pmr stage-2 kit...predator backfiring?

[robrose77]

So I bought a PMR stage 2 kit from hotrodminibike.com. I got the old air filter and muffler off the engine and put on the UNI air filter and straight pipe header. Also put a new jet in the carburetor(not sure what size....what ever jet and e-tube came with the kit). I fired the motor up(not on the bike its on a bench with no load on it) and reved it up, sounded really good. The only thing is at full throttle it tends to backfire here and there with a visible, but small puff of flame out the back...im wondering why?
Also even though I asked for a 13t clutch I got the standard 12t and Im wondering what sort of top speed I should expect with this...its going onto a motovox mbx11 bike?
thanks
Robert

 

[ProMod]

Running a straight pipe without a muffler can typically cause more backfiring on decell due to lack of back pressure in the exhaust. Carb adjustments may also need to be fine tuned for running a straight pipe verses one with a silencer or muffler that has some back pressure.
Weather/ temp,altitude humidity affect things too. A few things things to check if it still does it while riding are.... plug condition, make sure there are no air leaks and carb gaskets are positioned correctly. Idle mixture is set correct and there are no other ignition problems. Also would be good to recheck your valve clearance after running it for the first time. Do an oil change as well. I also would not advise running a brand new motor at full throttle with no load. It needs some proper break in time and in my opinion wide open throttle/no load or extended idle time is bad for a new engine. The engine needs a proper load and combustion pressures generated to seat the rings properly. Running the engine through its rpm range with moderate load and avoiding any lugging or high RPM works well. Warm it up to operating temp first of course.
With your 12t clutch and governor intact you should have a top speed in the low 20s.
A 13t will raise the top speed some but will be more sluggish on take off, and much harder on the clutch. Although the MV bikes do have a lower stock gear ratio which lessens this a bit. I could be wrong but I believe we may have talked about this on the phone and you were ok with trying the 12t at the time?, I was temporally sold out of 13t clutches. Either way if we made a order mistake I sincerely apologize.
If you would like to do a clutch or bell/sprocket exchange before you use it I would be happy to help out. You can easily swap the clutch bell/sprocket by removing the snap ring on the sprocket side of the clutch. This is a quick way to switch between a high or low gear ratio. You can always call us for tech assistance as well.
Brad
Mini Bike Performance Parts, Baja Doodlebug, Dirtbug, DB30 and MB165 Performance

 

[robrose77]

interesting the manual just talked about a break in period...I ran it and let it idle for about 2 hours before I put the new parts on it. I figure it will run better once it has a load on it.
Ill admit that I was hoping for a 13t clutch but knew I could get a 12t. I just figured the bike would be a torque monster so why not turn a bit of that into a bit more speed. Its no big deal, besides I think in my calculations the extra tooth only generated like 2-3 mph difference. In reality this is for my kids, I was just disappointed with the stock engine that wouldn't pull us up any hill and had really sluggish take offs. didn't want a jack shaft. I figured I had to get the stage 2 kit over the #1 kit because taking off the old predator gear and putting on the aftermarket stuff was the easiest way to get the motor to slide right into the frame...removal of the governor really appeals to me, but the price tag for the parts to do it safe was more than I wanted to pay.
Brad, I really appreciate the super fast shipping and quality of the parts. I will say that I thought the old chain would be good for the install and it wasn't, might be good to note that it is needed for a motovox swap, I thought it was just an option and not needed.
Do the new predator engines have different dimensions? I notice in your pics the engine sits almost midway on the motovox frame mount and on mine the motor is nearly all the way forward and the valve cover is hitting the rear fender...or did you adjust the rear fender?

thanks
Robert

 

[ProMod]

Thanks
I have not had a chance to measure up one of the new style Predators yet. They are still selling the older 69730 model at our local store. I wonder if the valve cover is slightly taller on the newer version?. Could you not move your engine far enough rearward with the mount adjustment to re-use your chain?
Also Something for you to try if you find that the bike is a little to peppy and front end light for the kids is to run one of the restictor plates sold on our website. For a more mellow play bike or pit bike I actually prefer the power delivery when running the R-plate. And with the governor functional it doesn't kill the top speed either. Just mellows out the big torque hit that makes the front end light.

 

[robrose77]

Just got the new chain on and since it was dark just rode it up the driveway and back twice to see what I had. WOW what a beast! I was trying to go easy on the throttle and still pulled a wheelie on take off in the thick gravel. once I saw how the throttle was with just a touch it took off like a rocket! and it still would pull the wheel up at speed. im impressed with this upgrade! I don't know what the stock predator would feel like but this is nice....definitely going to have to look into those restrictor plates for the kids though...my oldest might be able to handle it with some coaching but not the younger ones.
As far as the chain goes I had the motor back as far as it would go with the valve cover hitting the fender and couldn't even get the chain to loop around the shaft let alone the clutch, furthest back I could slide the motor was about 1/4 inch from the tube framing member in the front of the motovox mount plate. I think the chain needed about 1.5-2 inches more links. also the clutch spacer was just a hair too tight I ended up using a dremel to sand down the inside a bit to get it to slide over....other than that the kit worked great! Ill be putting in a review on your site once I ride it a bit more this weekend...thanks for the help.

 

[wartcleaver]

I'm almost done with my predator install on my doodle bug. It's been a pretty easy job. I have the old style predator. Does the new hemi have the same shaft length and will need a spacer between the clutch and engine case?

 

[youngengines]

I'm almost done with my predator install on my doodle bug. It's been a pretty easy job. I have the old style predator. Does the new hemi have the same shaft length and will need a spacer between the clutch and engine case?

They seem the same, I have two running. I have seen cranks vary from one to another in old Predators, and 196cc clones too, but not enough that a little shim here and there wouldn't take care of it.