Large Model Association Forum

My JC28Evo petrol engine is fitted with a Walbro Carburetor. I haven't flown the plane (1/4scale Cub) for at least six weeks if not longer.

I came to start the model the other day, spent a while getting it ready making sure it was safe to start after its rest. It started no trouble at all, a few extra turns of the prop to prime it and then a couple of flicks and started. I took it for a short flight and landed it after I heard the engine note fluttering a little.
Anyway, next day go to start it again to see if it needs a some tuning and it would not start at all. It would fire up for a one or two cycles but very quickly stalled.

There is a healthy and consistent spark and I noticed some air in the fuel tube. So I took the carb off, opened it up and found some dirt in the gauze filter inside, cleaned that out and removed the mix screws and blew through anywhere I could. All back together again and the engine still won't fire up, yes it runs for more than a few cycles this time, might be as much as a few seconds, but it will not rev and eventually stalls.

Does anyone have any suggestions. I'm convinced it's the fuel and the carb, but I not sure on exactly how to clean the carb.
BTW, the engine has been run in and has about 5 to 10 hours of flight on it with no trouble at all other than a small change to the mix screws when changing oil ratios.

Stuart, If your carb has had dirt in it then it must be completely dismantled to clean it properly.

The needles must be removed and so must the metering valve assembly and valve.

Do this on a table covered with a white sheet,as the very small spring tends to have a life of its own and if it pings onto the floor you probably wont find it. So when you unscrew the assembly always keep the the metering lever held down until the screw is undone and then release the lever slowly. Put the spring somewhere safe straight away.

You should blast through both needle valve holes with something like WD 40 with a plastic tube in the nozzle. Follow this up by blasting air through every hole. If necessary use a bicycle pump with a tube fitted on the end.

Do not alter the metering valve lever height when refitted as this has a big effect on the way your engine runs.

The following link has all the info you are likely to need and some excellent illustrations are included.


http://www.wind-drifter.com/technical/wg8walbro.php

Good luck.

Tony

I have now cleaned and rebuilt the Walbro three times and still the problem remains.

The only change I have made to the model is a new fuel tank that is now slightly smaller, further back and lower in the fuselage.
I've removed the fuel feed tube from the carb and sucked fuel through (YUK) and also blow through from the filler tube and fuel is flowing very easily.
Should the height of the fuel tank in relation to the inlet on the carb make any difference?

I've feed some neat fuel into the choke hole on the carb and the engine starts to splutter but won't stay running.

At my wits end with this now so any suggestions would be really appreciated.

as a quick trouble shooting process, assemble a new tank with new pipework, clunk etc and strapp it to the side of the model and try running the engine from this. if it runs fine, the fault is with the tank and feed in the model. if it is still the same its the engine side of things

Are you certain its the fuel system?
Check the ignition system - I had somewhat similar symptoms, turned out to be the NiCd which showed ok voltage off load, but couldn't deliver for more than a few seconds at a time.

Fuel system
Put another fuel tank on top of the stand off mounts and still the same problem, although it did fire for one or two seconds and then cut again and would not start again. Which seems to be the nature of the whole problem, the engine might fire for a couple of cycles and then stall with absolutely nothing afterwards.

Ignition system
I'm using Overlander 4.8v NIMH batteries, I replaced the battery this morning for another identical battery and still the same. However, I had questioned the CDI unit itself. But then I have removed the spark plug, cleaned it, checked the gap and turned the engine over to see if the spark was weak. The spark appears to the eye to be OK.

Hi Stuart, I had a similar sounding problem with a new DLE55 it was sorted when I changed the plug to a new one, worth a try.

Barrie

Sounds like it's worth a try as it's the only thing I haven't changed. Will order one tomorrow as I don't have any spares.

Thank you

New parts have arrived this week. Found this week that spark has no disappeared completely, so now fitted a new ignition module, spark plug and different battery. It now sparks again.
I've tried to start the engine and it seems to want to run but just won't keep running when I remove the starter motor, it is firing and seems to spin the prop up a little but not enough to keep itself going.
I've tried all sorts of different ignition timing and fuel screw settings, it does change except the engine will make more effort with the ignition timing fully advanced and doesn't even bother with it fully retarded.

Help, please!

Thanks

I had similar last year with a friends engine, just went off for no reason, crank woodruff key was sheared, and the flywheel had slipped, altering the timing, worth a look??

Sadly, it doesn't have a flywheel. It gets its timing from the crankshaft with a small magnet that activates the hall effect sensor.
Thanks though.

the aluminium adaptor the sensor is fitted into is the flywheel, the prop adaptor will be bolted to, it, its held in place by a woodruff key, key breaks, timing slips, still worth a look

Hi, It's hard to add to the useful advice you have had already but you may have a situation where all the adjustments are messed up now. The following may help:
Apache Aviation website has a section on timing the electronic ignition - the magnet should just pass the Hall sensor at about 28 degrees before the piston reaches the top of it's stroke.
Try hot wiring the battery to the ignition unit ie; bypass any switches in the circuit ( but if it starts you will have to disconnect the battery or petrol pipe to stop it ).
Make sure the spark plug cap is earthing firmly on to the spark plug. If not the ignition unit will switch itself off to prevent it destroying itself.
Make sure you don't have any broken wires in the low tension circuit. If a wire has been trapped the system may be going open circuit with vibration.
On the carburation side of things check if there is a pipe between the carb and the engine crankcase. If this has come off it won't pump fuel and so it won't go - It also pays to prime the system by turning the prop ( IGN OFF !!! ) on full thottle with your finger blocking the carb intake until some fuel drips out of the carb. If your memory is poor have a clean plastic bag to put your fingers in .
If the engine will run with easystart being sprayed into the carb you definitely have a carburation problem.
If it still won't go check the exhaust to see if the piston ring is inside it !! Regards, Peter Clare.

P.S from Peter Clare. Don't run for long on Easystart because it will not be adding any oil. Regards.

Thank you for your suggestions. Sunday got the better of me. I managed to start it but it wouldn't idle at all.
I've sent it on to Gashanger.com who are the UK dealers for JC engines. So just awaiting their verdict.