Cam timing - a bizarre issue

Discussion in 'Crazy-Awesome almost Dakar racers (950/990cc)' started by ShaiDanny, Sep 25, 2012.

  1. ShaiDanny

    ShaiDanny Adventurer

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    Hi fellaz,

    My bike is a 2003 950 S with Akra's and a pair of 41mm FCR's.
    Following a cam timing issue with a friend's '07 990, I decided that it was time to check mine, maybe I'm a tooth off or something...
    The mechanic who installed the cams on my friend's 990 rotated the crank only 75 degrees CCW to set the front cylinder timing instead of 1 revolution and 75 degrees. His bike started and idled fine (with a different sound though), but acted as if he only had one cylinder. Like a big XR650...
    Anyways, my bike always felt like it had more. It sounds totally different that any 990 (a much more metallic and "hollow" sound with less bass). I always had cold starting issues (both with the stock CV's and the FCR's). And mid-range was never as good as any 990, with or without the FCR's. With short(ish) 16-45 gearing, it would lay you on your back in 1st gear, but have real difficulty to lift the front is 3rd. Top speed was around 210km/h. Oh, and gas mileage was horrible...!
    So, I ripped it apart today and found a very interesting surprise. This is where you guys come in...:

    I don't have a spark plug socket (got lost) so I rotated the crank CCW and found the first locking hole in the crank. At this point, the rear cam lobes were pointing towards each other and their marks ("+") are perfectly aligned with the valve cover surface. Without measuring if the rear cylinder is at TDC (the plug is still in place), I can only assume that the crank locking hole is the boss in this situation. I then released the crank locking pin (used a metal rod) and rotated it just 75 degrees CCW. At this point I locked the crank again just to see the the front cylinder cams were perfectly aligned...
    Adding another 360 degrees CCW to the crank and locking it where the manual says is the front cylinder cam adjusting position, the front cams are way way way off.

    My (long-ish) questions are as follows:

    1. If the plugs fire every time the piston is at TDC, is it true that I can adjust either the rear cams or the front? What I mean is, when the crank is locked in the rear cylinder TDC and the cams are fine, I can rotate 435 degrees CCW and adjust the front ---- OR ---- from the position where the front cylinder is at TDC (crank is locked) and the cam marks are aligned, I can go 285 degrees CCW and lock the crank to adjust the rear cams?

    2. HOW FOR THE LOVE OF EVERYTHING V-TWIN, could my bike be running pretty well (not perfect) for 4 years with timing that far off???

    Any help will be appreciated here...

    Thanks,

    Shai
    #1
  2. ShaiDanny

    ShaiDanny Adventurer

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    I have a possible solution to my own question no. 2. Tell me if this makes sense:
    The reason that the bike worked pretty well although the front cylinder timing was off a full crank revolution is this - the spark plug fires at every TDC and since I'm carburetted, there was always air/fuel mixture to be drawn by the vacuum.
    The reason my friend's 990 worked on one cylinder only in the same situation is that his bike's fuel injector only works every 2 revolutions.

    Does this make sense?

    This could also explain why my bike sounds like a single cylinder at idle (and also sounds as if idle is slower significantly than the 990), because the power strokes are not spaced further apart like it should.
    #2
  3. spafxer

    spafxer Long timer

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    So, do I have this correct? You're risking bent valves and poor running over a spark plug socket? :huh

    Reminds me of all the old tractors and engines that are put on display at car shows. They are popping and farting and spitting,,,, but they are running ! :lol3 How can that be ?


    The only way to answer your question is to properly check the valve timing, and then see what you may or may not have been missing for 4 years! :eek1


    Good Luck. :wink:
    #3
  4. cpmodem

    cpmodem Orange Caveman

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    Makes sense. The Ninefiddy has been run as a "big bang" engine before by inmates. Haven't heard of a Nine-Ninety doing the same.

    BTW, you really need to get a spark plug socket if you're going to continue tuning your Katoom. See CJ. :p

    Sent from my SAMSUNG-SGH-I777 using Tapatalk 2
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  5. Chewy750

    Chewy750 Adventurer

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    No you are not correct, it is a 4 stroke engine, suck, squeeze, bang, blow, so stroke 1 downwards, fills the cylinder with air/fuel mixture, upward stroke compresses, it is at this point it fires, the downward part of the next revolution is the power stroke, the upward the exhaust, then back on a 'filling' or suck stroke. So from that simple primer how many revs per ignition?
    #5
  6. snowhawk jockey

    snowhawk jockey Slack Jaw Gaper

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    The angle not considered in this comment, is that the cam timing is what determines if its fueling or exhausting during any given stroke. The crank fires a spark at every rotation, not every other. The spark that fires during exhaust is the "lost" spark. This lost spark can have compression under it if the cams on one cyl are re-timed to be on a compression stroke, instead of exhaust.



    So, OP, did ya dyno it in the bigbang configuration? I wanna see the torque curve from that dyno sheet!:deal
    #6
  7. ShaiDanny

    ShaiDanny Adventurer

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    OK, so I can exhale now...
    So I've been running a big bang for quite some time now. It's time to do it right... How stupid do I feel right now...

    But regarding the fact that I can't remove my spark plugs - the two locking positions on the crankshaft are at TDC front & rear, right? So if I simply space the two power strokes correctly (by moving one of the cylinders exactly one revolution further as the manual states), I should be fine, yes? (Providing the fact that I'll turn the engine by hand after I'm done And see that nothing hits)
    #7
  8. men8ifr

    men8ifr Been here awhile

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    If it was configured so the power strokes are closer together I would have thought this should have zero effect on power or fuel consumption.

    The engine is still burning the same amount of air with the same effective spark timing and cam position relative to each cylinders TDC firing, the only difference is when the power is delivered with respect to the crank rotation.

    That is quite interesting as it kind-of seems like one big power pulse (single cylinder or 2,3,4 cylinder bike with each cylinder combusting at, or close to the same time as each other) gives more rear grip, or perhaps more predictable breaking of grip (spinning up) so you can use the power more effectively. - any-one know more about this?

    I guess downsides are
    1) more vibration (more like a large single cyl bike)
    2) more stress through crank and transmission (though not necessarily on v twin as power pulses are not happening at the identical time)

    Assuming the OP didn't fit the carbs and pipe I would say the fuel consumption will be due to being overly rich.

    [​IMG]
    #8
  9. Tangai

    Tangai Been here awhile

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    I ran my 950 on the Big Bang valve timing for about 30 miles. It made a lot more intake noise, because it’s sucking in twice as much air at once. I didn’t like that at all. The exhaust sound was more like a KLR 650 than a twin—did not like that. It was kind of fun, like a BIG single in the dirt but slow, kind of like a KLR 650, on the road. I didn’t want to idle. I changed it back right-a-way.

    Get or make a plug wrench and use a piece wire down the plug hole and re-set your valve timing. Take your time and go to the “HOW” and read about how to adjust the valve timing on the 950 as the timing marks on the cam shafts are different from the 990. It will run soooo much better as a twin.
    #9
  10. snowhawk jockey

    snowhawk jockey Slack Jaw Gaper

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    Tangai,
    You were running a stock airbox during your "experiment", correct?
    Just curious as I gather data on the subject.:norton
    #10
  11. geometrician

    geometrician let's keep going...

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    Big Bang engines aren't meant so much to put the power pulses TOGETHER as it is to spread them out- this system was used on many Formula 1 4-stroke roadracing engines as the 4-cylinders usually produced power with 90-degree crank throws- in essence, there was a power pulse every full rotation of the crankshaft- which had the marked downside in motorcycle applications that the rider must wait on partial/closed throttle for a turn to open up before they can apply power- for fear of the rear tire spinning up quickly once it has lost traction.

    As roadracing bikes approached & even exceeded 200bhp the tire wear was becoming more of an issue- by spreading the power pulses out (when viewed at the rear wheel) the tire compounds have more time to recover from the massive amount of energy (ie heat/hysterisis) they must endure.

    So in the end you have a more rideable bike with tires that will last longer, allowing them to use even stickier compounds that are synonymous with fast wear.

    As for trickle-down, Yamaha now sells their R1 with "Cross Plane Crankshaft Technology" making their bike that much easier to ride or race

    ----

    As for big-bang the difference in crank inertia pulse spacing will indeed affect fueling, esp in the overlap phase of 4-stroke valve timing when both the intake & exhausts valves are open. Overlap, not a piston drawing down & "sucking" air into the engine plunger-style, is what makes 4-strokes work at anything above a few RPMs- it's that important. Basically the exhaust gasses are super-hot as they first leave the combustion chamber via the exhaust valve- this creates a low pressure area via the inertia of the gasses expanding & cooling as they move down the header.This low pressure draws the next intake charge in as the intake valve is just beginning to open, even as the piston approaches TDC. Perhaps most importantly pressure difference is greater than would be available due to atmospheric pressure alone, which is <15psi @ sea level. The longer the duration of overlap, the spread of power will move to higher RPMs- the downside being a loss of low RPM torque. Extreme overlap is found in drag racing engines which operate almost exclusively at high RPMs. Lots of overlap is makes hot-rodded engine "lumpy" at idle, as they aren't operating at their most efficient RPM. In many case RPM must be increased to maintain idle.

    Adjusting overlap with slotted cam gears will allow you to entirely change the character of your engine to torquemonster or top-speed flyer without changing other elements like exhaust or intake, etc. Simply blueprinting your valve timing will allow you to match the cylinders for a smoother engine as well.
    #11
  12. snowhawk jockey

    snowhawk jockey Slack Jaw Gaper

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    Thanks for the comprehensive explanation of the power pulse vs traction as it relates to bigbang bike motors. This adds fuel to the theoretical fire!:1drink
    #12
  13. ShaiDanny

    ShaiDanny Adventurer

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    Sorted. It's like getting a new bike. Pheeew.

    As to the big bang - if any of you want to know how the 950 feels this way with 4 years of on-road and off-road experience, I'll be glad to share.

    Many thanks to you all.
    #13
  14. ST4s

    ST4s Adventurer

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    The latest theory in multicylinder race engine design is to NARROW the firing angle to allow the drive tire to regrip the race track (much disputed by Kevin Cameron with a lot of math to support his supposition). Yamaha's Cross Plane Crankshaft actually has all the firing pulses occurring in a 90 degree, I think that is the angle, period with the remainder of the crank rotations not powering the tire until the next firing sequence. Kawasaki newest ZX10R has a similar concept with the crank designed to fire one cylinder then shortly later two together then one again (Yamaha's design is patented). All of this to narrow the power production period from an inline 4's traditional 90 degree firing angle. I believe the AMA actually banned Harley Davidson from running big bang motors in dirt track racing because it gave them an unfair advantage over traditional firing V-twins.

    Cheers
    #14
  15. geometrician

    geometrician let's keep going...

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    exactly... placing power pulses into a smaller arc (narrow fire angle) increases the remaining arc length (regrip period)

    so, glass half full/empty,..

    ----

    I used to visit Kevin when I lived in Montpelier Vermont. He has one hell of a workshop on the property- half museum, half science lab. Remnants of engineering projects & collaborations from the major factories to the determined eccentric littered the shelves. The west wall had a black & white, mural-sized enlargement of a Suzuki RG500 Gamma engine that rivaled art exhibits I've viewed. Hell of a mind, when I started reading him in the 70's I was glad there were people out there whose mind was alive with thought- and able to put it down in words.

    Coincidentally his article on the 1982 Daytona 200 "war" between Honda & Yamaha discussed the very issue of high-horsepower, tire-spinning bikes having disadvantages when he calculated the engine horsepower required to maintain lean angle near the apex of the International Horseshoe, a 180-degree, flat curve that had riders waiting to open the throttle. He took into account all the frictional, mechanical losses & determined that the 150+bhp bikes were needing less than 30hp IIRC. (I'm gonna have to find that issue now- the article was about 6 pages of great reading)

    Cheers! :freaky
    #15
  16. ST4s

    ST4s Adventurer

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    so, glass half full/empty,..

    After typing in my little tidbit considered that your description may have been the other half of the glass.

    Cannot imagine personally knowing someone with the immense knowledge base that Kevin has. The guy writes articles drawing from the most diverse range of stuff known to man kind.

    Cheers
    #16
  17. KTMSER

    KTMSER Big bike friendly

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    Please share....:ear
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  18. Tangai

    Tangai Been here awhile

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    Yeah, everything but the exhaust was stock. I run a two into one with the Aka muffler.
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  19. gefr

    gefr Life is a trip

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    O.K. I understand the intake cycle drawing fuel mixture whenever intake valves open in a carbed motor. But the spark plugs would need a different pulse wheel (coil sprocket) to ignite compressed mixture in big bang mode. With the normal 950 ignition wheel he would get the one spark igniting during exhaust cycle. How was that working on ShaiDanny's motor? Or was it selfigniting?
    #19
  20. Goss

    Goss LC8 Adventurer

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    Our 950 fires the spark plug every TDC regardless if it is a compression stroke with fuel mixture ready to detonate.
    #20