Tuning issue 336, Dellortos, dual plugs

Discussion in 'The Garage' started by R90SS/6, Jul 27, 2011.

  1. R90SS/6

    R90SS/6 Adventurer

    Joined:
    Apr 29, 2011
    Oddometer:
    17
    Bike: 1975 R90/6
    Cam:336 (6 degree advanced been can type)
    Carbs: 38mm Dellortos (R90S jetting except for richer #65 idle jet)
    Pistons/Cylinders: (Siebenrock 9.5:1 1000cc pistons with Nikasil Barrels) New
    Heads are rebuilt with later rocker gear and dual plugging.
    Ignition is a crank mounted Omega. timing set at 3 degrees advance with 28 degrees full advance.

    I just put 300 miles on the bike since making all the modifications.
    For the fist 150 miles of breaking in the bike ran amazingly well with lots of smooth torque throughout the range.
    Since then I've noticed a drop in power over 4500 rpm and cutting out at full throttle.
    To try and remedy the problem I......
    I replaced the floats and adjusted the bowl levels.
    Adjusted and equalized the throttle pumps.
    Balanced the carbs
    tried richer and leaner main jets
    tried advancing and retarding final advance
    New plugs

    Compression is 140 psi on both sides.
    Leakdown test shows only 2-3% leakage, none on valves.

    I'm sure if I keep tweeking it I'll eventually find what's going on, but right now I want to ride.

    Any ideas?
    #1
  2. CycleDoc59

    CycleDoc59 Wrench Rider

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    Ignition settings sound correct. Have the valve settings been checked after those miles? They
    tend to change fairly soon right after a rebuild... Do you have good voltage/charging numbers? The
    original voltage regulator cut too soon, resulting in a chronically undercharged battery; and the coils don't
    like low voltage. And the current load is higher now with dual-tower coils.

    If it seems to be a miss-firing problem, a somewhat messy diagnostic is to tape a timing light on
    the gas tank, go ride, and move the pickup from one cylinder to another...... I found a bad coil wire end in a dual
    coil bike with that method last year.
    #2
  3. R90SS/6

    R90SS/6 Adventurer

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    Yes the valves have been reset a few times already. I'm using a Ducati style Enduralast Alternator, should be plenty of juice. I'll run a check on it though. Plug wires are new but the Dyna grey coils are used. I had not thought to check those, I've been so sure there was a carb or timing issue. Thanks
    #3
  4. ericrat

    ericrat Long timer

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    Perhaps a fuel flow issue? That bike should just be waking up at 4k. Snowbum says flow should be better than 350cc (~12 ounces) per minute. Could be clogged petcock/filter etc.

    I see you replaced the floats, but a low float level could be a cause too.

    Good luck,

    Eric
    #4
  5. craydds

    craydds Long timer

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    I offer no solutions for your issue, but you mentioned you have the Enduralast alternator. I am installing one on my '75 R90S and have some wiring questions. Perhaps you can help me. See my recent post, "Enduralast on R90S".
    #5
  6. R90SS/6

    R90SS/6 Adventurer

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    The ignition and charging system checked out fine. It appears that it was a fuel flow issue. Never had a problem with the 308 cam, but with the 336 and extra cc's... it wants more at full throttle and float levels are more critical. The floats were set a millimeter or so too low. I had followed Sno Bum's http://www.bmwmotorcycletech.info/dell.htm directions and set the floats so that 55cc's of fuel would drain out. This put the floats too low. Maybe the newer Dels (like the ones I have) hold more fuel in the bowls. When I moved them up to the exact Dellorto spec (I had to build a tool to measure it exactly) of 18mm http://www.ducatimeccanica.com/float_levels.html the full throttle starvation problem went away and even the idle improved. I'm really not sure why it worked fine for the first 150 miles of break in. Maybe compression increased as the rings set in and changed the fuel requirement. Any theories?
    Question: They make needle and seat sets that flow more volume into the float bowl faster http://shop.bevelheaven.com/detail.aspx?ID=474. I'm using the stock R90S Needle valve http://shop.bevelheaven.com/detail.aspx?ID=59. Since the floats control the fuel level, why wouldn't I want the largest flow volume possible? Why wouldn't any bike regardless of the fuel requirement?
    #6
  7. craydds

    craydds Long timer

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    Flowing more fuel is no problem since, as you mentioned, fuel level is controlled by the floats. I am sure the stock Dellorto R90S needle/seat flows adequately, but nothing wrong with more flow, and I'll bet the high flow set cost is cheap. Sounds like you've solved your fuel starvation problem by increasing the fuel level in the bowls (so ignore my previous response).
    #7
  8. supershaft

    supershaft because I can

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    I was going to guess ignition.

    I would run more ignition at idle. 6 to 28. It makes a pretty big diff down low.

    I never had a fuel starvation issue with my 1000cc/Dell's/336 and stuff but I just set the floats per Dellorto. I don't pay much attention to snowbum. So much of his advise doesn't work well if at all. Don't get me wrong, I do look up stuff there too but . . . . 12 ounces per minute? What on earth for? That would keep a 450hp Pratt and Whitney R985 happy and they burn about 25 gallons per hour putting along cranking out around 1000ftlb of torque.

    I started out with stock R90S jetting and gained a lot of power rejetting.

    I ended up with 62 pilot jets, disconnected accelerator pumps, 1mm richer slides, K9 jet needles, 262 needle jets and 130 mains. Compared to stock R90S jetting, that got me a lot more power from low midrange on up to over 8000rpm and into over rev. I don't recommend revving them that high without titanium valve spring retainers.

    Good luck!
    #8
  9. ericrat

    ericrat Long timer

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    It sure seems like a lot of fuel when I accidentally know off/pull a fuel line and it is spreading across the garage floor.

    12 oz per minute = 12 * 60 = 720oz/hour or 5.5 gallons per hour. That doesn't seem out of line.

    I don't know aircraft engines, but those number seems right, 1000ft/lbs at about 2200 rpm is around 450HP, not exactly putting around though. 25 gallons of fuel comes to about .3 BSFC, that sounds too good to be true. Must be putting out more like 350HP "putting around".

    An airhead making 70HP sustained, would need about 5.8 gallons of fuel, funny 'bums number is pretty good. .5lbs of fuel = 70HP/35lbs-gas. 35lbs-gas / 6 pounds per gallon is 5.8.

    I am not trying give you a hard time SuperShaft, but I knew that your maths didn't look right. These numbers are all a little theoretical since the time unit is hours and we don't (typically :evil)run 70HP output an hour sustained.

    I just checked my R100 and it is delivering about 20oz/minute on a full tank.

    Shaft is right about that not being enough advance down low, but that is the best your can do with the Omega to get at the 28 degrees total. To my mind Omega threw away a lot of options on the 42-47 degree range that we could have used on the other end.

    Glad you are running better now,

    Eric
    #9
  10. supershaft

    supershaft because I can

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    Busted! Sorry ericrat and everybody else out there about the fuel flow!! For some stupid reason I was thinking in pounds! I am so use to catching snowbum out that I dreamed this one up. My advise about his advise still stands AFAIAC. Overall, there is advise out there that you have to have both petcocks on and whatnot when most of it just isn't so.

    Thanks for catching me out and not going for personal blood. It says a lot!
    #10
  11. supershaft

    supershaft because I can

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    I forgot to add that I use to work on small radials. Mostly Wrights and Warners but more are familiar with P+W R985's. I only worked on those a couple of times really. One of my bosses had one in his G17 Staggerwing. That plane was bad to the bone!
    #11
  12. R90SS/6

    R90SS/6 Adventurer

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    Down low, and transition from idle to full RPM are actually really good. No flat spot, revs really fast. I'm attributing that to the 6 degrees advanced cam (later bean can style 336) and of course to the post 1981 clutch/"fly wheel". I've got the titanium retainers and new springs shimmed to 36mm..... but I'm trying not to over rev it until I've got at least a thousand miles on it. This is proving to be difficult, I guess I'm going with the hard break in philosophy.
    It definately likes the floats up at 18mm. Idle and top end have improved, but not perfect. At idle I'm having difficulty keeping the idle speed down. I'm running #65 jets the rest is all R90S. I've got the carbs all ballanced, but after I go for a ride it will suddenly be idleing at 2000 rpm. There's plenty of play in the cables (throttle and chokes). When I try to tweek the idle speed down the idle drops off suddenly. After some time I can get it ballanced at 1000 rpm again, but after a ride it goes back up to 2000. I may have a leak in the choke assemblies or the Omega ignition is set to advance to soon, I don't want to use richer idle jets because the screws seem just right at 1 1/2 to1 3/4 turns out.
    The other issue is at full throttle. The missing has gone away with the float adjustment, but It still seems a little flat under load at or above 4500. Not bad, but not as good as I had it. I wondering about adjusting the advance. With the bike set at 6 at idle and 28 at full, I end up with such a short, quick, curve. I've been following Dr. Curves article http://mailer.fsu.edu/~jroche/road_rod.html and he suggests that full advance should be around 6000rpm for dual plug bikes. Unfortunately, since I don't have bob weights I can't slow the curve down...Not sure if it's possible to get full advance at that high an RPM without retarding idle further. I definately need to play with the curves and timing more before I do anymore rejetting. I'm going to cut a hole in the front cover so I can change the settings easily and plug it with a rubber timing plug.
    #12
  13. Middleweightboxer

    Middleweightboxer Middleweightboxer

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    My wife built a R90S quite similar to what you are describing except for the charging/ignition systems. I tried every ignition timing combination that I could find and none worked for this bike. Some were just wrong.

    Through experimentation I ended up with idle timing set at the S mark. This is the only setting that would allow the bike to idle properly. Forgive me I don't remember the advance timing numbers but it was necessary to limit the full advance after dual plugging. With two flame fronts you can't use all the advance that a single plug setup uses. The burn is happening faster.

    I machined shims to limit the total advance on the mechanical advance unit. We would get detonation at the stock full advance timing. In addition, I find that with the crap we have to use for gasoline these days, my stock engines are happier with a little less advance.

    So if your running anywhere near the stock full advance try backing it off. We have run her bike hard for over 40k miles now and it runs perfect.
    #13
  14. supershaft

    supershaft because I can

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    Make sure your new carb spigots aren't leaking air after the engine gets good and hot.

    I suspect you might need the mixture screws richened (out) a bit more and a bit lower idle speed.

    Make sure your new valves aren't leaking with a leak down tester. It has happened to the best of them and it happens all the time to others.


    I suspect that 6000 rpm full advance was a typo but I have never asked him. I don't see it helping. I think I remember some people waiting until 4000 or 4500 for full advance? Personally, I don't like the timing options on most aftermarket ignitions. There is one from Germany that I like but I can't remember the name of it right now.

    Flat on top? R90S 155 mains are way too rich but so are the pumps! You have got some serious free hp coming your way when you get those things mixing right. Rich on top is a real power killer!
    #14
  15. Beemerboff

    Beemerboff Long timer

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    My G/S goes around 260 miles on 5.8 gallons, never managed to use that in an hour yet, but I will keep trying.

    A few years back I had a car with a fuel flow gauge on the dash, and this demonstrated the wide variation in fuel requirements depending on the throttle position, etc.

    So if you did indeed have an engine which averaged 5.8 gallons over an hour you would a maximum flow of 3/4 times that to cope with the peak flows. As someone has already said.

    I wasn't impressed with the Omega on my R100GS, made no improvement to the way the bike ran and couldn't cope with the Oz heat.

    Dont know the resistance of the Grey Dyna coil but I was running one Brown one.

    Replaced the Omega with a Boyer Micro power which comes with a matching coil and still costs less than the Omega.

    It only has one curve, but as that seems to be the right on I dont need any more.
    #15
  16. R90SS/6

    R90SS/6 Adventurer

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    Thanks to everyone for the comments, As stated before, I did a leak down test and was impressed with the fact that leakage on both cylinders was only 2 to 3 %. All of it (what there was) was coming past the rings, I could detect nothing at the exhaust or intake. The carb spigots are tight and sealed. After researching the Omega a bit I realized that there is no way to get full advance at any rpm except 3000 to 3200. You can adjust the advance up and down, but it will always max at 3000rpm no matter what. Maybe there's a Silent Hektik in my future...
    No big problem since my flat issues are above full advance anyway. I'm going to advance the idle to 6 degrees and just play with full advance until I get the best results. Question: I'm using the post 81 abreviated flywheel. Is the S mark 6 degrees (not 9) on those? Is full advance (Z) the same amount of degrees as on the older bikes (F)? Time to make a degree wheel.
    #16
  17. supershaft

    supershaft because I can

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    I missed the Dyna grey coils. I think they are 2.2 ohm? Too much combined coil resistance can weaken high rpm spark. Some people don't have the problem or don't notice it. Some people run two complete and separate ignitions and some run coils that add up to a lot closer to the resistance the ignition was designed for. I am going to run two .7 ohm Dyna coils on my dual plug setup if I ever get it back together!

    Set up your ignition on a dyno? That is one aspect of tuning that I think dyno tuning has a real advantage. Nevertheless, TONS of all different sorts of airheads have had their ignition set up on the dyno and they usually prefer right at the exact same settings. 6 to 28. Beemerboff reports his bike running best from 3500 to 5500 at 26. Completely believable but remember that some prefer to run around that much advance around that rpm range and then advance it a bit more to 28 degrees in the middle or high end of that range. Of course it all depends on the bike. I helped with a race bike that would make real good power at 26 total advance but it had real high CR AND 2.15:1 rod length ratio versus stock 1.91:1 rods and longer rods most always prefer comparatively less advance. Same with higher CR.

    Mileage? That depends on how you ride more than anything else. My mileage varies from high teens to low forties and is normally in the high twenties to mid thirties. I have never got 260 miles out of 5.8 miles and I run a lot smaller mains than most.
    #17
  18. ericrat

    ericrat Long timer

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    I think if you advance the idle to 6 then you will end up with full advance at 31 since 25 degrees advance is the lowest curve on the Omega (at least on mine). Of course that may work fine, perhaps it wants a little more advance on top and 31 isn't far off.

    This is one place I like a "dial" back timimg light. I can check the OT mark and then dial back to what I want and not have a wonder about timing marks that may not be where I want them anyway.

    Do you have a copy of the Dell Orto tuning manual? It is available here: http://www.ducatimeccanica.com/dellorto_guide/dellorto.html The "progression" parts may be helpful. Even when I think I "know" this stuff, going back and reading the Dell Orto or Mikuni tuning manual help me think about it better.

    You do have the dual ignition omega right? The one with two "z" outputs, one for each coil. Are the Dyna Grey coils the mini, dual output ones? The Dyna Minature coils MAY not be good in the application. They cannot handle the excessive dwell at low RPM operation. I don't know (Stephen Botcher may) if the Omega has dwell limiting, but I doubt it. Are they getting hot?

    Eric
    #18
  19. supershaft

    supershaft because I can

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    I just wrote a reply and lost it! Oh well. I gotta go right now but real quick I wanted to throw in that two degrees difference can all depend on how you line up your eyes on the timing mark so you have got to try to keep that variable out of the picture.
    #19
  20. supershaft

    supershaft because I can

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    Your stack/filter setup could be the biggest source of your flat spot. Beemers tend to like long intake tracks and that is the reason I run a late model box for their LONG velocity stacks. The length of the stack/filter setup is critical! Of course you have to jet around a stack length to give it a fair shake. I have spent at least a solid day or so experimenting with different setups and I run an almost stock late model setup. Even switching from one large and one small air horn to two large airhorns kills my midrange for very little gain on top. Sure I have seen and ridden a lot of bikes that had the horns removed completely but every one of them to the tee had a huge flat spot in the midrange.

    Jetting varies greatly just off idle depending on the exhaust and intake setup but I still highly suspect you are way too rich on top if not in more areas.

    Two into one exhausts typically favor midrange over two into tow setups. There have been plenty of race winning airheads with two into one exhaust on them. Sure they might cost you as little as a hp or two on top and get you big gains down below that. It all depends. Smart tuners always remember that before you get to peak power you first have to get there.
    #20