changing premix ratio and affect on jetting

[joshp]

I'm currently running 32:1 and I've got all kinds of spooge. My bike's jetting is off but I still would like to change to 40:1. I know I will have to buy leaner jets but how much leaner? This is a 25% reduction in premix or 25% increase in fuel, so what does that amount to typically for amount of jet sizes that I need to go down? When I say jet sizes I'm talking 1 step is 5 integers example 60 to 55. I'm just trying to get it to where I can start the bike and it will run. From there I can figure out what jets I need.

Thanks. I tried doing a search but didn't see anything directly talking about this. If it has been mentioned please provide link. Thanks.

 

[TimberPig]

Your math is off, it's actually 20% not 25%, but it matters little anyhow, as the change in the air/fuel mixture received by the engine is only about 1%. This is because the engine can only draw a fixed volume of fuel through the carb jets. Also by going to 40:1 from 32:1, you are actually increasing the fuel in the air/fuel mixture and thus making the air/fuel richer, which will equal more spooge not less. If you really feel the need for a premix ratio change, go ahead, either 32:1 or 40:1 is fine, but to stop your spooging problem, you need to rejet, not mess with the mix ratios. Pick a mix ratio, and then start setting up the jetting with it.

 

[crazyrydermx]

Your math is off, it's actually 20% not 25%, but it matters little anyhow, as the change in the air/fuel mixture received by the engine is only about 1%. This is because the engine can only draw a fixed volume of fuel through the carb jets. Also by going to 40:1 from 32:1, you are actually increasing the fuel in the air/fuel mixture and thus making the air/fuel richer, which will equal more spooge not less. If you really feel the need for a premix ratio change, go ahead, either 32:1 or 40:1 is fine, but to stop your spooging problem, you need to rejet, not mess with the mix ratios. Pick a mix ratio, and then start setting up the jetting with it.

Bingo! You need to rejet. If you need help, just tell us and we'll get you through it.

 

[motometal]

this is not something that can be easily calculated with math, guys...you would have to account for the viscosity of the fuel and oil, the differential between the two, etc

although ratio does affect jetting, I would doubt that it would affect it more than one jet size on the main or one clip position on the needle, going from say 32:1 to 50:1.

 

[joshp]

Bingo! You need to rejet. If you need help, just tell us and we'll get you through it.

Well I'm having problems jetting a 93 RM 250 and I've got all kinds of premix spitting out the back. It leaves speckes all over the ground. Part of my issue is that I have a PJ keihin carb that doesn't want to idle. I've seen other threads talk about switching to a PWK keihin carb but I just want to get it to where I can go out riding tomorrow. There is a lot of smoke coming out the exhaust so I figure it is running rich. I'm running motul 600 and I've read on this site I believe that it has a tendency to foul plugs so I figured maybe I should go leaner on the oil to try and prevent this by running 40:1. There currently is unsmooth/ hesitation until you get to 1/2 throttle and on then it pulls clean. Right after the hesitation it jumps. If you're hard on the gas it's great but with the fact that I suck at riding and that I'm riding trails that I'm not too familiar with I can't be flying down them. I'm finding that my adjustment have to be counter productive to get it close to idling for example I screw out the choke/ idle screw but at the same time I have to screw out the airscrew. I don't know if this is because essentially when I do this I'm increasing both air and fuel mixture going in allowing it to come closer to idling. I'm not too worried with idle as I figure I can compression start it going down hills if I kill it. The main thing is to make my low end power actually functional. Currently I've been playing with a 55 slow jet. A 60 is way too rich a 55 I believe is also too rich but when I go to a 52 it doesn't want to start. But if I recall right I believe I still had hesitations down low when I ran the 52 meaning I may need to go leaner correct? I printed out an article from this site about jetting but I don't know how accurate it was at telling what jets were functioning at what throttle position along with the fact that some of the jets I don't even believe my carb has like the air jet (the writer isn't referring to the air screw because he explicitly says air screw when he is talking about the air screw). I can tell the feeling you get when you're running to lean when you're on the gas it does that boowahh sound but if you're lean on the bottom side of the power band (low throttle) what happens?

Help appreciated. I may end up getting a PWK carb but I'm debating on spending $190+ because it's a 93 and it is only going to get harder to find parts for it. Until then does anyone have an idea of what to do on the bottom? I think I'll buy a 40, 45 and 50 since I'm switching t 40:1 and I'll buy a 160 165 and 170 for the main (currently running a 172 main) . I will be riding from sea-level to around 2,500 feet.

 

[TimberPig]

Maybe you should read this

Carb Tuning

as it sounds like you don't really know how to correctly approach jetting a bike.

 

[TimberPig]

this is not something that can be easily calculated with math, guys...you would have to account for the viscosity of the fuel and oil, the differential between the two, etc

although ratio does affect jetting, I would doubt that it would affect it more than one jet size on the main or one clip position on the needle, going from say 32:1 to 50:1.

Actually it could be calculated very easily with math, using fluid mechanics, but all he was basing it on is a simple reduction/increase in the amount of oil in the mix, which is not the amount by which the air/fuel ratio changes.

 

[joshp]

Maybe you should read this

Carb Tuning

as it sounds like you don't really know how to correctly approach jetting a bike.

I actually read over that article and what threw me was the 1/2 to full throttle using an air jet. Didn't know what this was. I got it jetted really close. I changed to 40:1 w/ the motul 600 and much cleaner smoke out the exhaust (very little if any at this point). I don't think motul runs good at 32:1 unless you are great at carb tuning and you have a good carb which I don't; I have the pj keihin carb. The bike now idles. I'm running a 48 idle jet and a 172 main jet. It is currently running better than it ever has. Air screw is at 1 and 3/4 and the idle screw is at 56 clicks. I think it is currently running as good as I can get it with the pj carb. Eventually I may switch to the pwk carb. Thanks for your input everyone. For the sake of testing I tried golden spectra at 32:1 and it ran better (less smoke power was cleaner) with the same settings as the motul 600 at 32:1. I may play with 50:1 later just out of curiosity; but for now the improvement is incredible.

Also try to explain this. During testing the bike made that boowah sound and died. I then attempted to start it and it wouldn't start. The plug fouled (would no longer spark). How do you foul a plug when you're running lean? The only explanation I can come up with is running oil rich with motul at 32:1.

 

[TimberPig]

A partially fouled plug could reduce the strength of the spark enough to make it bog as if it was lean. Sounds alone isn't a reliable tuning indicator. You need to combine sound, feel, throttle response and plug burns to get a good idea of how the engine is responding and whether the jetting is correct.

 

[gpoints]

A partially fouled plug could reduce the strength of the spark enough to make it bog as if it was lean. Sounds alone isn't a reliable tuning indicator. You need to combine sound, feel, throttle response and plug burns to get a good idea of how the engine is responding and whether the jetting is correct.

We checked the plug after the boowah and it dieing and there was no spark (we pulled the spark plug out and grounded it while another person kicked over the bike). The feel was like it was lean or running out of gas. It happened at just below half throttle. To me the plug smelled like oil but not like gas. The jetting was slightly off but we used recomendations for a starting point from other riders with the same bike from this site that were at around the same elevation. So the jetting wasn't that far off. It ended up being only one jet size on the main when we switched to 32:1 using golden spectro instead of motul 600. Then we changed the motul premix to 40:1 and rejected and it was much cleaner than previous jettings with motul @ 32:1. The rejecting was mainly to adjust one jet size for the gas richer fuel mixture at 40:1. It could be superstition but I won't run motul 600 at 32:1 again. It seems like it is better suited to lower oil based premixes.

 

[motometal]

Actually it could be calculated very easily with math, using fluid mechanics, but all he was basing it on is a simple reduction/increase in the amount of oil in the mix, which is not the amount by which the air/fuel ratio changes.

it depends what you call "easily", first you would have to know the viscosity of both fluids, which I don't believe we do. Or, given the viscosities, we would have to combine this calculation with the calculation above relating the differential in actual fuel content, which isn't actually accurate anyway because a certain fraction of the oil burns, which can vary according to combustion chamber temperature and of course specific oil utilized. The other assumption made here is that there won't be any type of chemical reaction between the oil and gasoline, altering the net viscosity of the mixture.

 

[TimberPig]

it depends what you call "easily", first you would have to know the viscosity of both fluids, which I don't believe we do. Or, given the viscosities, we would have to combine this calculation with the calculation above relating the differential in actual gasoline, which isn't actually accurate anyway because a certain fraction of the oil burns, which can vary according to combustion chamber temperature. The other assumption made here is that there won't be any type of chemical reaction between the oil and gasoline, altering the net viscosity of the mixture.

Yes, you are right, it is well beyond the math capabilities of virtually everyone here, but given the skills of an experienced fluid dynamics engineer, and the information you mentioned, it isn't a particularly difficult problem to solve.

The simple take home message is that the changes in air/fuel ratio from a premix ratio change are very small in relation to the amount of oil added or removed from the mix. You are better off leaving the extra oil in the mix for lubrication, and jetting the bike to obtain the correct air/fuel mix ratio. Trying to richen the air/fuel mix with premix will result in fried cranks long before it results in significant changes to the air/fuel mix. Trying to lean it out with more oil in the mix will coat spark plugs in oil before it achieves the desired reduction in air/fuel ratio.