Jab Plenum Chambers

[seb7701]

Well, I guess that's where the question arose - I have seen a pic only today of a CAMIT install where the hoses are all strictly cylindrical, as opposed to mine, where the circulars glass section just prior to the carb is flattened slightly aka just light a cobra head.

 

 

[jetjr]

thats it, Theres a few versions Ive seen, designed to have air enter carb straight without the bend influencing things.

 

It wont fit on 3300 as I recall, not enough room.

 

I have sourced but not yet fitted a smooth silicon bend, 45 deg to replace both scat and frp adapter.

 

As with all these fixes it works for some and not others

 

 

[seb7701]

Jet- that's exactly what I have sitting around to fit, but just the other day thought I might just cut a short section to replace the scat hose to start with. Things have still never been so good as when I deleted the external air intake, but who knows, might be even better??

 

Will be interesting for sure. Just trying to sort this damned oil temp first....

 

 

[Markdun]

On my set up (Jab 2200 in a Cygnet) I have the outlet from the airbox directly behind the carb....about 150mm gap between. I have a fibreglass tube about 50mm long with 3 flat flow straighteners (2 vertical, 1 horizontal) on the carb and a 'criss-cross' flow staightener in the glass tube that exits the airbox (it actually goes into the airbox by about 15mm). The glass tube from the airbox is attached to the glass tube on the carb are joined by about 100mm of scat hose. ..no bends. All this made almost no difference to egt spread. ...The only thing that did make a difference was the pressure at the inlet on the cowl and the length of scat from the cowl to the airbox.

 

 

[Bruce Tuncks]

I know of an installation with the flat "cobra head". If it wasn't flattened then why did it have this name?

 

Nothing I tried upstream of the carb did any good, the fix came from vanes downstream of the carb.

 

 

[Markdun]

One word of warning on the silicone hose. I once saw a vw engine struggle to reach full power. The inlet manifold had the carbie under the engine with 1" steel tube going up and over to the inlet ports which are on the top of the heads. Like the Jab, the tube was joined by smooth rubber tube. However the tube was crossing a gap of about 80mm between the steel tube. We observed that at full throttle the tube was collapsing and restricting the inlet. ....i assume because of the high velocity of the air/fuel in the inlet manifold even though manifold pressure was nearly ambient pressure. You can get fuel resistive rubber hose with a strong steel wire embedded in it so it takes suction. .,.but it's a bastard to cut and install as it doesn't stretch much. Btw we fixed the problem with the vw with a steel tube 'stent' liberated from a bicycle frame.

 

 

[seb7701]

Spot on Mark - I will be looking to see how much flex is in the hose due to lack of ribbing, but at this stage will try it just as the bridge between airbox and cobra head, as it will only be a short length, which will hopefully retain sufficient rigidity.

 

 

[seb7701]

Well, haven't posted for a while, so thought I'd touch base with some more findings-

 

1. Airbox to cobra head scat hose has been replaced with blue silicone intake hose. Much easier than the somewhat finicky scat hose and no issues. Short length, so very rigid. No interference with temps.

 

2. Strange fact - I can manipulate the temps between the rear cylinders by how much tilt in on my 'flattened' fibreglass style cobra head. Having noticed an unsual 9 degree spread between cyls 3 & 4 after a carby removal/refit, I landed and adjusted the cobra to level it more and went back to a 1-2 degree spread! After checking my records, this has happened a few times, so just trying to find out how to mark it for consistent fit.

 

HELP NEEDED-

 

Just fitted new carb jets etc to start trying to dull down fuel consumption a little and now have a 'Jabiru' needle with totally different jets.

 

Can anyone running a solid lifter 2200 help out with what needle/main jets they're using with success?? I have an array of choices, but figured you guys might be able to save me some trial and error!!!! Thanks in advance!

 

 

[jetjr]

There was a quite detailed tuning sb from Jabiru outlining jet sizes and combinations for each model engine

 

Has been superceeded by a pretty useless one

 

Maybe theres a copy somewhere

 

 

[seb7701]

Thanks Jet - got the table from the latest overhaul manual, but things vary depending on needle and the table isn't 100% on reality. As such, just after some 'real world' setups.

 

 

[Bruce Tuncks]

At full throttle, the CHT's were supposed to be cooler than at part-throttle.

 

In my engine this was not the case, so I drilled out the main jet up a size on my number-drill set. When this didn't do anything much, I repeated the exercise and this time I thought I lowered the full-throttle temp a little, and anyway I was too scared to continue drilling out that main jet.

 

This was all before fitting the EGT/CHT on all cylinders and finding there were bigger problems to work on.

 

I can look up the drill sizes etc if they would help. My engine is 2200A sn 425 solid-lifter, 32mm carb.

 

 

[seb7701]

Thanks Bruce - still interesting to hear any jetting comparisons, but I've got a 40mm Bing, so will be a bit different. Still getting over how sensitive the whole darn show is with every removal and re-fitting of the carb and intake bits - painful!!

 

 

[Yenn]

I am surprised that at full throttle CHTs would be lower than at part throttle. Now if you had said EGTs I would expect that.

 

 

[seb7701]

Well, it's been a while since I last checked in, so here's the latest -

 

After getting more than a little frustrated with my one problem child cylinder, I took a run to Jab in Bundy where a leak down identified a definite issue with one spot in said cylinder, most likely the result of a historical incident whereby that cylinder did get hot many moons ago.

 

Anyway, one new cylinder later, it'll be interesting to see what happens. Temp is up by 6 degrees on no.2, but it's only 2.5hrs old and back on Aeroshell 100, so this will be interesting.

 

Overall, it's still running just beautifully still, but just trying to keep CHT's where I reckon they should be for what little summer flying I will do!

 

In relation to my last post, new Jab needle and recommended jets are now fitted with terrific results - down from 18LPH to 15LPH!!

 

 

[derekliston]

According to Jabiru manual max. Continuous oil temp is 100C. In cruise my oil temp is around 98C but climbing in Qld summer heat might be a problem. Manual says that it can go to 118C max but does not say how long it can safely stay there. Any ideas?

 

 

[seb7701]

Hi mate - 100 deg is nothing and have been there many times in the trial and error of mine, so wouldn't get too worried unless it goes much past. That being said, you should be able to do better.

 

It might be worth taking a look at the sealing around it and a few others aspects. As an example, mine is back on running in oil, which will see up to maybe 5 deg hotter oil temp and was seeing 80deg the other day with 14 deg ambient at 2600ft. I have seen big changes in oil temp with cowl lip changes etc. What model Jab are you running and which oil cooler?

 

 

[derekliston]

Hi mate - 100 deg is nothing and have been there many times in the trial and error of mine, so wouldn't get too worried unless it goes much past. That being said, you should be able to do better.It might be worth taking a look at the sealing around it and a few others aspects. As an example, mine is back on running in oil, which will see up to maybe 5 deg hotter oil temp and was seeing 80deg the other day with 14 deg ambient at 2600ft. I have seen big changes in oil temp with cowl lip changes etc. What model Jab are you running and which oil cooler?

2200 solid lifter and also still on running in oil W100. I don't have a problem with the hundred, it is how long I can safely go over it that is the question. Once our daytime temps get up into the 30s I'm concerned about climb temps.

 

 

[seb7701]

Also on a solid lifter, now with 54hrs on the clock..

 

The running in oil will definitely affect the oil cooling, so you will get some improvement once you go to W100, but depends on how everything else is set up too as to how good you can get it.

 

I have given myself 'virtual' ambient temp goal of 30 degree ambient temp, which so far I won't be able to fly in, due to my wretched no.2 CHT which on a 30 degree day would have been 156, but now with a fresh no.2 barrel and piston would be more like 161deg, which is close to the limit I use on climb! To add insult to injury, have just modified one of my ram air ducts to now ENCLOSE the plug leads, like the newer ones, only to find a 3 deg increase in the front cylinder and, wait for it...btwn 8-11 deg diff on the rear cylinder!!! Go figure....

 

 

[derekliston]

I must admit I don't worry too much about temperature differentials between cylinders so long as they are all within limits which, thanks to Skyview, I can keep a good eye on. I just remember all of the Cessna and Pipers that I used to fly didn't even have CHT or EGT gauges. Often wonder how efficient the cooling really was on those. I'll change over to W100 at my next oil change, so we will see what difference that makes. I have pretty much sealed all the gaps, might try the idea of the deflector plate behind the cooler if I can work out how to do it.

 

 

[seb7701]

Agreed, I'm off to ffind some ally sheet to make up an oil cooler duct for sure...just hope I can get similar results!!

 

 

[derekliston]

Agreed, I'm off to ffind some ally sheet to make up an oil cooler duct for sure...just hope I can get similar results!!

Also remember my old air cooled VW Beetle. Who knows what any of the temperatures or pressures were on that. Didn't exactly have much instrumentation! But it worked.

 

 

[seb7701]

Yep, I know a few blokes still running one CHT monitor on their engines, in which case I guess ignorance is bliss, although that ignorance is what caused others to overheat my no.2 pot once upon a time, as there used to be a CHT ring on no.4 only!!

 

I'm sure VW's and Honda XR bikes are immune to the effects of normal thermodynamics!!!

 

 

[Bruce Tuncks]

Oil temps too low can be a problem too. Oil can get diluted with fuel and water ( combustion makes water vapor) and you need to boil these off. This is one reason why a few minutes of ground running is not good enough to keep off corrosion.

 

My old Jab 2200A had low oil temps so I partially blocked the lower-cowl intake hole. Then one day I was in climbout on a hot day ( over 30 ) and the oil temp went into the yellow.

 

Now I have put in more ducting plates etc to ensure that the air coming in to the lower cowl all goes through the oil cooler and not around it.

 

What would be nice would be a flap-plate to control the air going to the oil cooler but this would be too complicated I reckon.

 

 

[Old Koreelah]

...What would be nice would be a flap-plate to control the air going to the oil cooler but this would be too complicated I reckon.

A simple door controlled by a long cable should be easy to set up, Bruce. Only two problems: it would be easy to forget to open it, and (with the phasing out of carburettors) choke cables are getting harder to find.

 

 

[bexrbetter]

Honda XR bikes are immune to the effects of normal thermodynamics!!!

Been a couple of famous XR lemons actually, heat specific.