Track Tac

When to change jet

M

Mikyll

Member
This question would be for animal or clone. If you have your motor dyno'd, it is tuned for the best jet on that day under those conditions. Those conditions will most likely not be anywhere near the same on race day or night. If the motor is running fine at the track, how do I know if I have the best jet to give me peak HP? Does a jet change even make that much of a difference in these motors if it is already running good? I'm pretty aware of the symptoms of going too big or too small. But how do you know when to change jet just to get better performance?
 
The best way to keep track of this is to send your air density gauge in with your engine and ask your builder to place it in the dyno cell with your engine when it is tested (or wherever intake air is drawn from.)

When you get it back - you've got a baseline to go off of. Jet up by the % the AD increases, jet down when the AD goes down.

--
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cuts
www.CarlsonMotorsports.com
Celebrating 25 years of service to the karting industry
765-339-4407
bcarlson@CarlsonMotorsports.com
 
Just to add.

Longacre says a 1% change in air density requires a 1% change in jetting. I have a spreadsheet that shows if you go from a 38 jet to a 39 jet that's a better than 5% increase in flow. As far as I know Jets only come in even sizes. .038 .039 .040 etc. etc. you can buy reamers from MSC in increments between .0?? and .0??. You can make sizes like .0383, or maybe .0393. If you check the English and metric reamers between .038 and .039, you'll see maybe three or four different sizes. For your information, a 1% change in jet size, in that range, is about .0002 change in diameter. These reamers don't come cheap but when your friends find out you have them, there's an income opportunity. Reaming Jets is infinitely more precise than drilling. I'm sure if anybody did some precision flow testing they would discover this is true. After all, fuel is something like 700 times denser than air, so the slightest imperfection in the jet hole, can make a big difference in flow.

Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley)
 
The key to the use of an air density gauge as your decision maker as to change the jet or not is to establish a sound, repeatable, bona fide base line. On these low compression type 4 cycle engines, plug reading to determine air fuel mixture quality is very difficult. Like you said, the best way to determine the optimal jet size is with dyno testing. You MUST have the air density gauge there that day to establish this base line as Brian Carlson stated.. Its great to know that a jet size change from one to another is so many percentage change, but this is useless information unless you know what the best size is for a certain air density to begin with. These 4-cycle lower compression engines (less than 20 hp) don't seem to respond drastically with reasonable jet changes. Its sometimes hard to tell the difference between a 38 and a 39 even though it is a roughly 5% change. They seem to have a "range" that they will work equally well within. Finding this "range" is the trick. So the best and most accurate way to determine if a jet change is needed is with a quality air density gauge. Practice with it. It will surprise you on what affects it and what doesn't. Knowing there has been a 7% change in air density from this morning to tonight is of little value unless you know when the jet size is right to begin with. Good luck
 
jsstump#70 may be right when it comes to stockers with the low compression, but as a compression goes out, the air density ought to be more and more important. Some guys are running awfully high compression ratios. Some as high as 14 to 1. Jsstump, what do you think?


Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley)
 
bigdave55 said:
hot out go up cool out go down
Click to expand...

If it is hot out you would go from a 37-36, if it is cold out you would go from 37-38.

On our findings in our SA motors running 10.5-11.5 comp. one jet change up was going from 11.5afr to 12.5-13afr. One jet change down was 10-11afr. On 94oct. Gas.
On a 50*f day was running 13afr, on a 60*f day it was running 12afr.
 
Ambient air temperature is only one variable.
"Generally" hot air is "thinner" air, cool air is "heavier" air. Not always, just a generalization if all you have to go by is air temp.

An air density gauge is much more accurate. You can use a Computech weather station and use density altitude (AD) and be even more accurate. A/F ratio on the dyno probably the most accurate. Use all three of these tools together and you can really get down to brass tacks. :)
 
alvin l nunley said:
jsstump#70 may be right when it comes to stockers with the low compression, but as a compression goes out, the air density ought to be more and more important. Some guys are running awfully high compression ratios. Some as high as 14 to 1. Jsstump, what do you think?
Click to expand...
Compression is king! I have found as performance goes up (compression and flow) most every adjustment becomes more crucial. The more hopped up the engine gets, typically the more finicky it gets too. This includes but is not limited to jet selection. When I first saw this thread Al, I thought of you. In a positive way I assure you!
 
Wouldn't your spark plug and engine temp play a role as to indicating if jet is being too rich or too lean? Most decisions when your that far off are being decided at the track, wether we like it or not. Best way is to have two engines or carbs set up with the changes and then swap them out under same race conditions. Good Luck.
 
Not sure about anyone else's, but my spark plugs in an alcohol animal always look nearly brand new when pulled out. Not sure how they could possibly be read. Now the plugs in a gasoline burning clone are generally pretty black. I've been told that clones tend to burn rich and the blackened plugs are normal even when jetted properly. Is that true?
 
Mikyll said:
Not sure about anyone else's, but my spark plugs in an alcohol animal always look nearly brand new when pulled out. Not sure how they could possibly be read. Now the plugs in a gasoline burning clone are generally pretty black. I've been told that clones tend to burn rich and the blackened plugs are normal even when jetted properly. Is that true?
Click to expand...
every manufacturer's brochure that I've ever read about plug color says it's not normal. You need to start with a new plug to get a good reading.

If you had an air density gauge, it would be very easy to tell when you need to change the jet. A good air density gauge is about $100. Money well spent. You only need one at the track, so get your friends to chip in with you and buy the thing. You can order them from Longacre.

Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley)
 
depends on what clone you are referring too.
In my SA motor they are light brown, to a medium brown, tip is clean. at 11-1 comp ratio, using 94 pump gas. running at 12-13-1 afr.
running 33-34* timing.

And Al would be right with air density on this one.
Got carbs. from many big builders, they are all very close in altitude to me...but rejetting was always necessary.
 
You must log in or register to reply here.
Back
Top