CB900C 5000 RPM Bog issue fixed

[StagByTriumph]

Hi All,
I had bought this 900 and it had the bog issue from the original day I bought it.  

So I recently did a complete rebuild on my KEI Hin VB43's from my 1982 CB900C with 12K mikes.  I followed the MacGregor Instructions which were great.

Like many others, I put the carbs back on the bike, it started right up, but after warm up would not rev cleanly past 5000 rpm.
But if I pulled the choke, I could get 7-8 K, so I know it was not a fuel availability to the carbs.  I did all the checks for vacuum leaks - none anywhere.

One thing I noticed when I asembled the carbs was the Damper Cylinder springs (a.k.a. compression spring for the vacuum piston) seemed a bit too short to put pressure to keep the piston down.

I am not new to CV carbs, I have rebuilt many SU's and ZS carbs on Triumph and Brithish Cars, plus the ones on my 1981 CB900c I owned from new some 30 years ago.

Anyway, CV carbs need the damper piston to sit on the jet to create a venruri to pull fuel out of the jet.  
If the damper rides too high, the venturi effect - Constant Velocity function does not work.

Most people think the piston has to open wide like the throttle plate, that thinking is wrong.

So I also went on You Tube and watched "dawino6260" rebuild his carbs, noticed his Damper springs were way longer than mine.

I pulled the airbox and watched the dampers on throttle and then were raising too much and fuel flow was not being sucked out of the jet.

So Tops off, grabed the springs and doubled their length.  Popped them back together and voila, 7-8 grand revs!

Tops off again, made the springs about 9" long each, popped them back together and bingo, 10 grand revs up through 3rd until I needed to shut down cause I was running out of road quick.

Now on my Triumps cars, there is a spring pressure and free length specification for the damper valve springs, but I cannot find a spec for the KEI HIN VB43's compression springs.  Anyone got one?

Anyway, a little more tweaks are in order but the bike is now running the proverbial  raped ape.

Cheers!