OS 120 surpass pump engine conversion
 

Hi out there!
My engine is the Surpass 120 MK I. It has not run more than approx 25 hours all together, but it's been lying about for several years after a crash until a got my 1/4-scale CUB finished last year. After consulting this forum a year ago I have run the engine several hours, and flown the model some, but I can't get it to perform reliably. I've had a pump problem no doubt about it. There is a gear within the pump and because there was a slight instability there, the taps and notches (or whatever you call them) could suddenly start catching. This was the reason, I believe, why the engine sometimes overheated. There were no spare parts available and in Sweden, where I live. I could neither get hold of a MK I nor MK II pump for replacement.
So, I decided to convert the engine to a non-pump. I simply got a new back-plate and grinded off the extension on the .....(what the heck is "vevaxel" in English? The part that holds the piston in one end and the prop in the other)..You know. After opening up the fuel needle 7½ turns it runs beautifully between idle and half throttle. I've realized the obvious fact that you must replace the carburetor as well. But, then you must also replace the inlet manifold. But the diameter of the manifold on my pumped engine is 12mm and on the non pumped engine only 10mm, roughly. So there will be a mismatch when it comes to where the manifold enters the crankcase. So we've got a problem here.
As I recall it, I got an e-mail from somebody "over there" after my prior contact with the forum. This someone claimed there was a conversion kit when you wanted to do this. But at the time I thought I'd already solved the problem so I gave it no notice. Now, is there such a thing as a "conversion kit", or how do I go from here? Please just don't give me the advise to sell my engine which is what I got from the OS "expert" in my country.

Looking forward to get some feed-back

Peter Evers

RE: OS 120 surpass pump engine conversion
 

Hi Peter,
I have the same engine with the same problem. I fixed mine by fitting a perry osilating pump and it runs better than new. You'll have to fiddle with the pump pressure (increase it) and reset the mixtures. Just remove the old pump and seal all of the holes in the backplate. My setup is 10% nitro,15% oil, os-f plug and 15x8apc prop. The lower oil content really helps the idle and it still smokes in the air and runs great after 3 years.

RE: OS 120 surpass pump engine conversion
 

Hi Mono!
Thanks for your reply. In replacement for nitro in the fuel I have installed an ignition system for safe running (what a laugh!). I have the same amount of oil but semisynthetic. Neither of these differences compared to your setup will have any affect as far as I can understand, so I'll go ahead and try your suggested Perry pump. I'll let you know how it works out.
By the way, have you checked out my CUB in the gallery? That's where the engine is located.
Peter