little lean  as bolted on. If you can find a stage 3 jet kit for either one, it will really help throttle response. Idle circuits are especially off. With the right main jet and the needles on the right clip notch, they are seamless carbs. I suspect that they'd be fine for mild street clutch CB750 based motors. Not quite the high airflow that mechanical carbs offer. They're just inexpensive and easy to bolt up. I've run 34mm CV carbs in Hondamatic motors up to 1180cc. They limit top end horsepower with lack of big airflow, but they run great in the midrange that most street motors live in. Not for clutch bike racing or very high rpm street use. Other carbs will dust them on top end. Don't know if this will help, but I'm trying to answer Frank's response.

                                                             JW

[TurboD]

Thanks for the help, I will look into those. What is the difference with the Canadian carbs? Thanks for the jetting tips, if I have time to work this project in, It will be turbo.

Over the years I have converted a number of KZ 900/1000 over to the GSXR 36MM CV carbs, they just flat perform better all around. Idle, response, top end.

[jweeks]

     The US Nighthawks were 700 cc due to tariffs in the 80's. That resulted in a different carb setup than everywhere else in the world where they were making 750 Nighthawks (and whatever they were called outside North America). I don't know why the carb spacing was different, but the carbs that everyone got from the 750 Nighthawks fit stock CB750 intake manifolds.(with a little persuasion) That's why I reference the Canadian Nighthawk carbs since they are the closest available to US buyers. I bought a set off Ebay.ca from a Canadian bike yard for about $125 shipped.
     As Pops has reported on the Hondamatic forum, the stock Seca carbs that I recommended to him work fine for his 'matic with K manifolds. Since the 'matic head carb spacing is the same as the K head that the clutch bikes use, they just bolt up with a little lube and elbow grease.
     Idle circuits tend to be a little rich on the above carbs. Main jets tend to be a couple sizes too small. A lot of the setup depends on the altitude that you live at. I'm east coast with density altitudes around 2,000 feet or less. If you live near Denver, the thinner air requires different settings. (DA's between 5,500 and 9,000')
     The GSXR 36's have the wider/wrong spacing and they won't fit stock K intake manifolds. When you grind away manifold inside diameters to allow fitting bigger carbs, the strength that remains is reduced. That's where creativity comes into play... Adapters, bigger tubes, different manufacturer's intake manifolds, silicone tubing, etc.
     Turbos are a different world. Dry or wet? I've never seen a pressurized box around carbs on a CB750. Everybody is using really old technology and crude manifolds to have a turbo suck through small old carbs. More horsepower gets a hotter motor which limits boost. Nobody has figured out intercoolers so the lower boost results aren't that spectacular. I'd really like to see a Nitrous vs turbo dyno graph comparison for a CB750. I suspect that they could be similar with the inherent motor limitations. Neither gets used too often on the street. Appearance is better with the turbo, but the performance might not be. The cost of the nitrous might be less for hardware but with refill costs it will end up more expensive over the bike's lifetime.

      Just my opinions.

                                                             JW

[EnginebyAdam]

Thanks for the help, I will look into those. What is the difference with the Canadian carbs? Thanks for the jetting tips, if I have time to work this project in, It will be turbo.

Over the years I have converted a number of KZ 900/1000 over to the GSXR 36MM CV carbs, they just flat perform better all around. Idle, response, top end.

I have a set of BST36SS's sitting on my shelf. Have you had to re-space these carbs at all? I have thought about using them on this build, but spacing wise they go on the dohc Honda's easier.

[TurboD]

I remember the tariff costs back in the 80's. This explains when I searched Feebay for Nighthawk 750 carbs, they were most all in Canada. Lol. It sounds like the NH 750 carbs would be a better choice then looking for CB650's.

I am aware the GSXR carbs are spaced different, I was just saying that I have "updated" a few Kawasaki's with them and had good results.

If and when I follow through with this project, it will be "Blow through" (turbo blowing through the carbs). A BT system is more efficient, throttle responce is much better and ultimitely can produce a lot more power.

I have work with both Nitrous and turbos in bikes as well as cars, and overall turbos are easier on the engine. Nitrous tends to shock the engine with the instant burst of power, where the turbo comes on a little smoother.

Here are a couple of my current turbo bikes. The green one I built back in the early 90's and it made 210 RWHP in 92, and the white one is a current build, I am shooting for the 400 mark.

[TurboD]

Thanks for the help, I will look into those. What is the difference with the Canadian carbs? Thanks for the jetting tips, if I have time to work this project in, It will be turbo.

Over the years I have converted a number of KZ 900/1000 over to the GSXR 36MM CV carbs, they just flat perform better all around. Idle, response, top end.

I have a set of BST36SS's sitting on my shelf. Have you had to re-space these carbs at all? I have thought about using them on this build, but spacing wise they go on the dohc Honda's easier.

Just to be clear. If you are referring to the GSXR 750/1100 36mm carbs, I don't believe that they will fit a SOHC Honda very easy or they would be my first choice. They do fit right onto a Kawasaki KZ 900/1000/1100 when using the later 81 and newer "J" model intake boots.