Bing carb (3300) stuff

[ianboag]

A couple of years ago I installed the economy tuning kit. Never looked all that hard at anything after that as I figured - Jab plane - Jab engine - Jab kit. Has to be good.

 

Since the latest revision SB018 and the "don't fly lean" stuff in JabaChat I have had a closer look at some of my tuning stuff. I tend to cruise my J200 at 2650-2750 rpm which gives me 100-105 kts and 15-18 lph. The EGT is 700 C which falls in the recommended range of 680-720.

 

BUT when I open the throttle beyond this, the EGT goes UP not down. To about 750 instead of down to about 650. On the face of things it would appear that full throttle operation is a tad lean. I possibly need a bit more main jet (?)

 

I'm reluctant to drill my jet out as I have heard about how one can easily XXXX up all this fine drill small hole stuff. I would probably get another jet and drill that out or just buy a bigger jet.

 

Then there's lifting the needle (I don't know which notch it's on at present)... ?

 

Has anyone been there and done that ..... ?

 

Ian B

 

 

[Guest brentc]

Ian if you're on old 'lean' needle (the 20lph one), there is no adjustment from memory. The original needle (27 lph one) had 3 notches, but the later lean one had one only.

 

In my notes I have that 285 is the current needle jet and the main jet is 255.

 

Mine was still a bit lean on the 285 (burning 20 lph) so recently I put in the 290 jet (just unscrewed out of the bottom and put an new one in - only took 5 minutes). I don't have the matching idle jet to go with it so I get a little bit of hunting at idle, however that has definitely richened up my mixture.

 

I'm running at around 700-710 in the cruise at 2,800 and 680'ish at full power or possibly a little less than that.

 

Based on the shape of the economy needle (the 20 lph one that I have in mine) it makes absolutely no sense that it could run leaner / hotter at full power at the needle tapers off over it's entire length versus the old original one that has a huge taper in it at the 2,800rpm mark.

 

I'm wondering which needle you actually have.............

 

 

[ianboag]

Thanks Brent.

 

I'm not sure what needle I have either. It was a long time ago. We were shipped an economy kit that was sans a couple of parts, so gave it back to the distributor and got a replacement which I didn't install for some months.

 

The only way it makes sense (to me) to get leaner/hotter at WOT is if the main jet is the WOT bottleneck. So that 5-minute screw out of the bottom idea would be what you did to the main jet? or the (above it) needle jet?

 

I've never pulled a Bing carb to bits on my own so I don't know much. To me it looked like I should be changing the main jet, which is at the bottom, and looks (from the drawings) about as simple a job as you describe .....

 

 

[Guest brentc]

The 285/290 jet is the one right at the bottom, so the main jet if I've got that right. It really is as simple as I have described, just a couple of spanners, something like an 8mm and 10mm from memory. The needle jet must be the 255 if I've got that right.

 

I'm still thinking though that your needle might be wrong as it makes no sense that you are getting an increase in EGT with the application of power.

 

 

[ianboag]

If the main jet (the one at the bottom) is a bit small then as the throttle is opened the needle is right out of its hole - as one opens the throttle further, the butterfly opens more, there is more air going through the carb but not enough petrol makes it through the (too small) main jet so the mixture leans out a bit.

 

I think. Sort of makes sense to me - fits with what you have been saying. Thanks.

 

IB

 

 

[Guest brentc]

Sounds right, I hadn't thought enough about it.

 

It makes sense that if you are lean at full throttle, then you are simply not getting enough fuel in, therefore it must be the lower jet.

 

The bit that is confusing me is that it's rich enough at mid-range which is the opposite of what normally happens where it's usually leaner mid range, then richer up high.

 

Best check with the experts, they can just send you both jets to put in and you'll be safe. Shouldn't cost a lot. My 290 was around $25.

 

 

[ianboag]

Much appreciated.

 

 

[Guest brentc]

Ian and others. Cruising around on Saturday to Yarrawonga.

 

90% power climb to flight level 115.

 

290 carby jet.

 

Temps as shown.

 

CHT 225

 

Oil 80 or less (was 90 down low, it was a warmish day)

 

Oil pressure 2.8

 

EGT 610-620, down low, around 680-700

 

Indicated approx 102-105 knots

 

Flew non-stop Tooradin - Yarrawonga FL115, 2,900 - 3,000rpm

 

Yarrawonga local flight with Captain, around 15 mins

 

Yarrawonga - Tooradin - 8,500ft

 

Tooradin - Moorabbin - 1,500ft

 

3.6 hours - 79 litres

 

~ 22 lph

 

Not as much as I would have hoped for. Was hoping for 24.