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Who says single Amal’s don’t flow...

Discussion in 'Motorcycle Related Discussions' started by Fast Eddie, Apr 13, 2019.

  1. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    There’s meat in the body to bore them out to 40mm though Glen. Not done it myself but know folk who have.
     
  2. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    A common mistake which is made when building race motors, is to enlarge the inlet port to get more flow. Using a bigger carb on a normally aspirated motor is OK as long as the port is suitably tapered. Once the gas speed in the inlet port drops too much, most motors will not perform. Or if they do, often it is only at very high revs. With forced induction, you do not have the same situation. Using a flow bench to improve port shape can be deceptive, because what happens in an inlet port is often sonic.
    The main problem with porting cylinder heads, is that by the time you find you have gone too far, you have usually stuffed the head.
     
  3. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Well, back to this carb airflow discussion chap, I have interesting news for y’all...

    The bike in question that started this thread is well known as it turns out, and was quite successful, it’s not just something cobbled together.

    ABED23C1-5470-41C6-8255-0E754395F487.jpeg


    For those confused by the original pic, here’s the bike:
    4FED258A-8FFF-4412-8623-659C9E7FD37D.jpeg


    And, contrary to our discussion about the carb being a 1000 series etc, here it is, a humble old 30mm concentric:
    E2E52486-08A0-4614-83AC-9B331E00AF41.jpeg


    And before you ask, no, it doesn’t appear to have been bored out:
    2F1BFD07-FF58-41F3-99A8-A1EC180BE3E1.jpeg
     
    Last edited: May 5, 2019
  4. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Continuous flow is different from pulsed flow. A resonating pipe of tuned length flows less than one which has the continuous uninterrupted flow which you get when you use a blower. Mass transfer depends on the pressure as well as the flow. With a normal intake tract, the system is sonic - there are nodes as well as high pressure points in a standing wave at various frequencies - the blower stops all of that and you probably just get laminar flow. If the blower is efficient enough, the carb size probably doesn't matter much as long as it is within certain limits.
     
    Last edited: May 5, 2019
  5. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    One of the Vincent speedway sidecars my brother races is on methanol and blown. The biggest thing wrong with it is the valve timing. Much of the charge must be going out the exhaust port, because it is insanely loud.
     
  6. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011
    It may be worth your brother's while to get in touch with Colin Chapman of "Mighty Mouse" frame - he filed the cams for the Comet single in his shed and got it right at the second attempt.
     
  7. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    My brother mainly races an 880 JAP sidecar. It is on 16 to 1 comp. with methanol. It is quicker than any of the Vincents, including the supercharged one. Which is something I find pretty unbelievable. The 880 Jap is a 1929 year motor. He has built most of the Vincents which his mates race, and they are quicker than any which come from elsewhere. What he pointed out to me is the Vincents were craftsman built motors rather than industrial engineered. It is like the difference between Spitfire and Mustang aircraft. Every bolt on a Vincent motor is a special. It makes life more difficult. Currently he is shimming the cam gears and followers in a Vincent motor. You should not have to do that. However the 880 JAP motor is more fragile.
     
    Last edited: May 8, 2019
  8. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    I’d say that’s a fair observation Al. Remember though, Vincent were effectively built in a large shed, many hobbyists today have better equipped workshops than they had.

    Whereas JAP were the Honda of their area, meaning that they were the biggest producer of engines in the world. Their production and machining facilities would have been very different to the Stevenage set up.

    Most of the difference between a Black Shadow and a Rapide was selectively assembly and blue printed motors (apart from a small CR increase and a small carb size increase). However, when Black Shadow production overtook the Rapide, I guess most of that selectivity got lost.
     
  9. Lineslinger

    Lineslinger VIP MEMBER

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    Feb 26, 2017
    That beast is cool.

    Is that a version of the Hogslayer in the background?
     
  10. Fast Eddie

    Fast Eddie VIP MEMBER

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    Oct 4, 2013
    The bike in the background is a double engine BSA A65 with the gearbox hacked off of one engine.
     
  11. robs ss

    robs ss VIP MEMBER

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    Aug 16, 2016
    I have worked on modified engines in the past where the choke size of the carburettor was so undersize that the venturi effect caused freezing of the incoming fuel.
    That looks like a distinct possibility with the layout in Nigel's photos.
    I wonder if it's ever been run - in anger? I suppose it depends on the RPM it is intended for - although I suspect above 7000.
    I may be over-disparaging but it smells of "show pony" to me (if that means anything to you northern hemispherers)
    Cheers
    Rob
     
  12. Eljahara

    Eljahara VIP MEMBER

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    Jul 8, 2018
    We prefer our “show whippets” up ‘ere in’t North
    Proper flat caps and capstan full strength!
     
  13. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Did you not read the info ‘panel’ I posted a picture of? It’s a famous bike campaigned by a famous rider, Alf Hagon, latterly of shock absorber fame and fortune.

    Don’t think he’d take too kindly to the ‘show pony’ accusation!

    Ref carb freezing, don’t forget it’s a sprint bike, so only run for short bursts, maybe that super cooled intake charge is part of the reason for its success?!
     
  14. robs ss

    robs ss VIP MEMBER

    Joined:
    Aug 16, 2016
    Sorry Nigel - my bad!
    That's what I get from not reading carefully enough.
     
  15. acotrel

    acotrel

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    Jun 30, 2012
    Compression of a gas causes heating, expansion causes cooling. It takes heat to expand a gas - if you cause it to expand, that reduces the temperature due to it's latent heat of evaporation. In any case Alf Hagon would have been using methanol fuel for sprinting. That has a very high latent heat of evaporation, so runs extremely cold.That 880 JAP sidecar my brother rides on speedway is on 17 to 1 comp and uses methanol fuel. He is using Dellorto carbs of early JAWA solos with needles fitted. With out the needles, the motor was all or nothing - impossible to ride with. Two weeks ago, he took the bike to Broadford Bonanza gave it a bigger handful as he came onto the front straight - the front wheel skipped off the ground and the bike did a quick right hand turn and whipped into the infield. Our sidecars run clockwise on speedway and they have a very pronounced lean to the right.

     
    Last edited: May 15, 2019
  16. acotrel

    acotrel

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    Jun 30, 2012
    I have never had a motor which loves methanol fuel as much as my 850 Commando engine.
     

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