My ZDZ 60cc engine developed starting problems after backfiring through the carb and stopping it now will only give a few pops on choke and nothing else. I checked the ignition battery first which was getting low so recharged that and tried again but nothing so checked the spark and found none so I exchanged the ignition for a rexcel unit I had which is new and unused this got my spark back but apart from the pop on choke the engine still refused to start but I have noticed fuel is running back out of the carb through the trumpet and today I also noticed air bubbles going backwards along the fuel line back towards the tank. Would this be caused by a carb problem or is it in the engine itself. I removed the engine backplate to check the rotary valve and that seemed to be ok with no damage or swarf inside the crankcase I also checked the carb diaphragm's and filter and they seem to be ok but I didn`t remove the needle. Up until 2 days ago the engine was running fine with very easy starts and loads of power so after all my checks I am at a loss to know what is wrong. The carb is a Walbro WJ-71 fuel is 40 to 1 premium petrol. The engine as it is has been good for the last 3 years with no problems. I have also noticed than if I turn the prop anti-clockwise past TDC then rock it back to TDC it seems to be popping in the carb as if it is blowing back out where as I thought it would have sucked in? if I plug the overflow pipe while doing this it pressurises the tank which doesn`t seem right?

Any help would be much appreciated

The engines is not timed right if you used the existing mounting holes for the sensor, RCexl ignitions have to be timed at 28-30 degrees before top dead center.

Milton

Your Falcon ignition may be just fine. I beleive they require a minimum RPM to give spark just like the DA and 3W ignitions do. The Chinese built ignitions will trigger spark whenever the magnet moves past the sensor. Max is correct the the timing between the two ignitions is different. As to your issues right now. One make sure the sensor is located so that your timing is 28 degrees BTDC as Max states. Secondly, the backfire did not create your carb issue, the carb issue created your backfire. The metering needle in the carb is sticking in the open position. Order up a rebuild kit and Google videos on walbro rebuilding. The sticking needle let too much fuel into the carb and caused the backfire, it's also causing the fuel drip.

Ok thanks for that info I will recheck the timing at the moment I have it set at 6.5mm past the magnet but I am not sure what degree before tdc that gives so will check it out with a timing disc. I did suspect a problem with the metering needle but wanted it confirmed before I took any further action.
Thanks for all you help and I will post again with an update once I have carried out all the suggestions made and tried again.

Umm... The ZDZ ignitions will turn off if there is no activity for one minute. So if you're trying to start it and haven't flipped the prop for one minute, the ignition is off. They will also generate a spark by just rotating the crank by hand. Please assure that you have a sparkplug in the cap and it's grounded to anyplace on the engine or you can damage the module with this test.

And yes, if you're trying to use that RCexl ignition you cannot use the ZDZ sensor mounting holes on the engine.

Hi Zeeb
Thanks for your input but I have some news for you regarding the sensor of the zdz and the rcexl unit I was able to use the original ZDZ hall sensor by cutting the cable and fitting a futaba type socket to the original so that it would connect to the rcxel sensor cable then after setting up the ignition to 28 degrees BTDC I could still use the original ZDZ hole mounts. As it happened when I rechecked my timing today it was exactly 28 degrees BTDC using a timing wheel on the crankshaft and 6.5mm from the magnet.I am also aware of the 1 minute shut down time on the ignition unit which I believe is 90 secs on ZDZ units.
My problem seems to be associated with the carb as speedracerntrixie pointed out in his reply which I agree with as today while checking the ignition the small amount of fuel that I had left in the tank started to run through the carb trumpet which if the carb was functioning correctly wouldn`t have happened so now as I originally bought this engine second hand and the previous owner had disconnected and removed the throttle return spring and made up a pillar connection for the throttle I have ordered a new carb instead of a repair kit as I like the return spring to be in place and a new carb would give me piece of mind as to it`s correct operation.
Thanks again for your input.
John

I have removed the carb and checked inside for any sort of damage but found none and being as it is a new carb I didn`t expect to find anything really, the needle seems to be set ok and is not stuck open and no dirt to be seen but I gave it a blow out just to be sure I also removed the backplate from the engine to check the rotary valve for anything untoward but that seems to be ok also. I have attached a couple of photo`s to show both carbs and rotary valve.

My next step is to mount the engine to my test stand and see if I can get it to run on that but will need to wait for the rain to stop before that happens? The carb at the bottom of the photo is the old one.
If my next attempt to get the engine running fails I am at a loss as to what to do next particularly as the engine was fine before all this happened and as both carbs were dripping fuel I thought the fault may actually have been within the engine but examination has not shown anything amiss so any further help in solving my non start engine will be most welcome.
John

All sorted now, engine is back in my model and running really strong so apart from a bit of fine tuning when I fly again all is well.
Now a week of high winds and rain forecast so will have to wait for better weather now.