Exhaust choice

[gazzz]

As for colour: header made of stainless steel and finish is in fact "semi polished", thus after thousands of kilometres including rains and so on it will have coppery look. In fact I liked that look so I was never thinking about re-polishing. That's how Black Widow header looked just before I replaced it with 4in4 exhaust.

[andye]

I've also got a Black Widow on my D1. Not noticed any loss of power, and can still get the oil filter out.
Straight swap for OEM.

[David Richard]

hi gazza yes I can see you got the best out of it ,I seem to remember in the add it said no jetting required ,there are not much choice to have a 4 into 2 to keep it standard looking in pattern pipes,im sorted for the time being ,david

[David Richard]

Hi Andye Not impossible but a bit fiddly with the bolt washer and spring would have been earlier if it was a bit further back. Reference Gazza's photo, looks very nice and suits your bike. did you do all the work yourself? david

[David Richard]

hey sorry spelling gaf ,I meant easier

[Freddy]

I found that issue with balancing pipe is acceptable price for engine free breathing. However, rejeting is definitely needed to make it play in tune.

Here is picture from Dyno: black is for Black Widow header and red is for OEM pipes.

Question: As the black and red lines virtually overlay each other till the very upper rev range why is 'rejetting definately needed'?

Also, any factory specification I've every seen for a 750 has max hp (either engine or rear wheel) around the 9400 rpm mark. Why is it falling away at less than 8000 rpm. The red line on the tacho is at 10,000 (I believe) which also aligns with the generally accepted practice of setting redline a few hundred revs above peak hp.

[gazzz]

Question: As the black and red lines virtually overlay each other till the very upper rev range why is 'rejetting definately needed'?

Also, any factory specification I've every seen for a 750 has max hp (either engine or rear wheel) around the 9400 rpm mark. Why is it falling away at less than 8000 rpm. The red line on the tacho is at 10,000 (I believe) which also aligns with the generally accepted practice of setting redline a few hundred revs above peak hp.

I bought my 1991 Zephyr with 11000 km on clocks. On 22000 due to shitty fuel I've got an issue with hole burnt through the piston in first cylinder. Since then I rode Wiseco 810ccm piston kit. I rejeted carburettors several times and time to time run bike on dyno with wideband oxygen probe to check fuel mix.

In fact on that picture both graphs were made with same set of pilot/main jets but with different headers. And in both cases mufflers were stock but from D-model. The picture just show what advantages give pipes with larger inner diameter on higher rpm. The original mixture of carburettors usually is closer to lean so header with more free flowing pipes needs at least larger main jets.

Yes, specs says that C1 has 72,0 hp (53,0 kW) / 9500 rpm and 54,0 Nm (5,5 kgf*m) / 7500 rpm

As you may see, max. torque in case with Black Widow exhaust (black lines) is in its natural place, and max hp is on 8800. I believe this shifting of max hp caused by big bore forged piston kit.

OEM header (which was pipe-in-pipe) definitely wasn't sufficient for engine with 810 kit, so engine was like a man with finger in one nostril: no problem walking, but quite hard to run fast. I am curious, may it be that header for internal Japanese market might have smaller diameter of header inner pipes than those of models of other markets?

[gazzz]

Gazza's photo, looks very nice and suits your bike. did you do all the work yourself? david

Yep, this was pilot bike of my project Zephyr 750 cafe-racer kit. The idea was to make conversion kit that needs no frame modification.