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Zenith 28 RXZ carburettor printer friendly version
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Bob Grimstead
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Posted Wednesday, June 22, 2011 @ 06:56 AM  

Hi again everybody,

Earlier in this thread, Donald showed us how to drill out the plug at the end of the gallery feeding the idling bleed, to clean out this gallery.

Because this required removing the carburettor, dismantling it, and finding a plug screw of the correct size, I tried to find a different way of cleaning out that tiny hole which gets blocked so easily. It's the little hole just below my wleding-rod pointer (sorry the image is out-of-focus).

The answer is merely to lower the carburettor so you can get inside the venturi (as in the photo above), and then clear the hole with a Primus stove pricker (another great Swedish invention).

If nobody knows what you're talking about, a Camping Gaz stove pricker also works well.
If you're still unable to get your hardware store to understand what you want, right click on this photo, click on 'save picture as' and save it somewhere handy on your computer, then print it out to take to them.

Yours, Bob

[Edit by Bob Grimstead on Wednesday, June 22, 2011 @ 06:59 AM]

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Posted Friday, June 24, 2011 @ 06:21 PM  

Hi Bob, I know what a Primus stove pricker is but I'm not sure it's the best solution. My main argument against it is that it doesn't really solve the problem. If a small piece of grit or dirt has got down inside the gallery, using the pricker is only going to push it out of the way but the dirt will still be inside the gallery and may migrate to the port again.

Donald

Bob Grimstead
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Posted Monday, May 19, 2014 @ 07:57 AM  

Zenith 28RXZ needle valve adjustment

Hello Folks,

We've talked a lot about this beaut little carburettor, and I've been working on HDO's again.

Here is some info on needle valve washers versus float bowl fuel levels.

I don't always understand why things happen with this carb; I'm just reporting what I find.

To remove the needle valve jet, simply take off the cast plenum holding the air filter (4 bolts), drop the float bowl (2 screws), remove the circular grey platic float, its U-shaped brass pivot and the stainless steel needle valve.
Then removing the brass jet needs a slim 12mm socket. I have four of these but even the deepest one was not deep enough, so I drilled it to 12.5mm depth with a 12mm drill bit. That's still not quite the ideal depth, but If I had drilled it any deeper the wrench centre would still have interfered with the jet. Obvoisly, an extra-long socket would be best.

I measured the single aluminium washer under my needle valve jet at 0.75mm thick.

With that fitted, my fuel level is about 4mm below the float bowl's upper gasket face.

The book says it should be 9mm below this face.

I made up another washer, 1.75mm thick.
With this in place under the jet, no fuel at all would flow into the float bowl.

When I used two standard 0.75mm washers, measuring 1.5mm total thickness, still no fuel flowed into the bowl.

With no washer at all however, the fuel level was still only about 4.5mm below the upper face.

Having no really thin aluminium washers, I tried fitting an AN960-616L washer under that jet. I measured this washer as 0.8mm thick, but it also has a slightly greater inner and outer diameter -- 9.85mm ID, 15.9mm OD.

With this washer under the needle valve jet, the float level became about 7mm below the top face of the float bowl, and therefore nearer to the correct specification.

Time (Lambda sensor & flight test) will tell whether there is any beneficial effect.

Yours, Bob

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Posted Monday, May 19, 2014 @ 02:44 PM  

Hi Bob,

Piqued by your experience I went to the garage to have a look at my spare 28RXZ and am puzzled. The 13mm socket from my BAHCO ¼" drive socket set fits easily over the needle jet seat. The internal depth of that socket is about 13mm which gels with what you say about drilling yours to 12.5mm but that BAHCO socket is not what I'd call a 'deep' socket so yours must be super shallow. The height of the needle jet seat from the shoulder to the top is also about 13mm so mates well with the socket. From recollection the one on the RF3 is the same.

I have one of Eugenio's aluminium washers underneath the one in the aircraft which, if I remember correctly is just over 1mm thick and I believe my float level is quite close to the stated 9mm though it's difficult to measure. Certainly the geometry of the float/needle linkage is very sensitive to the height of the needle valve seat.

Donald

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Posted Monday, May 4, 2015 @ 07:38 AM  

Carb problems.......
Sometime in the last year or so we started to get terrible cutting and surging of the engine on BXLN during the take off roll particularly on uneven ground. Badly with say half fuel and still evident with full fuel. I realise that with very low fuel it could be possible to uncover the tank pipe briefly and that negative G will affect but even now in the climb it will also start running badly at attitudes achieving around 70mph but not so in a cruise climb.
Any thoughts folks?
Cheers
Paul.
ps any UK owners with a spare 28RXZ that would be prepared to do a loan to help troubleshooting? Hope someone can help!
Bob Grimstead
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Posted Tuesday, May 5, 2015 @ 11:07 AM  

Hi Paul,

I had exactly those symptoms on my Aussie Fournier several years ago.

From memory it was a partial blockage of one of the jets by very tiny dirt or grit.

I see you've searched for this thread.

Try searching for 'grit' or 'dirt' or 'debris' or possibly 'jet' and take a look at the (many) other threads discussing carb problems.

I also have an old, spare 28RXZ of unknown serviceability that you can borrow for a few weeks.

I might be prepared to let you borrow WGN's serviceable one if we could arrange for me to fly my daughter in JC for some gentle aeros? ;-)) (If you don't ask, you don't get).

Give me a call at the weekend on my home number. I shall, as usual, be working on WGN.

Yours, Bob

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Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

Bob Grimstead
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Posted Tuesday, May 5, 2015 @ 11:13 AM  

oops!

This is that thread I mentioned , but it's back on page 1.

Incidentally, I have used all three carbs on both aeroplanes at various times, and the Australian one just will not accelerate and take off if there are less than 10 litre in that tank, regardless of type of fuel or carburettor.

The British one doesn't suffer from that problem and will take off with only two or 3 litres in its tank.

Both tanks look identical and are identically aligned in the fuselages.

I have just learnt to accept it as a characteristic of the aeroplane.

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Posted Tuesday, May 5, 2015 @ 12:54 PM  

Paul,

What do you mean by 'cruise climb'? Slower than 70mph or faster?

I agree that your problem is, probably, one of fuel delivery but I don't believe it has anything at all to do with uncovering the tank outlet on take-off from bumpy ground. I've been in and out of some rough fields with fractional fuel remaining and I've never seen that, but since you say it's worse with partial fuel I wonder if your flexible hoses are deteriorating to the point of restricting fuel flow. Or perhaps the finger strainer in the tank or the fuel filter is partially blocked, though I imagine you've considered these 'usual suspects' yourself.

To avoid unnecessarily dismantling it might be a useful first step to perform a fuel flow test with the fuel line disconnected from the carb. I'm pretty sure Bob Grimstead has a post somewhere on this forum where he gives a target fuel flow to meet any conceivable regime. I've done that on mine and while I cannot remember the numbers the flow was about 3 times any demand I could ever place on the plumbing.

After that I'd suspect the float bowl fuel level which, under 30cm head of fuel should be 9mm measured down from the float bowl joint. In my view that has a strong bearing on proper carb performance but your surging is a recent development and I think it unlikely that any significant change has occurred there.

Is your main jet clear? The only time I ever had surging was on the ground whilst trying a different, smaller main jet. A partial blockage could, I suppose, produce the same result.

One other item that probably is not the cause, but worth checking anyway, is that the air bleed to the emulsion tube is clear.

It's a pretty simple system that does nevertheless seem to give rise to all manner of problems, most of which will probably be traceable to degraded materials or jet/float level settings other than those recommended by the manufacturer. Looking back on my own experience with the Zenith I would say that most of the issues I've had over the years have been down to me dicking with things I shouldn't have. Nowadays I mostly try not to interfere.

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Posted Thursday, May 7, 2015 @ 06:28 PM  

Thanks for the replies, much appreciated.
Bob, XLN has always been ok on rough ground and any fuel load in the past but I am aware of other aircraft (and you mention HDO) that do suffer. Thanks for your carb offer but I have an offer of one that should (?!) be nicely set up for the configuration of XLN and will let you know. AZJC is currently on a ground up rebuild planned to almost new status.
Donald, by cruise climb I mean full power but a higher than say max rate climb speed. I am sure there are many personal preferences out there but 80-90mph works for me getting me where I need to go with a good view ahead, better cooling and just accepting the reduced climb rate. Unless there is lift around of course! Last week it ran badly climbing at 70 mph but not at lower pitch attitudes.
Sundays job is to check the fuel flow, pipe, tank and take it from there.
Float bowl level has been recently checked at 9mm.
Cheers
Paul.


Bob Grimstead
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Posted Saturday, May 9, 2015 @ 05:15 PM  

Paul if it's not too late I would suggest the easiest way of checking your fuel flow is by simply removing the air filter casting (that's just four screws) and then unscrewing the main jet. If the flow out of there is adequate this then proves that everything in the fuel system is okay.
If the flow out of the main jet is less than 50 or 60 L per hour -- or in other words less than 1 L per minute, you can work upstream from there to find the culprit.

Yours Bob

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Posted Sunday, May 10, 2015 @ 01:10 PM  

Cheers Bob
I pulled the main jet and the flow is noticeably down, I would say well less than half a litre per minute. I then disconnected the fuel pipe and have it would appear unrestricted flow from there so I will strip the carb later this week.
Bob Grimstead
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Posted Sunday, May 10, 2015 @ 01:59 PM  

Hi Paul, I'm glad my advice worked out.

Do please let us know what you find.

Yours, Bob

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Posted Wednesday, May 13, 2015 @ 02:18 AM  

Paul,

Less than half a litre per minute still equates to between 20 to 30 litres per hour which ought to be plenty, so I'm guessing it's not the carb.
When you did the test how was the aircraft sitting? Tail down? Tail up and top longeron roughly horizontal? I'm going to guess the latter which would put it closer to your 'trouble free' flight regime which leads me to ask about the fuel hose routing from the tank outlet to the cock. Does that hose have any spare length to it, perhaps curving rearwards or maybe dipping below the level of the cock?

Staying with fuel hoses but on a slightly different tack, I have a friend who had new hoses professionally made for his aircraft (not a Fournier). One day on take off his engine lost power and the engine ran badly. He got back on the deck OK but investigation revealed that one of the end fittings had shaved off a small sliver of the hose lining. It hadn't detached but was free to move somewhat with the fuel flow and cause an intermittent obstruction.

Just speculating here.

Bob Grimstead
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Posted Saturday, January 21, 2023 @ 00:14 AM  

For the past ten years or more I have only used one carburettor on both of my RF4Ds because it runs well and gives good power whereas the original one fitted to HD0 just didn't. But I've got fed up with removing that good carburettor, wrapping it up in several polythene bags and putting it in my suitcase to smuggle through security & customs at both ends, so I decided to put the original carburettor back on HD0 and sort it out.

This carburettor clearly causes the engine to run excessively lean. In the cruise below 6,000 feet there will be an occasional misfire and with the fuel tank level below about fifteen litres it coughs and splutters during the takeoff roll.

On three flights in the past fortnight, each of something over an hour's duration, I measured that it used approximately 9.5 lph, 8 lph & 8.7 lph. Those figures are all less than Rectimo's published cruise consumption of 9.4 lph for a 1,200cc engine, and way below the 11 to 12 lph I would expect from this 1,400cc engine

The clincher was when I removed the sparking plugs and they were all completely white from overheating.

I fitted an enlarged main jet of 1.45mm, but it still ran lean. That's very odd because I only need a 1.37 jet on my English Fournier with its 1700cc engine.

So it's time to go back to basics — first check the fuel flow. And right there I find the problem. Removing the main jet, turning on the fuel valve and letting fuel run from the main jet housing into a litre oil container from which I've cut out one side. There's barely a trickle.

Rectimo quotes the 1,200's max fuel consumption as 13.5 litres per hour, so for this 1,400cc engine it will be 15.75. Let's say 16. The legal requirement for a gravity-fed system like this is 150 per cent of max, so I want to see a flow of at least 24lph.

The litre container wasn't even half full after four minutes, so that's a flow of around 8lph — just enough to sustain combustion but far too lean.

I dropped the float bowl and removed the float, its pivot & the needle valve. I checked the needle valve's internal drilling. It's 2.5mm, as it should be. Checking the fuel flow out of that, I get around 40lph — more than enough.

I checked the level of the fuel in the float bowl. The Zenith manual says it should be 9mm below the joint face, but on both my carbs it's always been about 6mm below (and it is difficult to measure precisely).

But I noticed that as the fuel float pivoted up and down it really didn't move very far. So maybe, even when the float was resting on the bottom of the float bowl, the needle valve was restricting the volume of incoming fuel.

Of course there isn't any proper adjustment, but as an experiment I removed the washer from under the needle valve body. The level of fuel in the float bowl went up to about 4.5 mm below the joint face but the float's angular movement was more than doubled.

Okay, so let's try our fuel flow check:

Now the litre container fills in just 2.5 minutes = 24lph. Bingo!

So now I shall try a test flight…

[Edit by Bob Grimstead on Wednesday, January 25, 2023 @ 10:11 PM]

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Posted Saturday, January 21, 2023 @ 11:05 AM    YIM

This sounds very promising Bob. Nice detective work. How satisfying it will be if HDO now runs as it should. - I bet it will!
Bob Grimstead
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Posted Saturday, January 21, 2023 @ 10:13 PM  

The test flight went well. Climb rate was 500fpm in a temp of 38°C (100°F) which isn't bad. Engine didn't get so hot and the misfire's gone. Fuel used in 15 mins (inc 5 mins climbing & 5 mins full throttle aerobatics) was 3 litres wich is much more as it should be. So now I'm changing the main jet back from 1.45 to 1.35mm.

I'll try making an hour-long flight sometime soon to check the consumption properly, but the mixture is definitely much better now. I think the basic problem is poor quality of machining of the original casting and needle valve jet or its seat, something I've noticed in the past with the float bowl vents – some work better than others. This aeroplane has always performed poorly on takeoff with low fuel levels and has been placarded for decades 'Minimum fuel for takeoff 15 litres', so I suspect this has been the root source of that problem.

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Bob Grimstead
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Posted Tuesday, January 24, 2023 @ 03:51 AM  

For the second test flight with the 1.35 mm main jet I took off with only 13.5 litres in the tank. There was no spluttering, stumbling or other mucking about.
It's still climbing at 500 fpm in 35°C. That's okay although I would like it to be like a little better.
The wheel warning horn is sounding at the right sort of throttle setting which shows that linkage is geometrically correct.
It's difficult to measure fuel usage accurately on a brief flight but this time it seemed to use 3.5 litres in 20 minutes which is much more appropriate.
But now the engine is occasionally popping and banging with the throttle closed which suggests it's running a little lean, so I shall increase the main jet size to 1.45 again.

Much more importantly, I had immense fun just hurling her around the sky in some basic aerobatics, finishing with a prolonged tail m-slide that made me feel sick after such a long time without flying.

More tomorrow…

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Bob Grimstead
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Posted Tuesday, January 24, 2023 @ 06:16 AM  

For today's flight with the 1.45mm main jet I took off with 10 litres of fuel and the engine behaved perfectly normally. The climb rate was still exactly 500 fpm although the temperature was down to 32°C. The only engine gauge apart from the tacho that I have on this aeroplane is the oil pressure gauge and the pressure stayed significantly higher today, so the engine was running cooler, suggesting the mixture is better, although it was still popping a bit during my final glide with the throttle closed, so perhaps it might be a bit happier if it's a little more rich. The sparking plug colour was about right too, although I have to allow for the final five minutes of throttled back cooling flight.

Fuel used in 25 minutes was only 4 litres, but I did do a bit of gentle stooging around to cool down after the aerobatics, so I guess that's about right.

I do find it interesting that the root cause of all the problems with this carburettor was simply a bit of substandard machining in the original Zenith factory – but then it was only ever supposed to be a motor mower carb.

[Edit by Bob Grimstead on Thursday, January 26, 2023 @ 03:29 AM]

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Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

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Posted Wednesday, January 25, 2023 @ 03:37 PM  

Hi Bob,

I find the description of your carb problem quite perplexing and cannot help but think the root cause is the float chamber level. As you know that should be 9mm below the float chamber joint surface. You tell us your's is about 6mm below.

So trying to rationalise your lean running with your higher float level and larger main jet I offer this hypothesis.

Could it be that your float level together with your enlarged main jet the aeration holes in the emulsion tube are immersed more than they should be giving a less aerated, denser fuel supply that the suction from the carb venturi is struggling to lift into the throat thereby leaning the mixture? As an analogy I know that when using a spray gun or an air brush if the paint is insufficiently thinned the gun will not suck and deliver the paint.

In comparison, my engine also has had the 1200cc - 1400cc upgrade but my float level is 9mm, my main jet is 1.25mm and it runs just fine. Certainly not lean. Plugs are slightly sooty but I kinda put that down to modern mogas as mentioned by Eugenio in another post.

I leave this for you to consider or reject as you see fit.

Either way, good luck.

Donald

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Posted Wednesday, January 25, 2023 @ 10:06 PM  

Thank you very much Donald for your suggestions.
I have discovered that, to get the correct fuel level in the float bowl of 9 mm below the junction, I need to put two washers under the brass needle valve jet.
Unfortunately, when I do this, the float and stainless steel needle valve are jammed completely motionless.
I don't have another carburettor to compare with this one, but I think the problem is that the seat in the bottom of the float bowl for the brass needle valve jet has not been machined deep enough. Alternatively it could be that the brass jet itself doesn't have its thread cut high enough or doesn't have the conical seat drilled low enough within its body.
Whatever the problem is, it has always been an issue with this carburettor and engine.
I am a pragmatist. Confronted with this situation, my only remedy is to adjust those things that I can adjust – basically the main jet's diameter – and accept any disadvantages that may ensue.
In fact, now that the fuel level in the float bowl is a mere 4 mm below the junction, the engine runs satisfactorily with this big main jet and the fuel consumption is about what it should be.
There is also a handy advantage for me. I haven't quite finished refining my inverted fuel nozzle yet, but yesterday I managed to get 10 seconds of inverted engine running from that additional fuel in the float bowl. That's a couple of seconds more than I've been able to get in the past, so I am very pleased with that.

Happy Hogmanay, and for those who have any association with our adopted home down under, happy Australia Day.

With very best wishes, Bob

[Edit by Bob Grimstead on Wednesday, January 25, 2023 @ 11:13 PM]

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Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

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