Jab engines

[Yenn]

Nothing wrong with my tiny tach except that it keeps reverting to one pulse per revo;ution. All I need to do is divide indicated rpm by 2 which is easy. I still have the tiny tach installed and have fixed the problem, which was a failing ignition coil.

Today I started the engine and no rpm on tiny tach as before. Turned of L mag and engine ran OK, turned off R mag and engine stopped. That just verified what the problem was. Replaced L coil and all works well.

[Bruce Tuncks]

Here's a puzzle I need to sort out...  On the last 2 starts, the engine has fired up almost instantly as usual, but then stopped after about 8 seconds.

It fires up again with the starter, then stops again. It does this about 4 times before keeping running.

I don't think it is the fuel supply to the carb, but I think this is the most likely thing... the fuel level drops in the float chamber etc.

Why don't I think it is the fuel supply? Because the fuel pressure gauge shows nothing unusual ( the gauge take-off point is between the mechanical pump and the carby inlet ) and after the engine stays running, everything seems normal.

But what electrical thing could cause this problem?

As regards the choke, the stopping seemed not to be effected by choke or no choke.

Once in the air, everything was fine. Mind you, I have stayed close to the airfield after this event.

The 2.2 engine is now running on mogas, as it has for a couple of months now.

[RFguy]

Bruce, I reckon float chamber needs inspection. float chamber fault / something stiff /stiction ?

 

is this after throttle is applied(choke doesnt work after throttle not closed with bing of course) 

 

I'd be getting a spark opinion. a clamp meter around the spark plug lead, ya know like old style timing light days.

can you find a timing light around ?

 

Edited May 16, 2021 by RFguy

[onetrack]

Intermittent problems are usually electrical in origin, consistent problems are usually mechanical in origin.

[Tasmag]

Unless both mags are intermittently grounding there shouldn't be any electrical issue that stops a Jab engine once it is running. 

[spacesailor]

I have heard of a small floating piece of Plastic, intermittently blocking fuel flow.

spacesailor

[facthunter]

I think it's electrical but I can't see 2 cutting out at once. Do it with only one mag on (if you can) and see what difference that makes. It has to  be fixed by a process of elimination. I think it would splutter a bit if it was fuel.. Nev 

[Yenn]

I doubt it is fuel level causing the problem as Jabs will run several minutes at low rpm with the fuel turned off. I like the idea of using a timing light to see if you are getting a spark.

I just had one coil stop working for a couple of minutes on startup, then it came good. One dud coil and a failing one could give the symptoms you describe.

If you have a multimeter handy you could do a quick check of the coils as described by Jabiru. Just check resistance of P lead connection to the body of the coil, which should be 0.8 1.0 Ohms and P lead connectiion to HTlead at distributor, which should be 5.9 to 7.1 K Ohms.

[planesmaker]

How old is the fuel? Perhaps try fresh fuel anyway I would not rule out fuel quality as the problem. 

[turboplanner]

4 hours ago, planesmaker said:

How old is the fuel? Perhaps try fresh fuel anyway I would not rule out fuel quality as the problem. 

Good point; if fuel containing ethanol (check brand but now most brands are blending below 95/98 Premium) has been used, it jellies up and clogs galleries, sometimes setting solid.

 

If Premium 95/98 is used, it's self cleaning, but the aromatics evaporate very fast; at times two weeks is all you get; I have the problem ith farm engines right up to the fuel injected Chev 250, however the symptom with Premium 95/98 no aromatics is the engine just will not start. Even a tiny amout of fresh fuel added seems to catalyse the mix for an instant start.

[RFguy]

A petrochemical mate told me to use 95 instead of 98 if it might sit around, because the 98 had more aromatics to evaporate off and turn back into junk.

 

I couldnt find any % specs for aromatic  on local ULPs, so I cant verify the reliability of that claim.

 

Might be able to get the MSDS .....

Edited May 16, 2021 by RFguy

[Bruce Tuncks]

Thanks guys. Lots of things to try out. I'll report back later today.

[lee-wave]

20 hours ago, Bruce Tuncks said:

Here's a puzzle I need to sort out...  On the last 2 starts, the engine has fired up almost instantly as usual, but then stopped after about 8 seconds.

It fires up again with the starter, then stops again. It does this about 4 times before keeping running.

I don't think it is the fuel supply to the carb, but I think this is the most likely thing... the fuel level drops in the float chamber etc.

Why don't I think it is the fuel supply? Because the fuel pressure gauge shows nothing unusual ( the gauge take-off point is between the mechanical pump and the carby inlet ) and after the engine stays running, everything seems normal.

But what electrical thing could cause this problem?

As regards the choke, the stopping seemed not to be effected by choke or no choke.

Once in the air, everything was fine. Mind you, I have stayed close to the airfield after this event.

The 2.2 engine is now running on mogas, as it has for a couple of months now.

My 2200 hydraulic motor also starts instantly but if there was the slightest move with the throttle or choke it would stop.  If your idle is set a fraction too high using the mechanical stops then the motor may display stopping after starting. Try lowering the idle a little and keep the throttle hard closed for at least 30 seconds after starting and also don't touch the fully opened choke for the same period of time....

[turboplanner]

Just now, RFguy said:

A petrochemical mate told me to use 95 instead of 98 if it might sit around, because the 98 had more aromatics to evaporate off and turn back into junk.

 

I'd partially agree to that depending on the word junk. If junk means an inert liquid, usesless to fire up an engine, but otherwise the petrol we've always known and loved, it's not going to cause any mechanical problems (as in blockages), and can be catalysed and used (in a car, tractor or stationary engine).

 

If junk means it accumulates a jelly in the bottom of the tank, I'd be more wary, but I've been using 98 for about tenyears now in cars, tractors, chainsaws, stationary engines, rotary hoes, trimmers, and the pattern is a nuisance but not a disaster (i.e. either tip the fuel out and put new fuel in or catalyse the fuel and the engine starts)

 

Where you have to be wary with 95 is to read the small print because some are tipping in ethanol for cost purposes and that jellies and blocks air galleries. In some carbies you can wash and blow it out, but in others where the Manufacturer has drilled a gallery and plugged it each end, to result is not so happy. I boiled a carb on the stove for a couple of hours once, but had to throw away the engine when I couldn't buy or fit another carb.

[RFguy]

agreed.  yeah junk was a loose term. I think he meant back to something not as good, wasnt specific.

 

As to whether it may be partially ethanol filled, well, yeah that's a certain problem and  may well be a problem since many cars specify 95 minimum and there would be an opportunity to capitalize on the high volumes.

 

and yep, a small addition of fresh fuel brightens the stale cocktail up. I dont know what the exact numbers are, people say 25% but I've never seen anything quantitive.

[Bruce Tuncks]

Yep, the fuel had been in the tank for 3 weeks. Also, it was bought in Edenhope and who knows how old it was when I bought it.

I don't have an induction type timing light but I do have a moving-coil meter and I'm going to try that with a coil around a plug lead.

But the motor easily passes the mag test before take-off so it seems to me that the fuel may be  the issue.

Its raining here right now so I cant do my chores till that stops then I can head out to the airfield.

[turboplanner]

45 minutes ago, RFguy said:

and yep, a small addition of fresh fuel brightens the stale cocktail up. I dont know what the exact numbers are, people say 25% but I've never seen anything quantitive.

I get an instant start with a couple of litres fresh 98 in the tank even up to 3/4 full with a fuel injected engine. 

 

 

[spacesailor]

One other thing that crossed my tiny mind.

Is there any play in the 'butterfly spindle'  letting extra air in.

Had one a long time ago that cut out at certain throttle positions

 ( didn,t seem that bad  but was the culprit)

spacesailor

 

[Bruce Tuncks]

Here's what happened today.  I measured the resistances of the coils and got 6.44k-ohms for each high-voltage line ( meter clipped between the HT lead and the earth)  and 1.0 ohms for the low voltage line. Both coils were exactly the same.

Then I tried seeing the spark-pulse. I wrapped a clip-lead 3 times around a spark-plug lead and measured the induced voltage with a moving coil meter.  Nothing! This electrical stuff is trickier than I am. I tried both ac and dc ranges. Maybe my old moving-coil meter is not up to much these days.

Next I tried a start using lee-wave's idea of not touching the choke after starting. It worked! Maybe this month-old mogas is trickier for starting than avgas was?  Anyway, I went for a circuit between the rain-showers, and the engine was ok. 

Next visit to the airfield, I will try to duplicate the stopping problem. Maybe I was opening the throttle too soon, something I tend to do because the idle is smooth at 1000 rpm but quite rough at 700 rpm. I was not conscious of stopping the engine by opening the throttle the other day though. I do have the idle set low as I don't like the idea of longer than necessary ground-runs on landing.

Thanks for all the comments.

 

[Old Koreelah]

6 hours ago, turboplanner said:

I get an instant start with a couple of litres fresh 98 in the tank even up to 3/4 full with a fuel injected engine. 

 

 

That’s been the common advice, but it should only hold true if you run the carb dry on shut-down (how would the fresh fuel get into an already-full float bowl?)

 

I only run my carb dry if planning not to fly for weeks or months.

 

 

[RFguy]

only half a teaspoon of fuel a second at idle..

[facthunter]

Bruce what  you say could be the problem. The choke becomes ineffective if you open the throttle too soon. It might be a worse effect on some of the colder mornings we are getting.  IF  the motor is LEAN it will just die without a cough or splutter. Nev

[turboplanner]

1 hour ago, Old Koreelah said:

That’s been the common advice, but it should only hold true if you run the carb dry on shut-down (how would the fresh fuel get into an already-full float bowl?)

 

I only run my carb dry if planning not to fly for weeks or months.

 

 

I did say that was in the fuel-injected engine; there's an electric fuel pump in the fuel tank which pumps up to full pressure as soon as you turn the key on, and I probably shouldn't have used instant because it still needs the starter turning the motor over a few turns, but a start on first key start.

 

I was going to say my collection of stationary engines, tractor, etc always take a bit of cranking, so it's harder to tell there but they all start with a small amount of new fuel as against having to drain all the old fuel out.

 

The new fuel seems to catalyse the lot; you don't have to shake the tank or try to stir it in the tractor tank; it seems to spread itself.

 

I think you're probably right about the carby bowl, but it would only need a squirt or two coming up the lines to catalyse that.

 

I used to tip the fuel out of the chainsaw, brushcutters etc, and syphone as much as I could out of tanks where engines were not going to be used for months, but the 98 premium with the extra shot of new fuel seems to solve my problems.

 

[Old Koreelah]

I’ve recently switched to 98 MoGas. Despite best intentions to fly regularly, it doesn’t seem to work out that way; MoGas gets stale. I have noticed she’s getting harder to start and I guess it could be more than the cooler weather.

That isn’t the only challenge; a mate once found lots of varnish in his float bowl after leaving it full of MoGas for months. Maybe just a crook batch, but another reason to run it dry.
 

I haven’t adapted my fuel management habits to the new fuel. Looks like I should turn off fuel as soon as I touch down, allowing the carb to run dry, in case the weather stops me from flying for weeks.
Then, as Turbs says, I can add a fresh batch of fuel to help make starting easier.
 

 

[Bruce Tuncks]

Yesterday I tried, from a cold start, opening the throttle a few seconds after the engine started with choke, and sure enough it stopped. 

In nearly 20 years of avgas, I had fallen into the habit of opening the throttle early on to get the revs over 1000 when the engine runs much smoother than with the 600 rpm idle I have it set to. Not once did it cause the engine to stop, so this is the only difference between mogas and avgas I have noticed so far.

The slow idle is in order to shorten the ground run after landing, and yes it is marginal. But on a warmed up engine, there is no more tendancy for stoppage than before the change to mogas.

So thanks again guys for the help.