Hi all,

This is not gonna be the usual thread on this, i have read them all and i understand about the air pressures and the drilling hole mod etc.. But some plum of a previous owner dint stop at 24 1/2 inch holes, oh no.. they did 40! The bike runs surprisingly well considering how much its been messed with.
it also has a straight through racing exhaust of unknown brand.

Problem: half way up the rev range it hits a massive lean flatspot.. i can roll the power on through it and then it pulls again but im aware this is not healthy for the motor and so i dont power through it.. Below half throttle/revs its smooth as butter and idles like a champ, starts as expected with choke etc..

What ive done: cleaned carbs totally, lifted floats to be a little richer, this helped a little. Blocked the air holes in the frame and removed the little snorkels.

What i plan to do: I plan on closing all but 20 of the holes, closing also the snorkel holes on the airbox. This will mean only 20 1/2 holes are feeding the motor and Eventually get bigger jets.

Question time! : My other interim solution was to cut a piece of card/metal the same shape as the air filter and cut a hole out the middle about 50% of the area as to limit incoming air and create more of a vacuum in the filtered side of the airbox, as to suck more fuel out the jets with less air (richer).. Would this work?

 

It might work but it assumes you have enough fuel in the first place. A straight through pipe has zero back pressure. That would be what I would concentrate on. Tuning for a racing pipe is very tricky. If the main jet size is close then you get into needle profile (shape). There are a multitude of needle profiles that will allow more fuel to flow at certain positions (like half throttle). One of the biggest problems with your strategy is you really won't know if you have the correct mix or not. In a case like this you need to dyno tune with an O2 sensor to insure the mix is not too lean. Engine will seem to run fine with a lean mixture but the potential to destroy the engine is there.

 

yeah the cardboard idea is just so i know if im going in the right direction, most definitely not a permanent solution! dynoing is too costly at the moment but if i get a chance i will do it, for now i have to simply read the plugs, which looked a little on the rich side but this bike has been owned for 3 years by someone who never even checked the air pressure and has put about 6000km on it with the carbs in this condition. I think ill do the cardboard trick as its free and easy.. if this works then i think ill lift the needles up with some tiny washers.... and get bigger main jets.. as i think we can agree, its better to run on the rich side!

 

A couple of problems with what your thinking. Upping the jets if you're running good except for the flat spot isn't a good idea because it likely will run worse everywhere but the flat spot. Raising the needle does nothing for half throttle. The needle is already fully open at the point you're trying to improve. Volume of fuel at that point is controlled but vacuum and butterfly position. It might have a tiny effect but probably not. Raising the needle with shims (washers) is usually done to help the transition from pilot to main which is about 1/8 th throttle.

Plug chops are not very reliable for determining state of tune because today's fuels burn very clean and rarely show any color change unless your running very rich. You can cut the sides of the plugs to expose the base of the insulator and sometimes see a change but you sacrifice a set of plugs each time and you may not see much.

Sorry to rain on your parade but the only way to really tell what the mix is with an O2 sensor. This is critical with an open pipe. If you have a calmer aftermarket pipe or a stock one, you might try it to see if the flat spot goes away. My guess it will...as long as other mods are not too radical.

 

Yeah agree about the pipe.. My old 1200 ran so crap with open headers, Even with a very free flowing can on it, it was almost perfect with no jet change.

I dont have any other cans to try on it, but it does have a DB killer in it which looks like it might give some back pressure, maybe i can wrap it in exhaust wadding it make it more restrictive and add back pressure.

The reason i thought about shimming the needle is cause every chart/article i read looks very similar to:



(this is from keihin)

Ive got a pretty big exhaust leak which i just discovered.. ill get that sorted as well while im padding out the db killer.. might just make enough back pressure..

I was also thinking about running the carb vents to the airbox so they see the same pressure as the intake part of the venturi, but the lean spot even occurs when its in neutral at 0mph so i dont think its air pressure related.

Thanks for you continued help mate! I will keep this up dated with all things i try out.

 

Not sure that's Keihen. Looks like Dyno Jet chart. Stock carbs don't have clips on the needles. There is a bigger effective area with clips because the needle moves more from top clip to bottom clip. What that chart is trying to show is overlap more than anything else. It's not very helpful in your case. It's not showing actual fuel delivery. If it were then where all those factors overlap you would be flooded with fuel. If you have Dyno Jet needles with clips you could move the needle to the highest position and it might help.

If you look at half throttle on up the needle taper and main Jet are the only real factors like I said earlier. The reason I suggested needle taper is because you said it runs good except for the flat spot which assumes the jets are probably close. A sharper (skinny) needle taper will allow more fuel thru the existing Jet.

 

Fix the exhaust leak before you do anything else. That could be your problem.

 

I see what you mean.. Yeah I was thinking of sanding the needles to make them thinner but there is no going back from that. At least putting washers under the needle head is reversible if its a total mess afterwards exhaust leak is number 1 on the list, together with tacho and temp gauge so ibcan at least know exactly when the problems in fueling come up and try again at the same conditions!

 

So.. I got the exhaust welded up, totally sealed now, but didnt make much difference. I also blocked quite a lot of holes that were drilled and left only 20, and also blocked the original intake holes so it has to breath from the drilled holes. Its pretty much exactly the same! Im starting to think it could be an ignition issue as its like a switch when it starts misfiring, although throttle position definitely sets it off...

I let it warm up properly today so the engine was too hot to touch with bare hands and as soon as i sat on it... it died... now when i put the side lights on, i get the cooling fan run and the ignition is dead and no starter button. Turn the lights off and it starts and runs but no cooling fan. The mess of wiring the previous owner did is gonna have to be addressed sooner rather than later i think .. hopefully its just a corroded block connector or something simple... all i wanna do is enjoy riding it!

 

I have put fresh plugs in it, do you think that gapping them a little smaller than spec could help with the lean issue?