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Zenoah 38 conundrum

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cymaz19/04/2019 22:21:43
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Doing some reading for another thread

 

Martin, if you think it’s carb related, have you seen this? Explains everything about the Walbro

Edited By cymaz on 19/04/2019 22:22:25

Martin Harris20/04/2019 00:47:29
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Thanks Cymaz - I hadn't seen it and it gives an excellent run through of the intricacies of the system. I believe I understood the basics but it certainly dotted a lot of "i"s and crossed some "t"s!

At the moment, I'm pinning my hopes on the inlet manifold gasket leaking between the inlet tract and the pressure port (I believe my theory ticks a lot of boxes) but Her Majesty's postal services have decided to let the replacement sit in a heap on the floor of the mail centre near the supplier for a couple of days if their tracking is to be believed.

Martin Harris23/04/2019 15:57:43
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Well it looks like I may have got to the bottom of the problem - thanks for all the helpful suggestions and it's probably as a result that I managed to marshal my thought processes into finding what appears to be the solution. I've run the engine today after obtaining and fitting the proper gaskets whereupon the engine has been started and run several times with only a slight hiccough when I forgot to open the tank vent before the first run!

If you recall, I'd found some softening (and wetness) of the inner composite gasket material between the inlet tract and pressure pulse hole and I'd found a small quantity of oil (which could only have come from the fuel) had collected in the pressure pulse drilling of the manifold bend at the gasket area. I'm theorising that the fact it ran OK when it was first fitted after the repairs and failed to even cough a few days later was due to the effects of fuel softening allowing leakage. The replacement, while of a similar composite material was a lot thinner so hopefully won't be prone to any similar internal leakage.

cymaz23/04/2019 17:03:46
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RESULT ! yes

Martin Harris21/05/2019 19:18:05
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Posted by cymaz on 23/04/2019 17:03:46:

RESULT ! yes

Yes, no and eventually (I hope) yes...

I took it flying a couple of weeks ago and although it started easily and produced decent power, all was not entirely well. The engine kept appearing to go rich and take extended periods to clear on throttling up during flight. Over the duration of the flight, the idle increased such that it was too high to land and after several circuits and eventually running out of throttle trim, I had to cut the engine as I came over the hedge.

On dismantling the model, I noticed that I'd used far more fuel than expected and there were black wet deposits where the model had remained almost spotless in the past. I concluded that there must be something wildly wrong with the carburettor, backed up the low end needle ending up completely closed.

Having invested in a needle valve setting gauge during my investigations and carefully following the Walbro instructions to use it to set the arm so the needle valve was just opening with the gauge contacting the operating arm, I knew it wasn't the adjustment at fault and I'd also tried a new needle as well in case it wasn't seating properly. I'd almost resigned myself to buying a new Walbro carb but hadn't got round to trying a Chinese knock off that I'd bought for not a lot a while back which was not going to be easy to modify for throttle linkage and a couple of other awkward factors.

A couple of nights ago, I was browsing the 'net and happened upon an official Walbro video of setting the needle valve. When I watched it, the penny dropped...their definition of the needle valve opening was not when it started lifting off the seat but when the operating arm started moving...

Having whipped the carb off, I opened it up this morning and checked - there was probably an 1/8 of an inch difference from the initial movement of the lever until the valve started to operate! No wonder the bottom end was running rich...

So, needles set at the recommended 1 1/4 turns out, the engine leapt into life and with a 1/4 turn richer on the low end and a smidgeon on the high needle was starting, idling, transitioning and running at full power beautifully. As I had some urgent test flying to do with my 1/4 scale Cub this afternoon there wasn't room to pop it in the car for a proving flight but I'm as confident as possible that there's every chance that with the gasket problem having caused the pumping problem and the needle valve now operating correctly, there's no reason not to expect success.

cymaz21/05/2019 21:06:16
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With the arm and needle there is quite a bit of “play” before the needle comes off it’s seat. The needle seat tool is an absolute Godsend.

I have a carb rebuild set and gasket set coming soon for my recalcitrant engine. On careful hearing I can make out a sucking sound when turning the engine over by hand. I’m still not sure if-its a leak or the pulse port

Martin Harris21/05/2019 21:48:34
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Posted by cymaz on 21/05/2019 21:06:16:

With the arm and needle there is quite a bit of “play” before the needle comes off it’s seat. The needle seat tool is an absolute Godsend.

If you use it correctly! blush

Martin Harris24/05/2019 21:49:15
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Posted by Martin Harris on 21/05/2019 19:18:05:
Posted by cymaz on 23/04/2019 17:03:46:

RESULT ! yes

Yes, no and eventually (I hope) yes...

...but I'm as confident as possible that there's every chance that with the gasket problem having caused the pumping problem and the needle valve now operating correctly, there's no reason not to expect success.

Famous last words!

I took the model to the field yesterday and told a few people that I could get to listen that I'd found the reason for its reluctance last time out, expressing my confidence before taking it on the long trundle to our "bottom" pits. I applied choke and starter...nothing.

Hmm, perhaps it didn't need choke...still nothing. Disable ignition and spin it over with the throttle wide open to clear any flooding...nothing. Here's where things got slightly odd, plug out and it looked a bit wet so borrowed a blowlamp to dry it out. Nothing. Check for spark...difficult in the bright sun but is there one? Borrowed a new plug from a clubmate. No, still not even a cough. OK, all I've done is add the cowling since it was running perfectly so although I couldn't think of anything that would affect a magneto engine, no electrical connections except the plug lead, but off with it to have a look "just in case". Nothing adrift.

By this time, the continuous returns to the other end of the field (each stage of the investigation needing one) were getting rather frustrating so I took the model back to the clubhouse pits. I took out the crankcase drain plug and injected a few drops of fuel...success...then the prime ran out and it stopped. Again to check with the same result. Fuel checks ended up with a piece of fuel filled tubing attached to the inlet which resolutely refused to move as I cranked so it must be fuel pump again.

Put some tubing on the pump diaphragm cover vent and gently blew and sucked - the level dropped. turned the engine over and the level dropped some more. Connected the plug again and cranked the engine - "Woo Hoo" it ran until the fuel in the pipe ran out. Connected the proper fuel pipe and away it went. Static run was fine so (luckily the wind had started to swing round and everyone had returned to the top of the field) taxied out for a very successful flight.

I have to wonder if this is a new problem or an intermittent one which has affected my investigations at various times...although I did change the pump diaphragm at an early stage - perhaps I'd better run a pipe from the vent system to apply mouth to mouth resuscitation in case (or perhaps I should say when) it happens again!

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