OS 90 FS timing
 

I recently bought an old OS 90 FS off eBay in England. This is the one with the cam gear at the back of the engine, not the newer generation Surpass where the cam gear is a clever worm off the main crankshaft. My 90 FS was grubby but a real bargain, the way I like it. I stripped the head off and whilst cleaning off the gunged internals with oven cleaner (careful - this caustic soda stuff attacks aluminium) I accidentally dismantled the timing gears. I search the web on an easy way to reassemble this but came to no avail. Hence I gritted my teeth on a trial and error approach. I looked hard and found no discernable marking on the wheel or crank for timing alignments. Oh dear, this could prove interesting to get it to run again.

Going back to the 4 stroke principle, the inlet valve has to open after TDC (top dead centre) and the gears have to mesh nicely around there. Also, just to make sure, the exhaust cam comes up around BDC (bottom). As it turned out, I found that the inlet cam opened just before TDC (about 5deg before TDC) and went on just past BDC, whereas the exhaust cam came up just before BDC and continued a little past TDC. I thought that, allowing for the inertia of the charge in the cylinder, this must be about right. Hence I found the gear mesh at about this point and reassembled the unit. And it ran beautifully! Well, here it is. I hope that this approach helps those out there when it comes to reassemble timing on these 4 strokers.

RE: OS 90 FS timing
 

Just for the record...

There's a timing mark on the face of the sun gear that faces the center of the big end slot on the drive disk with both at top dead center.

The notch cut on the camshaft rim is opposite the timing mark on the sun gear.

The screw on the back of the cam has left handed threads.

Enjoy,

Jim

RE: OS 90 FS timing
 

Hi teddykan,

Congratulations on your find, a great engine.
Here´s a sketch of the cam gear alignment that w8ye describes above

Incidentally the valve timing for the FS-90 is as follows:

Inlet opens at 40º BTDC
Inlet closes at 90º ABDC
Exhaust opens at 80º BBDC
Exhaust closes at 40º ATDC

Enjoy,


Tomas H.

RE: OS 90 FS timing
 

Dear Teddykan:

The PIPE Here...and I'm wondering...are you in need of a FACTORY MANUAL (the one that would come in the box with the engine) for your FS-90 rear camshaft OS four stroker?

If you are, please send a private message through RCUniverse WITH YOUR EMAIL ADDRESS in it...THEN I can start sending you the scanned pages of my FS-90's manual for your needs!

I very recently helped someone here at RCU with their need for a factory manual for their "pre-Surpass" OS FS-40 four stroker...and if YOU need one as well, I'm "right here" waiting for your reply!

Hope to hear from you "one way or the other"...by the way, the Perry P-20 "shaker pump" fuel pump (at http://www.perrypumps.com/prod02.htm ) works GREAT on the FS-90, with a 330º counterclockwise angle setting "from fully closed" on the hexagonal outlet fitting on that pump, as the setting I've used in the past with my FS-90!

"Hope to hear from you soon"...

Yours Sincerely,

The PIPE! ;)

RE: OS 90 FS timing
 

:)
Many thanks to all of you helpers from all over the world. I opened up the FS 90 and saw that I had it about 10deg too advanced. I have now reassembled it correctly and it runs great.
Many thanks
Check out my other problem with an OS 91 VRDF blown hole in the piston, which I subsequently repaired by hand. That awaits testing but I need somewhere desolate as re-bedding in an ABC engine will be very noisy!

teddykan

RE: OS 90 FS timing
 

Hello; While you were in the cam chest, was there a bearing on each end of the camshaft? Were there any bearings on the cam idler? I have two of those great engines but I have never been into the cam chest.

RE: OS 90 FS timing
 

There are no bearings listed in the parts list other than the front and rear crankshaft bearing.

Enjoy,

Jim

RE: OS 90 FS timing
 

There ARE...THREE...CAM bearings that exist back there!

Dear W8YE and Teddykan:

The PIPE here once again...and there ARE...three cam bearings..."back there" where the camshaft resides !

If they DO need replacement, Boca Bearings has these at http://www.bocabearings.com/main1.as...274&n=*CSK-004 ...I've had these already changed over (along with the CRANKSHAFT bearings, also from Boca) by John Sepe, on my FS-90, not very long ago!

If a rear camshaft FS-90 NEEDS "all" new bearings, remember that FIVE bearings must be purchased...two for the crankshaft, and THREE for the camshaft...for a "complete" bearing changeover...!

Just thought I'd let other know about this...!

Yours Sincerely,

The PIPE! ;)

RE: OS 90 FS timing
 

I have 3 of these. Great engines. The amazing thing is they are on 2oz heavier than the Magnum 90. I believe the Magnum 91 is also lighter than the OS 91? I have attached a picture of one I just picked up for $69.

RE: OS 90 FS timing
 

Pipe,

I just bought a NIB FS-90... haven't rx'd it yet, but don't think it's coming with a manual. I'd appreciate getting a scanned copy.

If you can, send here or to [email protected].

thanks.

RE: OS 90 FS timing
 

geez. I thought i'd gotten a great bargain... sounds like the FS-90 is an out of production engine?

and you just got one for $69!!??!! guess I overpaid by about $100.

:-(

RE: OS 90 FS timing
 

southlaketxflyer,

You should be very happy with purchase; a new-in-box OS FS-90 for around $160 is very good.
Most of these engines go for a lot more, I´ve seen a couple (also nib) that went for $250 or more on the bay very recently.

There´s a world of difference between new-in-box and used.
I´ve been bitten several times by used engines that I got off the bay at "bargain" prices.
There are many descriptions; "little used" - "hardly used" - "hardly run-in" - "like new" - "not sure if this engine has run", etc., etc.
It all means essentially nothing - what you get can be anything from like new to "in dire need of extensive and costly repairs".

Yes - the FS-90 is out of production, but there are still quite a few in circulation, and you can still find spare parts without too much problem if you look around a bit.

You have a top quality engine with wonderful handling characteristics and ample power for most applications.
Yours is NEW in the box, with all accessories. It should last a long time if you take care of it.

It´s a bargain - enjoy it!

Tomas H.

Mr The Pipe
I just received a very old OS FS 90.
Several days cleaning an dissasembling.
It will be very useful if you still have tha manual.
If so, can you share?
Regards.
Jorge Feliú

Haven't seen any posts from "The Pipe" for some time.....

I hope he is fine.
Thanks

I was thinking the same not so long ago.
Last I heard his mother had passed away and he seemed uncertain where he would be living
With the Wuhan Flu and all we lost contact.
Matbe I could look up his ham radio callsign for a current address (?)

Good idea. I always enjoyed reading his posts.

Yes, Like getting a letter from an old friend.