f2cafe racer

[MarkCR750]

Tacka tacka sound is tappets, clattery rustling sound is cam chain, carbs out of ballance make it worse, not forgetting there is clutch rattle to add to the mix, they are never going to be as mechanically quiet as modern engines, so bear in mind that if you are used to riding a more modern machine the honda might sound a bit agricultural, every time I go out on mine I become convinced that it's about to explode half way around the ride, then I get back to my driveway and it's humming away merrily on tickover, so I guess all is well, I've heard quite a few CB750's that are mechanically noisy but also one that a quiet as a mouse, Bytio's Phil read replica, he had completely rebuilt the engine and obviously made a very good job of it (or were his tappets clearances too small! 😧) my advice is check the fuelling is ok (plug colour) if it is just ride and enjoy it , earplugs help 😄

PS I've heard people say that oil splashes about if you remove a tappet cap with the engine running, mine never has, the components are all wet with oil but it's not like the North Sea in there!, anyway I've done over 2k miles and the top end seems fine).

[yozzer74]

Cheers I'm probably just being paranoid .has its first engine I've built like you say oil isn't splashing around but they are covered in oil

[Pops400]

Wow, this looks amazing. Another one i've missed keeping up on.

A real credit to you, great work. Really looking forward to seeing it in the flesh.

And I agree, 74 was a proper good year. 

[theshootist]

Truly inspiring stuff here!

I love the tank especially. Can I ask where you had this made?

[yozzer74]

One off welding& engineering

[yozzer74]

Got a few miles in on bike today she is still smoking .but does ride well .I've had plugs out and there black .& the bike starts up without the choke .does this mean it's running rich .what do I need to do to sort it out .the smoke is sometimes black too  thanks in advance for any help

[hairygit]

Try dropping your needles 1 notch and see how it is, also before doing that, triple check your float levels.

[yozzer74]

Can you do that on f2 with the needles

[MICK CLARKE]

No you cant drop the needles into existing differing positions on an f2 cause they havent got any, but you can shim the needle,I think theres a kit available.
Cheers
Mike

[K2-K6]

Some further thoughts on the Jetting.

Assuming you've got the accelerator pump working ok? Do you know which size main jet they are running? And which air filters have you used?  Also are you using the original bellmouths inside the filters?

[yozzer74]

I put 140 mains in and I'm running velocity stacks pump is working  cheers

[K2-K6]

Here's what I believe is happening, if I post it we should get some input as to how you can optimise it.

When you start with the original intake system it has a manufacturer set draw / restriction on which the carbs operate. If you open the throttle slides fully, then with the engine flat out, the draw through the main jets is controlled by that fixed process,  so the original jet size is set to supply demand under those conditions. They then use the shape of the throttle slide in combination with the needle taper to get a linear delivery of fuelling as you move away from wide open throttle all the way down until you get to the idle circuit.
This relationship is vital to jetting and throttle response being accurate throughout the engine's range. It's important to note that as the throttle slides get more closed, then the vacuum characteristics are progressively handed over from the airbox intake design to the throttle slide design as you move away from absolute maximum flow. It's this area I believe that's most changed by removing the original intake.

When you remove the original airbox, you remove a restriction which causes the carb when wide open to exercise less vacuum on the mainjet resulting in less fuel (running lean)being pulled from it. It's this effect that you're responding to when upping the main jet size just to get back to the same mixture for the engine, you are not running the engine at a different mixture from standard at wide open throttle. It's this point that people commonly don't consider.

So you've got the maximum throttle mixture something like correct by using bigger main jets. Now this is where it gets more interesting. Dropping away from wide open, you get much more into how the vacuum through the carb was controlled just by the throttle slide shape, which hasn't really changed. So you've now got very similar vacuum throughout made range but pulling fuel through a too large main jet. Unsurprisingly it runs rich.

If the original development engineers where doing this for that maximum flow they'd have had a number of different throttle slide shapes (different cut aways cause different vacuum gradient) along with a variety of differing needle shape to bring the Jetting and response into acceptable range. Without those options I doubt anyone would get close to optimum jetting over the whole range.

I'd start by just considering dropping main jet size and being careful to see how close you can get the wide open setting to optimum without running too lean at that point. This will get you closer too being right in made range.

You've got an additionally problem with accelerator pumps as well, as they're adding just where you've got it richest (they do nothing when the throttle and engine are at a constant flat out state). You may be needing snookers to truly get it right. It's also very difficult without previous experience or something like a dyno and exhaust gas analysis.

I'd start by trying something like a 120 main jet to give you a little better look at what it's doing.

[yozzer74]

Wow thanks for all that information k2. I will give it a try .

[onethumb]

Watching this with baited breath. When I rebuilt my carbs I used 120 jets because of the pods.
I saw somewhere about restricting the flow using an old tshirt.


Sent from my iPhone using Tapatalk

[NigelWilmshurst]

Hi Yozzer
I have not been on here for many months, just watched your you tube stuff on the F2 it looks and sounds the bowl lox! Hope you get the carbs sorted out soon...