box stock main jet

W5R

New member
29-31 should be fine,stock air filter wont let you run much bigger of a jet than that. if temps are too hot lower your timing
 

alvin l nunley

Site Supporter
29-31 should be fine,stock air filter wont let you run much bigger of a jet than that. if temps are too hot lower your timing
Not that I disagree with you in any way, just wondering why this engine uses such a small jet compared to what I’ve seen in most posts?
I see .039” recommended all the time. I’m guessing this must be a different type of engine or carb or something and I would just like to know what the difference is.
Comments, compliments, criticisms and questions always welcome.
 
Not that I disagree with you in any way, just wondering why this engine uses such a small jet compared to what I’ve seen in most posts?
I see .039” recommended all the time. I’m guessing this must be a different type of engine or carb or something and I would just like to know what the difference is.
Comments, compliments, criticisms and questions always welcome.

He is running a stock air filter, it wont use as much jet as an aftermarket air filter adapter/filter.
 

W5R

New member
yes it can al, the stock air box used on clones is basically an air filter inside an enclosed box, comppared to an aftermarket air filter that is out in the open when installed on the engine. The stock airbox breathes air thru a small hole on the bottom side of the airbox rather than thru the entire air filter. I would say the stock airbox gets approx 25% of the air that an aftermarket air filter gets if you were to compare the 2. I honestly dont see how the stock airbox even breathes enough air for the engine to run when used for racing, but it somehow does and works.
 

Jeff

Member
A 31 main jet and 19 low jet works perfect with small pipe and stock air box.any bigger jetting and it will flood out. Shoot for 5800 to 6000 rpm with 6000 if running a small bull ring.
 

alvin l nunley

Site Supporter
I honestly dont see how the stock airbox even breathes enough air for the engine to run when used for racing, but it somehow does and works.
Well, like my SIG says; if the data doesn’t match the theory, get a new theory. If that engine can reach 6000 RPM, with anywhere near the right gear ratio, it must be getting enough air. I could be wrong, but I sure would like to see comparisons of the two on a dyno.

Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.
Tuning is tough! (Al Nunley)
 

alvin l nunley

Site Supporter
A 31 main jet and 19 low jet works perfect with small pipe and stock air box.any bigger jetting and it will flood out. Shoot for 5800 to 6000 rpm with 6000 if running a small bull ring.
I'm still trying to figure this out. Just curious, how much difference is there in gear ratio between a filter and the stock airbox?
 

Racer14sl

New member
I honestly dont see how the stock airbox even breathes enough air for the engine to run when used for racing, but it somehow does and works.

Yes, it works very well. You can make one pull beyond 6000 very strong with the right setup...
Define what's a little hot for you? A 31 main jet should be just fine.
 

Jeff

Member
I'm still trying to figure this out. Just curious, how much difference is there in gear ratio between a filter and the stock airbox?
The stock air box is like a restricter plate. If you take the 3 litle screws out under the filter and lift the top off and look down in there you will see just how small the hole for airflow there is. If your aloud to modified the air box then take a dreamil and start reaming. Take the filter paper out of the filter and just use the outer foam filter with light oil on it. Then you can go to a 34 jet.
 

W5R

New member
The track TJ (thomas james) and i race at has strict rules for box stock, the only jets allowed are the stock 28 main and 19 low speed jets, stock flywheel, and stock airbox that cannot be modified. We can also use any CL series or any BSP series cams. So, with the jetting being stictly limited, those who have high engine temps dont really have many choices on how to fix that problem or get the temps down. I think Tj is asking how he can get the engine temps down without messing with the jetting or timing. His engine is stock, same as mine.

I have been talking to him about his problem and i/we believe the chassis is tight and bound up causing the temps to be high. Im gonna try and help him free up the kart the next time at the track.
 

buckster

New member
Jeff-The airbox you are talking about in your 12-24-13 post- Is the restriction that way on a Harbor Freight Predator?
 

alvin l nunley

Site Supporter
Very restrictive rules! Still, and not knowing the rules, there are things you can do.
Check the timing advance and let us know what is.
Tell us what spark plug you are using, and what heat range.
Tell us what fuel you are running.
As a last resort, I would go to the next club meeting and tell them how stupid it is to restrict the jetting. Tell them how air density can change, up and down, and with a change of only five points higher, the engine could go too lean and that’s not good. Tell them the stock jet was decided on with the engine turning only 3600 RPM, not a engine turning 6000 RPM while pulling a kart, don’t know what weight class you’re running, the heavier the kart, the more important the mixture. Big difference. Not that I know a whole lot about four cycle racing, but this mixture thing is pretty basic to all engines.
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.
 

sCREamnClones

New member
AL....'Down-boy, down'! I agree w/ ur theory, but i'm gonna guess that these folks are just out too have fun w/o alot of red-tape and i'm good w/that also. If their rules imply parity and maintain a level playing field under Any conditions....'job done'! (?); How 'hot' is it running.....what oil is he using and how much.....what rpm is he currently turning? As Al stated....where is the timing? Note: If this is a 'hvy'...the problem could very well be Not 'enough' timing! Current rpm's knowledge may support that concern given gear ratio is proper. jmo
 

Jeff

Member
If your stuck on stock jetting then there ain't much you can do to get temps down just run a cold plug range and run cool power medium oil. I can tell you by going from a 3910x to a 3933x my head temp went up 25 deg. If it's no hitter then 430 you are ok. Just keep the oil changed
 

W5R

New member
his temps are around 450, 3910x plug, stock timing, thor oil. this track mostly is for fun, they dont put alot of money into the box stock classes and it keeps the playing field level with good close racing, which is what keeps the racers coming back week after week.
 
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