Carbs or Ignition

Discussion in 'Airheads' started by Voltaire, Feb 26, 2012.

  1. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    I'm hoping to get some ideas.....
    My racebike is a 900 with a 336 and 36mm Dells running total loss electrics does not like to idle, dies if less than 2000 rpm
    The Dells were a swap meet special and I have rekitted them and recently tried some new( er) slides....still no good.
    I suspected it was an air leak or idle circuit issue....now not so sure.
    The ignition is the base model Boyer....and after doing a bit of net trawling I'm wondering if it is not really up to the job.
    Battery is a 32amp yeller Motobatt.
    Once bike is running its ok, but it won't pull past 6500 in top.
    Anyone had issues with these units? I gather they are pretty old tech and not that happy at low revs, any input appreciated....as I don't have an alternator mounting anything else could be a challenge.
    cheers
    John
    #1
  2. motu

    motu Loose Pre Unit

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    i tol ya bifor din i

    Any engine with cam,intake and exhaust mods will need more advance at lower speeds. Before you go changing too much try advancing the ignition at idle - if your idle speed improves and increases then you are on the right track. There is just not enough manifold vacuum, and ignition needs to be advanced. I've run engines with silly cams on 38 deg advance - and 24 deg at idle. A hot cam in a BMW is about a Briggs & Stratton grind, so it's not like you dealing with 120 deg overlap....but it will have less vacuum than a standard cam.
    #2
  3. Crisis management

    Crisis management Latte riders FTW!

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    Geez guys, I came to watch the blood letting and you're debating lawn mower cams?? I'll go and sit outside Voltys' house with R80 idling just to annoy him....
    #3
  4. trustme

    trustme Long timer

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    If I was you I'd put it all back to standard .:wink::wink::wink:
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  5. supershaft

    supershaft because I can

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    Dell's should idle perfectly and pull well beyond that RPM. You have troubles at both ends of the rev range and maybe in between? I have put a sport cam and Dell's on three bikes and never had to readjust ignition timing off of stock. One of them was a 900. Since you have got troubles all over, you need to start with the basics. Leakdown, compression, and testing the ignition. I highly suspect that a lot of the 336's reputation of not idling well if at all is from the cam exacerbating a problem that was already causing idling issues before the cam was installed: Leaking valves.
    #5
  6. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    Bit of background. The engine has been completely rebuilt with new bearings and rings. Valve guides have been relined and a 3 angle valve job done.The motor has only run for a matter of hours.
    The idle thing is more just annoying than anything as it should idle, once its running it pulls right into red line no worries. Its only on the very long back straight that it wont pull past 6500 in top.
    I won a race on it against two BSA Gold Stars and a Dunstall Commando so I am reasonably happy with its performance.
    Is certainly a different bike to ride than Maxheadrooms R90s, which seems to run a lot smoother.
    You are correct with saying start with the basics. I intend to pull out the Boyer and stick points in to eliminate that.

    I had issues with a Boyer 20 years ago in Germany on a /5 and sidecar.
    ( Pic in from back in NZ but you get the idea)
    [​IMG]
    when it was below 0 degrees it was hard to start and needed the box warmed up....put up with this all the way back to London , shoveled away the snow fitted the points and sent Mr Boyer back his ignition.....which they replaced.
    #6
  7. supershaft

    supershaft because I can

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    Relined guides? I thought people no longer did that to cars let alone bikes? The fact that your engine was just assembled by I guess someone that relines guides is all the more reason to start with the basics. Valve jobs leak and rings don't seat and that is the tip of the iceberg.

    6500 in top gear doesn't tell me much without knowing your FD gearing but I suspect it should be doing better. What is the last gear it will pull 7500 in? But before I ask that question I believe most all /6's should have the oil pickup galley to the pump enlarged? I would have titanium valve spring retainers in any race engine. I have them in my street engines! I couldn't rev my engines above 8000rpm without them. Even if you don't regularly rev your engine that high they are still good insurance for missing gears.
    #7
  8. mykill

    mykill odd

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    Idle: I suspect you just need to fiddle with settings or different idle jets to get it to behave. 36mm will most likely be different from the known functioning 38mm Dells in R90s, but i would think your idle jet numbers should be close. I would start with mixture screws around 1.5 turns out and overly raised slides, get it warm and start adjusting mixture then speed, rinse, lather, repeat until it is happy. If your mixture screw bottoms and idle was still picking up, you need a leaner idle jet. Conversely if you screw out more than 3 turns or so.
    It should behave well with stock ignition settings.. SS asked about lower gear behavior, that is needed info. You may be off on your main jet if you cannot pull through in top gear. Lean is mean, but too lean will not rev out, nor will too rich. If it is jetted right, you may just be up against physics. 38mm Dells and 44mm intakes will work well for your setup.
    My R75 runs 36mm Mikunis, std timing and idles embarrassingly well, yours can too.
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  9. mykill

    mykill odd

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    Oh, Lose the fat battery! Good God man! If you have to keep the starter, get a Shorai.it will work all weekend. Better yet, lose the starter and get a 3A to run your ignition. Make your lazy friends push start you. It is the least they can do for the show you give them.
    #9
  10. trustme

    trustme Long timer

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    Looks like I better get my roller starter finished.
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  11. pommie john

    pommie john Long timer

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    I can't remember are you running dual plug heads?

    I use an old Boyer analogue ignition running two Dyna brown coils to the twin plug head with 40mm Dells. It will idle happily at 800 rpm. I actually have it set so it doesn't idle when cold , but at full temp it chugs along at 800, just so I don't accidentally stall it on the grid.

    It does sound a bit like an air leak, but if that were the case, I'd expect it to not want to drop back down after a blip of the throttle. That's usually a good sign of an air leak.

    The one thing you might check is strobing the ignition. Despite what Boyer say, it keeps on advancing to 5000 so that;s where I strobe mine. It might upset the neighbours, but hey. With twin plugs I time it at 28 degree at 5000rpm.

    As for the 6500 in top gear, it may just be over geared for the engine.

    Here's a graph of speed vs RPM assuming a 130 rear tyre, but no matter what tyre you're running, it will be in the ball park. At a rough guess I'd expect to see maybe 120mph from a moderately tuned bike ?? Maybe??

    [​IMG]
    #11
  12. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    Cheers guys for your input ..:deal..I'll go and do some fettling....:huh.
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  13. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    I downloaded a Degree Disc, laminated it and fitted it to the crank with a pointer.
    Ran the engine up to 3000 and 'z' appeared. Degree Disc read 30 degrees.
    Advanced the pickup plate a bit more at 3000 'z' moved to top and Degree Disc read 34.
    Higher revs result in more advance.....:huh
    Reading the Pazon vs Boyer article it describes the operation of the Boyer analogue quite clearly....bit of a dog by the look of it.
    Sadly Pazon don't do one for BMW airheads.
    FD is 3.0:1 and it pulls 4th really well, tacho is a bit vague so I'm wary of going into the red, but the back straight is soooooo long you have plenty of time to look at it. I must get a rev limiter off Ebay.

    I managed to get it to idle at about 1500 with the timing advanced to 34 degrees and this gave 15 degrees.

    SS, whats your spin on guide liners, they look pretty good to me, and once shown I did some VW heads too.

    Next I'm going to try a set of points as I don't have a alternator to mount a crank fired unit on.

    I'm going to get a Pazon for my Norton after reading about the Boyer :deal
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  14. pommie john

    pommie john Long timer

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    I have no problem with the boyer. It sounds like the ignition is over advanced

    Use your degree disc to make a mark on the flywheel ( a blob of white paint is all I use) at 28 degrees if you're running twin plugs, 31 degrees if not, and strobe it at 5000rpm.
    #14
  15. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    Cheers, I'll try that tomorrow.
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  16. supershaft

    supershaft because I can

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    With a sport cam a good part of your best power band is in the red!

    Guide liners are stop gap crude IMO.

    I have never thought to run a degree while the engine is running! I would suspect that it is slipping!

    Something I learned from my dad while I was still just a kid. Every shop race bike he worked on the very first thing he did to them was degree the flywheel. Why run a degree wheel? Just degree the flywheel! The whole trick to that is not degreeing it off of TDC. Way to much crank movement per piston movement at TDC. Make any around mid stroke piston stop and bump into that one way and then back the other. Divide that by two and THAT is TDC. WAY more accurate.
    #16
  17. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    I'll find out soon what a sports cam does when I put the bike on the dyno. I noticed when I took the electronic tach apart that it wont physically go much into the red area, could just be that one.
    I don't see an issue with guide liners IMO
    My Dad was not mechanical so I taught myself on VW and Triumph engines.
    To find TDC on a Triumph I machined the centre out of a sparkplug and made up a tapered rod. This touched the piston and I read off the dial guage, combined with a degree wheel this proved quite accurate...Setting up a Triumph engine with no cam wheel markings is a lot more work than a BMW.not sure about your bump/stop divide by 2 method but each to his own.
    The degree wheel tightens on the end of the crank nicely and does not move, with the advantage you can read actual information without having to peer into the crankcase viewing port, you should try it.
    #17
  18. mykill

    mykill odd

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    I run an electronic tach from an oilhead. +,- and point wire connections and you are good to go. Usually pretty cheap on Ebay.
    #18
  19. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    cheers, currently running a mechanical one....retro...:roflmight have a look.
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  20. supershaft

    supershaft because I can

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    Think about the geometry of the setup. Piston movement varies from crank movement. The thickness of a mark on your dial gauge can easily be a couple of degrees at the crank right around TDC. At mid-stroke, piston movement is at its maximum in relation to crank movement. THE first thing that should be done with any degree wheel on any engine is using it to find TRUE TDC. Finding TDC at TDC is a shot in the dark. TDC determined from mid-stroke is WAY more accurate. Mark at your mid-stroke stop one way, roll around into the stop again and mark that. Count up the degrees and divide them by two. Go back to half way between your stops via the degree wheel and that is TDC. Without doing this first, the whole point of the degree wheel is pretty much lost. I am just trying to help. It's pretty much tuning 101 but a lot of people are in the dark about it.
    #20