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Zenoah 38 conundrum

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Martin Harris15/04/2019 21:44:35
9173 forum posts
242 photos

I've never been a great fan of petrol 2 strokes but the Z38 (older magneto version) in my Maher's Pacer has been a very civilised example, well silenced, easy starting and reliable.


A year or so ago, for whatever reason, the reliability came to an end. An awkward dead stick ended up with a heavy landing/removed undercarriage - and a removed spark plug complete with the remnants of the thread from the head. I repaired the thread successfully with an insert, ran the engine at home, went to the club, flew it several times and returned home with a smug glow of satisfaction.

The next time out - not the slightest sign of it firing. At home, all the symptoms were of a dry engine so I bought a carb kit, rebuilt the pump diaphragm and checked the needle valve height with the correct gauge.

Not a sausage. If I remove the crankcase plug and inject a small amount of fuel the engine fires and runs until it runs out. I've tried a jury rigged fuel tank to eliminate plumbing problems, confirmed the route for the pump pressure pulses is clear, and seen fuel disappearing from a short length of piping (probably) into the carb - it could have been sucked into the airflow of course. Earlier today after considerable cranking the plug was bone dry. After some experimentation with sucking and blowing the diaphragm vent at one point I found the plug quite wet. Dried it out - still wouldn't fire. Primed it through the crankcase plug and away it went again until the prime ran out.

OK - must be the carb so I substituted it with a new Chinese knock-off and got exactly the same symptoms.

Today wasn't the first time I've been through this and on the previous occasion the engine started and ran after some similar messing about so I left the cowling off so that I could run it up before the promise of some decent weather tempted me to leave some of the winter hacks at home.

So, what have I missed?

I can discount ignition as it ALWAYS starts on a crankcase prime so it must be fuel related.

Fuel used was fresh.

What can be causing these intermittent problems - the Walbro has been checked carefully for any foreign objects and given new diaphragms.

The tank has been checked (and substituted).

The choke is a physical plug, servo controlled inside the fuselage and is not obstructing airflow.

A completely different carb has been tried.

There is nothing blocking the pump pulse vent system.

The only significant history that may have relevance is that I suffered some plug whiskering problems in the weeks leading up to the cylinder head damage causing incident - this appears to have been caused by ingesting some balsa dust from the fuselage after doing a little "titivating" necessitating a stripdown to remove balsa fibres from the inside of the engine. However, once cleaned out the engine ran well until the poorly executed deadstick quite a few flights later.

I'm fairly confident that I've covered all the usual causes so can anyone suggest something "off the wall" that I might have missed?

cymaz15/04/2019 22:53:10
9047 forum posts
1192 photos

The only thing not mentioned was battery and switches. It reads that you have been methodical and thorough. So, I wonder if these have been inspected and checked?

Are you using the magneto version or CDI. I had a problem with inconsistent running and found it was the opto isolator switch playing up.

Edited By cymaz on 15/04/2019 22:53:38

Edited By cymaz on 15/04/2019 22:56:22

Martin Harris15/04/2019 22:56:12
9173 forum posts
242 photos

Thanks Cymaz but it's got magneto ignition and it always runs on a prime so it seems to be fuel related.

cymaz15/04/2019 22:59:17
9047 forum posts
1192 photos

Has to be some dirt the one of the carb galleries. Leaking bearings, letting air out , reducing the pulse to carb

Martin Harris15/04/2019 23:15:15
9173 forum posts
242 photos

That's certainly along the lines of my thoughts so far but I've checked extensively for any blockages and the crankshaft seals looked fine - also, it ran perfectly for several flights between the rebuild and the next outing a few days later when the engine flatly refused to even cough. It also ran perfectly on a further occasion.

Denis Watkins16/04/2019 07:24:36
4200 forum posts
86 photos

The Magneto Condenser can be a tricky little devil Martin

All can appear well, cracking a spark, but when the internals of the condenser break down, it is not often all at once,

And operation becomes erratic, with some control then none during the cycle, ie, some good sparks, then none

I don't know your design of mag, but cast an eye and meter over it, on the LT side, Low Tension

Martin Fraser16/04/2019 09:42:54
43 forum posts
23 photos

This is probably a long shot Martin but these symptoms remind me of the same problem I had with a brush cutter a while ago. I went through the carb rebuild and a swap routine but it would only fire and run on a prime through the plug hole.

Suspecting piston/cylinder problems I removed the silencer to find the exhaust port badly restricted with a carbon buildup. I'm wondering if your silencer has become partially restricted after the dead stick landing with crud or a loose baffle. I'm working on the principle that carburation begins at the air intake and ends at the end of the exhaust pipe and any restriction in the gas flow may not operate the carb diaphragm sufficiently.

Good luck, Marty

Engine Doctor16/04/2019 09:53:34
2417 forum posts
31 photos

Just a couple of thoughts as you seem to have covered most causes of non starting When it had the heavy landing did the engine/ prop take a knock/ If it did then check the gap in the pick up, that's the gap between the coil and the flywheel . The coil gets its energy from the magnet and if the gap is too big or the touches the coil the it wont spark . The gap should be the thickness of a piece of thick paper and is adjusted by slackening the screws holding the coil . Check that the shaft isn't bent or twisted before adjusting while your there check to make sure no metal filings have got onto the coil or magnet ..Might also be time for a new plug as its had a couple of knocks and whiskered . Whiskering can cause a carbon track to be arced onto the plug insulator that causes the spark to leak to earth under pressure. If you have access to a sand blaster then give the plug inwards a clean , this should remove the carbon track if that's the cause if it then starts ok replace the plug as it will probably foul again . Let us know what you find .

ps When you stripped the engine did you make sure that the gasket and insulator block hole all lined up with the pulse hole in the cylinder ? Often overlooked .


Edited By Engine Doctor on 16/04/2019 10:42:47

Engine Doctor16/04/2019 10:55:20
2417 forum posts
31 photos

Hi Martin just another thought. When yo stripped it down didi you re assemble with the old gaskets ? The symptoms you describe definitely sounds like poor crankcase or secondary compression. This is more important than primary compression on a two stroke as its the only way the fuel air mixture can transfer from c/case to the cylinder; hence it starting with a crank case prime that givies a rich or wet mix that can temporarily block any small leaks while starting.

Alan Gorham_16/04/2019 11:02:15
1182 forum posts
135 photos

How important is crankcase compression on a sideport engine like a ZG38?

I have heard similar stories about carbon build up in the silencer but the symptoms don't sound right in this case.

Assuming the flywheel/mag gap proves OK, is the cylinder bore fine because you say the engine has ingested balsa dust.

Ultymate16/04/2019 11:39:40
1701 forum posts
62 photos

I would suspect poor secondary compression as already mentioned by engine doctor you say it hit terra firma hard enough to yank the plug it may have damaged a bearing seal thus losing secondary crankcase compression !!

Martin Harris16/04/2019 12:28:11
9173 forum posts
242 photos

It was more of a wiping action on the plug - and the three bladed prop survived unscathed - but I was thinking of stripping it again to check the crankcase seals - more so now that ED has confirmed the effects. I would say the quantity of prime is very small - we're talking of literally a few drops and as the engine is inverted, there's no liquid getting anywhere near the seals.

And how does this fit in with it not firing at the point when the plug became wet on the one occasion yesterday? I would reiterate that I can't see that the problem is ignition related as it has fired and run at tickover for several seconds each and every time that I've primed it and I'm assuming it flooded while I was messing about with the choke and/or manually sucking/blowing the pump diaphragm?

There was a reason that I went off petrol two strokes shortly after I started motorcycling and have had four strokes ever since!

Edited By Martin Harris on 16/04/2019 12:29:45

Jon - Laser Engines16/04/2019 12:41:40
5222 forum posts
254 photos

While it might take a little gymnastics and 5 arms you could try running the engine off of a squirt bottle just to see if it runs normally when directly fed with fuel. If you can keep it running and get it up to full power, even for just a few seconds, it would at least give you a clue as the engine wont make it to full power if there is a problem with under piston compression etc.

Manish Chandrayan16/04/2019 13:46:54
622 forum posts
73 photos

Martin, Have you tried a different known good plug? Won't be a major effort and might as well rule that point out.

I know you said it starts on a squirt of prime so has spark etc, but we had a weird experience were on a OS 33, original NGK we had fuel, spark etc and the engine would not advance once started. Few short seconds of running and then it would die down. We tried almost everything to the point of the owner deciding to sell off the engine. And then on a hunch I fished out an Rcxel i had in my caddy, installed on the engine and it ran as if asking us all what was the fuss all about?. I have also had experience of other way round where the Rcxel plug had me frustrated for few weeks till I pulled it out and replaced with a NGK.

Martin Harris16/04/2019 15:19:29
9173 forum posts
242 photos

I can't see any logical reason why it would behave that way but it could be worth a try in the light of your experience. It is, however, a new NGK plug as the old one was damaged when it was ripped out.

Edited By Martin Harris on 16/04/2019 15:20:30

Manish Chandrayan16/04/2019 15:49:42
622 forum posts
73 photos

In the case I mentioned it was a brand new engine with brand new NGK CM 6. Having tried that you will atleast negate one more variable and it's easy to do

Jonathan W16/04/2019 16:03:59
112 forum posts
11 photos

I've no direct experience of the Zenoah, but looking online at the manual and parts diagram, it appears to be a very basic piston-ported 2 stroke. It doesn't even have a reed valve, so there is very little mechanically to go wrong, apart from general wear and tear. In my youth, I used to service chainsaws etc with similar engines. Once the external bits like carb, exhaust, ingnition had been eliminated, the main cause of poor running was lack of crankcase compression. The starting, idling, throttling would all go to pot. If you could eventually get them started, they would still scream away on full throttle but die as soon as you throttled back.

It has so few moving parts, it's hardly going to take long to strip and inspect. If it's losing compression, either through the crank seals or past the piston, no amount of tweaking the external equipment will help. Of course, it would be a nicer solution if replacing an external item would fix it, so do try to eliminate those first.

Engine Doctor17/04/2019 09:30:11
2417 forum posts
31 photos
Posted by Alan Gorham_ on 16/04/2019 11:02:15:

How important is crankcase compression on a sideport engine like a ZG38?


Extremely important . its extremely important on ANY two stroke engine The compression generated in the crankcase when the piston descends is used to propell/transfer the fuel mixture into the cylinder when the ports are opened and the subsequent vacuum when the piston ascends the cylinder is used to draw a fresh charge into the crankcase. Even a tiny leak will cause poor idle , low power and poor starting . The pulse of compression and vacuum is also used by the carb to pump fuel from the tank . The seals are affected by many things . probably the most common is the wear on bearings . They can also  harden with age or be affected by using cheaper oils . As its an older engine the seals are a prime suspect . Renew them and enjoy the reliability once again .


Edited By Engine Doctor on 17/04/2019 09:40:41

Martin Harris17/04/2019 11:27:45
9173 forum posts
242 photos

Thanks all.

There's one more factor I want to eliminate before removing the engine to check the seals - I made a new manifold gasket from "proper" gasket material (yes - I did provide a pressure pulse hole!) as the original ripped when I stripped the engine to clear it out and I have an original replacement in the post. I'm now wondering if it's possible that my "vintage" gasket stock has hardened or gone porous and is not sealing correctly between the inlet tract and the pressure pulse hole?

Unless there's been a catastrophic failure of a crankshaft seal, I'm struggling to understand why it flew faultlessly for several flights after the engine was stripped and cleaned out and then simply refused to fire after a week or less in storage.

Martin Harris17/04/2019 12:53:18
9173 forum posts
242 photos

Well, it's moved from a speculative idea to a definite possibility! Careful examination of the gasket has shown some softening of the inner material between the inlet tract and pressure pulse hole and I found a small quantity of oil (which can only have come from the fuel) had collected in the drilling of the manifold bend at the gasket area.

My (again very speculative) updated theory is that residual fuel may have softened the fibrous gasket material after the successful flying session and allowed leakage to occur between the reduced pressure in the inlet tract and the pressure pulse port preventing the pump diaphragm from working.

The proof will be in the pudding - watch this space in a day or two once the "proper" gaskets arrive!

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