YS 91FZ idle question
 

Hi all,

I'm a bit puzzled about the idle behaviour of my 91FZ. Main needle setting is okay at the top end. Set the idle per advice of running at full throttle, then idle, and if rpm drops slowly after a second or so, too rich, if speeds up then too lean. My issues are: i) transition - lags a bit as I advance from idle to full if it's been idling a bit like it's too rich, but okay if I've been using the throttle (like in flight, up, down, up, down) - ii) if I leave it on idle for too long, it just quits (idle too low? too rich?), and - iii) - here's the really weird one...sometimes (like 1 flight out of 3), power is great on takeoff, but after a procedure turn and maybe the first turnaround, when I advance throttle to full, the engine burples along happily at half throttle power setting only! When this happens, I land, and refuel, and restart, only to find everything is fine again. So I take off and the next flight is great! How come? I didn't do anything. I checked the throttle linkage, it's fine. Fuel lines seem fine too, though I haven't replaced them yet. (Expensive!).

Any thoughts?

Cheers,
Ken

YS 91FZ idle question
 

Have you adjusted the regulator? Sounds to me like it is set too lean. Open it 1 turn and see how it goes.

YS 91FZ idle question
 

Thanks Dave,

Regulator remains as per factory setting (flush with case). I'll try it out 1 turn and let you know how it goes.

Cheers!
Ken

YS 91FZ idle question
 

Hey Dave,

Okay, so I had 4 flights with the regulator out 1 turn. It seems to help the dying on idle bit. But my second flight - no power issue still remains! It's very strange because it always picks up again after that (i.e. third flight). It's a bit of a waste of fuel and effort...any thoughts? Someone at the field suggested that it was because the engine was hotter after the first flight, and that I took a longer break between #2 and #3, so the engine has more of a chance to cool down. Any comments? Thanks!

Ken

YS 91FZ idle question
 

Did I ever ask you what fuel you were using?

YS 91FZ idle question
 

Hi Dave,

Ah, fuel...yeah, a bit of a mistake on my part. I'm using 20% (15% and 25% equal mix) Omega fuel. I only learned AFTER I bought 8 gallons of the stuff that I should be using a straight synthetic fuel like Cool Power.

Ken

YS 91FZ idle question
 

Can you duplicate this problem on the ground?

YS 91FZ idle question
 

It sometimes happens when I start the engine and advance to full throttle, but then I go back to idle, and if I advance again it clears up (often slowly, like it's burning excess fuel - too rich?) If it doesn't work the first time, it does by the second time I cycle the throttle. So that's why takeoff is always good and with great power. If this problem does occur, it's usually after the procedure turn or after the first turnaround. Puzzler, ain't it?

Ken

YS 91FZ idle question
 

It has to be a regulator problem. How much adjusting have you done with it? Sounds like the reg is too rich.

YS 91FZ idle question
 

Hey dave,

Well, until you asked me to try the regulator out 1 turn, I didn't adjust it (didn't want to change something I wasn't very sure about how to adjust). But correct me if I'm wrong, wouldn't turning it out richen things? So if it's too rich now, then should I turn it in again, like 2 turns in? (To take into acct the 1 turn out - which was counterclockwise) Or is turning it out actually leaning things? (Like the idle mixture adj)...

Thanks Dave, you're a great sport for keeping on with this to help me.

Cheers,
Ken

YS 91FZ idle question
 

As we've said here before, the regulator controls the transition between low and high.

Try going in to where you were and then work in 1/4 turn increments. The regulator is not a fixed value, it has to be adjusted.

YS 91FZ idle question
 

Ken Choo,
Pull out the operator's manual and reset the engine to the perscribed settings per the manual. When my engine was new, I scribed a line on the casing where the factory setting was on the regulator. The setting was 3/4 of a turn from seated, or about flush with the casing body, set the low speed screw 1 1/2 turns out from seated, and set the high speed neddle 1/12 turns out from seated. Am using power master 20/20 fuel, a 16x6 master airscrew prop ,and a custom 3/8 i.d. exhaust about 20 in. long.With these settings and this combo, my ys.91 goes from 2000rpms to 8600 rpms static in about 1 second, has a super precesion idle, and pulls like a 747. Ys says not to rev the .91 past 9000rpm static on their web sight so keep this in mind.
One of the biggest helps for this engine believe it or not is the spinner. If you are running a plastic spinner, throw it in the trash and buy a true turn spinner. I'm running the combo in a h9p-51 and am amazed how much better the .91 runs with this spinner, and also picked up betweem 750 and 1000rpm's more. The true turn is litterly 0 balance and helped the .91's low idle emensly. It acts like a fly wheel and the idle is soooo sweet, can idle for a good minute then roll the throttle to full and instant 8600rpm's, no stumbling, hesation, or stalling. Have run my .91 with these settings for at least 70 flights, and have'nt touched the regulator or low speed screws, and have only had to adjust the high speed leaner or richer 1/8 of a turn.
Hope this helps....good flying.........