knocking under acceleration

[dgyver]

You are way over jetted on the mains and running very rich. I was running 145 and 147.5 with the 555. I kept playing with which one I liked the best. With 150's it would run flat. I know the new owner changed the jetting slightly but that have to do with location/elevation.

Are you running stock pistons?

[russomf]

stock pistons yes, decked heads, web cams.  help please

[dgyver]

I will need to look at my notes on the jetting I was using prior to boring for the 79mm pistons. Other than overbore and your milled head, our setups are the same.

[russomf]

dgyver thanks a lot i look forward to your input.  thanks to everyone else who has helped.
can anyone still tell me what who the air screw works?

[The Buddha]

OK dgyver you made a larger chamber ... and needed slightly smaller jets, he's got a larger (well what did you do ... dude I dont remember) ... but larger Chamber with same carbs = smaller jets ... remember in the old days suzuki used to fit similar carbs to 750's and 1100 and they'd jet the 750's with larger jets and smaller in the 1100. Bigger chamber = more velocity = smaller jets, now larger valves and milled head dont actually result in an increase in velocity like the larger chamber does (who knows my instinct tells me its not) ... so you'd have to jet up.
Now black smoke under 5K is cos pilots circuit is too rich ... whihc may be wholly different from the main. You drop to 40 pilots, you lean out the mains further (pilots affect the mains and Vice versa) ... so I'd rather you dont start with a jet size that give you max power ... start with somehting that is so over rich it runs like sheite ... and drop into the right jet sizes.
I dunno, I'd try 40/160's instead of 40/150's. A tweaked motor is more likely to blow a hole in the thing even if its slightly lean ...
Cool.
Srinath.

[dgyver]

Do not forget about the cams. High lift + longer duration = more flow

When I ran the 75mm pistons (the rest of the motor was the same as the 79's), 140 mains worked good. I tried as big as 147.5 and it had nothing on the top end. Try 140 and then you may want to go down a size or two.

Another question...are you using stock needles or DynoJet needles?

[The Buddha]

Do not forget about the cams. High lift + longer duration = more flow

When I ran the 75mm pistons (the rest of the motor was the same as the 79's), 140 mains worked good. I tried as big as 147.5 and it had nothing on the top end. Try 140 and then you may want to go down a size or two.

Another question...are you using stock needles or DynoJet needles?

You had DJ needles in right ... huge difference ...
Cool.
Srinath.

[russomf]

i have 3 different needles i can use. stock and the other 2 are as follows 1) same length as stock but the end is tapered down for the last 1/2" to 1/2 the diameter. then i have a set that are about 1/8"-3/16" shorter but have ther same taper as stock.  i am running the shorter set than stock right now so i do not have the washers in the system.  so should which pilot jet should i go to? 40, 42.5 (if they make it)?   i have to go buy some tomorrow.  thanks again guys much appreciated.

[dgyver]

Use the stock jets first, as long as they are in good shape. The shorter and tapered ones may open up too fast.

Use the 40 pilots. I tried 42.5 but it did not help but it did not hurt either.

Also, try plugging one of the holes in the bottom slides. It helped with my idle hang.

[russomf]

OK so go 140 mains, stock jet needles (no washers) #40 pilots and about 3 turns maybe less.  thanks for the starting point.  your input has been much appreciated.

[dgyver]

I do not remember the exact number of shims I was running, it was only 1 or 2. The needle height affects the mid range so start with 0 or 1 shims. The main jet size must be within 1 size (no more than 2) before adjusting anything else. It affects all the throttle ranges.

[russomf]

OK i will start with 140's and see if i can get a set of #40 pilots today, then set up the rest from there.  you have been a great help, how will i know if i am running lean or rich?  i keep getting told to run rich then lean out but do not know when i am running rich or lean, what are the symptoms for both?

[dgyver]

You can look at the plugs for their color. Black = rich, white = lean. Good is a medium/light tan. This can be misleading since they only show the running condition throttle position when the bike is shut off. Which is usually at idle. Without doing a plug chop at WOT, you will not know if the main is correct. Also, looking at the exhaust tip will show light carbon dusting if you are running rich.

[russomf]

thanks ill have to do all that to make sure i am not runnning rich or lean.  better safe than sorry.  so i have to be moving when i do this right?  that must be interesting to pull off, let me guess, full throttle, left hand on clutch, right hand kill switch and coast to a stop.  great as if riding were not hard enough as it is.

[dgyver]

Yeah they are kind of tough to do. Then you have to wait a little bit for the motor to cool off to pull a plug.

[Cal Amari]

These articles might help you to understand carb tuning / jetting better; jetting will always be a bit of a black art. Since you're starting with a milled head, you're dealing with higher compression, and the carbs will be more sensitive to the changes you make. Be sure you're tuning from rich to just-slightly-lean to just right, unless you like seeing daylight through pistons. Better to deal with fouled plugs / too rich rather than holed pistons / too lean.

http://justkdx.dirtrider.net/carbtuning.html

http://www.factorypro.com/tech/carbtune,CV,high_rpm_engines.html

Let us know if you need anything clarified or explained...

EDIT: Neglected to mention that the first link refers to the Kawasaki KDX (dirt-bike) two-stroke engine (don't worry about reed-valves etc.), but the animation and illustrations are good. IOW, ignore the two-stroke details, and focus on the specifics about carburetion, and you won't get confused.

[dgyver]

Thanks for posting the FactoryPro link. I did not have it handy. I keep several printed copies in my log books.

btw....if you think tuning a carb on a 4-stoke is a tough, try tuning a 2-stroke GP bike. The jets have to be changed as the temp and humidity change. A friggin pain! They are a blast to ride though. If it was not for the maintenance, I would get another one.

[russomf]

off to get my #40 pilots and 142.5, 137.5 and 145 mains.  will post up what i find as i go through this.  with the pilots set i do not mind changing the mains it is not bad and i can do them hot now.  just takes a rag top keep the hands from getting burned.

[The Buddha]

OK Let me know what works. I just have a feeling that milling the head, flowing it and higher lift cams dont actually significantly incread the velocity through the carb ... Higher velocity = smaller jets due to hiugher vacuum, and over boring it does accomplish that ...
Higher compression will need the same mix composition as normal compression, and its not a big step up to flow, taller cams will fill the chamber better at low revs, but sorta choke it up at higher revs duration and dont do much for velocity increase, higher duration cams will probably raise the velocity in the carbs to an extent ... and mix loss in over lap will make your motor lean ... and you'd run into limits in the carbs ... you'd need bigger ones. Now flowing ... OK that will make the flow through the carbs faster, but it will move more vacuum into the carbs and hence not actually make much of a diff in jet size ... See jet changes are needed when you alter vacuum in the carb for the same flow ... Take off the air box and paper filter and instantly your carb has a lot less vacuum, that lower vacuum will have to suck out the fuel needed ... so you;d have to drop in larger jets. You dont have anything that has increased the velocity ... a whole lot ... Anyway let me know what works.
Cool.
Srinath.

[dgyver]

The longer the valves are opened with the high lift cams will increase the amount of fuel supplied.

I wish I still had access to CFD software.