@RT325, nothing was missed on your part, but on mine. And with that, it's just me being anal. I thought I bought an identical part to replace my brake rod, not an "updated" part that looks different. I am sure it will work perfectly. I know I had to throw my dream of an all original bike when I ruined the tank, secondly when the CDI needed replacement. All that's OK, the bike still looks stock. The brake rod would be a departure from the stock/original look, albeit a very minor detail. Like I said, I am being anal. I am a detail guy. Eventually I will, or would like to get this ride looking really good in the near future, once I get the bugs worked out.

Sorry haha, i was forgetting the originality thing. All good.

Motor stuttering at high RPM

I made this video so you guys could help me with what I hope is a carb tuning issue. I set the idle, and lowered the needle one notch from center to the next to the last at the top of the needle and there was no change in behavior. The motor stutters from what I think is 3/4 throttle and up. The engine smokes maybe a bit more than needed, the oil mix is 15:1, maybe close to 20:1. I read the plug after break in, it is dark, but it is a brand new plug since the rebuild. I only has maybe an hour, maybe 2, at most of run time on it. If it is the main jet, which way do I go with it? I do not think it is pinging, but maybe still retard the spark a little more. It is at 2mm BTDC. It is an HPI ignition.

Ok, listened to the video & is just plain going over rich on full throttle. I get a good short listen when you head out the gate. The bike should be a rocket . So first thought on that is something is restricting the exhaust. Or something choking the inlet or air filter--or main jet way too big--but still guessing exhaust. Also the ignition should be nearer 3mm than 2mm btdc, just too retarded [like me at times lol].

Below is a snippet from the manual covering 250-36-500 & interestingly they say 2.3 for 250 but more for the 360 & even more for the 500. 360 will have the same internal cdi as the 250 with the early two stage pulser [no retarding once running] so timing stays put after starting. 500 is pretty much like that too but via flywheel & pulser like a DT360 a that holds stable after starting. So interesting the big bore 500 has 2.7btdc & has the same stroke as the 360. Anyway--i'd just set your at 2.5 & go try it but is more going on than that--like exhaust strangulation within even though its fairly loud on the video.


 

Here's a random video of someone giving an MX250 a pretty good rev but not for long. You get the idea of it revving right out with no struggle. Drag it to just before 1.28, any later & you'll miss it.

Thanks RT, I will bring the timing back up to 2.3. I was thinking it was too rich on the main jet, I just need to pop the carb to check what is there now and get a leaner one. Curious thing, it's not like this is a new bike or even just a new carb. It is the original carb that was working up to the time I put the bike in storage. I assumed the main jet would be the last thing it needed.

About the air filter, it is 100% new.

One last thing, is there a simple way to check the expansion chamber? I really thought it was hollow all the way through. No baffles, no nothing inside for anything to get blocked up, unless it is at the stinger, but everything else is such a large diameter. It seems a flashlight may do the trick? A borescope would work I am sure, but I do not have one.

I was totaly forgetting its a hollow chamber haha but have seen them build up carbon in the tail pipe leading to the tail pipe muffler. But "what tail muffler are you using & how restrictive is it". I used to burn out my chamber on my KX125 & could watch the color creep along the chamber but that wasn't badly carboned. I'd do it just to be sure. There's not a lot to cause it to go very rich on full throttle & exhaust is top of 'my list' & main jet but they don't wear unless someone's been drilling it. To be "sure to be sure" i'd go & hold it on full throttle & see if it clears or 'that's it' & stays rich.

No tail muffler, no spark arrestor.

How do you "burn it out"? I am thinking pull it and toss it in a fire????

Yes pretty big fire to take the complete pipe. But i used to get the carbon burning in the tail pipe in a bad case of build up when i had an oxy accertylene plant, then put the compressor air gun down the tail pipe but runs out of air pretty quick. My next one i'm going to try using a leaf blower in the front end & heat it from that end first. you mentioned a borescope & i've not got one, but sounds like a good idea to have a good look rather than burning when not neccesary. So no muffler or restriction on the tail pipe?, must be very loud.