Hey guys,

Jab 3300, wood 64x47 Zodiac 601 HD.

Recently rejetted from a .255 to .250 b/c I was running rich at high end.

Flew it for about 5-10 hrs with the .250 Main Jet in it, all was good, left bank was a little on the lean side and I was getting about 3100 rpm level at cruise on a good day.
Decided last night to try to remove the scat hose from intake and replace with smooth lined radiator hose 2 1/8" diameter, and nice smooth molded 90 degree bend from airbox.

I had expectations that I would get more power, and more even distribution of fuel, the exact opposite happened. One bank is now running even leaner, and much to my supprise I was lucky to get 2850 rpm today. I'm kind of bummed about this. What gives?

My thoughts are that now that I'm getting more air to the engine, maybe my mixture might actually be too lean. But the reality is I have one rich bank and one low bank with egts ranging from 1080-1150 on 1,3,5 to 13-1350 on 2,4,6. The average of them is about right, but there is a pretty big desparity. As a side note everything is very happy and even at cruise 2200-2500. The measurements I'm giving are MWOT, which is really where I want to dial it in and get the most pwr. It seems what I really need is better and more distribution of the fuel/air I am sending into the manifold, not necissarily more or less fuel.

So I've changed back to the .255 main. I did one high speed taxi like this but it was too windy to fly so I didn't get any good data.

WHAT ELSE COULD IT BE. Anyone else switch to a smooth intake hose and have to readjust?

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Comment by Stephen R. Smith on March 19, 2011 at 11:15am

Whack-a-mole is a great way to describe the first month or so of flying an experimental airplane.  If you keep whacking eventually you will get to the point where you can just fly and fly and fly and nothing seems to need your attention.


Regarding your oil temps: Last summer I flew my XL along side another XL for three days.  We flow from California to Texas.  Both planes were in good condition.  My plane has the earlier cowing, the other XL had the new cowling and oil cooler, like you seem to have on your plane (based on the pictures).  The newer cowling presumably provides better cooling.  My plane ran cooler and could perform expended climbs without needing to level off to allow temperatures to go down.  The other XL was frequently on the edge of acceptable temperatures and simply could not keep up with me.


The main difference was that my plane has a 4 inch lip on the air exit on the cowling.  When I installed this it helped the CHT and oil temperatures substantially.  If you fly in a hot climate I strongly recommend this modification.  It’s a bit ugly but not as ugly as a damaged engine.



Comment by Michael Herder on March 19, 2011 at 8:58am
EGT's are now much much closer to being even.  .255 main, intake hose from intakehoses.com and curved vertical divider installed.  Much better.  First hot 85+ day in Texas, oil temps got a little high, so I'm working on that not.  These first hours feel a lot like playing "whack-a-mole" at the arcade.  Just when you get one thing figured out, something else pops up.  It is the nature of the beast though, and I do find it rewarding when the problems are solved.
Comment by David Graham on March 18, 2011 at 6:32am

Hi Michael we too run a smooth hose from the airbox and we added a flange of metal upright in the tube just at the cary endwhich stops the swirlong motion and this improved our tempa and evened them out more... including the EGT's


Don't give up ..


Regards Dave Graham  

Comment by Mark Ertz on March 17, 2011 at 10:34pm
When addressing tuning of less than 200 rpm many factors come into play.  Going back and reading your original notes above, I see you are flying a different wing and a prop than I use.  Not all 3300s are the same either.  And of course, each airframe will trim out differently even of the same design.  So, continue being open minded as you have been and you will figure it out.  Trust more than the read out numbers on your computer.  Color of plugs, color of exhaust, burn rate, bully film, etc .....  Process over product. 
Comment by Ian McClelland on March 17, 2011 at 4:41am
Thanks for your input Mark. I have a smooth rubber intake hose and a small divider installed. I therefore believe that a rich mixture at only full throttle (confirmed by a big drop in egt on 2 oposing clyinders) is likely caused by the main jet. My reasoning is this. At full throttle the only jet in contension is the main jet. I will report back when I have re-tested with a slightly smaller main jet.
Comment by Michael Herder on March 16, 2011 at 9:44pm

I tilted the bowl, and put back the .255 jet.  A little better distribution.  I had a radiator intake hose on without a divider, but I was warned that a radiator hose could collapse when heated.  Today I used SCAT but I ordered a new smooth elbow from intake hoses.com and will report ack with my results. 


 Stephen/Mark your comments about not changing jets until airflow is addressed are 100% correct.  I rejettled because I thought I was running rich.  I was.... on 3 cyl and lean on the other 3 so clearly the airflow and distribution of fuel is the problem.  I feel like I'm getting closer and closer to tuned in with each flight, but it i absolutely amazing the difference these seemingly small changes makein performance and overall smoothness.  The jabiru felt like it was really purring today.  Still would be happier if I was getting another 100-200 more rpm.  I was barely getting 2950 in level flight, saw 3000 briefly. 

Comment by Mark Ertz on March 16, 2011 at 6:40pm

Stephen's foto of the vertical fix!

Comment by Mark Ertz on March 16, 2011 at 6:32pm
Ian, rough/rich @ full throttle is the because your sensors are unable to read the air pressure and inturn unable to set the diaframe to get mixture correct.  Leave your jets alone.  Read my post below 14 March for the fix.  Put the vertical divider in your 90 degree intake hose and you will get great full throttle and all ranges of throttle setting.  So many 601/Jabiru are flying with this issue.  Their pilot/builders are losing fuel effectiveness and power.  Fix your intake flow first!  The Bing is so misunderstood.....  Stephen is correct.  People really need to read the Manual on the  Bing carby to understand the mixture function.  No ram air or turbulant air flow over the sensors for the mixture to be work correctly.  No scat tubing, no leaks, no shape bends...  That is why the Jabiru aircraft uses a cobra intake tube.  There is no room for it in the 601 or the sonex.  So we adapt.
Comment by Stephen R. Smith on March 15, 2011 at 10:35pm

In the early stages of tuning my engine I did try a smaller main jet.  This seemed to help but more experimentation led me to the real problem; bad airflow in the intake system.  Therefore, I suggest folks fix all air intake issues before playing with jet size.


Comment by Ian McClelland on March 15, 2011 at 3:27am

I have a question about the Bing carb vent to the air box.

I have an old carb that didn't have the bowl vent barb installed when I purchased the engine. I fitted the barb and plumbed it to the air box as instructed by Jabiru. I have notced that there is an area next to the barb under the welsh plugs that is still vents the fuel bowl into the engine nacelle. Has anyone noticed this on their carb?

I don't see the point of venting the carb float bowl to the air box if the bowl can still be influenced by nacelle pressure via the other ports.

To the guys who have rough/rich running at wide open throttle have you tried going down one size main jet? This jet is supposed to be the only jet infliencing fuel flow at wide open throttle.

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