Fuel ratio

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11/28/2017 5:03 PM

I recently decided to change my fuel ratio on my RM 250 to 50 : from 38 :1… My first time out I noticed it had a bog from bottom end and it had a weird rev with the idol… Not to mention surging while going down a road at a steady speed… Is this an indication that I need to change my jetting If I’m going to run this fuel ratio? BTW I use redline synthetic oil .

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11/28/2017 5:29 PM

It is probably running rich now that u have taken oil away and added more fuel so u would need a smaller jet. With the bog on bottom u could try adjusting your air screw out some depending on how far it is already out or u will need a smaller pilot jet. Why have u decided to change oil mixture?

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11/28/2017 8:18 PM

ktm-5 wrote:

It is probably running rich now that u have taken oil away and added more fuel so u would need a smaller jet. With the bog on bottom u could try adjusting your air screw out some depending on how far it is already out or u will need a smaller pilot jet. Why have u decided to change oil mixture?

Hanging idle and surging are lean conditions. A small change in oil ratio will not have enough of an affect to cause such a drastic problem. Sounds like a left side crank seal went away, or you have some type of air leak.

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Tomac and/or Anderson for 2020.....

11/28/2017 8:29 PM

ktm-5 wrote:

It is probably running rich now that u have taken oil away and added more fuel so u would need a smaller jet. With the bog on bottom u could try adjusting your air screw out some depending on how far it is already out or u will need a smaller pilot jet. Why have u decided to change oil mixture?

BobPA wrote:

Hanging idle and surging are lean conditions. A small change in oil ratio will not have enough of an affect to cause such a drastic problem. Sounds like a left side crank seal went away, or you have some type of air leak.

Your right I must be missing where he said a hanging idel. I didn't think the oil ratio would make much of a difference either but depending on how it was jetted b4 it could as if it was already on the fat side maybe the more gas would have affected it?? But definitely a good idea to check for air leaks

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11/28/2017 8:30 PM

I tried spraying carb clean around flywheel and no change in idle... not sure if it could still be leaky seal

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11/28/2017 9:34 PM
Edited Date/Time: 11/28/2017 10:08 PM

Just go back to the old mixture and see if it still does it. Then you know thats the culprit and you can jet it according to your new ratio. Not that its much help but i switched from pump to VP T2 on my RM250 and had to lean the jetting out. Idled higher on pump and had slight detonation at high rpm.

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11/29/2017 5:44 AM

OK… Long story short my bike was running fine at a track a couple hundred miles away had a little bit higher elevation and when I came in it started surging with a high idle and I didn’t think nothing of it I thought maybe it could’ve been the elevation or it was running hot something along those lines… I’ve been meaning to change my fuel ratio because I knew I could run it a little leaner with synthetic oil and it seem to smoke quite a bit at the ratio I was using… I change the ratio and that first ride out After track day I noticed it started running real temperamental and had an excessive surge at a high idle also when I would go down the road at a steady speed it would start lunging and surging so I kind of have two different issues going on here crank seal seeming the most obvious but I did pull the flywheel cover off and squirt carb clean around it and there was no change in the idle at that point ...I also tried spraying the boot, going to have to go back to old ratio and eliminate that possibility for now, still I’m skeptical it’s the ratio.

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11/29/2017 8:09 AM
Edited Date/Time: 11/29/2017 8:13 AM

There is also the right side crank seal that can go bad.when mine went it didnt have those symptoms. It just leaked tons of oil and wouldnt idle. Wouldnt hurt to check the reeds.

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11/29/2017 8:35 AM

Spray some starting ether around the reed cage. Sometimes they don’t seal well or start sucking air.

If you use carb or brake clean, wipe it off or it’ll eat at any rubber.

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If you're not mixing gas, you're not haulin ass.

12/4/2017 4:31 PM

Ok, I did notice oil at the crank seal but !! Not only was there no change in idle with carb clean I replaced the seal and it’s doing the same thing.

Any ideas?

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12/4/2017 4:32 PM

Btw... I did spray either all around reed cage and boot as well .

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12/4/2017 5:44 PM
Edited Date/Time: 12/4/2017 5:45 PM

Changing your oil ratio does not make your bike run leaner. The amount of fuel decides whether you are rich or lean. maybe u are sucking air from your exhaust

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12/4/2017 6:13 PM

ktm-5 wrote:

Changing your oil ratio does not make your bike run leaner. The amount of fuel decides whether you are rich or lean. maybe u are sucking air from your exhaust

Actually the oil to fuel ratio does affect the fuel to air ratio, albeit not by a significant margin. The jets only pass a finite amount of fluid, so when there's more oil, there is less fuel which makes the air fuel ratio leaner.

More premix oil makes the air to fuel ratio leaner.

That said, unless you're jetted so perfectly Mike Gosselaar would be impressed it's unlikely going from 50:1 to 38:1 will make a noticeable difference.




Many years ago I had an issue with RedLine 2 stroke oil separating. Don't know if that's the case here, but might be worth looking into.

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12/4/2017 6:16 PM

Yes I have realized this, this went from the question of the mixture to the crank seal which I thought was it from what I researched, I’m going to tear the carb down to eliminate that avenue, also have to look at the exhaust, I know it drips a lot of unburned oil out of where it meets the cylinder??

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12/4/2017 6:34 PM

ktm-5 wrote:

Changing your oil ratio does not make your bike run leaner. The amount of fuel decides whether you are rich or lean. maybe u are sucking air from your exhaust

Matt Fisher wrote:

Actually the oil to fuel ratio does affect the fuel to air ratio, albeit not by a significant margin. The jets only pass a finite amount of fluid, so when there's more oil, there is less fuel which makes the air fuel ratio leaner.

More premix oil makes the air to fuel ratio leaner.

That said, unless you're jetted so perfectly Mike Gosselaar would be impressed it's unlikely going from 50:1 to 38:1 will make a noticeable difference.




Many years ago I had an issue with RedLine 2 stroke oil separating. Don't know if that's the case here, but might be worth looking into.

Umm I think in a way we are saying the same thing?? I was saying if a guy was adding less oil to lean out their bike that it doesn't work that way as much as they want, like u said it doesn't affect it as much. If someone wants to lean their bike out because it's running rich changing the oil ratio is not going to do it and it's not running rich because of oil it's rich because of too much fuel. So I'm not sure what I said that u misunderstood but we are pretty much saying the same thing....lol

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12/5/2017 4:26 PM

It was dirt in the carb, pulled it apart blew it out and it purrs like a kitten again !!! My crank seal was leaking anyway and I was able to check my crank bearings in the meantime , thanks everyone .

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12/5/2017 4:28 PM

Glad to hear u got it figured out

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