ScottMcM

My Feedback: (66)

I'm got an issue with a pre-surpass OS 91fs. It is running way rich on the low end, and any effort to to lean it fails. I raised the idle speed, and it continues to run, but it smokes badly, like it is running rich. When I lean the low end even a little, it quits. Also, to get the high end to run right, the high speed needle is open only 1/4 turn. It runs fine on top end, but I'm a little concerned that I may have carb problems. I have disassembled the carb, and can't find any obvious problems. I have several more OS 4-strokes (this is the only pre-surpass), and none of them exibit this problem. If it were running lean, I'd guess that there was trash, or an air leak....but I'm lost with it running rich. Any ideas??

w8ye

My Feedback: (16)

To my knowledge, all the OS 91 engines were Surpass.

There was the OS 91 Surpass I that was around during most of the 90's and then in 2000 the OS 91 Surpass II came out with the black valve cover and low speed and HS needles on the same side.

Prior to the OS 91 Surpass four stroke, there was the OS 90 four stroke which was a rear cam. it was not a Surpass engine.


In your case, I would tune to get the engine to run correctly and ignore the smoke at low speed.

ScottMcM

My Feedback: (66)

I didn't realise all 91s were surpass (obviously). It has the old style valve cover, and the low speed needle is on the center of the throttle arm. I have been running it as is for several months, but was curious as to why it smoked so much and why the high end peaked out at only 1/4 turn. I will, however, take your advice and fly it as it is. Thanks for your help.

w8ye

My Feedback: (16)

The high speed usually runs at around 1 1/4 turns.

There is a part of the needle valve holder that becomes damaged in these.

As long as you can still get it set OK on the high speed, I wouldn't bother with it.

estradajae

Hi Jim

I did a rebuild for a club mate of a 91 surpass like the one described. It runs very well on the top end but have found the same sympthoms described by the op...it tops with 1/4 hsn turns, and no matter what you do with the needles, to make it run at idle you have to open the barrel much more tha usual, and it does idle very rich, smokes like crazy and it tends to quit...leaning the low end helps a little with idling, but if i lean it to do it right, after some time it begins to load up, and runs so rich that it dies. At this settings the engine cant transition from low to high....i thought it was the hsn and lsn needle interactio ...so i worked on it but no....it wasnt possible to tune the low end, it will never run leaner on the while idling.
Took off the carb and everything seemed fine, cats eye was clean and ok..and so on...despite of th hsn working on only a quarter turn.

Im sure the engine itself is not the problem, is a carb problem, but i just cant find what it is...any ideas?
I have tuned succesfully many 91s on the field... i usually know how to make them run right, but this thing was not possible to tune!

Best regards

estradajae

Cmon Guys

Does anyone else has had this problem? the low end being too rich and if you lean it it won't pick up??

Any thoughts??

hellcat 1

I have had two 91 surpasses that got to where you couldn't lean the high speed out with the needle all the way in. I think that what had happened was that the high speed seat, which is pressed into the carb body and is part of the cats eye that the idle adjustment covers, either moved inward in the carb body or became wallowed out because of needle vibration. I have solved this for a while by grinding or filing the the high speed needle holder down so the needle would go in farther, but the real reason for needle vibration is a worn o ring on the needle. The best solution would be to replace the carb body, but I don't think you can get it any more, unless you can find a good used one. If the part with the cats eye moves inward as well that would make the idle adjustment off. You do need good o rings on the high speed needle as well as the low speed adjustment. That part comes out by taking the barrel out and screwing in all the way in. You can tell if the seat has moved in by making sure it is flush with the flat area that it is pressed into. Take the high speed needle and holder out to check this out. If so you might be able to press it back in with a brass tube placed over the cats eye part. I think what can cause it to move inward is trying to screw the hig speed needle in to hard. Good luck. Sam

estradajae

Hi Hellcat

Sorry I haven't replied earlier.

This engine is driving me crazy!

First I took the carb off, and made sure it was celan (cats eye and so on) o-rings etc and checked if there were missaligned parts, bad seats, parts that may be out of their place and no...the carb, apart from the stains, is in great shape.. Then I put it back on the engine. The engine still wants to run rich on the low end. I had a new magnum 91, and put it's carb on it, and when I tested it it ran much much better (new carb with new o-rings btw). But yesterday, we tried to fly that plane again, and the engine went back to the issues. I noticed this funny thing of the engine running with onle about 1/4 turn on the HSN on that magnum carb, and not being able to lean the low end where it does not run rough...because it runs so rough when idling that you see the plane shaking badly. If you lean the low end the engine just stops, and is not possible to start the engine....I though that maybe I was tunning it wrong or that the needles were out of whack...so I opened about 1.5 turns on the hsn and opened another bit on the lsn, but no... the engine wanted to run only on 1/4 turn on the HSN (10.000rpm on a master scimitar 14x6).
Before I did the maintenance to the engine, the engine had the cam timing off, and it wasn't able to reach even 9000rpm on a 13x6 prop...so clearly now the cam timing is ok.
I've got to find the problem because it is driving me crazy!!

jaka

Hi!
When an engine doesn't run as expected ..take it apart and check all things! First of all it's very seldom a carb problem! The only carb problem is the O-rings in the low and high speed needle, but those O-rings are very seldom a problem. I have hundred of engines which I have used over the 36 years I have flown R/C and non of them have had any carb prioblems.

Carbon build up on the piston top and on the valves and on the vale steam must be checked! Carbon build up is very common if you have run the engine on castor oil.
Also check the cam position.
Check the ball bearings and change them if they are rusty and worn.

14x6 is a suitable size for a .90-.91 four stroke.

estradajae

Hi Jaka

I actually did a complete maintenance to the engine, it has new bearings, now a very clean piston, valves, valve seats, compression is great, valves are adjusted, new glow plug (o.s F) cam timing correct (it was off as stated on the previous posts).

Fuel I use is Omega 10%, which makes my other 91 4 stroke engines run very good without this specific issue.

Best regards.

jaka

Hi!
Here is how the cam should be set on all OS, ASP,SC, Magnum and TT four strokes.

estradajae

That is how it is, engine has the timing set correctly...if not, it wouldn't hit 10.000 rpm on that prop.