ululi1970

I am starting to get myself acquainted with a 91 ac that I got used a while ago. I am running a 15x8 MA prop, on 15% nitro.
It starts without too much fussing. Transitions reasonably smooth. However, I cannot get it to run past 7500rpm. Is this ok, or
should I get something more with this prop?
Also, I noticed fuel coming out of the carb barrel, especially at mid range. By the end of the run, there is quite some fuel all over the carb.
Again, since I am new to 4-stroke engines, not sure if this is normal.

Any help is appreciated...

Alberto

MrShoes

My Feedback: (15)

I have the same engine and I get 8500 rpms with the MA 15x8 K-series using Powermaster 20/20 fuel; running rich. The way this engine works (supercharging/rotary valve), you shouldn't be getting fuel out the carb.

petec

My Feedback: (58)

Alberto,

You should be getting more RPM out of that prop. A little fuel mist is normal but a large amount of fuel is not and points to a regulator that is not closing properly. A misbehaving regulator will cause you a lot of grief. If you are comfortable poking around in the YS you might want to disassemble the regulate and make sure it is flushed out with raw fuel to clean out any debris. If you are not, then send it into YS and have them give it a once over. Given the age of the 91 it may need a little TLC and Richard and his guys run each engine before they send it back.

Also make sure your fuel is 20% oil content.

Pete

ululi1970

I disassembled the regulator, and flush it with fresh fuel. Everything seemed fine. Tried again. Working on both the regulator screw as well as the HSN I was able to get it to spin at 7700 RPM, with some misting coming out of the carb, especially during transition. We are not talking
loads of fuel, just small droplets. It holds idle well at 2000... What else could be wrong?

Alberto

ululi1970

One more thing. I remember now that when I took apart the regulator, the crankcase side of the diaphragm was full of fuel (or oil, hard to tell). Is this normal? The engine was stored upside down... IF not, what parts do I need
to rebuild the regulator? My understanding is that the pressure applied on the crankcase side pushes the diaphragm against the plunger, and when that happens fuel under pressure from the tank is allowed into the carb. Is that so?
Thanks for your help.


Alberto

ululi1970

I started thinking that if the regulator does not close properly, it should affect the low end as well, should it not? Yet it idles very well without any tendency to slow down. Also, when I took the glow plug, the element was a nice gray. I would not expect that from an engine that runs rich.
So what else could it cause loss of power at high rpm? A worn out ring? Compression seems good, though this being my first ring engine I do not have a baseline to compare to. Also, when it is warm it starts as well as when it is cold. In my experience with automotive engines,
warn pistons/rings loose compression as the engine warms up and the cylinders expands, making them harder to start...

Also, could it be the 15% nitro is too low? I will try to get a jug of 20/20 for comparison...

Alberto

Four Stroker

Well Alberto;

You are not getting a lot of help here. You say the engine has good compression. Do you have good tank pressure ? Should be about 6 psi.

Old Dave Shadel once said that the main culprit of air chamber engines was air leaks around the chamber. This would include the gasket and O-rings on the transfer tube. Are you spewing fuel around the chamber instead of out of the carb ?

ululi1970

I do not have a way to measure pressure in the tank. However, after shutting off the engine, when I disconnect the vent line I hear a hiss for about a second. So there is some pressure in the tank, though I do not know if it is enough.
Fuel spews out of the carb intake. I am fairly certain of that. The air chamber is clean, while there is fuel dripping from the bottom of the carb assembly. Not sure if there are other leaks besides the fuel that comes out of the carb intake.

Alberto

petec

My Feedback: (58)

Sorry for the lack of help guys, I work for a living and have less time these days than I used to have for answering questions due to the wonders of downsizing.

Now about fuel coming out of the carb, a fine mist or a few tiny droplets while running is nothing to worry about but a big drops are an issue. We need to know which is it is. The spring in gut regulator could be just old and not giving you the proper pressure. Do not think that stretching the spring to give you a little more pressure is good idea...it's not and you will drive yourself nuts. I bought a used YS years ago and never paid attention to the spring and it was giving me grief, sent it to Richard who did nothing else but swap out the spring and it ran like a top.

What are the settings on you HS needle, LS needle and where is your regulator set? Flush, above, or below the housing? The regulator controls the total VOLUME of fuel to the engine. the HS needle cuts the amount back to what it needs for WOT and when the engine transitions it needs more fuel volume to keep from going lean. Remember the old carb on you car had an accelerator pump to help for just this reason.

Four Stroker

I'm sure the lack of concern for your problems from the usual YS experts is because you bought a very old used engine. It probably needs a complete rebuild. Used engines are always sold because they don't run - worn out, crashed. You can check the regulator action on any YS engine with fuel tubing from the regulator to carb by:

1) Pinch off the fuel line to the carb. Take out the glow plug and spin the engine up at WOT for 60 seconds to pressurize the tank.
2) Un-pinch the fuel line to the carb. Pull off the end at the carb and turn the engine over slowly.

The regulator should open and close. The result is that fuel will squirt 10 feet when open and should not run at all when the regulator is closed. If the fuel will not stop completely at some prop position and drips, the reg. needs rebuilding.

ululi1970

I am surely gratedul for all the help I can get. I get the best RPM with the HSN ~ 3/4 out, and the LSN ~ 4 1/8 out.
I set the HSN by starting at 1 1/2 out as per manual and slowly leaning while checking speed with a tach. The tach btw works, as it gives be 3600 when I point it to a flurescent light in the shop.
The regulator screw is slightly out, but I did not see much difference with it flush or even a bit in.

The guy who sold it to me said that it had a new ring in it, new diaphragm, bearings, gasket set and needle valve assembly.

I will try Four Stroker test to make sure the regulator closes as it is supposed to. Though last time I ran it, I did pull the line from the regulator to the carb with the tank still pressurized, and I did not see any fuel coming out.
I may also be able to jury rig a pressure gauge to the tank, to make sure I get enough pressure in the tank. Not sure though, depends what we have in the lab...

Alberto

tande

My Feedback: (48)

You got this engine used from a guy who said it had new pieces installed......so we can assume it has been disassembled.....do we know by who?, at what skill level?.....the cam timing could be off.....

petec

My Feedback: (58)

Good point about who put the parts in the engine and the cam timing.

One other thought, before we go any further, have you checked the valve clearence?

Alberto, considering a YS can be temperamental due to is high performance and not knowing the history on the repair of the engine, you might want to consider sending it to Richard at YS. You said you are new to 4 strokes and the YS is not like other 4 strokes so making sure you are starting with a good baseline to learn with might help a lot more than chasing symptoms. Once you get the YS figured out they are a great engine, but starting with a problem child makes a lot of people shy away from them.

Four Stroker

Everyone should have a tank pressure gauge from McMaster-Carr:

3846K1 Multipurpose Gauge, Steel Case, 1-1/2" Dial, 1/8 NPT Bottom, 0-15 PSI at $10.70

Just JB Weld a YS brass nipple in the bottom. They claim it is calibrated to 2% at midrange which is 7.5 psi - perfect for us.



3/4 on the main needle ain't right and points to the regulator.

ululi1970

Here's what I found so far:

1) The valve clearance is about 0.05mm, so within specs.
2) at TDC, the timing mark on the camshaft is at the top. Based on the manual, that is where it is supposed to be.

BTW, to inspect the timing I removed the cover to the camshaft and rotate the cranckshaft. Can the timing be accidentally
changed without taking the camshaft out? I do not hink so, and I checked before closing the cover, but just in case.

I will do Four Stroke trick as soon as I can get a pressure gauge from the lab...

Alberto

ululi1970

Well,
I think I am onto something. I noticed a hissing sound coming from the top of the engine when it was being turned slowly. It turns out that the valve cover has a small crack near one of the screw holes, and air escapes off it.
As it happens, the seller mentioned a crack that had been taken care of. In fact, it has been epoxyed (or JB welded), but clearly it did not stand the conditions during run.
Fortunately, a new cover should not be too expensive. Since I am after parts, anything else I should order just in case? I was thinking about the regulator spring, a new diaphragm and a gasket set...
Also, I read somewhere to slightly roughen the bottom of the valve cover to ensure a good fit with the gasket. Is this a good suggestion.

One more thing. Is it normal that, without glow plug and turning the prop slowly by hand the regulator opens only every other revolution of the prop? My understanding is
that the pressure to open the regulator is controlled by a small opening on the crankshaft, hence my confusion...

Hopefully, I will be able to get the cover in a few days, and I'll get back to you with some good news.
I thank everybody who contributed. I learned a lot...

Alberto

Four Stroker

Well the obvious. A new valve cover gasket. Also get the plunger and silicone nipple thing for the regulator - YS0176. Sounds like you may have found the problem. A YS engine will not run if there is a pressure leak absolutely anywhere - gaskets, O-rings, seals.

Well I do usually sand the paint off of the bottom of the valve cover on a flat surface.

tande

My Feedback: (48)

Not on any "YS" I've had & I've had a bunch (but never a 91AC).....TDC as far as I know is NOT used for cam/timing reference.....the reference dot on the crank/shaft while at BDC is not the same as piston TDC.....FWIW....

Four Stroker

That's a good point but I don't know if it is a whole tooth off. Eyeballing it, it looks 10~20 degrees off TDC. Check that out ululi1970 ! He probably meant the timing dot and just called it TDC.

ululi1970

Ok, the manual states to set the mark on the drive washer with the mark on the engine case. I did notice that at this point the piston is not quite at TDC. I guess this is what tande alluded to. Then check the dot on the camshaft. As Four Stroke mentioned, the difference between true TDC and the marks aligned
does not amount to a whole tooth off on the camshaft. At this point, I'd stick with the manual. I will set the valve clearance so that the intake is at the low end of the recommended range (~0.002'') and the exhaust at the high end (~0.004''), to account for the higher temperature the exhaust
valve is subject to, unless your experience suggests otherwise.

With any luck, I should have the valve cover here by Saturday...
Alberto

petec

My Feedback: (58)

A crack in the valve cover will certainly cause you to be lean, but it usually has other issues like idle irregularity and loss of pressure in the fuel system.

Some guys "true up" the bottom of the valve cover on 400 grit sand paper. If you decide to di that make sure the sand paper is on a good flat surface. As for other parts, regulator spring, diaphragm, plunger, and get a spare check valve. The check valve is always a good spare to have in the field box.

Let us know how it turns out.

ululi1970

Well, it took much longer for the store to get the parts, but at last I got them. At this point, I have replaced the valve cover and gasket, together with the carb and plenum gaskets. Also, replaced the plunger/spring assembly.

Unfortunately, things did not change when run. At WOT I can get ~ 3 PSI in the tank, and still no more than 7500 RPM with 15% nitro. Basically, nothing changed, including fuel misting out of the carb during transition.
Best performance is obtained with the HSN ~ 3/4 out, and the LSN about 3 turns out. At this point, I am inclined to try to list the engine for parts, and get something new...
Alberto