OS 35 won`t reach max revs

[christopher small]

I have an old OS 35 FP runs ok but doesn`t reach maximum revs,you keep turning the main needle valve in waiting for the revs to pick up but they don`t ,then it cuts out from lack of fuel,its running slightly rich all the time.

Would the plug have anything todo with it?

Chris

[Tim Mackey]

Unlikely but possible. Cheap and easy enough to replace a plug and find out.

If its an old 'un, then maybe the needle is simply worn and the fine taper that is needed is now too coarse.

Also, check very carefully for any leaks of fuel or air around the carby. 

[christopher small]

Thanks again Timbo,could be air leaking round carb,i expect theres an o ring in there somewhere.

Chris

[Former Member]

[This posting has been removed]

[christopher small]

Thanks,I shall check it all out.

Chris

[Chris Channon]

Hi, What prop are you using ?

What fuel are you using, anything from straight castor to 10% nitro will be fine.

Let us know then we can go from there.

Chris C

[david field]

you should be running a 9x6 or 10x5 propto get good revs. If its running rich it should be tending to 4 stroke,  turning the needle out will mke it richer.  A good starting point should be 2 1/2 turns out from fully in . If it tend to 4 stroke up to 1/4 throttle turn needle valve 1 click at a time waiting 3-4 seconds between clicks. If it was rich surplus fuel can take afew seconds to clear the crankcase.  When running smoothly slowly open to full throttle.  Carefully turn neele valve 1 click at a time until it no longer speeds up, if the motor speeds slightly  move 1 more click the same way ,if the motor slows 1click at a time the other way until it nolonger speeds up. in both cases 1 clck back once you go over max revs. Lastly hold model vertical if it cuts its eiher too week or you have an air leek. As soon as the motor starts to cut  bring the nose horizontal this should restore power if your close to right setting. Just turn needle valve out 1 click and try again to hold vertically until steady running obtained. Once set it should not need altering for sbsequent flights , or if it does only one or two clicks at the most. Good luck

[christopher small]

Thanks Chris and David,not sure what prop was on there,as it has been taken off some time ago,fuel was 5% nitro,i think i have been too quick on the needle valve setting,and not taking my time.also it doesn`t seem t have much compression,how can i check for this?

[JohnP25]

I wonder how clean the carb is. You did write that it was old.

[david field]

my suggestions were made thinking you were running a 2 stroke , if your motor is a 4 stroke the same principles apply.. If you can remember cars with manual chokes , they would 8stroke if the choke was left out too long. The length of the inlet tract makes a difference to the time to respond to a change on the needle valve. Do you use an inline filter?

[christopher small]

Hi David,I have an in line filter on the fuel pump from the fuel can,but not on engine.Would the engine be worn i have had 25 years.

[david field]

i hav some 2 stroke motors this age which run ok. should run cleanly and be only a bit down on power. How does compression feel? Does it still pop over tdc with no electric to glo plug? Compression will be noticably down if badly worn.

[christopher small]

thanks david i shall go and check later,would be realy nice to get itgoing again.it would be a shame to bin it,or use it as a doorstop.

[Jack Bagley]

Hi Chaps,

                  Sounds interesting to me, but when I have indulged in this sort of exercise, running up old motors from the "box" under the bench, I usually find that they are just "shagged out" with age and use and I shouldnt't have kept them. But we all do it, just can't bring oneself to actually chuck them away, so back in the "box" they go untill next time I feel like, " oh, I can't remember when I last ran or used this one so I'll just run it up to see" etc and around I go again! I'm glad that I don't appear to be the only one who spends hours just pointlessly flicking props over in the vain hope that a great powerful roar will emit from said object! Best of luck anyway, we old prop flickers must stick together, (there's a pun in there I think!)

                     Best wishes to you all, Jack.

[Jack Bagley]

HI Ron 2,

                 Sounds pretty cool to me, Funny but I actually got a few revs out of an anchient twin plug Merco 60 on Sunday. It ran splutterinly for about 30 seconds, never to be heard again! So like youRon, I put it back for another day!  Will I never learn?

    Best wishes to you, Jack

[christopher small]

I have just got my old OS35FP out of its box in the workshop,all sticky and horrible covered in dust,I am not going to throw it out yet.

Should I strip it down and clean it in petrol?

[Jack Bagley]

Hi Ron & Christopher,

                                        Many eons ago, I used to "boil" out old motors that had b ecome gummed up with castor oil residue, as instructed by our local expert and it actually works! Saves on smell, castigations from one who has to be obeyed etc. Re oiling with a thin machine oil also works wonders afterwards. Of course, once I started to use Synthetic oils, this problem hasn't occured. I would try boiling in a saucepan with just a hint of detergent for about 15 mins, followed by a thorough drying and oiling.  Keep on "prop flicking" chaps, One day the results may astound us!

                                 Best wishes to you all , Jack
 

[Colin Hostler]

Hope you haven`t had the fuel for 25 years!!!

Colin

[Jack Bagley]

Hi Colin,

               Like it!  I'll check the label for best before date!

                                         Best wishes to you all, Jack.

[The Wright Stuff]

Hi all,

Am having exactly the same problem with my OS 46 LA, so I thought I'd resurrect this thread rather than start a new one. It just seems to be always running rich - sounding like it's four stroking (without the high pitched scream), and closing the needle valve just stops the engine. This started after I crashed it - it was fine before then! Cleaned it all up, and seems to start ok - nothing seems to be bent - just won't hit peak revs!

Am checking for leaks, but I have a question. If air was leaking IN, then surely it would exhibit the symptoms of running lean. Is it possible air is leaking OUT during compression?

Cheers,

Ian

[Shaunie]

My first thought with both of these engines is are they over propped?

Second thought is plug problems.

Remember airleaks have a decreasing effect as throttle approaches W.O.T. They are more significant at lower throttle settings due to increased vacuum. You could picture it as at low throttle an air leak is a larger proportion of the total airflow than at full throttle. What this means is an airleak round the carb base will cause a poor idle but is unlikely to have much of an effect at full throttle.

Hope this helps, Shaun

[Martin Harris - Moderator]

Posted by The Wright Stuff on 03/09/2012 12:47:37:

Am checking for leaks, but I have a question. If air was leaking IN, then surely it would exhibit the symptoms of running lean. Is it possible air is leaking OUT during compression?

 

Yes it's quite possible.

Have you checked the backplate (and crankcase) for cracks or loose screws? LAs are well known for loosening their backplate screws.

If you lose crankcase compression on a 2 stroke it will affect the fuel transfer to the cylinder and drop power.

Edited By Martin Harris on 06/09/2012 00:30:32