Box Stock

Predator build

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needbout350

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Hey everyone noob here please forgive me if I am asking a question already discussed. I searched but either don’t know the right key words or couldn’t find what I think I needed.

I am getting into mini bike riding as a hobby with my kids and am working on my first engine build.

I have a non hemi gen 3 predator that has been run for about 45 hours in stock form with the governer. I am wanting more power for wheelies and general putting around. I’ve recently done some modifications (about 5 hours with governer removed, intake/header, billet flywheel, and 18lb springs) and am looking to take it to the next step.

I want to add a mod2 cam (is this a good choice?), flat top piston (.570 compression height), +0.020 arc rod, .670 boar stock appearing carb with low jet, e tube and bigger main jet, and maybe 1.1 raitio rockers. I may try to port/debur the head as well.

I’ll be dropping the gear raitio to turn about 5500-6500rpm. I’m currently using a tav2 2.5k engagement with a 10t and 50t sprocket for 5:1 raito hitting about 4000-4200rpm. I’ll be going to a 60t sprocket 3.1k engagement and shooting for 40ish mph.

Given that my engine has roughly 50h on it will I need to hone/boar the block for the new piston? Should I be concerned about the crank journal?

I’m also considering only adding the cam, carb and rockers if the block/journal are too far gone. Then running until the stock rod goes pop and starting with a fresh block.

What do you guys think? Will my mods get me where I want? Should I get the piston and rod or leave them out for now.

Sorry for the long post and thank you for any input.
 
Well I blew apart the standard 30 series clutch today so I ended up ordering a juggernaut which is currently on sale for the same price as the rod and piston so. I will have to wait for them anyway. The mod2 cam is on its way as well as all the other parts I mentioned. Hopefully the stock rod lasts for a little while.

If anyone could tell me if I will need to hone/boar the block for the new piston that would be awesome.
 
You can replace a piston of the same size without boring the cylinder but if you put new rings in a engine that has time on it you need to hone the cylinder or the new rings will never seat. Bigger piston normally needs both.
 
Well that didn’t go well. I put the .680 carb and mod 2 cam in and it’s all over now.

The side cover came loose on me (torqued it to 18ftlb). Some oil dumped out while I was riding and I didn’t notice till I got back in the driveway and the rest of the oil puddled up.

I torqued the side cover back down but all the end play was gone and it will not turn over unless I really force it. I’m guessing the crank and rod got bent when the side cover came loose. I’m hoping my cam is still ok.

Is it possible my cam is done for as well? Would you risk sticking it in another motor?

Also does any one know why my side cover would have come loose causing this issue. Is it possible the stock rod had something to do with it? The max rpm it saw was 5800-6100 in short spurts while riding on the street. I had also installed a new juggernaut that had a slight wobble and seemed to add vibration between 4500 and 5500 in rpm. (Go power sports said the wobble was normal).

What do you think caused my side cover to come loose?
 
The vibration is more than likely the cause .. The rod is more than likely seized to the crank . No big deal just replace rod an polish the crank
 
Wobbling parts are never good .
Thats likely the issue .
Did it have end play ? New gaskets ?
Cam is probably fine .
Crank and rod not so much .
 
flattop1 said:
Wobbling parts are never good .
Thats likely the issue .
Did it have end play ? New gaskets ?
Cam is probably fine .
Crank and rod not so much .
Click to expand...
Yes it had new side cover gasket and the end play was a hair under 0.01in when it was initially torqued.

Now there is no end play. It’s tight and hard to turn over even with the plug out.
 
Curflo said:
The vibration is more than likely the cause .. The rod is more than likely seized to the crank . No big deal just replace rod an polish the crank
Click to expand...
Would this still be the case if it spins fine with the side cover loose.

It would be nice if I could get away with replacing the rod since that was the plan anyway. Is there an easy way to test if the crank and block are ok? If not I’ll just start over with a new motor and put in a billet rod from the start.
 
Been there, mine would last about 20 laps and then puke, dover set me straight and hooked me up with gaskets and studs ,mine did it three times and never hurt bearings or cam
 
Depending on how long it ran without oil . The block and piston should be fine the rod galls to the crank . Inspection is the only way to know .
 
Pk in oregon said:
Buff sharp edge on side cover it cuts the gasket , then set play at .012 thousand minimum
Click to expand...
Well I checked and the gasket had been blown out my guess is it got cut like you are suggesting. Once it was gone the cover slapped around until it came completely loose.

About 1/4 of the gasket is gone. I’m guessing the end play is gone because the missing gasket is allowing the cover to get to close.

It still had oil in it when I got back but not much. It never ran totally dry though and I was only riding for 10-15minI’m not really sure how long the cover was loose for. I’m hoping I can replace the gasket and keep plugging along.
 
It should look like a stripped bolt .
Metal transfer from the rod too crank . Streaks of aluminum on the crank journal and missing and or grooved rod surface .
 
Well I decided to start over with a new block. The rod was galled and I wanna put a flat top and billet rod in it. Im not confident I can properly hone the boar or polish the crank so new is better.

I’ve got the rod and piston on the way. I’ll be using my other go fast bits from the failed engine.

The build is
Mod2 cam
.670 carb
18lb springs
Header pipe
+.02 arc rod
Hemi flat top piston
Billet fly wheel
And a new juggernaut free of wobble and free of charge courtesy of go power sports.
 
Ok I built every thing up and broke it in. I seem to be having a small issue with my .67 stock carb tho.

I cant seem to get it right. I started with a .035 main jet and it was acting lean but the sparking plug was reading rich. I went up to a .038 main jet and it ran pretty good but the plug was still black with carbon. I put the stock carb on with the .038 jet and it ran strong for a short time then got to hot and started braking up and shutting off. Checked the plug and it was showing clean and white... to lean. So I went back to the .67 and put my .038 with the stock low jet and the idle was erratic regardless of how I set the fuel screw. I put the modified low in (builder doesn’t say what it is I think it’s a .026) and she runs good but the plug is still rich and the bottom end is not what I expect off idle.

So I think I’m rich on the low end and about right up high which is why the plug shows rich. As soon as I sit and idol the plug is getting covered in excess carbon. My question is should I just leave it alone and run it. Or try to get a .022-.024 low jet. What do you guys run for low end punch.
 
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