A sync is always a good idea , maybe do the sync first rather than take the carbs off again, can always sync again afterwards , the sync is the easiest option first

 

If I can add a little something to the mix, if you are thinking your carbs might be the issue, I would focus on throttle position, not RPM. If you know at where the throttle is when the flat spot occurs, you can better focus on what part of the carb you should be focusing.
With some tape, while the throttle is completely closed, put a piece somewhere towards the top of the grip. Place another piece directly across from it, where it will remain stationary. Now twist the throttle wide open. Place another piece of tape across from where the grip piece is. This will give you a visual reference of where wide open and closed is. Now, just divide the space in half, and you have your mid-position. Do that again for your 1/4th and 3/4th.
Now, see where your flat spot occurs. Use the chart to see where in the carb you may be having issues.

 

thats a handy chart!

I can tell you straight away that the problem is between 1/2 and full throttle.

Ill remove again, and double / triple check every setting, as theres not much sense in adjusting other stuff, if the settings arent all exactly as they should be.

I am hopeful that once everything is returned to normal settings, the problem will go away.

fingers crossed

 

I imagine that you will see some nice improvements when you sync your carbs. That can only help.
Another thing to check is your valve clearances. Having these in spec is the 1st step in properly tuned carbs.

 

OK, so the manual recommends the K model has a 105 main jet, wht is fitted currently, is a 138!!

I am assuming that has been done to get more fuel in for racing.
The pilot jets in the manual suggest 2 1/2 turns out, but they were all different, some were 2 turns, some were 3.

So, I am assuming (hoping someone can correct me if my thinking is wrong here) that the 138 jets are providing too much fuel, and too much fuel is causing the issues I have - i.e. running way too rich.

Am I correct in assuming I could test this theory by removing the air filter / top half of the airbox and zooming up the road to see if its better with a lot more air coming in?

I would assume that I would also need to wind the pilot screws out further as well, once its back together, as I assume more turns out on the pilot screw = more air / leans out the mixture?

I will still do a balance etc as I had planned, but this seems like the issue - large main jets, plus the pilot screws being mostly too far in, means running too rich, and I get a stumble when I get into the upper half of the rev range

 

Sounds like you are on to something with the jets. Good job finding that.
I am guessing you will need to adjust the pilot screws too.

 

ok, so after some more mucking around today, I set the pilot screws to factory settings, removed the air filter, and took it for a quick hoon.

success!! started easily, idled well, took off like a rocket, and wound itself all the way to redline without any issues.

Decided to balance the carbs - I was told this had just been done very recently - BULL****! probably the worst Ive seen! and we couldnt get them to balance at all.

We checked the diaphragms at the top f the carbs - theyre all fine, so tomorrow we will have a go at checking the valve clearances. Im assuming they are out of spec, since they carbs just wouldnt balance, so we will check those tomorrow, and go from there.

 

Outstanding. You are making progress. Do yourself a favor and do a bench sync after you check the valve clearances. If you need help in doing that, just give a shout.

 

Hi Tim just caught up, I think we are 12 hrs behind you here in GB , but looks like your getting it sussed

 

yes we have already bench sync'd, as the engine was warm after the failed attempt at a vacuum sync, and it has to be left overnight if we are gunna check the clearances (engine must be totally cold) and we wanted something to do.

Its in the ballpark now.

Thanks, in part to some of teh help I have had on here, it does feel like Im getting closer to something i can race with.