ys engine surging at full throttle
 

My YS 91 will not run at full throttle when it has warmed up. The engine keeps surging at full throttle (pulsing) even though it is running slightly rich. I have been informed that this engine can suffer from a bad seal at the supercharger plenum chamber or a cracked chamber. I have lapped in the plenum seal face down to a few microns and replaced the gasket, and the chamber is not cracked, the engine still will not maintain full throttle. It sounds as if the throttle is being open and closed every second but the engine does not cut. The pressure in the tank maintains for at least a week so there is no leak either in the tubing or the check valve and I have checked the tube from the regulator to the carb. I have changed the plug and am using a proper YS 20% nitro fuel mix. The pressure in the tank at full throttle when it is pulsing is about 8 psi, no more so this indicates that there is no blow back past the piston. Any ideas?

Regards Tony

RE: ys engine surging at full throttle
 

Valve seats/seals worn. That's my best guess...

RE: ys engine surging at full throttle
 

Are you sure the servo is not breaking into oscillation? I have to ask since a surge is sign of a lean condition and you state the engine is running rich.

RE: ys engine surging at full throttle
 

Piston ring.;)

RE: ys engine surging at full throttle
 

Thanks for the input, the throttle servo is stable and although I am by not familiar with this make of engine, the compression is incredibly high compared to my other standard 90's, SC's, OS etc, so I can't understand how it can be a piston blow past problem. I have also inspected the piston liner for black marks which might indicate gasses getting past the piston ring and the bore is prisctine and has still got the original manufacturers honing marks. Thanks for any further ideas you fellas might have.

RE: ys engine surging at full throttle
 

The problem is, you won't really be able to tell on the ground if there is an issue with blow-by on the piston or the valves and valve seals. They might be fine below normal operating temps, but once the engine is at full operating temperature the metals expand allowing for small leaks to affect the engine's performance. If you can't figure it out, I'd say send it to YS parts and service.

RE: ys engine surging at full throttle
 

This was quite a typical YS problem some years back
I've had the problem 4 times on my old 91FZ.
Ring change cured mine every time. Even though the old one I took out looks perfect.
Was recomended to change the liner as they belived it was causing the probems. ie it wasn't perfectly round.
Nothing you could measure without some serious equipment... but enough to case problems once the ring beds in and wears some.
Strange but the info came from YS themselves.[sm=50_50.gif]

They started gass porting the rings some years back to help cure this.
Mind you, mines been good for some time now... still with the old liner and no porting.[sm=thumbs_up.gif]

Get a new ring and try it. If it doesn't work... least you'll have a spare ring.

And hey, you're in the UK!
Just send it to Phil at Pro Build if you're uncertain.
Give hime a ring.
You'll have it back in a flash.

RE: ys engine surging at full throttle
 

The liner still has the cross hatch honing marks. How much fuel has this engine seen? Did it ever run well in the plane it is in now?

Two other things to look at, first is the o-ring from the carb body to the air box.
Second would be cooling for the engine in the current plane. I wonder if it is getting hot enough that you are seeing a bit of fuel perculation.

Other than those thoughts a ring would be a cheap enough piece to replace to try.

RE: ys engine surging at full throttle
 

Hi again,
I have just taken the engine out of the airframe and noticed that the 'o' ring between the carb' and airbox is out of position on one side such that I can roll it away from the airbox with my finger nail The seat in the airbox is a 45 degree chamfer so there is nothing to stop the pressure from forcing the 'o' ring out of position, in fact the chamfer helps this happen. I cannot think of a reason why YS have not put a recess in the airbox to retain the ring properly rather than a chamfer. The 'o' ring that was fitted has a 2.5 mm section which seems too fat for the chamfer to hold ( it tends to try to squeeze out). I am wondering if the preveous owner replaced with the wrong size seal, I cannot check at the moment as there are no sizes given on the parts list. I would imagine that as the engine came up to full throttle (pressure) the seal became unseated, reducing the pressure in the chamber, and then reseated when the revs dropped. The 'o' ring would therefore act as a high pressure valve causing the rev instability. I will source a new 'O' ring and try again. If the seal becomes unseated again I will have to machine a proper recess location in the airbox. Thanks to all who have helped me I will let you know the results of the next test. Regards Tony

RE: ys engine surging at full throttle
 

Surging at high power on a YS is almost always an air leak. I can hear it a mile away. Pressurized air/fuel mixture building and bleeding off. It doesn't really go lean as it leaks air and fuel, it just loses air/fuel mass, then builds pressure again, until it has enough pressure to blow past it again. ... Surging.

Usually, it is the valve cover gasket. ...But it can be any seal.

Lapping the valve cover flat with 280 sandpaper or finer on a flat hard surface plate works wonders. You will see how the valve cover gasket area is anything but flat when you do this.

I do this (among other things) before I ever even try to run any YS. First thing that gets done out of the box.

Some say 2 gaskets, I say nope. Fix the distorted valve cover, and you will only need one gasket, ever.

Cheers...

RE: ys engine surging at full throttle
 

This engine is quite frankly driving me up the wall. I have lapped in the rocker cover seal face and the carb seal faces, changed the ‘o’ ring seal on the air box and cylinder liner. Today I ran the engine at full throttle on the ground for a full tank without any problem. Left the engine for half an hour, re-fuelled, warmed up took off and after a couple of minutes at full throttle the engine lost power. I immediately chopped the throttle and the engine stayed at idle but would not throttle up again for quite a time, ( I presume while pressure built up again). The only seal I have not replaced is the front bearing oil seal but I can’t believe that it’s the problem. It has been suggested that the problem is with the piston ring or liner but the compression is so good I can’t believe that this is the cause. Still persevering !!

RE: ys engine surging at full throttle
 

What check valve are you useing? if its not the original YS get one and try it. If it is the YS type make sure theres no debries in it and its working correctly. I know you said it was holding good pressure ,but a tiny bit of grit can cause alot of problems at times.
There was some issues with the 91's piston ring not sealing good, thus the gas porting come about to add more pressure to the ring as it come up to TDC to fire, the gas porting solved the problem and its not hard to do. YS did my 91 FZ for free. Theres some detailed threads by Dave Shadel on how its done.
Which engine do you have the FZ or AC?
Good Luck let us know how you make out.

RE: ys engine surging at full throttle
 

Hello,
I am due to do a further test with my engine this evening. The engine is an FZ. I own an aluminium diecasting foundry and have good tool room facility so I would be interested in doing the change to the porting. I cannot find any reference to 'Dave Shadel' by putting a search on this site. Can you give any more information on how to get to the relevant thread? Kind Regards Tony

RE: ys engine surging at full throttle
 

I told you, try a new piston ring.
There is always a chance it won't help but...It's the only thing that fixes mine.
Surging a full throttle with a 91FZ is a CLASSIC problem.
This has been the only fix I've found with that engine in 10 plus years of operation.[&o]
And yes, it has nothing to do with compression, or at least normal compression type tests... so don't bother thinking about it.
It's about the hot fuel igniting, blowing the piston down and some of the burning hot gasses creeping passed the ring.
Order a new ring and try it, if no-go send it to Probuild for a full service.
Least you'll have a spare ring.
I normally carry a couple, plus complete gasket sets, plus O rings for all my YS engines.
If you've been operating these engine for a long time... you'll know why.

RE: ys engine surging at full throttle
 

Here is the link to the thread by Dave Shadel

http://www.rcuniverse.com/forum/fb.asp?m=230086

RE: ys engine surging at full throttle
 

Hello Petec, I have seen the thread about piston porting. It mentions drilling four #60 holes. Is this an American notation. Is it 0.6 mm, 60 thousands of an inch ( .060") or 60 microns which is equivalent to .008" 0r 0.2mm? Hope you can help. Kind regards Tony

RE: ys engine surging at full throttle
 

A number 60 drill bit is actually 0.040 inches or 40/1000ths. AFAIK, the drill number has little if any relation to the size of the drill. Hope this helps.
Paul

RE: ys engine surging at full throttle
 

Tony, I'm sorry I didn't make note of actual drill size for you.

Thanks Paul, you saved me from pulling out my drill index and putting a micrometer on the #60 drill.

Pete

RE: ys engine surging at full throttle
 

Thanks for all the help. Because of the time zone difference I made the holes .007", which are smaller than the #60 size. I only had time to make one flight this afternoon as I was trying out two other airframes but the YS seemed to work O.K. at least it did not do anything nasty. I will persevere and report when I have had a few more flights. The one thing to come out of all this is NOT to buy a second hand engine!! King regards Tony

RE: ys engine surging at full throttle
 

.007 ?????? being a jorneyman machinest I know you can get drills that small, but most the time you need a special machine to use this small of a bit because of the breakage factor. Also a .007 will not come in a standard 1-60 drill index, a 60 is the smallest which is .040 A drill that small is only twice the size of a average hair .
Also if you did use that small of a drill the holes will fill with oil and carbon deposits in no time and won't do there intended function, if indeed there large enough to work at all.

RE: ys engine surging at full throttle
 

Hello,

drill sets from 0.3mm to 1.6mm are available here for about £6 , 10 dollars. I used a 0.3 drill in a high speed dremmel drill. My conversion is wrong .3mm is .010 not .007 but it is still very small and I take your point about oil or carbon blocking the holes. I made the holes that small because I was waiting for a reply from other members about what a #60 size American notation meant and I think you will agree that it is easy to enlarge the holes rather than make them smaller. thanks for your input Regards Tony

RE: ys engine surging at full throttle
 

Success, the engine now works perfectly. When I originally had a problem with the engine surging I spoke to Phil at Probuild and he told me the cause was either a leaking rocker box gasket; a leaking airbox gasket or a crack in the air box casting. I had noticed previously that on turning over the crankshaft, bubbles of air could be seen escaping around one of the cap screws securing the airbox. I assumed that a new gasket would cure this as there were no sign of cracks on the airbox casting. Having replaced the gaskets the engine continued to surge so I took the airbox to my foundry and crack detected it with a die penetrant. The penetrant came straight through the corner of the casting even though visually there was no sign of a crack. Having replaced the airbox the engine ran much better than before but I was still not happy with it so I decided to do the modification suggested by Cyberwolf ( piston Porting). Success at last! I also had a rear bearing cage failure which stirred up a bit of the crankcase. Fortunately it did little damage to the engine but on examining the bearing race, corrosion could be clearly seen. The moral of the story 1) Don’t buy a second hand engine.
2) If you do buy second hand replace the rear bearing.
3) Replace the paper gaskets they are easy to make with 0.5mm gasket paper.
4) Try piston porting ( 4 X 1mm holes drilled through the top (crown) of the piston exiting at the back of the piston ring groove) if you have the facilities.
5) Go for the easier option as suggested by DAVID BATHE, replace the piston ring. Many Thanks to everyone who has helped me.

Tony

RE: ys engine surging at full throttle
 

Good show Tony ,i'm glad you got it going to your satisfaction, Enjoy .

RE: ys engine surging at full throttle
 

So are you saying a valve cover gasket leak will appear as a "general fuel system" leak???

I can see fuel leaking out of my gasket cover when the motor is running. I adjusted my valves awhile back and the gasket looked very worn. I've seen similar problems to this.

RE: ys engine surging at full throttle
 

If you can see fuel coming out of the valve cover gasket area when running you have a leak in the fuel system that will cause erratic running.