Box Stock

Timing Advice

MRG29

MRG29

New member
We have been running the Predators in our adult class for two years now and are allowing the JRs to run them next year. A couple timing questions.

Motor - Stock cam (sealed) 10.8 springs, single stage pipe, re jet carb, air filter adapter been running 5 degrees advance with stock flywheel. Next year both Adults and JRs will be running the ARC Billet.

What timing should we shooting for in:
Adult
Green Plate
Blue Plate

Thanks
 
More timing = bigger jetting, keep that in mind. I would try 32-34 for the adult engine and 36-38 for the plate engines. You will also want to jet the plate engines richer to make up for the restrictor plate. Possibly .040/.028 as a starting point and tune from there, you may end up as big as .034 on the low speed jet. For the adult engine, I would start at .039/.024 and tune from there, may end up with a .026-.028 low speed jet. These are just a starting point, would be best to have them dyno tuned if you can afford it and have access.
 
Weddle Racing said:
More timing = bigger jetting, keep that in mind. I would try 32-34 for the adult engine and 36-38 for the plate engines. You will also want to jet the plate engines richer to make up for the restrictor plate. Possibly .040/.028 as a starting point and tune from there, you may end up as big as .034 on the low speed jet. For the adult engine, I would start at .039/.024 and tune from there, may end up with a .026-.028 low speed jet. These are just a starting point, would be best to have them dyno tuned if you can afford it and have access.
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it just amazes me how big the swing is between recommended jet sizes. .032" – .038" equals a 41% change in flow. On the low speed; .028" – .034" = 47% change in flow. (Jet bore area)

The difference between a .038" and a .039" jet is only a .001" increase in diameter, but it's a 5.4% increase in area.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
 
Weddle Racing said:
More timing = bigger jetting, keep that in mind. I would try 32-34 for the adult engine and 36-38 for the plate engines. You will also want to jet the plate engines richer to make up for the restrictor plate. Possibly .040/.028 as a starting point and tune from there, you may end up as big as .034 on the low speed jet. For the adult engine, I would start at .039/.024 and tune from there, may end up with a .026-.028 low speed jet. These are just a starting point, would be best to have them dyno tuned if you can afford it and have access.
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just to be clear here, at the beginning of my post, is said 32-34 for adult and 36-38 for the plate engines, that was referring to timing, not jetting.
 
Weddle Racing said:
It also amazes me how you keep chiming in about engines you have never worked with nor have any experience with at all Al. Those of us who do this stuff everyday know what these engines take to run and perform at their best. Have you ever pulled a clone or predator apart? Ever had one on the dyno and tuned it? Played with jetting and timing? Alot of us on here trying to help have, and aren't just pulling numbers out of a hat or throwing darts in the dark based on mathematical theories and air density changes. If you knew anything about these engines you would know they are completely different than your 2stroke smokers when it comes to jetting, carb setup and everything else, air density does not affect these engines enough to be constantly changing the jetting during a race day unless you just like to be chasing your tail at the racetrack. Not trying to be harsh here but you really should stick to what you know and stop confusing people who are trying to learn these engines and how to work on them.
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I can see now where I misunderstood, thank you for that. It seems people use the numbers 38 interchangeably with .038. My mistake.

As far as jetting is concerned, if even the smallest change makes a difference, (for example; different gear sets with the same ratio making a difference) I guarantee you that changes in the air density will make a difference.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
 
Weddle Racing said:
It also amazes me how you keep chiming in about engines you have never worked with nor have any experience with at all Al. Those of us who do this stuff everyday know what these engines take to run and perform at their best. Have you ever pulled a clone or predator apart? Ever had one on the dyno and tuned it? Played with jetting and timing? Alot of us on here trying to help have, and aren't just pulling numbers out of a hat or throwing darts in the dark based on mathematical theories and air density changes. If you knew anything about these engines you would know they are completely different than your 2stroke smokers when it comes to jetting, carb setup and everything else, air density does not affect these engines enough to be constantly changing the jetting during a race day unless you just like to be chasing your tail at the racetrack. Not trying to be harsh here but you really should stick to what you know and stop confusing people who are trying to learn these engines and how to work on them.
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Comment deleted, it ain't worth it...
 
MRG29 said:
We have been running the Predators in our adult class for two years now and are allowing the JRs to run them next year. A couple timing questions.

Motor - Stock cam (sealed) 10.8 springs, single stage pipe, re jet carb, air filter adapter been running 5 degrees advance with stock flywheel. Next year both Adults and JRs will be running the ARC Billet.

What timing should we shooting for in:
Adult
Green Plate
Blue Plate

Thanks
Click to expand...

As stated our flywheels come set at 32 degrees with a stock key. I would leave it there and play with jetting because you are using a stock cam and stock carb and may not get the full benefit of a lot of timing. One thing to consider with Predator engines is some engines came with carbs with a .570ish venturi and some came with a .610ish venturi. The jetting will be slightly different between the two carbs along with what has been done to them.
 
alvin l nunley said:
I can see now where I misunderstood, thank you for that. It seems people use the numbers 38 interchangeably with .038. My mistake.

As far as chastising me for what I say, I don't think you have that right, or the authority. I could be wrong.

I'm confused, while we're on the subject, you telling me I'm wrong leaves me with the feeling that you think you are right. Nobody's right, 100% of the time.

As far as jetting is concerned, if even the smallest change makes a difference, (for example; different gear sets with the same ratio making a difference) I guarantee you that changes in the air density will make a difference. You can go on believing your right about it not making a difference, but in this regard, you're absolutely 100% wrong. And one other thing, you're telling newcomers that it makes no difference, has got to be just as equally wrong compared to anything I've told them

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
Click to expand...

Dont get me wrong, I'm not saying air density changes wont require jet changes, I am saying that such small changes in air density dont seem to affect the performance of these engines as much as you would think, not enough that you would need to change jets on a regular race day. Now, there may be days where the air density will change drastically from the start of the race day to the end of it, and those times are when you would be right and would need to change jetting, but even then, its not even a full jet size that you would change most of the time unless its just really poor weather. Usually, you would only change jetting half-size if you change it at all, if that makes sense. I have been to alot of big shows, and have yet to see any of the big teams or front runners and winners with a dyno or air density guage at their trailer playing with the carb jetting. Occasionally you might see someone swap jets, but its not very often at all. Carbs are actually painted at the races I have been to, so you cannot take the carb off or make any changes once you qualify.

I am not saying air density has no effect on jetting or won't require a change in jetting, I am saying that the numbers you are giving people are not something the average racer is going to have to worry about at a regular weekend local racetrack, thats all. Yes it does make a difference, but not as big of a difference as gearing or tires, prep, air pressure. What I'm saying is you can be half or a whole jet size off the "ideal" jetting and still have a good running engine, good enough to win the race. Of course nobody wants to be off on jetting, but it is usually not enough to hurt performance as bad as you make it sound.
 
JPR57 said:
Comment deleted, it ain't worth it...
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Go ahead and speak up, I know your probably just itching to say something to discredit what I said or something like that, or how I dont know anything about what I'm talking about like you have done before on here. Go ahead and get it out, might as well. I know I'm not right about everything 100% of the time, and I don't claim to be, but I'm also not a professional or a kart shop owner who has anything to gain by helping people or trying to push products I sell on people. I just try to help if its something I have experience with or know something about. I have successfully helped hundreds of guys on this site with building their engines or putting a build together and have never asked for anything in return.
 
Now Zach, why would I down talk one of my biggest sales generators..? Like I said, it ain't worth it. You just keep right on keeping on.
 
Non Hemi Predator Tuning
I am looking for some advice or recommendations in regards to tuning my 212cc non-hemi predator engine. I am building the engine for off road use and looking to make good low to mid range power (RPMs not to exceed 6000). I have made the following modifications.

Removed governor & low oil sensor
Removed spark resistor & replaced plug boot
.010 steel head gasket
Port matched and polished head & manifold
Lapped Valves
Filed throttle butterfly screw (stock carburetor)
1.3:1 ratio rockers (stock cam)
22 lb springs, lash cap & exhaust valve retainer on intake valve
Billet guide rod plate
4 degree timing advance key
3019 racing spark plug
Valve cover breather
Performance Air Filter
Jet kit (140 emulsion tube, 36 or 37 main jet)

I have two questions.

1.How big should I drill out the low speed idle jet for this build? I have seen .020 for stock engines and up to .025 for modified.

2.Which exhaust pipe would work best for this build? I have been considering the 1 inch header tube with Briggs screw in muffler. I have also been considering the Box Stock Header and Muffler Honda/Clone with the mini 91 muffler. However, this is only a 1/2 inch exhaust pipe.

I appreciate any advice or recommendations that you all can offer.
 
I would go for a better less restrictive pipe and muffler, like the Southern Express Advantage pipe and Rlv muffler. I'd also make the low side jet .024-.026 if your looking for that low-midrange power
 
Weddle Racing said:
I would go for a better less restrictive pipe and muffler, like the Southern Express Advantage pipe and Rlv muffler. I'd also make the low side jet .024-.026 if your looking for that low-midrange power
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I set the timing to 29 degrees + or - a degree and installed a .036 main jet (the .037 was too big. I could smell gas, the exhaust would back fire when it idled down and it fouled the plug). I started with a .021 low speed idle and it seemed to run ok but I couldn't tell if I should go bigger or not. I eventually ended up going to .028 low speed idle jet when I noticed black smoke coming from the exhaust. I noticed that the plug would foul when it idled even with a small low speed jet. Is this normal or should I use a a smaller low speed idle jet?
 
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