2 Stroke Jetting 101

[Kenneth S (222)]

I just read through the 'Needles' Thread by Wasp and Lozza

Extremely interesting thread and professionally put together Youtube clip. Thanks Guys.

Lozza, my knowledge of jetting is basic at best, only ever changing the occasional main jet, adjusting the needle clip position and the air screw, never looking at slide or needle profile or understanding the correct sequence of thought to analyse those finer points. I'm sure it must be possible to put together a simple plan of attack, a preferred sequence of analysis or perhaps you are you aware of a publication which would assist me in a step by step process of understanding these finer points and sorting out the jetting of my 2 strokes?

For example, on the 79 CR250 when I received it, it was misfiring badly in the top end. A friend suggested I check the main jet size. The main jet it came with from the US was one size larger then standard and the plug was a dark colour so we tried the standard size. This improved the misfiring out of sight but it still seems to be lacking horse power up hills when flat out bogging down when I change from 4th to 5th. I also have a flat spot at about 1/8 to 1/4 throttle, right where I want to be as I accelerate out of corners. I tried dropping the needle one position which made the bike response crisper but didn't seem to fix the flat spot.

I don't know where to go from here, have I given you enough info to comment?

[bazza]

1st check your float level and float needle is in good condition. Your flat spot seems to be between pilot jet and needle taking over. Is your air screw in correct possn?probably about 1.5 turns out.Are you running Honda carb or a Mikuni?

[Kenneth S (222)]

1st check your float level and float needle is in good condition. Your flat spot seems to be between pilot jet and needle taking over. Is your air screw in correct possn?probably about 1.5 turns out.Are you running Honda carb or a Mikuni?

Thanks Bazza,
I'm running the standard Keihin Carb. I'll check the float level, not sure how to do it but will work it out.
I adjust the air screw each time I ride. It seems to change depending on conditions. Standard is 1 1/2 turns out and most times I start from there and try a little leaner until it seems crisp off the bottom end. I guess my current belief is it the bike seems to work better out of corners when the pilot screw is adjusted to run a little lean down low rather than rich. Not sure whether that is right but it seems crisper. Perhaps I am forgoing torque though!??

[Lozza]

Without hearing it I would not be surprised to find your ignition is weak.

[Kenneth S (222)]

Yes, my mate suggested that could be it as well. I have been searching with Google to look for a process to check this but have not found one yet.

Is this something I can check with a volt meter? What is the first step to checking it?

[bazza]

leaning on the air screw is because you  are compensating for rich carb on bottom half of throttle

[Lozza]

Yes, my mate suggested that could be it as well. I have been searching with Google to look for a process to check this but have not found one yet.

Is this something I can check with a volt meter? What is the first step to checking it?

basicaly finding the amount of air gap the spark will jump. The greater the gap the better the ignition

[Kenneth S (222)]

Have checked the float level and it was too high. I don't understand why that would effect how the engine ran down low though. Doesn't the pilot jet govern the fuel flow for the bottom end? How can too high a float level effect it?

[Freakshow]

Pilot jet is ALWAYS on..... so if its rich then sos the rest if its lean sos the rest and you masking that with you main at full tap. hat does the manuals say you pilot should be ? i would replace it anyway if its an old carb any perhaps with one either side to see what changes you see.

The main is the last thing you do and thats just to stop it blowing up.  The riding part is all the other stuff.  I have had bike run shit on the same day with differant riders, not skill but weight bias.  Standard jetting for the japs was based on a 7o kg jockey, in most cases where in the 90 kg brackets..... if the ignition and timing is right go back to your pilot and needle.

[Slakewell]

I got this guy to Jet my YZ he gave me a few tips. 

[Noel]

I think its a bit like Warragamba , when its near the top It flows over abit,

If the fuel level is high it is easier and quicker to lift and I think also affects the air correction holes in the cross drilling's   (maybe)  

I also seem to find with older carbies that the factory float level seems to be a bit to high for proper running,
particularly down low.
perhaps floats loose some of there buoyancy, or all the little bits of wear give a false static  setting,

Noel

[pancho]

A simple thing to do to see what effect a high float level has is to stand the bike in the normal upright position, remove the car by top and slide if possible, turn the petrol tap on, and look down to the needle jet where you will see if the float level is too high fuel will be overflowing out from the needle jet. Consequently this will affect the mixture making it rich right up to full throttle when the flow is only effected by the main jet size.
 Cheers.