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

It's not going to be an electrical issue, unless the CDI is really acting bizarre and messing up the timing when RPM rises, but that's really really unlikely. I have a 1978 SR500 with the stock CDI and the timing is still spot on.

I'd be guessing at the CV diaphragm, as it controls the slide going up and down. I don't claim to fully understand them, which is why I ditched the pair on my BMW R75 and replaced them with mechanical carbs. But most people keep the CV carbs, so I guess if they work, they're good.

What happens if you raise the slide with your finger? Lift it a bit as you open the throttle. Think you still need the butterfly valve to open, so you need to twist the throttle and help the slide go up. If that works, my guess is the diaphragm might be leaking. I think you can test that with liquid (assume gasoline?)

How's you're choke/enrichener w/ cable ? the choke may still be on causing it to run rich;the plug looks black and it's running rich.

Tim said:

It's not going to be an electrical issue, unless the CDI is really acting bizarre and messing up the timing when RPM rises, but that's really really unlikely. I have a 1978 SR500 with the stock CDI and the timing is still spot on.

I'd be guessing at the CV diaphragm, as it controls the slide going up and down. I don't claim to fully understand them, which is why I ditched the pair on my BMW R75 and replaced them with mechanical carbs. But most people keep the CV carbs, so I guess if they work, they're good.

What happens if you raise the slide with your finger? Lift it a bit as you open the throttle. Think you still need the butterfly valve to open, so you need to twist the throttle and help the slide go up. If that works, my guess is the diaphragm might be leaking. I think you can test that with liquid (assume gasoline?)

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Tim I'll give that a shot.

grcamna5 said:

How's you're choke/enrichener w/ cable ? the choke may still be on causing it to run rich;the plug looks black and it's running rich.

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Grcmana5 It seems to function properly. I start the engine with it pulled all the way out, then half way, then once it's warmed all the way in and this seems to work like it's supposed to. Rpms drop with each movement towards the bike on the choke. Again my first motorcycle so I understand the concepts but not necessarily what to look for.

jjacks31 said:

Grcmana5 It seems to function properly. I start the engine with it pulled all the way out, then half way, then once it's warmed all the way in and this seems to work like it's supposed to. Rpms drop with each movement towards the bike on the choke. Again my first motorcycle so I understand the concepts but not necessarily what to look for.

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The choke/enrichener 'plunger' on your bike has a cable? if so then sometimes the cable sticks and the little return spring doesn't always have enough pull to return the choke/enrichener to completely off.I would remove the entire mechanism and lube that cable w/ light,slippery fluid until it runs out the other end of the cable and completely stops dripping so it's all out and then remove the choke/enrichener 'plunger'(#32 on 'jetmechmarty's' diagram) and get that working as smoothly in it's bore/hole in the carb. as possible while making sure the return spring has plenty of tension before putting that all back together.Those cables are notorious for hanging up.
edit:also,when taking apart your carb. make sure you tighten all the little brass jets enough to properly 'seat' them but not too much;they have to be snug but not So tight you can't remove them again later.The large main jet also might have a brass washer underneath it and make sure that gets put back in before you install it.

Ah misread that the first time. My choke does not appear to have a cable. Missing the little plastic housing to push/pull it but it still works.

jjacks31 said:

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.

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Trash in the fuel? What did the stuff look like in the jets? Looked inside the tank?

re clean the carb then run it from a separate fuel source so yoiu can see how it runs

john83 said:

Trash in the fuel? What did the stuff look like in the jets? Looked inside the tank?

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The tank is the source of fuel and all the stuff in it gravity feeds down into your carb. I always love to make my tank as clean as possible so it won't foul the carb.

Here's a good guide to your carb. Ignore the bits (like synchronizing) that are for twin setups.

http://www.amckayltd.com/carbguide.pdf

Sooooooo All I did yesterday was take it apart and put it back together (yet again) and it magically worked. Was able to ride it around for a bit, but only did for like 5 min as I didn't want to suck any garbage in the engine. The vacuum thing doesn't seem to be an issue anymore but I noticed some differences in riding. For example when I start the bike with the choke all the way out it revs relatively high and is very responsive to throttle inputs. After I put the choke all the way in it starts to bog a bit. If I pull the choke back out and let it rev for a bit it becomes happy again and I can run it with the choke all the way in for a bit. I'm wondering if maybe I was wrong and the blackened spark plug was just an old tell tale and Im possibly too lean now? Here's me getting it to be able to act up again. Very curious.

Working: https://youtu.be/_vYKYL6AKBE

Bogging: https://youtu.be/1Sicd5mSiJo

your carb should be run thru a ultrasonic cleaner some of the passages are still not clean so its running lean

this is what is indicated by it running better with the choke out

and how/where do I do that?

ask if any members near you have a ultra sonic cleaner or phone around most shops have them today

or if you have interest you can get one for about 70 at harbor freight that will do the job

Just saw this thread.

I had the same problem with my SR and I had the same pod "filter" (more like blocker)

Good to see you ripped it off. You might be having problems now as it is in taking too much air. Buy a k&n filter and stick that on then time to tune.

You can get it clean with spray carb cleaner. Running with the choke open tells me the pilot circuit is plugged up.

Here is a tutorial for you:

https://youtu.be/95nlrP-yn2I

Ultra sonic cleaners are great, but they are in no way at all required, and absolutely NOT a substitute for manually inspecting every passageway to know with certainty that everything is clean and working properly. since you have to do that anyway, you might as well just get a can of carb cleaner and simply clean it manually. It looks in your pics that likely it will clean up pretty easily. If you take your carb completely apart, clean every passageway, and properly reassemble it, it will work perfectly. Period - presuming it is not otherwise damaged. I have "rebuilt" hundreds of carbs. Rebuilt is in quotes because it really amounts to meticulously cleaning and assembling not so much rebuilding. Most of the carbs I see were "just rebuilt " (usually by a so called pro but often by the owner) "so I know that's not what's wrong with my motorcycle" and also "they used an ultrasonic cleaner so you know they're perfect". Invariably, that is in fact the problem. Again, ultra sonic cleaners are awesome, just not magic.

It is tempting to want to simply determine THE problem, and just go fix that. The problem is that (as you are discovering) finding THE problem with certainty is often not that easy, and you can spend a crazy amount of time chasing geese. I find it simpler to just be very thorough with the carb(s) and KNOW that they are not the problem. Your carb is really a pretty simple device, but like anything you are unfamiliar with, can be pretty daunting at first. The key to cleaning them and being sure they will work correctly when back on the bike is to understand how they work and methodically clean each passageway for each system one at a time. All carbs work on the same basic physics principles and most motorcycle carbs are very nearly identical to each other function wise. Lots of stuff on the interweb explaining it. Just take each system one at a time and go through it. Learn how the pilot system works, then find all those same parts/passageways on your carb, and spray carb cleaner in where the air goes in and follow the path with the cleaner all the way through the carb until it sprays out into the main bore. Do the same for the fuel path in the pilot system. Then do the same for the air, and then the fuel for the main system. Same for the choke. That's pretty much it. You have a CV carb, so you need to make sure the air passageway that supplies vacuum to the top of the diaphragm is clear, that the diaphragm is good (a little trickier than it might seem) and that it installs properly and seals up. See something on your carb and not understand what it does? Go find out on Google or ask here. Figure out how it all works, know everything is clean and put back together properly, and chances are your trouble will be gone.

So been a bit busy with the holidays and just got back home today. Turns out it 'magically' runs because I forgot to put the big spring that keeps the diaphragm from getting stuck up. Not to toss in the towel, but for about $100 bucks would a new 34mm VM Mikuni carb be worth the lack of headache?

Even with a new carb, you will still have to tune it.

I kept my CV. If I were keeping my SR I would change the carb, but that is just personal preference.

jjacks31 said:

So been a bit busy with the holidays and just got back home today. Turns out it 'magically' runs because I forgot to put the big spring that keeps the diaphragm from getting stuck up. Not to toss in the towel, but for about $100 bucks would a new 34mm VM Mikuni carb be worth the lack of headache?

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Are you sure you haven't got holes in the diaphragm? Is it seated properly under the top of the carb? Did someone stretch the spring?


The vacuum should easily override the spring when it's running.