Moe's Motovox drag mini.

#1
I bought a Motovox with a predator 212. I want to drag race this bike. It's certainly fast now, but I want to go just a little further with it. I am wondering in what order to make some of the mods I am considering. Below is the description from the ad:

- High Flow Air filter
- Larger Main Jet
- Larger Pilot Jet
- NR Racing Camshaft
- Upgraded valve springs (do not remember spring force but they match the camshaft)
- Governor removal
- Aluminum Flywheel
- Flywheel Key advance (6 degrees)
- Straight pipe header (with header wrap)
- Gold Oring Chain
- Quick Change Rear Sprocket
- New centrifugal clutch with proper spring and weights
- New Street Tires (much better grip)
 
#3
I verified with him that he did not install a billet connecting rod. So first thing I want to do is put in a billet conn rod. While inside I want to try my hand at porting and polishing. I've watched enough youtube videos so you know I'm an expert. : ) Seriously though I know I can do it and at the very least have a mild P&P. After that, where do I make my gains? New mikuni carb (vm22)? More advance, it's at 6 degrees now?

When I crack it open I want to check the in hole measurement right? So I know which conn rod to buy? I think I read that it's typical to get a +.020 aiming for a zero piston to deck height. If I do that do I still use the .010 head gasket? If I do these things it would raise compression correct? What other changes might I expect to do after a change like this?

Is there anyway to tell what weight valve springs he installed? I would imagine it was something that matched the cam. I have the box to the new cam and the stock cam at home. So I will check which cam it is running.

Miscellaneous.....I need a seat! This thing gives me a very uncomfortable buzzing in the nether regions! Has anyone ever used the rear seat to a crotch rocket? How about rubber mounting the seat to eliminate some of the vibration?

I've been lurking around here for a while reading everything I can. My goal with this is a bike I will race at the drag strip, 1/8the mile. I'd like to be 60mph+, even better if I could break 70mph! Not too worry, I've been riding my whole life and also race a 1978 KZ1000. So I'm not new to wrenching and racing.
 
#4
Moe,
Sounds like you have a good foundation for a build, and a good plan for the engine build as well.
I made my own seat using a piece of 1/2" plywood. It's a lot stronger than the plastic crap that comes on these bikes.
I'll see if I can find a pic of it to post.
 
#5
You need to get the rod and flywheel for safety concerns. You need to have .030 clearance between piston and the cylinder head at tdc. Your motor appears to be a hemi so it must be measured at the outer edge of the head. Best to use clay. You can stack the .009 metal head gaskets till you get your .030 clearance. Other option is if it is .020 in the hole with a stock rod get a stock length billet and use the .010 gasket.
 
#6
You need to get the rod and flywheel for safety concerns. You need to have .030 clearance between piston and the cylinder head at tdc. Your motor appears to be a hemi so it must be measured at the outer edge of the head. Best to use clay. You can stack the .009 metal head gaskets till you get your .030 clearance. Other option is if it is .020 in the hole with a stock rod get a stock length billet and use the .010 gasket.
The latter makes more sense to me. I do plan on the conn rod. No sense in investing money in this only the have a stock rod come flyin out the side of the case. It already has a billet flywheel. Why the PO installed one and not the other is curious. I can only imagine he had some max rpm in mind that he felt the stock conn rod was ok.
 
#8
The latter makes more sense to me. I do plan on the conn rod. No sense in investing money in this only the have a stock rod come flyin out the side of the case. It already has a billet flywheel. Why the PO installed one and not the other is curious. I can only imagine he had some max rpm in mind that he felt the stock conn rod was ok.
A stock carb (from my experience) cannot pull stock cast rod past its "locking up" or breaking point(7-7500rpm depending on how loose the engine is) I did a 390 carb instead of the 212 and it immediately locked the rod up like 3 pulls into the new carb. Swapped for another stock rod with the crank (polished the journal) same deal...
Put a rod in it and it wings up past 8k fairly easy.
 
#9
*now on a kart where you are WOT all the time... I can see a stock rod laying over at less rpm but for drag racing ... There's load and stress for a lot less time.
 
#10
I checked the cam he put in it and it's a Dyno CS grind.

Walt thanks for the insight. Any idea how many rpm I can do with the cam I have?

It sounds like the stock carb is more restrictive than I thought. I will have to give that thought.
 
#11
Google is your friend, I looked up that cam and it says max 7000 rpm. So here's the million dollar question. What kind of mph can I expect from 7000 rpm?

I know I need the billet conn rod, valve springs, carb? What else do I need given what I have already and the three things above that I will add?
 
#12
I'm a newbie myself, but I would verify the timing to know where its at. The ARC flywheels are 32 degrees already, so 6 more puts you at 38. I recall reading that 34-36 was the sweet spot for a drag setup, closer to 32 to sustained kart use.

gear ratio calc:
Gear Ratio Speed Calculator (MPH)

your tire looks like a 15x6-6?
 
#13
I checked the cam he put in it and it's a Dyno CS grind.

Walt thanks for the insight. Any idea how many rpm I can do with the cam I have?

It sounds like the stock carb is more restrictive than I thought. I will have to give that thought.
With the stock carb you are limited to 7-7800rpm from my experience. With a carb... The cam is the limit then... I have a mod2 cam on nitrous spinning up past 9500
 
#14
Just checked and it's geared 12/58. My tire is 15". At 7000 rpm it should do almost 65.

Also checked the clutch and it is a max torque that has the number 4 stamped on it. I wand to figure out the stall speed of this clutch.
 
#16
Wow Walt! Is a Mod2 cam a drop in or is there more work relieving it?
First off, nice bike! I like the direction you're headed and have built/am building a few Motovox's myself. Unfortunately, no cam is exactly a drop in, in a Hemi Predator... The Hemi Predator crank will require removal and clearancing for the larger cam lobes on an aftermarket cam. These are the areas in question that need to be clearanced, ground down and removed;

After "clearancing" and removal of governor gear...

Even with a basic original CL1, the exhaust lobe still made contact with the crank, before clearancing...

And wow is right... I am still trying to wrap my head around how it's possible to turn 9500 RPM with an otherwise basic Mod 2 motor, only addition being nitrous. It's an otherwise 7000 RPM cam. I can see a big boost in power up to 7,000..Maybe a few extra RPM, but 2500?! I have a hard time believing a stock Predator crank is capable of turning 9500 RPM... Now, I will admit I don't know dick about nitrous, I'm just not seeing how it's physically possible. Would love to see that on a tach on video though. I'd be sold on nitrous instantly.
 
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#17
If you've never used nitrous it would ruin your life instantly. Either you'd love it and never live without it or you'd blow your engine because the concept escaped you and you'd hate it like the plague and tell everyone you know about how it kills engines.

I have a tach with recall on it... With 13/69 on a 13" tall tire it turned 9360 in the 330' with a mod2 ported and milled head stock valves and a 390 carb.



Also, the newest of predators don't need to be clearanced for most cams I've installed for myself and others. The new ones have really low journal shoulders ... First time I saw this I got really excited.



Apparently the Chinese are seeing some of the warranty jobs I've sent back and are taking notes :lol:
 
#18
Before I changed the rear gear it did 9680 in the 330' at 59/60mph

Raced my buddy in his m5 bigger cam bike. Lost by 7-8bikes on motor and will beat him on nitrous easily if I don't have to "pedal" it.

The damn thing started to knock the tires off after I stretched it. Gotta reposition myself on it and get comfortable all over again.
 
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#19
Walt,

What springs are you running, 26 lb? What kind of tach do you use? How much did you mill your head? Incredible rpm on that! How do you fell it is holding up?
 
#20
Made a few changes over the weekend. Skateboard seat, straight handlebars, and best of all was the high rpm spring for the max torque clutch. Take off is much better now. It use to bog before, now it seems to get to the bottom of the power band before taking off. I was curious about how fast it could go since my old test has been while holding my cell phone so I could see it. I had my brother clock me in his pickup and it was 52 mph. Not bad for now. I'll race it at the dragstrip at the end of the month. Then over winter make some serious mods. I am curious about what kind of rpm I'm turning. I think it's time to get a tach.

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