85 Atc250r wont run smoothly

[anthony250]

So I got this 1985 Atc250r and its been running good for three years but in that whole time it ran great except for 1/4” throttle was so boggy, after that it ripped the rest of the range . Same for every gear. . I decided it was time for a rebuild, and after it I would rejet it to get rid of that issue. For more info its a bike that the last rebuilder set up for 91 gas reliability in mind. “I have all specs “As mentioned ran good no issues besides the bog . Which i was figuring out was a lean bog. Riding at glamis. Weisco 2mm over bore piston . 38mm pwk air striker carb DGH needle 3rd clip , full exhaust. K and n intake no lid. V force 3 reeds. 40:1 castor 927 . Br8es plug. So that whole time it was on 48 pilot, 172 main.

After the rebuild i kept all specs the same as the rebuilder did, as in gaskets and weisco piston 2mm over. Light cylinder hone. And tried 58 pilot same main. And rode it on my street in socal, and it was so rough, in the midrange and kept backfiring through carb and exhaust. Long story short took it to El mirage this past weekend and every thing i tried I couldn’t get it smooth in the whole range. It always was backfiring like “umph” . Best i got was 172 main , 58 pilot and needle on 4position. Br8es first on .27 gap then changed to .18 which seemed to get better. It ran great but not perfect. Brought it back home to Near sea level to rejet for glamis and get it close. Rode it on the street and it was terrible boggy and backfiring issue agian about 1/4 throttle.

I hope anybody has insight as im not sure what to do to this thing. I have checked ohms on the coil and was

[ATC King]

Is the piston in backwards?

[shortline10]

I have seen this a few times atc king lol !

In my experience if you have flames coming out the exhaust and back firing on acceleration that’s usually electrical related .
I would be checking the stator , CDI and ignition coil and of coarse all electrical connections .
If it’s more of a miss or cough issue then possibly reeds or the carburetor .



Is the piston in backwards?

[anthony250]

Piston not backwards, but will triple check. I have check the Ohms values on the ingition coil primary and secondary and with cap off and got perfect numbers. Need to check stator, but what do i check there? Cdi i looked at connections and all good. Cant test that. It just started doing this after I rebuilt it last week.

[shortline10]

CDI isn’t testable , just need to try a known good one .
The 85 model is notorious for bad CDI’s , lots of threads on this site about them .

[anthony250]

CDI isn’t testable , just need to try a known good one .
The 85 model is notorious for bad CDI’s , lots of threads on this site about them .

Just find another I guess to try. What other models or years fit?

[shortline10]

85 is a one year only CDI , no other years will work and the aftermarket ones do not work either , OEM only .

[anthony250]

What do i need to look for on the stator?

[El Camexican]

Not at all suggesting that you don’t follow the troubleshooting advice of others, but I’m going to suggest that you also take the needle out and roll it on a smooth surface to see if it’s perfectly straight. I bent one once installing it in tight confines and it almost drove me to drink (more) till I found the issue.

[350for350]

I'm no expert on the subject, but I'll have to stick with the same line of thinking that El Camexican was using. This started out with a lean bog at 1/4 throttle. He went from a 48 pilot jet to a 58 pilot jet. That seems to be a big jump to me. It would try putting the needle, spark plug gap, etc. back the where it was and go smaller on the pilot jet to see how it acts. I would try a 50/52 first.

[ATC King]

I have seen this a few times atc king lol !


Yep, it happens.

I wanted to bat one into left field on the first pitch, just to cover all the bases.

[ATC King]

As far as testing the electronics, it's got to be completely thorough, including a wiggle test of the harness if nothing else test bad.

Wire harnesses can and do go bad inside while the outside can still look fine. Only by loading each individual wire and moving the harness around can problems be found without untaping the entire thing.

The other thing with electronics is parts that test bad are typically bad, but parts that test good can still be bad. Without testing at operational temps and with a load applied a part that test good still may not actually function. That is where probes and scopes make the difference in being able to actually see what's going on in a intact and operating system. Most people don't have a scope though or know how to interpret the data. An ignition scope can tell an experienced user a whole lot about an engine in a short amount of time without having to detach a single component.

Vehicle diagnostics - a stitch in time saves nine.

[ATC200X4716]

The comment above about moving 10 sizes on the pilot is true. That's a big jump; maybe go to 50/52. The transition where your idle/pilot circuit goes away and your needle/main jet take over is right around 1/8-1/4 throttle. You could have a lean spot there. Did you try raising your main jet size? Are you sure your jet sizes are real i.e no one drilled them out? (been burned by that one before). Is the bowl full of fuel? I wouldn't hesitate to try the 48 pilot with a smaller main just to determine which direction you are going proving reducing fuel makes the symptom worse. It seems like you do have jetting off a bit, but could also have an electrical issue as well. For what it's worth and the comments earlier are good, I usually make a first pass at all the electrical first. Ohm out the coils in the engine from the plug (flex the wires with the ohm meter attached. Been burned by a bad wire right where it enters the engine case). Ohm the ignition coil and cap. Better to swap in a spare just to rule it out. Check to make sure your ground wire is actually grounded to the frame/engine. Yours does run, but usually I would check the on/off switches but more so for a no start condition. Don't trust a new plug out of the box, been burned multiple times by that one. NGK is better named NFG. The backfiring and spitting out the intake does seem like you have a spark, but sometimes it is at the wrong time. Has the keyway on the flywheel sheared and moved? I've clipped a timing light on the plug lead connected to a 12 volt battery for power to watch the spark to see if it's smooth, or erratic and missing.

[anthony250]

Going to try setting back original settings i had , 48 pilot and 172 main, dgh needle 3rd spot. And see if my original lean bog returns and jet from there. If i continue to have backfire issues then I do have electrical issues. I got a spare cdi and coil to test too.and set back spark plug gap. Needle is not bent, very new. Piston is not backwards.

[anthony250]

Well i took the bike for a test with a spare cdi and no luck, tried a spare ignition coil and it seemed to be hell of a lot better. And i took it to the desert today, and about 30 minutes into riding it blew up. And locked up. It had 50 pilot 1 turn out dgh needle 3rd . 172 main. The range of power was really good except some hesitation at about 1/8-1/4 throttle. . Not sure it broke becuase it was lean or maybe i didnt warm it up as good as i could have.
Pictures uploading

[anthony250]

https://ibb.co/ZWBX23w
https://ibb.co/NNChf6w
https://ibb.co/GkKV8sP
https://ibb.co/9TvF5jp

[anthony250]

Looks like the piston expanded from the skirt outwards on the exhaust side. And broke one skirt ooff the cylinder

[El Camexican]

Ouch. Sorry for your loss.

[anthony250]

I know, any ideas ?short warm up? Lean?

[ATC200X4716]

9 times out of 10 the blown up engines I've gotten in for repair were too much piston clearance. If you were lean you would have melted the top of the piston right at the exh port but it doesn't look like it.The rings would usually be stuck in the grooves because the piston melted slightly around them. A properly fitting piston will not have any rock in the bore. If you have .005" clearance your piston to bore clearance is about double what it should be, and those brittle cast iron fingers that hang below the cylinder take a beating and break off. I've gotten bikes in that ran (not real well) that had a stock piston in an overbore cylinder. It will run believe it or not. I've bored a ton of cylinders over the years and I always request sending the old piston so I can see the clearance and I measure .006", .010", .015" sometimes .020" and I'm amazed this stuff was running. Revving a water cooled bike before the cylinder and water temperature are up will result in a cold seizure where the piston expands and seizes, or at lease severally galls itself against the cylinder wall. Always warm your stuff up before romping on it. The engine clearances are only correct when the engine is at temperature!

[El Camexican]

What he said /\

A loose piston beats the heck out of itself and the cylinder at the bottom of the stroke when everything is coming to a stop and changing direction. That’s something I like about putting in a new sleeve, they’re thicker and stronger than the cast in stockers, especially after the stock sleeves have been bored to the max.