Zenoah G320(340) not starting.

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aperson

Active Member
Messages
25
Hello everyone, I have a very peculiar issue with my G320rc out of my DBXL 2.0. It only had about a gallon ran though it when it suddenly stopped and wouldn't fire back up. I took it apart and found a scored piston and cylinder wall. I believe this was caused by me using an alcohol based air-filter oil and running in wet conditions causing the filter to dry up.

I ordered an OBR full mod top end kit and 2mm stroker crankshaft along with new ceramic crankcase bearings to turn it into G340. Fast forward two weeks and I've received my parts and rebuilt my engine and now I just can not get it to fire at all, no pops burbles or anything and I honestly do not know where to go from here. I've tried every conceivable thing trying to get this engine to start. Including swapping the carb, coil and plug from the zenoah and putting them on an old CY(?) 28cc from a smartech thunderbolt (remember those) that hasn't ran in YEARS. That engine was coaxed to life and idled fairly easily without any hassles. So surely I must of missed something on the zenoah I'm not aware of.

I know it's getting enough fuel as I've flooded it once already. Spark is good. Compression I am curious about as I didn't put an actual gauge on it but it seems that the zenoah is a tad bit easier to turn over by hand than the 28cc CY engine. It's not stupid easy to turn over but the CY has a better "kick" as it passes TDC. I'm not a total newbie to engine building having built a few full size engines before and also a lot of nitro rc engines but I'm not a total expert on how piston port gasoline two strokes work. So any suggestions no matter how simple or silly they may seem would be appreciated.

So far my troubleshooting list includes:
The basic checks for air, fuel, spark ( I didn't check compression with an actual gauge but it passes the finger test)
New spark plug
checked piston orientation (arrow towards exhaust)
Changed base gasket
Re-Redo of the sealant around the transfer ports
checked and messed around with the coil and spark plug gap
Tried starting without carb
squirted gasoline straight into intake and spark plug hole
Made sure the little ports in the carb and isolator were clear of debris
 
Curious question did you have to grind the case for the stroker crank?

Did you check to make sure you didn't flood the bottom end with fuel? I know once they get too much it's a nightmare to start. I just rebuilt my g340 with an esp top end and it went together butter smooth. My compression also is high enough that I can lift the back of the 5t with the pull start cable.

If its a new cylinder and piston I cant see that being the issue unless something went wrong there. You should have plenty of compression with a new top end. Hows the carb? I know from the factory the metering springs are too high with pop pressure.
 
I did have to grind the case for the new crankshaft. Is it possible that being too generous with my clearancing could cause issues? I did also disassemble the bottom after the first flooding and it was pretty soaked in gas but I cleaned it up and re-assembled it. Carb is a 1107 with the ball bearing mod from ddm haven't checked pop-off pressure yet though.
 
So I may have found the issue. I sprayed some soap and water around the crankcase halves and noticed some bubbling around the bottom. I did use a small amount of sealant along with the gasket so I'm thinking maybe I nicked it while clearancing for the crank and didn't notice it. Hopefully this is the problem.
 
So I Left this thread on a bit of a cliffhanger but I did eventually get the engine running. I ended up swapping the stroker crank for the factory one and put an esp hi-rev piston with a different cut compared to the obr piston and it fired right up. The only thing I could think of is that the longer stroke combined with a piston with more intake timing was starting to overlap into when the spark plug would start firing. Not sure if that makes sense with a valve-less small engine but it would in a normal valved 4-stroke application.
The obr piston on the left compared to the factory piston.
20221218_144407.jpg
 
Can't compare 4 stroke operation with a 2 stroke. A 2 stroke Is and has been more efficient due to it only has 2 cycles, the engine would be on its intake stroke while firing because the air fuel charge is going under the piston where it's being compressed to go through the transfer ports. That's generally why the pistons are being notched in place of a cam because that the only thing to change air/fuel and exhaust timing and duration.
 
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