Two stroke trouble shooting

Hey guys, I'm trying to trouble shoot the throttle response on my 2 stroke engine:
95 YZ 250 freshly rebuilt with mad compression.
I removed the airbox and using a K&N filter

Starts first kick, runs strong off the line and in the beginning of the mid-range, then drops off flat. This is for the engine I put on my Derby GPR, could it be that redoing the exhaust routing could have something to do with it?

I pulled the choke and rode with choke on and no difference so I'm pretty certain it isn't the jetting.

Any help appreciated

 

Mad compression? How high is it?

Changing pipes can drastically affect jetting. So can changing the air filter. You're main jet is way too lean, and your needle/needle jet probably is as well.

 

Sounds like the powervalve is stuck closed or not opening ,could be exhaust you said you rerouted it did you change the length from the headpipe to the chamber any?

 

2 stroke tweaking

The carb was jetted for the pipe on the last bike it was on, so if it was a jetting issue, usually full choke will help trouble shoot? I'll raise and lower the needle this weekend and see if it helps.
Powervalve...I'll pull the cover and see if its working correctly.

Mad compression - sometimes I'll be kicking it over and it becomes as rigid as a CR 500 (I know I've had a few of those) and lifts my planted leg off the ground.

I'll report back after this weekend, I had a death in the family so I'll be tied up the next couple days.

 

Mad compression? On a 2-stroke? Why? If you want that thing to rev out I don't think you want high compression.

 

2 stroke tweaking

it was really high at first probably due to the fresh rebuild, its coming back to normal gradually

 

If you are way off, you'll probably need more than just needle up/down. New pipe and no box could put you in the pretty far off category. Yes the choke should help you diagnose somewhat. Do a plug check if you can.

+1 on the powervalve as well.

you could also be simply running out of gas, ie. sticking closed float, bad fuel petcock, bad vent...

Check the easy stuff first. Then again, I guess its all pretty easy on a 2-stroke

 

Sounds like classic symptoms of way lean on main jet. Quit being lazy with the choke test and upjet it for real, maybe 3 sizes.

 

Fooling with the "choke" at WOT does nothing on a modern 2S carb. The choke only works with the throttle closed otherwise there is no low pressure to pull fuel from the choke orifice down in the float bowl. This is left over from the days of Bings with actual chokes behind the slide hooked to a lever on the handlebars.

My RM250 powered bike had a 380 mj. Stock rm was like 290 or so. Also had to go to a far different jet needle. You may need to lower the CR to get the motor to really rev. The 82 RM was famous for unreal topend and had a hemi head. The 83 was more of a short shift motor with the exact same cyl and pipe and had a squish band head. Putting on the 82 head was supposed to restore the unreal topend pull. Never got the chance for back to back comparo. Having too much fun riding . You may also need a bigger diameter stinger.

 

Thought most chokes were now "enricher circuits." I could easily be wrong, havent looked at a diagram of one in while to see what they actually do these days.

 

check the color of the tip of the pipe. also pull the head and see what it looks like. the tip of the pipe should be a light tan with maybe just a little gray. the piston should be a light brown, a gray indicates too lean, and a dark brown being rich.

oh yea, check your reeds too. and check for leaks between the carb and the intake port.

 

It is. What it is not is a venturi.

 

Well since it pulls air from before the venturi adds fuel then releases the fuel mixture on the depression side of the venturi it will richen the mixture substantially at WFO.

If the engine goes flat at upper revs AND has a lot of intake noise then it could be a Lean problem .If the engine won't free rev to upper RPM its probably another issue causing the problem. The 95 YZ didn't rev much to begin with it was a midrange motor.

 

Well, why didn't his motor run ragged as shit with the choke on and all the extra fuel going through the motor all the time ?

The choke passage is basically a straight 6mm passage through the carb body. The choke plunger blocks off the choke emulsion tube and this passage. The fuel has to be drawn a long way from the bowl. Air is always going to take the path of least resistance and I don't think the contorted choke path meets that criteria with right angle turns and an exit at 90 degrees to the bore.

 

Trouble shooting

if it makes a diff, the engine is a 93, the plug is a little wet neither black nor tan because I just run it around the block

 

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You are wasting everybodys' time until you can run the thing through the gears a few times with no idle time in between and get all the excess fuel out of the crankcase. Then you need to put in a fresh plug and do a couple of laps around a track and then really get the thing wound out and do a plug chop.

My RM250 motor set up as a stock 83 with a big K&N signed OFF at 7900 rpm. I never got around to trying different reeds and the 82 head before I got hurt. The Rotax 257 in the second bike I built was supposed to turn 9500 +.

 

The exhaust is very critical on a 2 stroke. If you have changed the exhaust drastically that is probably it.

 

Like RB sez, you gotta get it close enough that you can to run it hard for a while. I have a 93YZ and I dont think its never been a high revver but upon changing reeds I did end up 2-3 mains down(yes thats leaner) from what it was(it might have been +2 stock) before it would rev out good. I do mine on an abandoned running track or big ass field so you can sort of hold it open for as long as you want.

BTW its always been a good woods motor, for a 2stroke. Good low end and nice midrange, not a big top end hit though.