Correct fuel ratio

[procircuit21]

Is it ok if i run Yamalube 40:1 in my '91 KDX 200? It has an FMF fatty, powercore 2 silencer, and a boyesen RAD valve on the motor. As far as I know it has stock jetting. I had been running 32:1 but i was fouling spark plugs a lot so I thought i'd try and switch. I am going riding tomorrow and I'm gonnna try it then. What is the best type of pump gas to get? The highest octane i have found around here is 92 at the amoco. Are there any places that have higher octane gas? Also, I am pretty sure the carb has stock jetting. Should i change it since the motor mods? If so, what should i change it to? Oh, i also tossed the airbox lid.

 

[wibby]

Changing your ratio is not gonna solve your plug fouling issue. Changing your jetting will.

If I've learned it right, going from 36:1 to 40:1 is actually making your bike run richer.

Just wait until all the brains come in here tomorrow they will set you on the right path

 

[JasonWho]

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[CaptainObvious]

Nothing wrong with Yamalube @ 40:1. Everything is wrong with your jetting!

KDX's are notoriously rich from the factory. If you are fouling plug and spittin’ spooge it’s not your premix ratio, it’s the jetting.

Checkout C-Dave’s site for jetting suggestions. Remember, in order to find the proper jetting you will need to…

1) Run the same fuel and mixture;

2) Use a fresh spark plug;

3) Use a clean airfilter;

4) Have a clean silencer (one that isn’t soaked in spooge). While you’re at it, inspect the powervalves and the inside of your pipe and clean as necessary;

5) Have a lot of jets in hand. I would suggest mains in the range of 150, 152 and 155. Pilots in the range of 38, 40, 42 and 45.

As mentioned above, 40:1 is a richer mixture than 32:1. We’re talking fuel, not oil here. For a given amount of premix, 40:1 contains more fuel than 32:1.

 

[DVO]

Just for fun, until you get around to messing with the jetting, you could try using a higher ratio of oil ( 30:1, 28:1) which would lean it out and maybe stop fouling plugs until you can take care of the jetting issue. Just my opinion.
Use the search function to the max!
Good luck.

 

[CaptainObvious]

Originally posted by DVO
Just for fun, until you get around to messing with the jetting, you could try using a higher ratio of oil ( 30:1, 28:1) which would lean it out and maybe stop fouling plugs until you can take care of the jetting issue.

That doesn't work. You'll be amased by how well the bike will run with correct jetting.

 

[JasonWho]

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[CaptainObvious]

Because the volume of fluid is too much for the amount of combustion. Any fuel/oil not burned is going to foul plugs, clog the powervalves and run out the pipe.

I'm sure that Rich Rohrich or one of his many disciples can give a technical answer.

 

[procircuit21]

OK, tell me if this is right: It's ok to run 40:1 i am just gonna have to change my jets. It doesnt feel blubbery or lean anywhere in the rev range, but i dont know too much about carbueration. Also, am i correct in that the pilot jet, main jet, and needle position correspond to different throttle openings? I thought i heard that somewhere but i wasnt sure.
Today i went out riding and i bought a new plug. I had previously been using a B8ES, but i used a B9ES just to experiment. It did feel like it had a little less power, like it was a little bit lean. Is a 9 colder than an 8?

 

[G. Gearloose]

Yes, the 9 is a colder plug. That means the electrode is cooler..
You really must digest the FAQ and CD's JustKDX website and review old posts to build this foundation...

 

[JasonWho]

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[cfr1970]

So far, so good. You've recieved all good information so far. Make sure when your jetting experiments begin, that each time your conditions are as close to the same as possible. You won't be able to control the air temp :| . I have the same year bike with similar mods. I run synthetic at 50:1 at sea level with an 8 plug. My main is a 155 and pilot a 48, needle 2nd from top. But if your conditions aren't exactly like mine, then I just gave you an algebraic formula for which you have no way of finding the answer. There are a ton of posts here for jetting and fuel mixtures. The only definite I can give you is, you don't need high octane in this bike. It runs just as well on 87 as on 97. :thumb: