bhp

[ukpco]

Hi all, just got back from a dyno run on my 700S mk2. 32bhp at the back wheel - and that's after setting up the carb and ignition.

Anyone else had their bikes dynoed? Is 32 bhp typical?

[Smokey_Paul]

are you on a 36 or 38mm carb and what tuning is your barrel?
Engine no. will give me a clue.

[ukpco]

Hi Paul

engine number is 080. It has a 34mm carb with standard 320 jet. It's slightly rich at full throttle and slightly weak in the mid range.

Paul

[Smokey_Paul]

I am surprised that the dyno people did not tune out the carb weak /  rich running as that should be pretty easy. 47 Bhp was the factory figure when new so 32 Bhp at the back wheel is not far off the 20% transmission loss gestimate!  I would try a 36 mm carb with my jetting on the carb page on the website and see how that compares.
Also what filter do you have I found Ram Air foam filters gave much better results than the cone type.
I was thinking of Port tuning as some later bikes had the Peter Green porting, you will have standard port timing unless tuned but by the sounds of it not.

[hendred]

Assuming the engines really did develop 48 bhp then I believe the rear wheel bhp should be closer to 40 bhp.  Phil Irvines books Motorcycle Engineering and Tuning for Speed have some interesting data on transmission efficiency and speed/bhp calculations. Although quite old books they are of the era of the Silk technology.  According to the books, 32 bhp at the rear wheel should give a bike capable of about 100mph.  Clearly you are not seeing this.  This suggests to me that the engine is down on power at lower revs so won't pull through to give higher speeds (did the dyno people produce an engine/speed power curve?).   If this is the case, will the bike go faster in third? The gearbox is quite close ratio and the data in the handbook shows a max speed in third of about 90mph depending on gearbox sprocket at 6000rpm.
 Two strokes are sensitive to exhaust tuning - have you checked the silencer for blockages and is the wadding in the tail of the silencer in good condition?  Replacing the wadding made a significant difference to the performance of my Silk.
One final thought - are the rear chain gaiters new?  If they are and the chain was tight they will absorb some power until they "run in".  

Hope this helps