Get a single piston 4 stroke they said... SR250 Engine Trouble

[jjacks31]

Hey guys, pretty green on the engine stuff but I've been using my service manual like my meemaw uses her bible and I'm stumped. The bike starts and idles happily, but the second you get above 20% throttle it misfires and refuses to rev. I've already taken apart the carb and cleaned it (multiple times) and cleared the clogged main jet (which I thought was the problem) and no change.

I have also reset the fuel mixture to factory settings and no avail. The stock airbox was replaced with a pod filter and both jets are a size larger than factory, but the previous owner rode 600 miles on that setup before this problem started. The spark plug would lead me to believe it's running rich, but even leaning out the floats and the mixture screw has changed nothing. The previous owner believes it's a CDI issue but I want to be certain it's not a mechanical issue before I drop $200 on a new OEM unit.

Le Spark Plug

Here's a link to the video of the issue. help me Do the Ton! You're my only hope.

https://youtu.be/X5vTEptrBDs

 

[cxman]

is the low speed/ pilot jet clean?

 

[jjacks31]

Yes. the bike has no problem Idling.

 

[cxman]

if the pilot circuit is in anyway restricted the bike wont accelerate

 

[jjacks31]

pulling the carb off now brb

 

[jjacks31]

So pulled off the carb and cleaned both jets again. This time when It warmed up I actually get it to rev! 1 time :-[ then it went back to sputtering at quarter throttle again. Pulled off the carbs again and the jets looked like they were never cleaned out. Coudln't tell if it was just left over gas or something substantial. The plot thickens.

 

[jetmechmarty]

You didn't clean it like you thought you did. That's a CV carb, correct? If so, put the airbox back on until you have it operating and become familiar with it. CV carbs like an airbox. Post a picture of the carburetor and we'll know.

Take the top off and make sure the diaphragm doesn't have any holes in it.

 

[jjacks31]

It's the factory carb

And I'm not sure the difference in types of Carburetors but this is a 34mm Mikuni BS34-III. Ill need to put a new filter on the airbox as its pretty disintegrated.

 

[jetmechmarty]

OK. That appears to be and likely is a Mikuni CV carburetor. That pod filter could end up being an issue. Put the airbox back on. Get it running properly before you mess with it. Stock jets wouldn't hurt, either.

Many times these pod filters have a lip inside. It blocks all the vents at the back of the carb and it's never going to run.

You may be blocking all these ports. In addition, CV carbs just plain work better with an airbox. You may want to later on opt for a different kind of carb to run with the pod.

 

[jjacks31]

Popped of the pod just to see if it would run and this happened... supposed to happen? This is at 20 percent throttle.

https://youtu.be/fDEeWFw6YWM

 

[jjacks31]

According to my friend that means it's not holding a vacuum. So I checked to make sure there was a good seal between the bike and engine: Check. and the diaphragm doesn't have any cracks or holes or cuts in it. :-\ I mean at least I'm learning a lot about carburetors.

 

[carnivorous chicken]

You may be blocking all these ports. In addition, CV carbs just plain work better with an airbox. You may want to later on opt for a different kind of carb to run with the pod.

This. The airbox on SR250s is very restrictive, and you need to go up several sizes if you want to try to make a pod work, even a crappy Emgo pod. Putting the airbox back on will require going back to stock jetting though, although if you're one size off it might be OK. The advice to get it running well stock is sound. It could also be running rich at low idle but then very lean at more throttle.

 

[jjacks31]

But would that explain the weirdness above?

 

[jetmechmarty]

Put it all back together as stock. Start it up and let it idle. Open the gas on a propane torch (not lit) and see if you can find a spot where the RPM increases. A can of carb cleaner or WD40 should work, too. Any inflammable aerosol should do the trick.

 

[jjacks31]

Will do. Excuse my lack of knowledge, but I assume you spray the aerosol directly into the intake. Also weird that the previous owner was able to ride so far on larger jets and pods before this happened. If the aerosol thing works would that definitely rule out electrical problem?

 

[jetmechmarty]

No. Spray the aerosol around the outside of the carburetor, especially around clamps. Your throttle cable works directly upon a butterfly valve. That shaft has seals at the ends. Spray there an look for RPM increase.
The red arrows show where those seals are.

 

[jjacks31]

Will have to try it tomorrow. Thank you!

 

[carnivorous chicken]

Will do. Excuse my lack of knowledge, but I assume you spray the aerosol directly into the intake. Also weird that the previous owner was able to ride so far on larger jets and pods before this happened. If the aerosol thing works would that definitely rule out electrical problem?

I'm curious about this too -- did he tell you he rode all those miles, or do you know for a fact that he did? The two can be very different...

 

[carnivorous chicken]

Shit, nobody (including me) has mentioned float height or blocked cap vent as possibilities too -- fuel starvation at higher RPMs.

 

[jjacks31]

Sir Chicken he's a pretty reasonable guy that has stayed in contact with me almost daily after the purchase answering history questions. He seems to want to see it run as bad as I do. I mean no guarantees I guess.