Newly built Predator 212 quits running for absolutely no reason and won't start again. About to either scrap this POS or sell it

RB55_2018

RB55_2018

New Member
#21
#21
UPDATE

I finally found the culprit. Out of all the most difficult problems to find in an engine, this is probably the hardest issue imaginable to find.

I pulled the cylinder head and carefully examined everything.
The bore looked to be in near-perfect condition, indicating that this likely wasn't a piston ring failure of any sort (the bore would be scratched).
The piston was in perfect condition and showed absolutely NO sign of piston-to-valve contact.
The head seemed fine, it just had some regular carbon deposits in it.
The head gasket was fully intact.
The valves SEEMED fine and showed zero signs of piston-to-valve contact.

I went ahead and pulled the valves off the head. They still looked perfect. Didn't look bent or damaged in any way. I did a quick valve lap/grind job and reinstalled everything back into the cylinder except the pushrods and rockers. Since I don't have a compression tester and can't get one now since it's Christmas I figured that I could test compression by turning the engine over without the rockers and pushrods installed to give me a full engine seal (spark plug installed as well).

To my surprise, with everything sealed shut on the engine (head gasket installed and head torqued, valves fully closed), it was very easy to turn it over. I know that it takes an incredible amount of force to turn over an engine with good compression without the compression release on and this one was way too easy to turn over. Therefore, it had SOME compression but nowhere near enough to run properly. I also know that an engine won't fire if it has low compression.

I spent a while turning the engine over and could eventually pick out the sound of air hissing somewhere, seemingly in the head. I put my finger over multiple locations such as the exhaust port (would indicate a faulty exhaust seal), spark plug (bad threads on plug), pushrod holes (would indicate some sort of piston ring and/or bore problem), and finally, the intake port. Although it was very faint, I could feel a bit of air flowing past my finger on the compression and intake strokes along with the hissing. I'm almost certain the low compression is caused by a faulty intake valve seal.

I have absolutely no idea why the intake valve suddenly stopped sealing properly when accelerating under load at low RPMs. I had the engine spinning up into the 7,000 RPM range the day before with no issue. Maybe the colder weather had something to do with it? Although that wouldn't explain why it stopped sealing AFTER warming up (things expand) rather than before (things shrink). I'm pretty disappointed in this engine though. Don't plan on building any more if these are the kinds of issues I'm going to be dealing with. I was extremely careful in reassembling the engine so I'm sure that this wasn't caused by me being stupid by dropping the valve on the floor or hitting it against something.

I'm going to try to aggressively lap and grind the intake valve to see if I can get it to seal right. I'll report back in a bit. If this doesn't work I guess I'll order a new cylinder head or buy another engine entirely from Harbor Freight.
 
RB55_2018

RB55_2018

New Member
#24
#24
Rooster63 said:
So, you HOPE that you found the culprit?
Click to expand...
Yes, how am I supposed to know anything for certain? Without access to a professional engine lab and extremely advanced tools none of us here can know many things for sure...engine building is sometimes more of an art than a science.
 
RB55_2018

RB55_2018

New Member
#26
#26
UPDATE

I lapped the intake valve very well with Permatex valve grinding compound and assembled everything back together. I turned off all the lights in my garage and used a super powerful handheld flashlight to check that the valve was sealing. On a sidenote, the install height for the 26lb springs was .850 and I measured it at almost exactly .850 for both valves so I know they are getting more than enough spring pressure. On a non-hemi you would definitely need to shim them to get the correct .850 height since they are pretty short springs but on a hemi they work fine with no shims. After lapping the valve I cleaned everything off very well with brake cleaner, changed the spark plug completely (as per many recommendations), and did my compression test again. I know that if it pulls a vacuum it won't make compression when it travels back up but I specifically reinstalled the plug at BDC before reinstalling it and rotating the crank so it had plenty of air in the chamber. It was definitely harder to crank over this time.

I cleaned out the pilot jet on the GX390 carb and tried to start it again after reinstalling everything EXCEPT the carburetor. I sprayed lighter fluid down the open intake and pulled the engine a few times. Suddenly I got a nice pop and it started and ran hard for a few seconds before dying (it used up all the ether starting fluid). This gave me a lot of hope so I went ahead and installed the entire carb and throttle setup. I tried to start it again and it started up great after a few pulls. Messed with the idle a bit and got it idling very stable. I completely removed the centrifugal clutch and drive chain so the engine had no load and I could rev it freely.

After a few minutes of idling I hit the throttle hard and it revved very high without issue. Sounded good as well. Again, it was definitely idling a bit on the rich side since I could smell the NO2 fumes, unburnt gas, and see black smoke coming out the exhaust but it was running which is all I really care about. Revved it a bit more and shut it down. I stored it INSIDE my garage this time since I'm worried that the cold had something to do with it not running right. It's 18 degrees outside.

I'm happy I got the engine running but I'm still not out of the woods by any means yet. Yesterday the engine died when I actually put a load and ran it so who knows if the same thing will happen again. It's going to be 42 degrees tomorrow and I'm storing the bike inside so if this is somehow a sub-freezing problem hopefully I shouldn't have it tomorrow. For good measure I will completely drain the gas later on and let the tank air out in my warm garage for a while so any water or freezable fluids should evaporate.

I'll update tomorrow when I reinstall the clutch and chain and take it for another test ride. REALLY hope that we won't have the same issue where it randomly dies out again. If it does I will jump straight to changing the carb pilot jets; I have two very rich jets but again I can't see how that could be the issue since it's already idling rich now. Merry Christmas to everyone for now!
 
RB55_2018

RB55_2018

New Member
#29
#29
SAS289 said:
If I were you I would get a thicker head gasket in there.
Click to expand...
I have seen people running 0.009" metal head gaskets on these Predator 212 Hemis with the head milled .050" running a .308 lift cam without issue. I'm only running .265 lift with no head milling so I think I'll be fine. When I had the engine apart I checked for any signs of piston to valve clearance and saw none. I understand that everyone has different clearances they're comfortable with so I respect your advice but I feel comfortable with this setup. If I decide to run a .308 cam one day I will definitely use a thicker head gasket in there though.
 
RB55_2018

RB55_2018

New Member
#30
#30
UPDATE

Today I tried to take the minibike out for a proper test run. Did nothing from yesterday except relocating the breather filter, installing the air filter on the carb, shortening the fuel line a bit, and adding some gas to the tank (from the same gas storage tank that I used to fill it up originally). It was 43 degrees outside. Started up within two pulls with a bit of help from the choke and idled fine, just had to decrease the idle speed a bit. I notice that these Predator 212s they always idle fast when the engine is cold but then slow down a lot after it gets up to temperature. Must be something to do with the idle air/fuel mix being a tad lean when the engine is cold then becoming richer as it warms up due to the fuel atomizing better.

I let it warm up for a minute and took it down the road. I have a nice 1 and a half mile of straight road section outside my house where you can easily do full throttle runs. Great for tuning carbs. I hit the throttle hard and it picked up speed nicely but the engine was cutting in and out for some reason and jerking me back and forth in the seat. I was really worried that this was some ignition issue but leaned out the air/fuel mix a bit by adjusting the main jet (I have an EC Carburetors adjustable main jet on the GX390 carb) and it disappeared completely. I've never had that kind of behavior when running a rich AFR on other engines with Mikuni or flatslide carbs but I guess every engine and carb setup behaves differently.

I took it for a 10 mile ride and the engine performed incredibly well. This is a DBX 10 or some other kind of little kids 100cc minibike and the power-to-weight and power-to-size ratio with this engine is absolutely absurd. Definitely NOT for kids anymore lol. The front end lifts up at the slightest touch of the throttle and it will wheelie at 50 miles an hour like nothing. Acceleration is way beyond what the small wheelbase of the frame can handle. I got the engine to rev to at least 7,000 RPM and I had no issues so at least everything seems to be somewhat ok. I'm happy with it at the moment.

In terms of future upgrades, the highest priority is better brakes. I have tried very hard getting the crappy brakes that came with it to work better but they simply lack the stopping power required to slow down a minibike with a 212 Predator and 130 lb rider above 20 miles an hour. Friction alone almost does a better job then the brakes. Really not safe to be using on any public roads with traffic at the moment but I only ride on deserted neighborhood roads so I'm fine for now. I plan on installing some hydraulic brakes that have enough power to lock up the rear wheel in the near future.

I'll also get a digital tach for it so I can see exactly what RPMs I'm getting. This will allow me to gear the minibike perfectly. Right now I'm running a 12T clutch with a 70T rear sprocket and 13.5" diameter wheels. It's working fine for now so I don't think I'll mess with it for a while. Might also get a better chain and clutch one day (just using the cheap max-torque clutch and chinese chain for now).

On a final note I REALLY like the Dynocams CM camshaft. I'm using a 21 mm GX390 carb and I absolutely love the powerband. It's got tons of low-end torque (more than enough) to get you started and it just keeps pulling and pulling. Excellent midrange and top end of course. Not like other "performance" or "racing" cams that I have used before which have poor low end torque, pick up well over a short RPM range, then suddenly drop off and quit producing power like you've hit a governor limit.

Hopefully I won't have to continue this thread but I will come back if I run into any more confusing issues. In the end, I'm almost certain that a bad intake valve seal was what caused the engine to quit. Thanks to all of you for the help!

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Thedoggo34

Thedoggo34

Member
#33
#33
Built a mini bike very similar to yours with a predator, and finally after all my work of getting the engine onto the bike, I took it for a test ride. As soon as I gave it gas I wheelied and crashed into the pavement. The stock sprockets ratio on the centrifugal clutch and the bike is crazy.
 
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