weird jetting with high compression?

9 replies to this topic
  • tool

Posted September 05, 2004 - 06:48 PM


i was reading an article about 4 stroke hop ups, and it said that most mods (including high compression) require richer jetting. well, i think they are wrong. :thumbsup:
after i put in my 13.5:1 piston my bike wanted less fuel. :awww: the bike had a rich spot until i leaned the fuel screw from 2 1/2 turns to 1 3/4 turns out, at which point the idle would hang a little but there was no decel popping.
how do i tune the jetting now? these rich and lean spots dont make sense to me. :devil:

  • felony

Posted September 07, 2004 - 08:56 PM


i went to a 170 main since i did the hi-comp piston,my sparkplug was showing signs of running lean so i did it to be safe,since i did a complete crank rebuild i wasnt taking any chances,seems to pull pretty good up top as well. :thumbsup:

  • tool

Posted September 08, 2004 - 06:48 AM


i went to a 170 main since i did the hi-comp piston... seems to pull pretty good up top as well. :devil:

thats odd, my bike torques HARD now in the very low mid, but it falls flat on its face when i get it up on top. :thumbsup: before the swap the bike had a rich pop, and afterwords that was better... but the jetting still seems too rich.
170 main? im using a 150 MJ <-- is that why i dont have any top end? when i was changing pistons, i noticed the plug was black and there was carbon on the valves. it must need leaner jetting, yes?

  • Fastest1

Posted September 08, 2004 - 07:45 PM


Colorado is part of the answer. What altitude are you? Ohio would run bigger mains just because it is lower. I would still go up with the main until you get top end too. No harm in trying bigger. Have you played with the needle at all? Changed pilots?

  • tool

Posted September 09, 2004 - 01:59 AM


i ride at 8000-10,000 ft. the engine was rough at a constant speed and seemed rich, so i leaned the needle 1 clip position. that helped clean things up and the bike pulls hard, but i really need to try it at the track to see.
my main problem is a bog on hard landings, which is more tiring than anything. the jetting is in the ballpark, maybe i need to adjust my leak jet or diaphram? maybe the high-comp piston requires less squirt?

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  • YZ250F_Rider

Posted September 09, 2004 - 03:30 AM


yeh, sounds like you need to screw with the ap. But make sure your idle is high enough to begin with. It makes a big difference in how pronounced the bog is.

  • Fastest1

Posted September 09, 2004 - 06:01 AM


Check your float level, sounds like it is too high and flooding on landing. Sloshing.

  • tool

Posted September 09, 2004 - 06:22 PM


i made several changes to my bike since the last time i went to the track... i leaned the needle one clip, added 12 oz. to the flywheel and reduced the endcap opening 1/8 or 1/4 inch. killer response and traction.
ive got the top end back, which seems to be from the combination of all three changes. the bog was from trying to lug the engine too much with the light flywheel. the torque is still there but the engine revs like before, with more zip in the top end. :thumbsup:
the bike is dialed. :devil:

  • grayracer513

Posted September 11, 2004 - 09:50 PM


I don't have your complete answer, but I can make sense of at least your rich idle. One of the side benefits of higher compression is a stronger intake stroke at low speeds. Since the difference in the volume at TDC and BDC is greater than it originally was, there is more vacuum generated during the intake stroke, which sharpens the "signal" to the carb somewhat. Could explain that part.

I'm sure you already checked this while the head was off, but I discovered on my son's 250F that tight intake valves can cause the idle to flair, or hang up as you mentioned.

  • tool

Posted September 12, 2004 - 12:44 AM


that does make sense. the higher compression seems to help every aspect of my bike. it does require more accurate jetting, though. but that does explain why i was rich after the piston swap.
my valves are all on the tight side, but still within spec. im planning on a complete overhaul of the head this winter - new cams, chain, valve work... everything.
once again its greyracer with the wisdom. :devil: :thumbsup:

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