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High compression piston (re-jet)

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Hey guys, i just got my bike up and running yesterday. We did the replacement on the valves for the recall. I also installed the wiseco high compression piston at the same time. The first thing the mechanic noticed when they fired the bike up, was that it was popping a lot, they checked the carb and realised the needle clip was in the 2nd position, so they moved it up a few clips, i think to the 4th and then it cleared up nicely. So yesterday i took it for a spin down the road, and i noticed that when i tryed to lug it in 5th gear, it was sputtering a bit or something. It didnt seem right. I am thinking maybe the engine still needed time to warm up properly so i will test it at the track this week.

My question is that do you guys think i will need to re-jet, go back up on the main? Because i switched from stock which i believe is 178, to 175. And i changed the pilot to a 45. also the leak jet is a #40. We are thinking i may need to go back up on the main jet to stock 178, if we are having top end power troubles. Let me know what you think. Also, it could be that the new piston isn't broken in yet? I really do need to test the bike on the track to really see how its running, but i just thought i would ask here in advane. Thanks

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First off the bike needs to be jetted stock. Sounds like you are now finding that out.

The stock main is a 182S. You lean the bike out to a 175-178. Pilot is stock 42, you usually have to increase that to a 45. My 06 has been poping on decel now that it is 80-100* outside and had to run a 48 pilot.

needle 4th clip and a 4 LJ.

with the piston you wont need to rejet, but depending on the fuel you are now running it may require a rejet.

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First off the bike needs to be jetted stock. Sounds like you are now finding that out.

The stock main is a 182S. You lean the bike out to a 175-178. Pilot is stock 42, you usually have to increase that to a 45. My 06 has been poping on decel now that it is 80-100* outside and had to run a 48 pilot.

needle 4th clip and a 4 LJ.

with the piston you wont need to rejet, but depending on the fuel you are now running it may require a rejet.

yes i know it needs to be jetted when stock. I did, sorry about the bad info there, i guess you dont remember but i actually tryed the exact same jets you recommended a good few months back now. I went to a 45 pilot, 175 main and a #40 leak jet. Where i am, its summer weather (like florida baically) all year long. We are thining i may need to go back to the stock main jet. Which if i remember clearly, was a 178, or maybe that was the spare one that i got with the bike, but what do you thinkk?

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First off the bike needs to be jetted stock. Sounds like you are now finding that out.

The stock main is a 182S.

Stock main is a 178.

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Hey guys, i just got off the bike (literally, i have a practice track in my backyard). And the thing is, while i liked the power it was clear that the bike is not running as crisp as it should. You can hear it sputtering almost as if it looses power sometimes. I noticed that when you are being aggressive and racing the motor, it works nice, but as soon as you lay off and relax a bit, its kind of choking up or something. Hope that helps i still would like more suggestions, thanks.

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Hey guys, i just got off the bike (literally, i have a practice track in my backyard). And the thing is, while i liked the power it was clear that the bike is not running as crisp as it should. You can hear it sputtering almost as if it looses power sometimes. I noticed that when you are being aggressive and racing the motor, it works nice, but as soon as you lay off and relax a bit, its kind of choking up or something. Hope that helps i still would like more suggestions, thanks.

I already told you,put the needle in the 3rd pos(count from the top),that is your major problem.4th is too rich,you can thank me later. :thumbsup: Another thing,where is your location? Do not depend on a mechanic to jet your motor correctly on a stand.If there is no riding area and they jetted your carb,you can bet, they are just guessing. Put that needle in the 3rd pos and test.

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They moved the needle from one position to the next. It was in the second position, then they moved it to the next one, still not good, then they moved it again and thought that was the best it would get with that main jet (175). They are saying the bike is running too lean with the high compression piston and that i should pur back in the stock 178 main or even maybe the 180 that came as a spare with the 06s.

Just a quick note, air temparature here is usually between 85-95F OR for the guys who use celcius, thats about 30-35C.

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Go up on your jets.

I found indoor in the winter the 45/175 worked well. Now I am 48/178.

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I am a bit confused after reading your recommendations, then looking at the manual. From what i understand, if outside is humid and hot like it is here where i am all year round, then you may need to go with a leaner jetting overall, and then the opposite for when it is cool and dry (richer). But Matt you are saying in the hot conditions, going richer actually helped?

Oh and bigred55 you are saying i should go leaner, back to the stock 3rd clip position, the reason they moved it from the stock clip position, was because it was popping too much.

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I am a bit confused after reading your recommendations, then looking at the manual. From what i understand, if outside is humid and hot like it is here where i am all year round, then you may need to go with a leaner jetting overall, and then the opposite for when it is cool and dry (richer). But Matt you are saying in the hot conditions, going richer actually helped?

Oh and bigred55 you are saying i should go leaner, back to the stock 3rd clip position, the reason they moved it from the stock clip position, was because it was popping too much.

I tend to stay out of jetting recommendations because there is usually too many variables for each bike, its various mods and the rider's location.

Adam, you are correct. Typically cold weather requires richer jetting while hot weather requires leaner jetting. Matt is racing INSIDE, so perhaps, it isn't so cold? :thumbsup: He is also messing with BOTH ends of the jets to compensate for his throttle response (i.e. changing the MJ & PJ).

Anyhow, the jetting for the 06 YZ250Fs seem to be on the rich side stock. If you go richer on the PJ to get rid of the popping, your starting will suffer a bit and you'll probably have to compensate by going leaner on the MJ. If you go leaner on the PJ, your starting will be easier but then you'll have to go richer on the MJ. The needle adjusts the middle stuff depending on how big your MJ is, i.e. if the MJ is large, you'll most likely will have to lower your needle to clip position #2, if your MJ is smaller, then your needle may need to be at position #4 to get more fuel into the mid throttle range.

To me, a little popping on deceleration, although it sounds annoying, is OK since it allows for easier starts. Too rich of a PJ to get rid of the deceleration popping makes my YZF hard to start - I'd rather kick and go and hear the popping on decel then kick, kick, kick, kick, etc. to start a hot or cold bike. The main thing is that your needle and MJ must work well together since these 2 things control the power range that most of us will ride in, i.e. middle to upper rpm. Adjust the PJ and idle circuit so that your bike will start easily, then adjust the needle & MJ so that your bike is just slightly on the rich side but still revs easily and quickly building explosive power. If you are too lean on those circuits, your bike will cough as it tries to gain rpm, and if you're too rich, your bike will bog as if your spark plug is fouling and it will have sluggish throttle response - once you know the difference of how your bike feels when lean vs rich, then you'll want to adjust it right in the middle but slightly on the rich side and you should have your best throttle response as well as max output - easier said than done though, the art of jetting...

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Thanks av, i have a 175 MJ in right now. I will put in the 178 tomorow and see how that goes. I will leve the PJ as the 45 that is in right now. If the 178 doesn't fix things, then i will try the 180 MJ.

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Indoors in the winter.

Washougal is not indoor. Nor Albany or Madras or Eddysville.

Weather effects jetting and I keep on top of it.

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I will leve the PJ as the 45 that is in right now. .

I think the PJ is your problem. I have had zero luck with a 45PJ on both my '03 and '05. The stock 42PJ ,while riding slow in the 1/8 throttle range, had a weird misfire to it. A 40PJ cleared it up. A 45PJ wouldn't hardly let the bike run.

After this recently new HC piston and some flow work to my head, my bike needs a lenaer MJ, leaner clip position, and I've installed but not tested a 38PJ as it was running fat right off the bottom and hard to start, especially hot.

Every bike seems to have different needs.

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I think the PJ is your problem. I have had zero luck with a 45PJ on both my '03 and '05. The stock 42PJ ,while riding slow in the 1/8 throttle range, had a weird misfire to it. A 40PJ cleared it up. A 45PJ wouldn't hardly let the bike run.

After this recently new HC piston and some flow work to my head, my bike needs a lenaer MJ, leaner clip position, and I've installed but not tested a 38PJ as it was running fat right off the bottom and hard to start, especially hot.

Every bike seems to have different needs.

You need to go leaner with the HC piston?, we are thinking you would need richer with the HC.

From what it seems like, the bike is great off the bottom, so i don't think the pilot jet could be the problem. The problem starts more on half to full throttle so we are thinking its the main jet right now. I will know today for sure though, i am actually going right now as i finish typing this to change the MJ back to a 178 then maybe 180 and from there we will know a little more on where we are headed. thanks guys i'll be sure to post later on today on my findings, your help is appreciated.

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Hey everyone, we installed the 180 MJ this afternoon and the bike is working way better now! Maybe there is some fine tuning still left to do but the 180 is running much much better than the 175 was. Thanks for the help guys.

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Well after riding at the track yesterday, i discovered some more bugs. The top end runs fine, but now i have to come into corners a gear lower than i used to it seems, it bogs and sputters and won't pick up down low now and i had to use the clutch to get it out of the bogging quite a lot. Any recommendations? We are going to try to move the clip position on the needle this week (to a richer position). My friend thinks the bike is running lean overall because of the high compression piston, so that is why we will try going richer and see what happens. Any recommendations i will be greatful for.

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it bogs and sputters and won't pick up down low now and i had to use the clutch to get it out of the bogging quite a lot. Any recommendations? We are going to try to move the clip position on the needle this week (to a richer position).r.

Bogging and sputtering sounds too rich to me. If it's lean, it doesn't sputter, it sounds anemic. If your bike, while doing the bog/sputter thing, sounds very low and throaty, you're too fat.

Lean the clip 2 positions and test. If better, lean it 1 more clip.

Whatever direction you choose to go, go in fairly large increments and see if your changes were better or worse. that way you can tell if you're going in the right direction or not Then fine tune.

Mine was bogging a little down low, so I installeed the 38PJ and it cleared that up nicely with the added bonus of making mid throttle even crisper. Mine ran great yesterday in 6" of sand with no hesitation, no bogging and easily pulling 3rd thru the sand corners and revving 4th.

I still may be a little rich as I don't need the choke to cold start. fuel screw is 1.75 out.

Keep in mind I've had some flow work done to my head.

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agreed to rich down low. But also it could be your AP setup. Timing, squirt and duration.

I would not lean out the pilot beyond stock. Try the needle position and then take off the carb and bench test the AP circuit. also check the float level

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Let me just clarify something here, the gerneral consensus is that with a high compression piston, you should re-jet to go richer? Correct? Because that is the idea that we are following here, assuming that the bike will now be lean with the wiseco piston, so we are going richer on the jetting now to solve the issue.

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