2 stroke gurus help me? 250r surging once warm?

[toki]

mmmkay.

83 250r.
new topend and rebuilt crank, maybe 5 hours on them?
all new gaskets, seals, bearings EVERYWHERE.
34mm flatslide (I cant remember what the jetting was i ended up settling with)
stock exhaust.
uni filter with lid.


heres the scoop. starts GREAT. starting from cold it takes 2-3 kicks. if its below 40 degrees i might use the choke and it all works as it should. starting from warm. 1 kick. no problem at all. once running and up to temp it power wheelies everywhere, throws chunks of sod at small children, tries to kill small woodland critters, and refuses to leave your shoulders in their sockets.

plug is a nice cardboard brown color. i went through probably 10 plugs doing plug chops. the head is a little dinged up from the previous owner grenading the rings and piston. i smoothed out what i could and lapped the head flat. i've preformed numerous leakdown tests at about 5 psi and its doesnt drop 1 psi for 15 minutes. I even did one when it was HOT... ever try to remove a blistering hot expansion chamber as fast as you can? NOT fun. the pipe does have 2 microscopic rust holes in it. it IS sealed on the exhaust flange and does not leak (black rtv) at the flange.

now. after about 15 minutes of riding. after its good and warm. it will start to surge at low rpms and the idle will hang a few hundered higher then usual. almost like its skipping a beat "dun duh dun duh dun duh dun" instead of the "dun dun dun dun dun". scientific i know. but thats what its doing.

what the hell am i doing wrong? carb still not jetted right? pinholes in pipe cause this problem (they are seriosly PIN holes. the only reason i know they are there is because once dirt gets on the pipe there are 2 quarter sized oil spots that form)? would a pitted head cause some diesel effect? it does NOT ping. ever.

[barnett468]

Howdy!



can you post photo of plug?

your pilot might be rich because you should typically need to choke it somewhat at around 70 degrees.

what is your fuel mix screw set at?

have you tested for air leaks at the crab where it enters the boot?

what happens when you turn your fuel mix screw while it is idling high?

did you check the float level?

[toki]

i'll have to get back to you on some of that lol.

no clue on what i ended with the pilot. my memory is trying to tell me its actually a size or 2 smaller then what the orignial carb took which was odd to me.

mix screw. just played with it. was 2 turns out. turn in and the idle drops. i turned it all the way in and then out slowly till it was at its highest idle. about 2 full turns out. havent tried while idling high after hot. ill give that a go in a while

i should really hook a hunk of clear line up and actually see where my floats are set. i just eyeballed them with a caliper as best as i could, i dont remember what it called for but i had it pretty darn close to spec.

i have the stock carb boot, its a tight fit getting that 34mm carb in there. i've sprayed it with starting fluid several times (as well as everywhere else) and never found a leak.

[El Camexican]

All of what Barnett said (except the part about the crab, he has no edit button and really meant lobster)+ what are you running for an air filter? Is the rim greased and well sealed, did you perhaps over oil it?

As far as it changing tones and varying in RPM while idling I experience that once in a while on my dirt bike. It is jetted fairly lean on the pilot, it starts first or second kick cold on the choke and will idle off the choke within seconds and behaves the same an hour later when hot as heck. However in the attached video you will hear it rev up a bit at 1:44 by a couple hundred RPM. The video ends right after, but it maintains that higher RPM until the throttle is blipped and then it drops back down again for another few minutes. Is this what yours is doing? I’ve never understood this phenomenon, but suspect it might have something to do with the temperature of the stationary engine rising, or oil pooling up in the crankcases and reducing its volume, any ideas anyone (yes, you too Barnett). Can you post a video of yours toki?

[barnett468]

All of what Barnett said (except the part about the crab, he has no edit button and really meant lobster)

Bad crab...bad!



Answer zee questions toki!




just curious, what brand and type of oil are you using?

what ratio do you mix it at?

do you know what the compression is with a gauge?

[El Camexican]

just curious, what brand and type of oil are you using?

what ratio do you mix it at?

do you know what the compression is with a gauge?

On that engine I was using PJ1 Gold Mix @ 32:1. I think the engine had about 80 hours at that time and the compression (on a cheap auto gauge) was 160psi.

It was torn down at about 100 hours and the rings were right at the suggested minimum, but the piston was noticeably worn (smooth, no scratches) on the exhaust side.

At that point the crank was pulled apart and straightened, and the main bearings and seals were replaced along with the piston and rings. The combustion chamber was reshaped, did some case stuffing and have been running Motul 800 at 50:1 since. I intend to pull it down soon and will post the differences in piston and ring wear between the two engines & oils.

I don’t know the current compression ratio, but I was able to start the old engine with my arm, this one with the torque head is hard to kick.

Check out the piston wash. I was very happy with it considering it’s a trail bike, but the pipe had 1/8” of deposit on it.

PS. toki, you’re thread has been temporarily jacked!

[toki]

my thread! bad! doooown. doooooooown.

Anyhow heres a pic of my plug. looks rather dark to me... but that doesnt mean much lol. I got it good and hot and doing its whole rev dance crap and started turning the air screw. i was able to make it come down and behave a little better but it didnt make much sense to me. seems like a bandaid. i had to turn it in about half a turn, which dropped the idle as well, and made it stop doing its silly rev bounce thing.

[82 250r]

lean pilot jet or a vacuum leak will cause the low rpm problem

[barnett468]

PS. toki, you’re thread has been temporarily jacked! ...PJ1 Gold Mix @ 32:1.

Holy Mother of Mary...He not only jacked the thread he mentioned OIL...I'm sorry, I can't bear to watch and Barn's still doing his 12 step program.

"Hi...my name is Barnett and I have an oil fetish."

[CRAZY70MAN]

Lean pilot. My 86 r did the same thing at the ice races in cod cold weather.

[250rRoostmaster]

my thread! bad! doooown. doooooooown.

Anyhow heres a pic of my plug. looks rather dark to me... but that doesnt mean much lol. I got it good and hot and doing its whole rev dance crap and started turning the air screw. i was able to make it come down and behave a little better but it didnt make much sense to me. seems like a bandaid. i had to turn it in about half a turn, which dropped the idle as well, and made it stop doing its silly rev bounce thing.

Defiantly lean pilot or air leak. Check for a leak and if none are present then go up one size on the pilot!