For off road and off track purposes you are jetted way too fat. For your use and that 140 emulsion tube I would go down on the main jet I would not go over .033" and go back to almost stock size pilot jet. You are getting advice from racers that are basically accelerating to 3/4 to full throttle and staying there for the duration of a race. Your application is completely different you are jetted too rich and loading the engine up which will not be much fun, off road vehicles can take you quite a distance in ten minutes before they quit, and it is a long walk(push) home. Take from someone that used to try and run a factory racing built snowmobile in a recreational environment. You need to go reliability rather than performance.
Timing Advice
- Thread starter MRG29
- Start date
Bone Stock
Member
Thanks I appreciate the advice. I did notice that when I would run run WOT for about 20 seconds and cut it off without idling the plug would be tan with no black soot. But if it just idled or ran partial throttle the black soot would build up. I'm going to start over and ease it up from stock and see what happens. I'll post the results. Should I try to clean out the combustion chamber? If so what should I use and how can I do it with out taking off the head?
alvin l nunley
Site Supporter
And just exactly what "small changes" in air density are you talking about? 1%, 2%? Do you even have an air density gauge so you can tell? Without an air density gauge, you're just going by the seat of your pants. If you're not real serious about tuning, (which it sounds like) that's okay, but if you want to get serious, you'll get an air density gauge.
While you might not want to change a jet size by .001" (.038" to .039" = 5.4% change) with such small changes in air density, if the air density changes by 5% you might. I've seen the air density change by 8% on a given day. You go from a hot afternoon to a cool evening, or a cool morning to a hot afternoon, and you might be surprised at the air density change. Over a year, from winter through summer to fall, it might change even more. Thing is, if you don't have an air density gauge you'll never know how much it changed.
I've told before how we had 200 jets, very similar to carb jets, made on a Swiss CNC lathe and how they varied in flow. It was a real eye-opener for me. Then I hear people talking about drilling jets! Drilling jets is a very crude method for sizing jets. If you drill a jet, I can guarantee you you'll have no idea what the flow rate is on that jet. Fluids, as I've said before, are 700 times denser than air, so the slightest imperfection in a jet can cause a significant difference in flow.
You know these are not ideas that I came up with, these are ideas that real professionals have come up with, I'm just passing them along. If you go to Longacre's and read the stuff they have to say about air density, you may change your mind about how important air density is.
wihtout adding to the arguement, air density does come into play when the heat of the day or the cool of the evening changes. i think that everyone agrees with that....but are you talking about checking the air density for each race and making adjustments based upon your readings at that moment? it seems that you would be constantly cracking the carb open to make changes in the fuel flow. at my track, we have a little bit between the heat and the feature to look into making those changes, but at your average short track, you might only have 15 minutes to get everything ready...i don't have a unlimited pit crew to devote to tires or chassis or checking the air density and making the appropriate changes. i got me and my son.....
so, based on the above, what would you consider as a reliable way to make the adjustments without spending all night trying to determine which djet would be correct for any given air density reading? do you have a method to say that if you detect a 5% change, then you would put this jet in? what do you set as a base guideline on jetting before you leave the shop? you have to have a baseline or are you relying solely on the readings at the track minites before you hit the track to determine the jetting? i'm not trying to sound all knowing or even to start any crap. i honestly want to know. last season it was 90+ during the day, but that evening the temp dropped drastically into the 50's.....i remembered you talking about air density and i swapped the carbs to one with bigger jets....had a good run from last to second before running out of laps....your info was spot on....but i would really like to know what baseline you start with.
if you start with a base of 38 and 21 on the jets, how much of a swing are you talking about with the density changes? what is the bottom line of leaving them alone or changing them based on air density? does 5% warrant a change? or is it done in increments? change of +/- 10% and you change jetting by XX? see what i'm asking? what consitutes a needed change?
sometimes the seat of the pants method makes up for lack of knowledge in the exact measurements. i can tell when the rear tires aren't inflated enough or too much without having a air guage in my pocket, simply by the feel of the kart on the track. i think that racers who run consistently can tell when the engine is sucking too much air or fuel without the jibber-jabber. versus trying to explain with math on everything, can you put it in simple terms on when and where you would draw that line between leaving the jets alone or changing them?
again, i'm trying to learn, not be a smarty pants......but what i'm looking for is a baseline on what i should start with and then what you would consider enough of a change to warrant a change either up or down.
so, based on the above, what would you consider as a reliable way to make the adjustments without spending all night trying to determine which djet would be correct for any given air density reading? do you have a method to say that if you detect a 5% change, then you would put this jet in? what do you set as a base guideline on jetting before you leave the shop? you have to have a baseline or are you relying solely on the readings at the track minites before you hit the track to determine the jetting? i'm not trying to sound all knowing or even to start any crap. i honestly want to know. last season it was 90+ during the day, but that evening the temp dropped drastically into the 50's.....i remembered you talking about air density and i swapped the carbs to one with bigger jets....had a good run from last to second before running out of laps....your info was spot on....but i would really like to know what baseline you start with.
if you start with a base of 38 and 21 on the jets, how much of a swing are you talking about with the density changes? what is the bottom line of leaving them alone or changing them based on air density? does 5% warrant a change? or is it done in increments? change of +/- 10% and you change jetting by XX? see what i'm asking? what consitutes a needed change?
sometimes the seat of the pants method makes up for lack of knowledge in the exact measurements. i can tell when the rear tires aren't inflated enough or too much without having a air guage in my pocket, simply by the feel of the kart on the track. i think that racers who run consistently can tell when the engine is sucking too much air or fuel without the jibber-jabber. versus trying to explain with math on everything, can you put it in simple terms on when and where you would draw that line between leaving the jets alone or changing them?
again, i'm trying to learn, not be a smarty pants......but what i'm looking for is a baseline on what i should start with and then what you would consider enough of a change to warrant a change either up or down.
alvin l nunley
Site Supporter
I don't think everybody agrees.
I hear people talking about changing tires between heats. There seems to be enough time for that. There seems to be enough time to change corner weights. There seems to be enough time to change gear sets, (same gear ratio) between heats. Is changing the jet in the carburetor all that much more time-consuming? When to change gear sets sounds like black magic to me, while air density, and the need to change jets, can be accurately measured, no guessing, no black magic. I'll take proven, verifiable, fact I've found, against black magic any day.
And yes, check the air density at every race, and all during the race day. Keep a record of it!! Do this all year long! Looking at the NO GOATS printouts, (very good by the way) I see people keeping track of all kinds of things. I don't know if there is a place to record air density, but if there's not, I'm sure there should be.
alvin l nunley
Site Supporter
Have you ever heard of, "horsepower correction factors"? Testing an engine on a dyno, without correction factors in the software, gives you nothing but useless numbers. A good dyno will have a built-in barometric pressure gauge and temperature gauge. At the very least, the dyno room should have these two instruments hanging on the wall and the numbers entered, into the software, after each run. Some even have a gauge for humidity, but humidity is such a small factor in calculating corrected horsepower, it can be ignored.
"Simple terms"? What could be more simple than reading an air density gauge and changing jets to match any changes? A 5% change in air density would call for about a 5% change in jet size. Changing the jet from a .038" to a .039" is about a 5% change in area.
You first need to find the best jet at a given air density. This is really easy to do on a dyno. It's a little harder on the track, but still possible. When you find the jet that gives you the best performance, at a given air density, that is your baseline. Anytime the air density goes up or down, you change the jet accordingly. A 1% change in air density, with a starting jet size of .039", would call for a change in the jet size of .0002". Or .0392". Not very practical. Still, that is what the math calls for. Do the math on the area for jet sizes .020" to .040" and you'll see that the smaller the jet, the bigger the percentage change with an increase, or decrease, in size. For instance; .020" to .021" is a 10.2% change in area. .038" to .039" is a 5.4% change in area. Pi x R squared is the formula. I have a spreadsheet that does the calculations for me.
On the subject of jet sizes; drilling jets is a very crude way of opening them up. As I've said before, fuel is about 700 times denser than air, so the slightest imperfection in a hole can cause a big change in the flow rate. If it was me, and I was racing LTO four cycles, I would be flow testing my jets. And even though reaming jets is better by far than drilling them, I would still check the flow rate. I find it strange that people go to a lot of trouble to flow test carbs and heads but give no attention to the Jets. Or air density. The very best flowing carb, or head, is of little advantage if the mixture is too rich or too lean.
"Not enough time" is just a rationalization in an attempt to justify why you don't have an air density gauge. ....nope.... looked into getting one...unfortunately in this case, it's not rationalizing....it's the cost.....I can't justify spending that much on a tool for changing jets. while I understand more about getting a good baseline down within known parameters, changing jets will still have to be by my butt in the seat. thanks for explaining a little more and pointing me in the right direction for information (google, ebay and amazon)...it's an expense that I can't justify with my wallet and racing budget. if I traveled to other tracks for bigger money races, then it might be one of those things that you shouldn't leave home without, my traveling days are all but over and i'll stick to one track and do the best that I can with what I have.....thanks al!
alvin l nunley
Site Supporter
I totally understand. Been there done that.
Maybe you're not quite understanding what I mean by a rationalization. "Not enough time" you say, but the truth is, by your own words, you can't afford it. Your initial statement, "not enough time" is the rationalization. Not a problem, we all do it at times.
still...thanks al....i understand a bit more than i did on air density and looking at other articles on it so that i can understand further. although the budget won't extend for me to get one, i'm looking at alternate ways to keep track of the density to allow me to adjsut from there....don't have any ideas on the alternatives, but i'm looking!!
alvin l nunley
Site Supporter
Let me make a guess; if you needed a set of tires you would find the money. If you were going to do some extensive traveling to out-of-town races you'd find the money. I don't think I've convinced you of the benefits of the air density gauge.
This has to be the first time I have ever seen anyone refer to gearing as black magic lol. There is no black magic with gearing, just have to rely on experience to know when and how much to change, same thing applies to jetting. Alot of guys can change the jetting at the track without an air density guage, just using a little common sense Al. At most of the races I go to, the carb is painted after qualifying and can't be removed from the engine, so we can't change carbs like Mikey did. Its easier for me to pop the carb bowl off and swap jets than it is to change the entire carb anyways, but I'm not gonna be changing jets everytime air density changes 1%, its simply not needed with these clones and predators. Now if it were to change by 10-20%, I would agree with you that its time to make some jetting changes.
alvin l nunley
Site Supporter
I was referring to the practice of changing gear sets, with the same ratio. Knowing at what point this would be beneficial appears to me to be black magic.
alvin l nunley
Site Supporter
I agree, 1% is hard to adjust for, (.0002" in jet size starting with a .038") but I don't agree that it would make no difference. Let me show you some numbers for 10% – 20% changes in air density.
.038" – .039" = 5.379% difference in area. This would cover 5 points in the air density.
.038" – .040" = 10.847%
.038" – .041" = 16.402%
.038" – .042" = 22.134%
I'm going to suggest that 1/2 size jets would come in handy.
And remember, as the jet sizes go up, the percentage change decreases between sizes, as the jet size goes down, the percentage change increases.
Example;
.020" – .021" = 10.191% difference in area.
.020" – .022" = 21.019%
alvin l nunley
Site Supporter
It's nice to see someone doing it right. Although change comes slowly, with persistence, it happens.
MSC has reamers of almost all sizes. They're kind of expensive, but well worth it. You can offset the cost by selling jets to your friends.
A suggestion; half sizes in the smaller jets because the jump from .020" to .021" is a 10% change. The change from .038" to .039" is about 5%. Half sizes in the high-speed jets is still a good idea.
madd dawg racing
Member
It's hard for me to ream a .001 much less .0005 steady i guess...
alvin l nunley
Site Supporter
Speaking from experience, I don't understand that. I spent 20 years as a CNC machinist and programmer and it doesn't appear to me to be all that hard a task.
What kind of equipment are you using. As is said; the right tool for the job always makes it easier. I've heard of people sizing jets with a vice and a hand drill. I'm not saying, or implying, that's what you're using, but I've heard of people doing it. Holding a tolerance of a .0001" isn't all that easy a thing to do, but with the proper equipment, it's doable.
Actually, I think doing it is easier than verifying that you did it. LOL. I get nervous when people talk about drilling jets. And nobody flow tests jets, at least I've never heard of anybody doing it. It's been my experience that even the most precisely manufactured jets, with the "same size hole" have a difference in flow. I don't recall the name, but one of the big manufacturers of carburetors doesn't mark their jets by size, but by flow.
al....my budget is more or less set in stone with a modest amount for contingences other than the initial outlay for the start of the year. i do this because i refuse to "borrow" from the household budget....which includes all utilities, bill payments and food. I've seen too many racers lose more than a race by betting the bank on winning and coming up short. 've also seen them win big money but spend it the next day without paying back what they borrowed which leads to more problems....
all i'm saying is that a $200 expense isn't in the budget so i will search for other alternatives and go from there. I don't have the luxury of having a deep pocket, so i operate inside the budget that I've set so that i can have fun racing without getting into trouble with momma.....cuz, as we all know, if momma ain't happy....ain't nobody happy......
all i'm saying is that a $200 expense isn't in the budget so i will search for other alternatives and go from there. I don't have the luxury of having a deep pocket, so i operate inside the budget that I've set so that i can have fun racing without getting into trouble with momma.....cuz, as we all know, if momma ain't happy....ain't nobody happy......