PWK carburetors for Norton MK3

Discussion in 'Norton Commando Classic Motorcycles' started by nortonisthebest, Apr 13, 2018.

  1. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    Hello,
    I have a pair of PWK 30mm carburetors taken off of a 750 commando. Can I use these carburetors on my 1975 MK3 850? I found a Bill Getty article and he showed on a chart using PWK 30mm carburetors for a 850 commando that was set up with a main jet #132 and a pilot jet #35. It also showed a 32mm carburetor with the same jetting for a 850 commando.
    Both the PWK carburetors I have are jetted with the same jetting as above.

    Thanks
     
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  2. RoadScholar

    RoadScholar VIP MEMBER

    Joined:
    Dec 28, 2008
    Sure. You could also use a pair of lawnmower carbs if you had the infrastructure to attach, control and feed them. The keys are tuning and the level of performance you are trying to achieve. You said they came off a 750, so they should have the infrastructure to be attached, controlled and fed on the Mk3; if they ran well on the 750 they should be close, in tuning, to provide a good starting point for the Mk3 tuning. The main jets should be the least of your concerns initially, most motorcyclists use main jet capability infrequently, regardless of make or model.

    Your home work should center on the needle jet and the needle taper and position, they should be close, but you'll need to identify a source for these parts, and with this forum you may get lucky and have someone chime in that can send you a "recipe".
     
  3. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    Thanks for your reply.

    I installed the PWK 30mm carburetors and they work at idle and low end. But when I accelerate the engine starts cutting out. The plugs are on the sooty side but it doesn’t smoke. Stars good with a good idle. The slide needles are in the middle position. So I will set them one notch higher and give it a try
     
  4. RoadScholar

    RoadScholar VIP MEMBER

    Joined:
    Dec 28, 2008
    Good plan, but a very rich idle circuit can soot up plugs rather quickly.

    How many turns out from seat are your airscrews, or your mixture screws if the adjustment is a volume control?
     
  5. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    The air screws on the PWK's are turned out one turn.

    The Amal 932's that were on my MK3 didn't idle but ran good at 1/2 to full throttle. These PWK's are acting just the opposite. I thank the Amal carburetors my have a plugged idle circuit, but not sure. Don't know why I can get it to idle without dying. Also the throttle would stay up and not drop down when I let off the gas.
     
  6. Steves

    Steves

    Joined:
    Oct 16, 2016
    "Also throttle would stay up and not drop down", you have over tightened the 2ba screws on the carb top.
    All fastners on Amals should be gently nipped up with Spring washers otherwise the carb bodies and/or float bowls and/or manifolds distort leading to stiction, air and fuel leaks.
     
  7. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    My MK3 has 17K miles and the Amal carburetors probably needs replacing. I am trying to find another alternative before spending three to four hundred dollars on new ones. What carburetors would you suggest if I have to purchase new ones? I did replace the Amal 930's on my other 1972 Norton with a single Mikuni at a cost of four hundred. I don't want to do that again. I will continue to try and tune the PWK's and see how that goes. The throttle cables going into Amal carburetors, as a whole are not good and what you suggested is a good start.
     
  8. Steves

    Steves

    Joined:
    Oct 16, 2016
    To be honest with a little TLC 17k old Amals should work.

    1. Give them a good clean, use carb cleaner, compressed air and a suitable implement, I use bits of lock wire. Check orifices are open by directing carb cleaner in one end and observe other end end......protect your eyes!! Remember to clean tickle mechanism.
    2. Replace the needles and needle jets plus gaskets and O rings.
    3. If feeling flush with cash, put anodised alloy slides, stay up floats, alloy viton float valve and new main jets. I adjust the floats so the end furthest from the valve is just below the float bowl edge. There is loads of stuff about this on the Internet. I reckon if I count 1001,1002, 1003 while holding tickler down on a full float bowl and fuel comes out around the time I have said 1002 or am saying 1003 then I am not far off.
    4. Put back together and don't over tighten. When carbs back on the bike, I synchronise the slides by placing side by side in my left hand and turn the throttle with my right, adjust so they start to move at exactly the same time.
    5. Drop the slides in the carb bodies, press down with a finger, screw slide screw in until the slide just moves against finger pressure then screw them in 1.5 turns from that point. (Replace O rings)
    6. Screw air screws all the way in then back out 1.5 turns, (replace O rings). Carb tops back on, don't over tighten
    7. Run bike, get it warmed up, adjust both slide screw by small and even amounts 1200 ish rpm, adjust air screws for fastest idle, back off slide screws small and even amount (lowering slide) to get about 1200rpm, adjust air screws for fastest idle, (you may find at this point not much difference). Adjust slide screws by very small and even amount to get an idle around 900.
    8. If bike pops and bangs at low rpm (when riding) in one cylinder or two adjust the associated air screws, by turning in a little to richen the pilot mixture.
    9. If it struggles to pull away as the clutch is released raise the needle by one.

    Have fun. Be aware when you adjust screws you need to make sure you do so by small and equal amounts, wait for a response I.e. Don't rush it and don't get out of sync between carbs. If you have one a manometer can be useful at this point. All of the above assumes ignition good, tappets set, spark plugs gapped and clean, clean air filter and no manifold leaks. At any rate that's how I do it, but as they say more than one way to skin a cat.
     
  9. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    OK,
    I'm not sure how much cheaper the new parts will be compared with buying a new premier 932 carburetors. New slides, needles, "O" rings, stay up floats, jets and needles and seats. Do the new Amal premiers have all the latest up grades you mention above and are they worth the cost and are they better then say Mikuni 36mm?
    As far as cleaning my 932"s I cleaned them in carburetor cleaner, used high pressure air and even spray can carburetor cleaner in the mall orifices. The slides look an little worn though and probably in need of an upgrade. I live at 4000' elevation, what main jets would be a good start along with the idle jets?
    Oh I forgot to mention the MK3 was rebuilt with new black diamond valves and new 20 over pistons with Hastings rings. No smoke with good compression at 75 miles.
     
  10. Steves

    Steves

    Joined:
    Oct 16, 2016
    Well you will have to do the maths (which is English for math ;) ). I think the Premier come with all the upgrades. However you might get away with just replacing the needle jets to see how you get on with the old carbs, they wear out with needle bouncing around. I get dizzy at 4000 feet so can't help really other than to say one normally jets smaller as the air density is lower. So you need less fuel to get close to the stoichiometric mixture. Bear in mind carbs are not really precision instruments!
     
  11. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    Right, They aren't. I tried raising the needles one notch and the engine keeps cutting out at 1/2 throttle and above. I also lowered the needles one and then two notches with the engine still cutting out. I'm going to remove the PWK's and put the Amal carburetors back on after looking into installing the new parts you mentioned. I will try replacing the needles first and stay with 160 main jets. My problem with the Amal carburetors is not dropping down to idle right away and then not idling. I'll keep working with the Amal 932's again.
     
  12. Steves

    Steves

    Joined:
    Oct 16, 2016
    So I think you mean 260 main jets, the brass needle jets size 106 are what wear out, if replacing needles get the correct ones for Norton as you will have the step cut out on your spray nozzles. I can't remember without looking it up how to ID the needles, like in number of rings there should be, maybe someone else will know.
     
  13. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    if you have stepped spray tubes you will need the 4 ring needles
     
  14. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    No I'm using 160 main jets with 106 needle jets. At my altitude it seems to run good. I will pick up new 106 needle jets. The exhaust pipes are new and are staying chrome with no bluing and plugs are slightly dark to light brown.
    I don't know what stepped spray tubes are?
    I'm pretty sure the needles are stock with 3 groves. The needles were set in the middle grove.
     
  15. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    The rings are thin etched rings denoting what length they are they all have 3 grooves
     
  16. Mr. Rick

    Mr. Rick VIP MEMBER

    Joined:
    Feb 6, 2009
    Some folks living at altitude are having good results w 105 needle jets and 170 mains.
    Make sure your pilot (idle) jet (bush) is clear by poking a .016" wire or (longer than jobber-length) #78 drill bit in there, after removing the mixture screw.

    Different types of spray tubes are shown here:
    http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter
     
  17. Steves

    Steves

    Joined:
    Oct 16, 2016
    What Baz says. There are ID rings or shallow grooves at the top of the needle. You will always have 3 grooves just below that the needle clip slots into, you choose. Ok so I don't have any experience with jetting for altitude so if they were running on 160's then 160's it is. The link Mr Rick provides is excellent bear in mind there is some Premier carb stuff in there I think. Just to confuse matters I run two stroke chamfered spray tubes in my bike lol. Experimentation is what it's all about, but start off stock as Norton already did the work. Any local Norton riders in your area?
     
  18. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    I have owned over eight Norton commandos and most of them have had carburetor issues and the two I have now are no exceptions. I haven't ever liked the fuel tickler feather on the Amal carburetors and most of the issues have been either running rich, smoking or throttle response cutting out and not idling and don't forget electrical! As far as knowing other local Norton riders in my area there aren't really any. You can say I am a work in progress. I have learned a lot over the years and your responses have been very helpful.
    Most of the bikes I have purchased have come from people who don't know much about Norton's and have done so called improvements to the bike that have caused more headaches then good. So getting back to the carburetor idling problem.

    Question: There is a small threaded hole at the base of the 930 and 932 Amal carburetors. You can see it when the float bowel is removed and set aside. Then turn the carburetor up side down and it will be the only hole that is open leading to a passage way. On the right carburetor it is located at the right rear mating surface and on the left side on the left carburetor. Is this part of the idle circuit?
     
  19. Bob Z.

    Bob Z.

    Joined:
    Mar 30, 2012
    That threaded hole is the idle circuit fuel pick-up from the float bowl.
    The threads were for the removable brass pilot jet installed in the late 1960s, then the jet location was moved
    to the front of the carb behind the idle air screw.
    Something about low vacuum at kick-over engine speed.
     
  20. nortonisthebest

    nortonisthebest

    Joined:
    Aug 1, 2014
    After seeing Mr. Ricks illustration, that is the hole I was asking about. It is the "Pilot Jet" location. In all my years I haven't ever seen a pilot jet installed in any Amal carburetor I've owned. They have all been missing that jet! How impotent is it? Is it necessary? Thanks Rick for showing me that.
     

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