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  1. #1
    SeamusG's Avatar
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    Saito 91S Lacks Power

    Hi all

    I have a used Saito 91 that I purchased on eBay last year. I removed the back plate and verified that the guts were pretty clean and the bearings were smooth. I left the valve adjustment as it came (unsure what that clearance was). I ran a couple of small tanks of fuel through it on a bench.

    The engine is installed upright in my 8.5 lb SIG 4Star60 kit. Fellow club members (Denver area) fly this combination with a 14x6 prop (APC preferred for its additional weight & resulting fly wheel effect or MAS). Fuel is fresh 15%. Glow plug is an OS F. The original exhaust (beat, loose fuel pressure nipple) has been replaced with a new style cast exhaust after the maiden flight (the nipple came off mid-flight). The exhaust tip is pointed down at (maybe) 15 degrees. The fuel line and exhaust pressure lines were both split with tubing inserted at the split. This was done so that I could disconnect both lines and reconnect them together for transport and storage - keeping leftover fuel (tank has been emptied) from running into the carb or exhaust.

    The engine will idle smoothly at 2000-2100 rpm. Excellent off-idle throttle response. With an MAS 14x6 (or wood 13x8) the engine max rpm is limited to 8000-8100 rpm. With a wood 13x5 it will turn 10500-10900 rpm. The maiden flight was with the 13x8 and it barely had enough power to take off. This was confirmed by one of the clubs more experienced "trimmers" on its 2nd flight.

    Since then I have added a 3rd line to the tank, replaced the fuel lines without splices, adjusted the valves to .0025 (.002-.003 tolerances), replaced gaskets, o-rings and washers (except for the gasket between the cam gear housing and the crankcase).

    What would YOU do to get this engine running up to expectation? New ring? Valves? How can you diagnose poor compression? New bearings (I have a set on the bench)? The experienced trimmer was confident that the needle valve was good.

    TIA for your input.
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  2. #2

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    RE: Saito 91S Lacks Power

    Well, turning a 14 X 6 at only 8,000 to 8,100 is a clue. You should be well into the 9,000s like around 9,200 to 9,600 and considering your altitude maybe even a little faster. So me thinks it is in need of a new ring or a general overhaul. She sounds like she is worn out.

    If you do go in to replace the ring, replace the bearings too. They don't cost that much.

    But before you do that make sure the muffler is clear. make sure that the pressure tap is open so you are getting pressure in the fuel tank. I would take it and cook it in a crockpot filled with common anti-freeze compound. A restricted muffler can cause redued RPMs. It is the new style muffler, there is a hex head screw that holds the aft part of the muffler in place. Use an Allen wrench and go up the exhaust and unscrew it. There is a baffle of sorts with several small holes to let the exhaust gases out. Make sure that is clear. I even drilled mine out to open them up some.

    Go to http://saito-engines.com and read up there.

    Good luck,

    Cheers,

    Chip

  3. #3

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    RE: Saito 91S Lacks Power

    Looking at your photos, the barrel looks very new, wonder if somebody has rebuilt the motor and got the valve timing wrong, easily done on a Saito if you are not carefull.

    Yes they will run with the timing out but not to well.

  4. #4

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    RE: Saito 91S Lacks Power

    Your altitude will zap 15%-20% of your power. I fly at 4'000 foot level, and we experience it here. Figues are generally 3% per 1,000 ft. So if you are at 5,000 to 6,000 ft. ASL, figure 15% to 18% loss, plus factoring in humidity, barometric pressure, temperature, etc...You will need to compare that Saito 91 to another one with the same prop, fuel, etc... at your altitude to see if the motor is off that far. It's probably in the ball park judging by your numbers. A 15% loss in power will mean a 15% loss in RPM, thus, 9200 at sea level, means 7820 RPM at altitude. Thats why WW II fighters had Superchargers or Turbochargers, as they are atmosperically / altitude imune within reason. The two engines should be within 3-5% rpm of each other. Engines do not spin faster, nor produce more power at altitude like some have referenced.

    But what do I know, I only work for NASA as my day job...


    http://www.rotaryforum.com/forum/sho...d.php?p=163198

    \"I ran out of ideas and altitude at the same time...\"

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  5. #5
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    Thx Chip & futaba,

    Guess it's time to get up close and personal with this little ole 4-stroke. Gonna need a magnifying glass to be able to work on those tiny valves. [8D] I'll need to source a ring and figure out some way to true up and hone the cylinder. Hope that the valves are simply gunked up and don't need any attention.

    Cheers,

    btw futaba - had a chance to visit the NEC in Birmingham a few years ago (motorcycle show) - what a great facility

  6. #6
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    PW - everyone knows that NASA is just east of Clear Lake City - NOT way out west

    All of the references that I made to props are what guys in my club are flying at my club's field. Same plane, same engine, same location, my engine's performance just ain't cuttin' it. I've built and road raced motorcycle engines up here for a bunch of years. Spent plenty of my hard earned $ on flow benches and dynos. Performance stinks up here. Kinda fun to get down to high-oxygen content places like, well, like anywhere but here .

  7. #7

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    RE: Saito 91S Lacks Power


    ORIGINAL: SeamusG

    btw futaba - had a chance to visit the NEC in Birmingham a few years ago (motorcycle show) - what a great facility
    The NEC is about 30 miles from me, my local airport, to us Brits it's a huge place

    Cheers.

  8. #8
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    Chatted with my LHS - he said "20% for that 4-stroke" at twice the price. Emmm, wonder where his priorities lie?

  9. #9
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    RE: Saito 91S Lacks Power

    ORIGINAL: Prop_Washer2

    A 15% loss in power will mean a 15% loss in RPM, thus, 9200 at sea level, means 7820 RPM at altitude.

    But what do I know, I only work for NASA as my day job...

    Based on a 9200RPM starting point 15% reduction in power equates to a 8714RPM. 7820RPM would be 39% power reduction.

    I've seen it written more than once, but have not done the math to verify. The reduction in air density reduces the load on the prop almost as much as it reduces the amount of oxygen filling the cylinder. The end results is that altitude doesn't change RPM for a given prop that much. The engine is making less power at higher altitude, but not turning significantly slower because the torque requirement for the prop is reduced.

    But what do I know? I don't have a college degree.
    Greg

  10. #10
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    RE: Saito 91S Lacks Power

    As was mentioned above, I'd be lookin at the cam timing. I've put mine back together a tooth off and it ran but just wouldn't make the power up top. Luckily I had a Saito guru at my field and got me lined out on timing it. No more problems and it turns a MAS 14X6 9900 on 15% glow and 9400 on gasoline/spark which is what I am currently running it on.
    Spektrum DX8i, DA DLE SuperTigre OS FOX Saito Enya Jett TT: John 3:16

  11. #11
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    15 minutes later (using 1.5, 2.0, 2.5 allen wrenches, adjustable wrench, torch, wood dowel and leather gloves) ...

    Guess I have to check the cam timing [&o]

    I had these replacement bearings, see - and I had this big hammer ...

    My Ex-wife used to say that I was impatient - what can I say - that was the only time that she was right

    I'll keep ya posted
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  12. #12
    Moderator Hobbsy's Avatar
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    RE: Saito 91S Lacks Power

    Seam, is the outside edge of the piston ring black as it appears to be in the pic, if so that says the ring is not broken in, it will be silver if broken in.
    Case 570 Diesel, 188.4 Cu. In. HP==36 @ 1,900 RPM, CR 17.5 to 1. Bore==3.8125, Stroke==4.125

    As competition improves products, the differences between them get smaller and smaller

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  13. #13
    TimC's Avatar
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    RE: Saito 91S Lacks Power

    SeamusG, I'm using a .91 Saito in my 4 Star 60. I also live at 4000' elevation. Your rpm numbers are way low.

  14. #14
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    I noticed that one pushrod is 1-2 mm shorter than the other. According to the parts explosion these are supposed to be the same part suggesting that one wears more quickly than the other. Only after this revelation did I figure out that I don't know which side it came from. Easy enough to match the rocker.

    On cam timing, it is fairly easy to determine TDC by looking at the crank from the rear. There is no mark on the front of the crank where splines are cut that drive the cam gear. At best you need to approximate the spline trough that is at the same point as (matches) the crank's piston rod pin. The cam gear has a mark on the face that is at 6 o'clock when the crank's piston rod pin is at 12 o'clock. The mark locates a gear point. There are 10 gear troughs on either side of this point so that you can be assured that it is truly at 6 o'clock. HOWEVER the darn cam gear spins very easily. I am not sure at all that I have installed the cam correctly! Any tip / technique would be much appreciated.

  15. #15

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    RE: Saito 91S Lacks Power

    Take the back plate and valve rocker covers off, and watch the operation of your valve rockers in reference to the position of the crankpin. When the piston is all the way at top dead center on the exhaust stroke, your valves should be equally open.
    Club Saito Member #370
    AMA #853194

  16. #16
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    For the morbidly curious ...

    Here's the crank / cam components

    1st pic - you can see the piston pin on the crank nearly at 12 o'clock. You can see a small dot just above the splines and a black mark (which is mine) - denoting the gear trough closest to aligning with the center of the piston pin.

    2nd pic - slight variation showing the black mark and its alignment with the piston pin.

    3rd pic - top shot of the case and the crank. You can see mfg marks on the back of the case (horizontal line) and the front (arrow). They appear to be alignment marks for 12 o'clock.

    4th pic - front of the cam in its case. Note the small dot above a gear "point". The goal is to position that specific tooth in the spline trough noted above.

    5th pic - rear of the cam in its case showing the intake (forward) and exhaust (rear) lobes. One of them is really wearing its pushrod (I added another copy of this pic because the original does not appear to correctly expand to full size)

    6th pic - intake and exhaust pushrods (don't know which is which) - note the difference in length. There is just enough adjustment on the rocker to make up for the difference but not for much longer.
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  17. #17
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    Here's a couple of pics of the cylinder and head (single piece). Note, there is no liner.

    1st pic - top view of the head (kinda looks like my ex) with rockers and valves removed

    2nd pic - bottom view of the cylinder - trying to show wear - darn autofocus digital camera won't let me choose depth of field to improve focus

    3rd pic - side view of the cylinder wear

    4th pic - other side view of the cylinder

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  18. #18
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    Here's the piston and ring ...

    1st pic - front of piston assy

    2nd pic - side of piston assy

    3rd pic - top of piston

    4th pic - piston ring groove

    5th pic - ring (yea - I need to have my nails professionally done)

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  19. #19
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    Finally, here's the valves ...

    1st pic - exhaust valve top

    2nd pic - exhaust valve bottom

    3rd pic - intake valve top

    4th pic - intake valve top

    5th pic - intake bottom
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  20. #20
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    In summary - I will throw a new ring on the piston and call it good. The piston doesn't appear to be worn. I will try to score a couple of new pushrods to allow for some more wear. From what I can tell both valves and their seats look good

    Let me know what you would do (other than frame it and buy a new one )

    Thanks,

  21. #21
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    RE: Saito 91S Lacks Power

    I think some new valve springs wouldn't hurt also Seamus.

  22. #22

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    RE: Saito 91S Lacks Power

    ORIGINAL: SeamusG

    HOWEVER the darn cam gear spins very easily. I am not sure at all that I have installed the cam correctly! Any tip / technique would be much appreciated.
    The way I do it, is to Take an old piece of Sulivan snake inner and cut a chamfer on one end, then remove the cam followers. You will find that the snake can wedge the cam when pushed down the cam follower hole, so as to hold it in the correct position for replacement on the crankcase.

    Cheers

  23. #23
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    futaba - sounds like a very good idea. The follower (tappet or lifter) is easily removed and any inner nylon control rod that fits down the holes will work - tape 'em in place - install the cam - waaaayyyy to easy. .

    Tim - I'll see if I can score a pair.

    Thanks

  24. #24
    SeamusG's Avatar
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    RE: Saito 91S Lacks Power

    Items on order: ring, pushrod pair, valve spring/retainer pair and needle valve

    Btw - Harry Higley's "All About Engines" is a pretty good source for shop procedures.

  25. #25
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    RE: Saito 91S Lacks Power

    I use an old exacto blade and lay it against the cam carrier and slide it into a tooth on the cam gear with everything in correct alignment. After setting the cam carrier assembly into place the blade can be slid out from between the engine case and cam carrier. I know there are other ways but it's how I was taught and it's always worked for me. There is also a small hole in one cam lobe face and a tool can be slid down into that hole to keep everything in correct alignment. That's supposedly how the factory designed it to be assembled.

    Edited to add: The tool is slid into the hole in the cam through a lifter bore to hold it in alignment for assembly.
    Spektrum DX8i, DA DLE SuperTigre OS FOX Saito Enya Jett TT: John 3:16


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