Power Jet-Plug Reading-Timing-2000 RM250

[ckassen]

Tuning and tinkering

I have just rebuilt my top end and put a 265 kit in my 2000 RM250(porting and all). I am messing with the tuning and such and have a few questions.

My bike seems to have a lean idle sound(when the bike warms up), I have adjusted the pilot jet but when I make it too rich(so that the lean idle sound is gone) the bike likes to bog down. If I have that lean running sound the bike runs great. Is the lean running sound bad?

Power jet-Is that the electric thing in the carb, and if so, what does it do?

Plug reading-I have read alot on this and looked at alot of diagrams. One thing that I have never heard is the fact that to check the jetting you can look at the flat part of plug?(at the end of the threads). Mine is black(to consider this valid do you have to have a new plug?), but the porcelan is a nice brown light brown color....?

Timing-I advanced the timing a little so that the lighter ring/spot on the electrode is just a liiittle bit(.5mm) down from the tip of the electrode. When at stock there is not a lighter spot on the electrode. Is this ok?

Ping- considering that the ignition was perfect(no random spark) is it even possible for a bike to ping IF it was low on compression(130psi)? In other words can a bike with an INDICATED 130PSI suffer from pre-ignition...Considering that the timing is not too far advanced.

One last thing, the choke knob. It says idle adjustment on it(manual says 12 clicks out) but the manual does not call this the idle adjuster. It just says choke adjustment. The manual says that the idle screw is the black plastic one with a spring on it and controls how far down the slide can go(makes more sense).....What does the choke knob control?

Thanks,
Chris

 

[whenfoxforks-ruled]

The power jet may well be your problem, it comes off idle. I believe"ping" or detonation can occur from advancing the timing too much also. The adjustment on the choke is like the air screw. The base of the plug is not gospel, just reference. You have not fiddled with the exhaust valve adjustment?

 

[ckassen]

Oh, of course... :) I have it set to approx. 1/2 turn...It seems to work well...The powerband kicks in nice and low and allows me have a fair amount of power without having to rev. the crap out of the bike.... ;)

 

[whenfoxforks-ruled]

I had ours set to 2, it adjusts how hard the powervalve hits.

 

[ckassen]

Does it change how hard the powervalve hits, or when the powervalve hits? I am for a harder hit.... :)

 

[whenfoxforks-ruled]

How hard it hits. At a half turn would be for trail riding, maybe!

 

[ckassen]

I guess I misread somthing :bang: ....I am going to set it to 2 and see what happens....

 

[ckassen]

You said that the adjustment on the choke is like the air screw. Would turning it out make it leaner or richer?...I just get confused trying to tune the idle by using all 3 adjustments....

I just figure I can have that lean idle sound so it runs good throughout the rest of the powerband and then try to richen something els up....(choke knob)

 

[whenfoxforks-ruled]

I am under the impression the power jet works like the ap on a 4t. That said, try a smaller numbered power jet. It should dump more fuel in off idle if you are lean. If it is cold wherever you are, then your settings should be richer than stock.

 

[ckassen]

I wonder how much a powerjet costs?...Do you have to buy the whole assembly?

 

[whenfoxforks-ruled]

It is a very small brass jet. http://www.prosourcemotorsports.com/fiche_section_detail.asp, Say hello to Rusty.

 

[AV8R]

Power jet-Is that the electric thing in the carb, and if so, what does it do?

The powerjet is controlled by the selenoid valve "electric thing". The selenoid valve is open and allows fuel into the carb directly from the bowl through it's own "jet" (really just a calibrated hole in brass) until a preset RPM is reached (maybe 8500 or so) then it closes and fuel is delivered only by the conventional jets, mostly the main. This is assuming that the powerjet is the same as on the Honda's and Yamaha's.

Timing-I advanced the timing a little so that the lighter ring/spot on the electrode is just a liiittle bit(.5mm) down from the tip of the electrode. When at stock there is not a lighter spot on the electrode. Is this ok?

Advancing the timing makes detonation more likely to occur but if you run sufficently rich or with high enough octane it may not be a problem. I would leave the timing alone and work on the jetting. Get it dialed in then worry about fiddiling with timing.
The only way to check the spark plug is to check way down at the base of the plug. Look for that ring to be about 1mm or so thick. If you check it after riding a while you are getting the average of all the circuits and not just a specific circuit.

Ping- considering that the ignition was perfect(no random spark) is it even possible for a bike to ping IF it was low on compression(130psi)? In other words can a bike with an INDICATED 130PSI suffer from pre-ignition...Considering that the timing is not too far advanced

.

Ok the way I understand it is that "ping" is from detonation which is different than pre-ignition.
Pre-ignition is the fuel igniting before the spark from the spark plug. Some thing in the combustion chamber has reached sufficient temperature to ignite the fuel as a glow plug would. This is very detrimental to engine wear. As you can imagine the fuel is burning and expanding at the same time that the piston is still compressing the gasses creating extreme pressures in the combustion chamber.
Detonation is the uncontrolled burning of the fuel. Rather than burning evenly it has areas that explode creating local areas of very high pressures. This causes the pinging sound.
Detonation is from running too low of an octane fuel for the compression ratio of the engine. Or from getting too hot or lean when on the edge of detonation from sufficently low octane fuel. Or from the inefficency fo the squish band leaving unburnt fuel to be detonated. There is more to it than that but that kind of sums it up.
I would not think that and engine that tests out at 130 psi would be likely to detonate, but it could given the right provacation to do so. ie. advanced timing, lean and low octane fuel, running at high loads and rpm's.