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OS 90 4S valve springs

Old 10-21-2005, 01:48 PM
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donkey doctor
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Default OS 90 4S valve springs

Hello; does anyone know if I can use 91 surpass valve springs in my old FS 90? I have good compression and the valves are properly set and timed. The engine won't rev over 8000 rpm. I have an 14 x 6 Kseries MAS on a 4 star 60. Otherwise the engine runs great, nice low idle and easy starts. It used to take 5 feet to get off the ground, now it takes about 30 feet. There's no throttle restriction, it just won't rev up.
Old 10-21-2005, 08:16 PM
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William Robison
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Default RE: OS 90 4S valve springs

DD:

If your engine accellerates cleanly to 8K rpm and then immediately starts bucking/shaking/missing it could well be valve float.

On the other hand if it reaches 8K slowly, sounding as if it were straining to get there, look at another cause.

You said the inlet was not obstructed, don't be offended, but did you check the exhaust? Pull the muffler off and look at the exhaust valve. Can you see the stem, or is there a large blob where the stem should be?

If the valve is not badly carboned run the engine with the exhaust open.

If no joy with any of this, give the springs a try. Never hurt the engine to replace them.

Sorry I can't tell you anything about an interchange. Measure the outside diameter of the springs. The mid block Saito uses aprings with a coil diameter of 7.5 mm, if you can use that size spring they aren't expensive.

Bill.
Old 10-21-2005, 08:23 PM
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Default RE: OS 90 4S valve springs

Current springs have a free length of 13.8 to 14mm. The OD is around 7.3mm
Old 10-21-2005, 10:57 PM
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Kimhoff
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Default RE: OS 90 4S valve springs

Why don't you just order the correct ones from Tower?

Part number 45460200
Old 10-22-2005, 12:11 AM
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Default RE: OS 90 4S valve springs

That number brings up the valve springs for the 120/160 twin and the 240/320 twins
Old 10-22-2005, 07:11 AM
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Default RE: OS 90 4S valve springs

According to the parts diagram sent to me by Bax, the correct part no. for the OS90FS valve springs is listed in Kmhoff's post - 45460200 (same exhaust and inlet). It may be the same part for some OS twins.
Old 10-26-2005, 08:18 PM
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Default RE: OS 90 4S valve springs

I too have one of these old queens. Please let us know if they fit. Have been told they will by local engine guru but still have to try myself.

Eric
Old 10-27-2005, 01:39 AM
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William Robison
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Default RE: OS 90 4S valve springs

DD:

The reason I knew the coil diameter on the Saito is that the older ABC mid block engines used a coil diameter of 6.95 mm. They are no longer available, and the 7.5 mm springs wont go into the cup.

To solve the problem I made a 7.6 mm fly cutter to overbore the cups, allowing the newer springs to be used in the older engines. I've done several now.

If the early/late difference in your OS twin is similar, perhaps you can effect a similar solution.

Bill.

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