Too lean at low power settings

tcone1 · #1 10-01-2007, 09:28 AM

I'm perplexed.

I had to take the carb down to the folks at Lycon. It was way too lean in stock trim as a 10-3878. Lycon fixed the lean-ness at high power settings, but I still have problems with high EGT's at anything less than 20 inches.

My engine starts fine, idles smooth, and makes good power at high power settings now that it's been richened up by Lycon. At low power settings, the EGT goes through the roof, and the engine starts sputtering as though it wants to quit. I can't get any rpm rise at idle cutoff, and have tried enrichening the idle needle to no avail. I have checked for induction leaks...can't seem to find any bubbles when pressuring the induction system and squirting with soapy water. The primer solenoid isn't leaking. The fuel system will support 45+ gal/hour on the aux pump. The fuel screens are spotless and the bowl has no debris in it. The timing is spot on. All parts are new from Superior.

I have a Superior XP360, MA4-5 (pn 10-3878 flowed and modded by Lycon), Van's FAB, Slick mags, Whirlwind RV200 prop. I've only got single channel EGT and CHT.

Help, what else can I check? I don't like seeing 1500 degrees on the egt on short final, or while taxing.

Bob Martin · #2 10-01-2007, 09:47 AM

Where is your egt probe located? 2 to 4 inchs below the head?
What is your CHT doing while you see 1500egts?
Was it doing this before the Mod?

Ironflight · #3 10-01-2007, 10:03 AM

Absolute EGT's are kind of tricky, especially if you only have one sensor Tim. The temperature you are going to see is dependent on how close the sensor is to the exhaust port - 3" is about the middle of the prescribed range. Bob's questions are right on - especially in checking our CHT. If it's normal, then you might have a bad EGT sensor, or it's just too close - the danger there being burning up the sensor.

I'd try another sensor if the CHT is normal, or check how close the EGT is to the port.

Paul

Russ McCutcheon · #4 10-01-2007, 11:37 AM

"At low power settings, the EGT goes through the roof, and the engine starts sputtering as though it wants to quit." Quote from original post.

If this was at full rich then it is not the position of the EGT sensor but rather there is a problem, I don?t have any answer but those that do might rethink there?s if they missed this part of his post.

Russ

Ironflight · #5 10-01-2007, 11:53 AM

Oops....Russ is right, I forgot that part - you definitely have more than an instrumentation problem if it is running rough, and like Russ...I don't know what to suggest for that.

Paul

vlittle · #6 10-01-2007, 12:03 PM

Assuming Lycon fixed the full-power mixture by installing a larger jet, then the next thing to check is your mixture cable travel.

Your mixture cable when fully "in" (defined as 1/16" before hitting the panel), should cause the mixture control arm on your carburetor to hit the hard stop (at the carburetor end). If it does not, you should adjust.

V

tcone1 · #7 10-01-2007, 12:41 PM

My EGT probe placement isn't likely the issue. I don't care about what number of degrees the guage tells me it sees. I guess I misspoke. I don't like seeing peak EGT's while at idle or low power. The CHT's are climbing...I normally see 400 in the climb, 350 in cruise and let down, but start climbing when power is reduced. Yes, it's been doing this since first flight.

I have battled the mixture cable adjustment to death. I can get full travel at the carb...it hits both the rich hard stop, and the lean hard stop. I'd like to have a little more slack at the quadrant, but it's as good as I can get it.

I can get 200 degrees rich of peak at WOT.

Walt · #8 10-01-2007, 01:50 PM

first check for induction leaks with the engine running and spraying the areas with something like carb cleaner, you'll get an immediate rpm rise if you have a leak (of course extreme caution is advised, watch that prop). Induction leaks are always worse at low power due to the high vaccum. If you don't find anything there, a check of the cam timing would be in order next.

mahlon_r · #9 10-01-2007, 02:05 PM

Sounds like either and induction leak, primer leak or a piece of crud in the idle tube of the carburetor. If you are certain you don't have an induction leak and that the primer isn't the culprit then if it were mine, I would remove the carb, disassemble it and blow it out, put it all back together and then try again.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

Bob Martin · #10 10-01-2007, 02:23 PM

Quote:

Originally Posted by mahlon_r

Sounds like either and induction leak, primer leak or a piece of crud in the idle tube of the carburetor. If you are certain you don't have an induction leak and that the primer isn't the culprit then if it were mine, I would remove the carb, disassemble it and blow it out, put it all back together and then try again.
Good Luck,
Mahlon
"

I like the idea of the idle jet being the problem as it ties right in with your comment ,
"I can't get any rpm rise at idle cutoff, and have tried enrichening the idle needle to no avail. "

But your opening statement said the "engine starts up and idles smoothly"!
Doesn't seem like it would do both.

Seems to me the enrichment kit comes with a new idle jet!
Do you know if Lycon installed the enrichment kit and if they installed the entire kit?