Replacement Carb for OS FS70 Surpass?

Hi

I have an OS FS70 Surpass and I'd like to replace the carb.

Finding the correct part (OS44081000) for this engine is not easy to say the least.

I have, however, found a carb that is available for the OS FS70 II Surpass. The FS70S II carb (OS44082000), looks like it will fit but I don't know if it's truly compatible.

I've searched the internet high and low for documentation but drawn a blank.

As such, does anyone here have any experience of such an upgrade?

Many thanks in advance

Regards to all.

Pete

Edited By PWLT on 22/11/2020 19:53:40

• Tried HERE yet?

Alternative is Just Engines, that's where I found a look a like carb that works well.

Maurice

Out of curiosity why are you changing the carb? about 90% of the folks who call me for a new carb dont actually need one and a few O rings and a bit of cleaning is a bunch cheaper.

I've just checked on mine and the 70 Surpass II carb has a larger bore where it fits on to the inlet pipe spigot, so it is loose on the old Surpass inlet pipe. The inlet pipes are different shapes and cannot be swapped. If you had access to a lathe, you could turn an adapter sleeve to make the original Surpass inlet pipe fit snugly into the larger bore Surpass II carb.

I hope that is clear!

What is wrong with the old carb?

Thanks everyone for the replies, much appreciated.

To clarify, I have already bought the Carb (from World Models)

Just Engines were my first place to try, unfortunately, they too are unable to find spares (even aftermarket compatible components) for such an old engine. They did, however, offer the same advice that cleaning the existing carb would be by far the best option.

In the interests of full disclosure , it was too late for me, as I'd already tried and my inexperienced hand had damaged the mixture control valve. I had no idea at the time, but this is removed in the opposite direction to what I had expected. Since it's only made of soft metal, the screw head did not survive.

It was only later, that I learned here (How To: Setup A Carburetor), to remove it, you have to turn it in the opposite direction!

This probably explains why I decided to replace the whole carb

Many thanks for the info re the compatibility and the inlet valve having a different dimension. I don't have access to a laithe, unfortunately. This is all a huge learning curve for me, having only just returned to the hobby after 40 years!

That said, I now have the time to start tinkering and I'm actually thoroughly enjoying the learning processes. I'd love to know what the best way to learn all this is, other than trial and error. Until then, I'll perceiver!

Thanks again, everyone.

Best regards

Peter

Peter

For an adaptor / sleeve, you could try contacting ken king on this forum, he advertises for metalwork jobs of this nature.

As a temporary measure to get the thing set up, a good number of turns of PTFE tape might provide enough of a seal for you to run the engine. I have done this one an engine or two in the past, simply to get the thing running.

I can probably turn you a sleeve, since I have both engines to hand as a reference. Let me know if you need it.

Hi Jonathan

Yes please, that would be great!

I was worried how I would provide an exact specification in order to commission one (as suggested above) but if you already have the two versions of the engine (and therefore carb) that would be ideal!

I'll DM you so you can let me know what you would charge.

I also have one other avenue to try as I have (or rather I think I have) sourced a replacement mixture control valve for the Mk 1. That said, even if it is the correct part, (OS26781309 - which I derived from a rare exploded parts diagram of the Mk 1 carb), it still leaves me with the problem of how to remove the old one without a screw head!.

Many thanks in advance

Best regards

Peter

Peter

Out of interest, where did you find the carb diagram?

The copy of the manual I have came from dieselrc.com which does not include the parts diagram.

They are all here Nigel

Drop-down your motor description

Then download the PDF

Click on the Discontinued List, to see other motors

**LINK**

Edited By Denis Watkins on 24/11/2020 11:52:27

Thanks to the skill and generosity of Jonathan W in this thread I now have a working engine once again.

The sleeve that Jonathan made to adapt the Mk2 carb to the Mk1 OS FS70 fitted perfectly and today I put the engine back on the test stand for it's first run.

There is a short video here on YouTube.

I'm no expert on engine tuning but at least I now have the opportunity to learn more about the dark arts of setting idle and full throttle mixtures. The current settings are still factory defaults so I'm hopeful it can be adjusted to run more smoothly at low rpm. That said, I promise not to break this one, so I might just leave it alone!

Many thanks to Jonathan once again, it's really appreciated.

Regards to all.

Peter

Happy ending

It seems a trifle rich on acceleration, but quite difficult to tell properly via youtube.

I'd tune as per OS instructions - the step by step guide is pretty good. Engines need to be tuned. And if you're going to run them much it really helps to learn how they respond to *small* changes in carb settings. Just take it slowly, don't make big adjustments without retesting. Again, the OS guide tells how much is the right adjustment.

Tuning sounds ok, but the bearings are making very unhappy noises

I'm happy to bow to your far greater expertise Jon

May I ask what specifically should I be listening out for that tells you that the bearings are not in good shape?

It sounds like I have a whole new avenue of learning to go down and try and source some replacements!

Thanks in advance

Peter

Its hard to tell on videos as you can never tell what the mic is picking up but under the engine note, especially at idle, there is a gritty drone i can hear. Its most clear around the 1.29-1.31 mark in the video where you blip the throttle and the engine stops firing briefly. You can hear a gritty sound that decreases in pitch as the engine revs drop and could potentially be worn bearings.

As its only a 10 minute job i would whip the engine apart and get the crank out. You can then put your finger in the bearing and see if it feels gritty when you rotate it. If so, grab some new ones (r6z front and 6002 rear if memory serves...double check before you order) and get them in.

Dead bearings can also sometimes make a sound like a washing machine on spin. That pulsating whirr is clearest at full power and usually on 2 strokes, on 4 strokes its more the gritty cement mixer noise that gives the game away.

For the time and cost of changing the bearings it is well worth doing. If a cage fails you may destroy the engine completely and even if the bearings are wearing they are shedding little pieces of steel into the crankcase. The rotation of the conrod will throw these up the cylinder and once between the piston and cylinder wall they work like a grinding paste. Its why you see loads of really fine scratches on bores of clapped out engines, often with worn piston rings as well. Its not like the engine is full of sand, we are talking really small particles, but its still not good for the long life of the engine.

Many thanks for taking the time to write such a detailed reply Jon.

I shall research how to replace the bearings as you suggest.

Kind regards

Peter

no problem. They might be ok, it could be the camera picking up something else but as i say, its easy enough to open it up and check. As a first port of call just take off the backplate. If the bearing looks all clean and shiny its probably fine, if its discoloured at all or has mottling or rust spots then its worth going deeper.

Remember to set the end to tdc and take a photo of the cam position before you take it all apart

Good advice again, thanks Jon.

I've gone ahead and ordered the two new bearings, but I'll only attempt to fit them if I really have to.

Thankfully, JustEngines.co.uk had them in stock. Parts are hard to come by for this engine as it's now so old.

Best wishes and thanks again.

Regards

Peter