ASP 91 FS end float?

[Simon Lumsdon 1]

Have a newish ASP 91 fs which i have fitted in a Mustang and have been struggling to get it to idle nicely, below 3-4000 it just cuts. Have adjusted the low speed idle to optimise things but it still wont reliably idle? Runs fine at WOT and plenty of power, just wont idle slowly which makes landing the Stang a bit trixxy.

 

More worringly i notice that once its hot then it develops a noticeably amount of end float on the crank which doesnt feel good. I think whats happening is the crank is slipping on the bearings and moving in and out! Just stripped the engine and there are witness marks on the inside of the backplate and the conrod. I also notice that there isnt a shim behind the tapered prop driver, it just faces up to the front bearing. This isnt the same design as the os 91 which uses a woodruff key and does i think have a shim.

 

Have stripped the engine and it has minor rust on the crank and bearing but all still seems smooth, ( I might replace the bearings but not sure they need it)

 

Quite tempted just to fit an OS 91fs which i have but its frustrating to have problems with something that could/should be a good engine.

[J D 8 - Moderator]

The ASP91 has its prop driver kept in place by a tapered split collet, the rear of it nests against the inner race of front bearing and when prop is tightened the collet holds both the driver and the crank in place.[no shim] The only end float would be as a result of movement in bearing. Just Engines have a good diagram of the engine.

[Jon H]

high but smooth rpm idle is symptomatic of a very lean slow run needle. 

 

The exploded view of the 91 on the just engines site shows no thrust washer between the taper collet and bearing. The end float is likely down to a clapped out bearing. Oys possible front bearing boss might have either a hairline crack in it or the bearing has spun and worn the case causing the whole bearing to float in and out. I have a saito 45 with this exact problem. 

 

As the front bearing takes all of the thrust load any loose fit here will show as end float. 

[Simon Lumsdon 1]

Thanks Jon, noted on the lean low speed needle. I bough a tacho, very useful, full throttle on a 14x6 gives 9300 but low speed difficult to get below 3000. Noticed engine would bog down in flight when idling and throttle opened, took some time to pick up

 

think you might be right and front bearing is becoming loose in crankcase, witness marks on outside of bearing are suspicious

 

new bearings ordered

[Glenn Philbrick]

I have this engine in a corsair and noticed end float when hot, it also had a bit of a whine. It would run well and tick over ok but I changed both bearings. The rear bearing felt a bit rough. The end float has gone so I would change them, maybe you also need to tune the slow running screw as well.

 

[Simon Lumsdon 1]

Thanks, yes, new bearings ordered, i will then have another bash at the low speed tuning.

[Jon H]

12 hours ago, Simon Lumsdon 1 said:

. I bough a tacho, very useful

 

dont try to tune with it. They dont react fast enough to be of any use when tuning and your ear will always pick up changes much quicker. They are good for reality checks though which leads me to..

 

12 hours ago, Simon Lumsdon 1 said:

14x6 gives 9300

 

Thats a bit slow. I expected 9200 out of laser 80s when testing with a 14x6 apc. But if the bearings are dragging a bit that is not a complete surprise. Depending on the model you are flying a 14x7 may offer better performance and i wouldnt use anything smaller than 14x7 on a 90. 

 

12 hours ago, Simon Lumsdon 1 said:

ow speed difficult to get below 3000

 

1800-2000 should be easy enough to achieve with that engine and is what i would expect from a 90

 

12 hours ago, Simon Lumsdon 1 said:

Noticed engine would bog down in flight when idling and throttle opened, took some time to pick up

 

Did it leave a trail of smoke? I am not normally an advocate for diagnosing engine tuning with your eyes (ears are better generally) but in this case if the engine is running cleanly with little/no smoke trail, bogs on acceleration but also leaves no smoke trail it is more confirmation its lean on the slow run. Heavy smoke trails below half throttle and on acceleration suggest rich. 

 

To be fair though, all of this tuning chat is wasted breath until the mechanical side is sorted out. Get the new bearings in, strip/clean/rebuild the whole engine (carb included) and maybe even treat the carb to an O ring or two. Get it on a test bench and set it up, then put it back in the model and set it up again. Why set it up twice? Because it shouldnt take long anyway, and if the engine is easy to set up on the bench with a known setup, but then flatly refuses to behave in the model you can logically assume there is some issue with the tank or plumbing in the model. The bench also has good access to everything, you can see what is going on, check for loose bolts after the run...its just good practice to isolate the engine from the model and get it sorted by itself before adding further variables. 

[Engine Doctor]

Hi Simon Try re positioning the split collet tightly against the inner bearing race before tightening the prop first ensuring the bearings are seated properly in the crank case and crankshaft is fully forward . If Crankshaft moves due to excessive end float it will effectively change the valve timing ,hence the poor low speed running . My son had a old SC 70 fs years ago with the same issue some time spent fixing the end float sorted it out completely .

Also does your engine have good compression ? I ask this as many of the SC, ASP etc engines have poor comp due to poor cyl head manufacturing. this will also affect performance mainly noticeable at lower revs and at idle . 

As Jon says tuning is a waste of time and effort until any mechanical problem is sorted is sorted,

[Simon Lumsdon 1]

Thanks to all for this fantastic advice. Agreed two likely separate issues

 

have ordered new bearings, £10 so no reason not to. Suggestion of ensuring taper is firmly against front bearing is gold advice, if not securely seating that would allow end float.

 

low speed running, will set it up on the bench once above all sorted, agree it should easily do 2000 revs (and i now have a tacho to check)

[J D 8 - Moderator]

Hi Simon, Just another tip, if you find new bearing is a bit loose in case and likely to spin, some bearing lock fluid Locktite 641 or similar will keep it in place. Cheers, John.

[Engine Doctor]

Just be VERY careful not to let any bearing fit/lock..  get into the bearing 🥲

[Simon Lumsdon 1]

I had thought about thread lock but how would i get the bearing out next time? However might be forces to look at this

[Martin Harris - Moderator]

If the bearing is spinning, you have little choice.  
 

Loctite 641 is designed for servicing. Heating to 250 degrees or sensible mechanical persuasion should break the bond.  You can also use a suitable solvent apparently. 

[Engine Doctor]

The heat you use to remove bearings normally is enough to melt/ soften the loctite bearing fit to remove the bearings easily.

[Simon Lumsdon 1]

Thanks Engine Doc, fitted new bearings and rebuilt engine, compression seems fine. Thinking about it cannot see how end float occurs unless taper behind prop driver not properly seated? The crank is pulled against the driver so no float unless that taper wasnt seated. Dont think it can be the bearings floating etc

 

Anyway will bench test it this week and hopefully sort out the low speed running

[J D 8 - Moderator]

           Just a check Simon, I had similar issue on a two stroke. Turned out there was a crack/split in the prop driver so it did not grip the taper collet properly. Good luck with test. John.

[Mark Hewett]

I seem to have the same or similar issue with an SC91FS at the moment Simon. It had been running well since the last bearing change nearly 2 years ago, although I think that engine always had detectable crankshaft end float when hot. On the last session, after a normal flight, and a normal warm up and power, idle, pick up check, it then died twice during taxi out, so I stopped flying and it's stripped on the bench. With the prop driver removed I could see that the end float was caused by the front bearing moving in it's housing. On stripping the engine, there is evidence that the front bearing has been spinning in the housing. I'm just waiting for bearings to arrive and this time I'll use bearing fit on the front one and hope that sorts it. Good luck with yours.

[Engine Doctor]

In an earlier post i related the end float saga on an sc 70 my son had. The front bearing had been turning in the case and was quite loose but that wasnt the cause of the problem it was the result. End float in this design also allows the valve timing to change enough to make noticeable change in performance The cause was due to the fact that the crankshaft was too long between the bearings or you could say the case was too short this allowed the movement to develop as one or other of the bearings outer race could not seat properly/ fully into the case as the shaft would when fully seated at the rear push the front bearing out of it seating slghtly . This allows the movement to develop .

The easiest fix was to shorten the shaft at the front where it butts up to the front bearings inner race. I measured the end float and skimmed that ammount off of the shaft. Very easy on SC etc as metal is soft. Re asemble and checked float .... I found i had gone a whisker too far so made a shim washer to fit the shaft behind the bearing . This was ground until end float was just percievable to allow for case expansion when hot. A lot of assemling and time consuming but engine ran really well afterwards and not a job I would do on a cheap engine again but it was an itch that had to be scratched . Not a common problem on the ASP, SC etc but like the leaky head issue it does crop up .

Never had this problem on an OS , Enya or Saito etc but they are made to better tollerances to start with.

[Simon Lumsdon 1]

Interesting, dont think front bearing sits on a shoulder in this instance, just a plain shaft. Suspect the taper collar isnt seating back far enough and leaving a gap allowing crank to float in and out. If it does it again i am going to buy some 8mm shims of ebay and pack between from bearing and taper collar which should fix it

[Simon Lumsdon 1]

Sorry 10mm

[RottenRow]

I wonder why SC / ASP used the split collet method to retain the prop. driver on the 70 and 90 (and presumably the 80 as well).

 

I have the engines sized either side of those, 52s and 120s, and these all use the woodruff key in a slot method.

 

All appear to be based upon the same basic engine design (copies of OS) just varying in size.

 

Brian.

[Martin Harris - Moderator]

27 minutes ago, Simon Lumsdon 1 said:

Interesting, dont think front bearing sits on a shoulder in this instance, just a plain shaft. Suspect the taper collar isnt seating back far enough and leaving a gap allowing crank to float in and out. If it does it again i am going to buy some 8mm shims of ebay and pack between from bearing and taper collar which should fix it

Surely the collet can only start to grip when it’s compressed between the prop driver’s taper and the face of the front bearing’s inner race?  

[Engine Doctor]

7 hours ago, Martin Harris - Moderator said:

Surely the collet can only start to grip when it’s compressed between the prop driver’s taper and the face of the front bearing’s inner race?  

Unless there is a defect on the shaft just big enough to allow it to grip before it presses against the inner bearing race.

[Martin Harris - Moderator]

Thanks - yes, the thought had occurred but it seemed so unlikely that I dismissed it. 

[i12fly]

11 hours ago, Engine Doctor said:

....... Not a common problem on the ASP, SC etc but like the leaky head issue it does crop up .

Never had this problem on an OS , Enya or Saito etc but they are made to better tollerances to start with.

Sorry bit off topic, but I have an SC 70FS where the valve inserts leak in the head. Only happens when it is hot but enough to cause the engine to stop. Is there any cure other than a new head?