Hemi Predator Build #2

[bob58o]

I ordered this throttle cable. Every flatside throttle cable I found was 75".
It was only $7 free shipping and NOT shipped from China.
I hope it screws into the standard minibike hand grip throttle thing.

http://www.ebay.com/itm/172476479709

 

[bob58o]

Flywheel selection and Ignition Timing

Did you all know that as RPMs increase on these engines, ignition timing is retarded???

I've read the timing is retarded ~4-6 degrees (up to 8 degrees at 8,000 RPM). So stock timing (around 24* BTDC) may become 20* BTDC at 6500 RPM. So as you need ignition timing to advance, it is retarding.

So If you are looking for actual ignition timing of 28* BTDC at 6500RPM, you would advance the timing ~8 degrees. Because the ignition system will retard the spark ~4 degrees at higher RPM - an 8* offset key, for example, will only actually advance the ignition timing 4 degrees at high RPMs.

But at low RPMs the timing is not retarded. The same key you need to achieve proper timing at high RPMs will mean you are too far advanced at low RPM. If around 24 degrees BTDC is good for 3600 RPM, the 8 degree key has you at 32 * BTDC.

Obviously you would want a system that works in reverse compared to this. Like a car or something where initial timing was around 24 -26 degrees BTDC and at higher RPM it would advance to 28 - 30 degrees BTDC.

Not a big deal for racing when you keep it above 5k RPM the whole race, but something to think about on fun/yard karts.

I guess these expensive Briggs PVL coils ($95)
http://www.arcracing.com/555681-magneto-pvl-for-animal/
are the only ones that don't retard timing with high RPMs. Needs a PVL flywheel too for $160 (Hemi)
http://www.arcracing.com/6600-ph-predator-212-hemi-pvl-lite-flywheel/

These are set to 29 degrees BTDC. They are 29 from 2k to 12k RPM.

A 32* flywheel with stock ignition system will be around 32 at 2k and ~28 at 6 or 7k RPM

The PLV system helps reduce the compromise between tuning for low RPMs 4k-6k RPMs and tuning for higher RPMs (6k-8k+). If the timing set for 7000 RPM, then it is only 4 degrees too advanced at low RPM, instead of 8 degrees too far advanced with the stock magneto.

ARC Racing
June 2, 2016 ·
Just want to shed some light on the subject of our 6600 Ultra Lite Billet Flywheels that use the Briggs PVL ignition for the Honda GX200/Clone 196cc engines.
First off all these flywheels come set at 29 degrees of timing with a stock key, not the traditional 32 degrees of timing used in our 6619 or 6602 flywheels. This is because the Briggs PVL ignition does not retard the timing as the RPMs increase like the standard ignitions do. Meaning if you set the timing at 34 degrees static, it will still be at 34 degrees all the way to 12,000 RPMs. The standard coils can lose as much as 8 degrees at 8000 RPMs depending on the type of flywheel used. Therefore alot of timing advance is not necessary with the PVL ignition.
Secondly the firing point on the flywheel is a little different with the PVL ignition than the standard coil. As you see in the photo the firing point with the PVL is when the trailing edge of the second magnet (as the engine rotates) is in line with the right hand edge of the second coil leg. The PVL coil has 4 legs, two short and two long. This is one area we get the most phone calls about. People using a timing light to find the firing point are not seeing the same as the paperwork we send with the flywheel. That is because they are usually not spinning the flywheel fast enough. This coil will automatically retard at very low RPMs to help the engine start. Therefore you will need to spin the flywheel at least 1800-2000 RPMs for it to fire in the right spot. Most drills will only spin 800-900 RPMs and will give you a false firing point.
This will be the same for all of the PVL ignition flywheels we sell for the Animal, Flathead, Predator, Predator Hemi and Big Block engines.

 

[bob58o]

Reinforced rockers.
57 grams.

Stock rockers are 38 grams.

 

[bob58o]

More stuff.
black is 32mm ID, 1.65" OD straight

Blue is a reducer. 35mm ID to 32mm ID.

Not sure if I'm going to use either yet.

 

[bob58o]

I built this engine because my last minibike did 55mph at 7000 RPM.
Now, could I really have the next one be slower??? I think not!

So this engine was built to go fast. Big Carb, Big Ports, Big Valves, Big Cam....
Clutch engagement at 4000 RPM.
I'm not really sure why I'm fighting myself about the ignition timing.

I should just advance it for max power at high RPMs right???? Just so long as the octane is high enough and I jet the low end rich enough to keep it from pinging, I think it will be ok. The whole engine is designed for high RPMs so it is would be dumb to set the timing for the low end of the powerband. I think. What do you all think???

I'm going to be right around 11:1 static CR.
I will be using 100 octane gas.
Going to start tuning with a cold plug, the AR3910x.

I might get a 2, a 4, and a 6 degree key as well. It will give me 0, 2, 4, 6, and 8. Its a pretty good set to have. The 32 degree flywheel with a 2 degree key gives me 30 and 34. With the 4 degree key, I have 28 and 36. With the 6, I get 26 and 38 degrees BTDC.

Pretty confident that being able to adjust from 24 to 38 degrees BTDC covers the entire range from low end builds to spinners, from 8:1 to 12:1 CR, from 87 octane, to methanol, to nitro and beyond.

I still may decide on the PVL setup. What is your opinion?

---------- Post added at 11:08 PM ---------- Previous post was at 10:54 PM ----------

29 degrees (no retarding at high rpms) 1.6 lbs - lightest on the market - Needs Briggs PVL Coil - $260 total
https://www.ombwarehouse.com/predat...byPaq1euw3rYJgs24q9C_6A2OSYABXnEaAgDDEALw_wcB

30 degrees (will retard at high rpms) 3.7 lbs $90
https://eccarburetors.com/estore/sk100-sk-slipstream-predator-hemi-flywheel.html

32 degrees (will retard at high rpms) 3.5 lbs $110
http://www.arcracing.com/6626-predator-212-hemi-kohler-ch270-billet-flywheel/

32 degrees (will retard) 1.75 lbs $130
http://www.nrracing.com/product-p/6602-ph.htm

34 degrees (will retard) 2.6 lbs ($120)
http://www.ebay.com/itm/Predator-212CC-Hemi-Flywheel-/322497989677?_trksid=p2385738.m2548.l4275

I'm leaning toward the 32 degree 3.5lb.


So many options.
I've got lots of time to study up on this topic. Mostly cause all of my bills are due and cant afford to bust out $90 to $260 for a flywheel (and possibly an ignition coil). Going to look into flywheel weight. Guessing bigger is better for torque and lighter is better for throttle response and high rpms, but I've never personally looked into it. I just came across some stuff before.

 

[ezcome-ezgo]

Definitely go for the lightest weight, otherwise you will be driven insane by the possibility that a few grams are keeping you from achieving that last 1/5 of a mph.

 

[Noseoil]

What are you building for, is it for all-out top end speed, pulling hard to accelerate or something else? Until you have a real target for the build & your expectations, it's not going to be what you want. Light flywheel is fine, but it might shake a bit at different rpms as you get there. If you aren't racing in a "class" against other similar motors, it doesn't really matter.

The iron flywheels are made to help reduce vibration & dampen the effects from a single cylinder's inherent "out of balance" running. Really enjoying this thread by the way, just keep going!

 

[bob58o]

The rocker arms seem to bind on the shaft mounts. I'm guessing they got twisted a bit during welding. Not much, but enough where I feel I would need to bend them or sand/grind to make work rotate freely. I'm afraid that might create too much slop.

The seller was 100% cooperative and willing to take a return if I want. Jody from ARC says he NEVER broke a stock hemi rocker arm. He said some were breaking when the head first came out, but not since people figured out about the shortened Hemi Valve spring install height.

I told him what cam and springs I was using and he made me feel more confortable using the stock rockers. They are lighter anyway. LOL If they don't hold up... Next time I'll get 1.2 rockers and cut the piston deeper.

So I think I'm going to return the reinforced rocker arms and use the stock ones.

 

[ezcome-ezgo]

Alas, GAB-ing the rocker arms was an exercise in futility.

 

[bob58o]

So the money that was for the rockers is now going to the exhaust header.

Ordered!

Robertson 3 Stage Minibike Header
https://smallenginecams.com/


Just the flywheel left to order.

 

[bob58o]

Alas, GAB-ing the rocker arms was an exercise in futility.

Life is an exercise in futility. Engine Building is Just Fun!!!!

 

[ezcome-ezgo]

The link to the header told me too much about you. You'd better fix that.

 

[bob58o]

The link to the header told me too much about you. You'd better fix that.

I'm glad it wasn't Pornhub. LOL

 

[bob58o]

Removed the bolt from the gov arm hole and am using the hole for an additional crankcase vent.
1 K&N style filter = Fast
3 K&N style filters = Time Travel?

You have no idea how much it bothers me that all three filters are different. I wish the colors matched exactly.

 

[bob58o]

I never checked cam lobe to crankshaft clearance.
Guess who will be grinding away at the crankshaft tonight???

Thinking about modifying your Hemi Predator???? Watch this video.
I learn so much from these videos. Thank you so much Jody!!!!
https://www.youtube.com/watch?v=QuKsRRrDeXk

 

[KartFab]

howd you check it? I ended up doing this for my gx390 stroker as the cam lobes can be in different positions depending on how far in or out the cam is, even when you have the dot lined up.

 

[bob58o]

The guy who sold the rocker arms offered to let me keep them and would still refund, but I had already shipped them back

---------- Post added at 08:08 PM ---------- Previous post was at 08:05 PM ----------

https://www.youtube.com/watch?v=QuKsRRrDeXk

at 36:50 he mentions the crank.

Let me see if i can get a picture.

 

[bob58o]

It seems to pass without hitting, but really close. Even without the side cover. Lifting the end of the cam a bit will bring it to where it contacts I think.

I'll take the crank out and see what I can grind.

Good idea if you have a spare side cover KF!

 

[Whitetrashrocker]

Run a Hardline from your crank breather to the valve cover and put a T fitting. Use one less filter to mismatch your fettish.

 

[bob58o]

Crank clearanced.
Took maybe 0.050" off.