OS 91FS gas conversion?
 

Hi,

can a fs91 be converted to gas? If so, what kind of power can I expect? Is there a pumped carb for it?

Ameyam

Hi, yes, but Walbro/Zama carbs are physically quite large. You can make an adaptor easily, see thread 180FS glow to gas conversion on this board.

A 20/64" or 24/64" should be the right size. Power will be less than glow, take 1" from the diameter you are using for glow. Guessing 14*6 or 13*8 about right size on gas.


P.S. I was in India (Chennai) last year, I hope i get the chance to visit Mumbai :cool:

Yes, as stated, a "proper" carb will be needed if you want a good result.

You should also find that the glow plug will provide ignition, without the complexity of a separate spark-ignition system .... well, my little OS runs on petrol, just like a normal glow engine runs on glow fuel. I expect that your .90 will work just as well.

So your saying that you don't need a spark plug and timing to get the thing running right? I thought the power came from running a bit on the retard side because of pre-detonation of the gas... Please elaborate more on this setup Bob.

Took my OS .30 two-stroke and ran it on a 10% petroil mix. Engine ran pretty well; started easily ... but the carby adjustment was hyper-critical.

So, the glow plug functioned in the same way that it does with methanol ... ie, as a catalyst.

Nothing more to tell you, 'cos that's the extent of my experimentation.

10% gas is not a big deal. There's a big thread on this forum about the gas/glow mix and the percentages that you can run while still continuing to use a glow plug. Link:

http://www.rcuniverse.com/forum/engi...-glow-how.html

You will not be able to use a glow plug if you run 100% gasoline unless you use a glow plug driver that is set to be on all the time. At that point, why not just do it the right way and use a CDI system?

No, that's 10% oil, 90% petrol.

And it does work, without a glow driver.

That seems impossible, but apparently it isn't. I would think that it would be detonating all over the place. What type of plug? Are you using a gas (Walbro type) carb?

Ordinary glow plug.

Standard OS carb.

Carb adjustment very critical ... almost impossible to get it right. But, the engine does run and sustain itself. Can't say that I noticed detonation, but I spent about 20 mins with my mouth agape.

There's a thread somewhere, about a commercial version of this ... .40-sized two-stroke, running on petroil mix and glow plug. Buuuuut, that engine has a carb which is made for the job, with appropriately-fine metering.

Well, congrats on a very cool project.

I think the engine you're referring to is a Norvel 40. Apparently is has a specially coated liner (and maybe piston too) and you have to run a ton of oil but a few guys are reporting pretty good results.

Hi,

my engine needs a new head because the head muffler connection threads have slipped. Plus I need head screws. OS went for an odd size of 3.5mm for these engines and these arent available locally. I will need to get those from OS as well. With a FS72a, a FS91 running and another FS91SII, FS110A, 65ax, 95ax and 120ax sitting in the box (those are just the glow engines I have:o), there was no point trying to restore this one. Thats why the gas conversion question. But with a blown head and no easy way to convert to gas, there is not much point in the gas conversion either.

By the way, the engine has a neoprene seal o-ring in the carb and the HSN. Wont those need to be replaced if I run gas in it?

Ameyam

Well, if the glow plug' threads are ruined, you could close off the hole and try the engine on model diesel fuel.

It might surprise you.

My Enya .60 four-stroke runs on diesel, without any modifications. To be fair the CR is a tad low and a Cetane additive helps with reliability. The CR on your OS might be higher than 8:1 ... which is the CR on my Enya.

Anyway, you don't have much to lose by giving it a try.

No, threads for the silencer are spoilt

Ameyam

OK, so run the engine without a silencer.

You should be able to fabricate a short stub-pipe, which attaches to the head ... it may be that you can cut a completely different thread to that which exists; a Metric Coarse thread might do the trick. Once you have a short stub, permanently-fitted, you can lead away the exhaust goo.

It all depends upon what facilities you have.

I run several engines on straight pipes and I find the sound quite acceptable.