ORIGINAL: MTK
ORIGINAL: sps3172
Matt,
Are you still chasing a rich spot in the midrange with this carb?
Just out of curiosity, how far open is the butterfly held at idle (expressed in number of turns on the idle speed screw from fully closed)? Also, is there any sort of 'half round' cut from the throttle butterfly (near where the plate meets the inner wall of carb venturi)?
Depending on answers to the above, I may have a suggestion to lean out the 'fuel curve' of the carb in the midrange.
- Steve
Steve,
Well, it is ever so slightly rich at just above idle, maybe around 3-3.5K rpm. It clears up fairly quickly so I'm not sure if it is just getting warmed up and still clearing its throat, so to speak. True mid range, around 5.5-6.5 K is very close. Judging from the plug color
it is just about right. That's where I fly practically everything. Throttle curve I run is a backwards and sideways ''S'' almost linear from point 0 to point 2, fairly flat curve from point 2 to point 5 and nearly linear again to point 7 which is max
There is never any need to go to full revs even in strong wind (20+). I check full revs every now and then, but only with the nose straight up to check to see how well the engine is cooling under full load. In 85 degree environment the plane can still be snapped with full authority after a vert climb of 600-700 feet, without sag. That's plenty, but we haven't had 95 degree temps yet so we'll see..... Baffling is a must and mine comes within 1/4'' of the fins
The idle speed screw was pitched long ago so to answer the second question, I really don't know. I close the thottle butterfly within 1/32'' and usually that works pretty well. I tend to run the idle mixture based on the load I'm running. For example, the 20x10.5 needed about 1/16 turn (about the blade width of the screwdriver) richer on the LS screw such that it would settle down better in flight (downlines). The 18x10 3 blader and 19x11 2 blader require a leaner idle speed.
One thing I've noticed that I haven't been able to identify cause for is the fact that inverted downlines (45s usually) the engine seems to want to rev a bit more than when upright. On the 45 down snap that works to my advantage but on the square on corner, it's a disadvantage. I suspect plumbing but not certain. The carb is not set-up for equalization and that may also be a contributor.
Does all this make sense? Did you get my earlier response?
Matt,
[DISCLAIMER] Info below may be old hat for you.....I do NOT mean to suggest that I know better, by offering suggestions. I just thought I'd throw this out here as I didn't see it mentioned elsewhere in the thread, and more info is usually not a bad thing
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Not sure about an earlier response..... I did send a PM earlier, but I presumed you'd get to answering that when you had some time....no reply here yet.
On to the carb....
"Most" Walbro carbs have some sort of small cutaway in the butterfly that forces air to enter the carb right over the low speed/progression jets when the engine is at idle. In an ideal world, at idle, ALL of the air would be entering the carb via that cutaway (i.e. butterfly is closed fully), instead of allowing some air to bleed by the slightly open butterfly. When the butterfly is held slightly open to achieve desired idle speed, the vaccum over the aforementioned jets is reduced and one has to open the low speed needle to compensate. As the throttle is opened, the velocity and vacuum increase in the carb and jets now flow too much fuel for partial throttle.
Provided you can kill the engine with the ignition, I would suggest that you start enlarging the butterfly cutaway. You need to creep up on this.....but as the cutaway is enlarged, you will have to close the butterfly to maintain the same idle rpm. You'll also have to re-lean the L needle as you just improved the vacuum over those jets. In the end, with the engine recieving ALL of it's idle air through that cutaway, you will have the leaneast midrange possible with your particular carb jet sizes. Does this make any sense?
Also, you mentioned adding fuel to get the engine RPMs to settle during a downline.....
Here's my .02 on that. Rather than add fuel to an engine that 'hangs' before settling to idle RPM, one should lean the L mixture such that the RPM never settles (stays 'hung', if you will)....then mechanically reduce the throttle to lower RPM to desired idle speed. Tuning this way means, of course, that you'll have to make up for that missing fuel on the top end by opening the H needle. Again....did this make any sense? Also, I'm not positive this is what's happening in the air, in a dowline.....but it might be another tool/trick to keep in your tuning qiver none the less.
- Steve