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Some more Zenith carb questions

Discussion in '"Stock" Alpine' started by clhiller, Dec 13, 2019.

  1. clhiller

    clhiller Silver Level Sponsor

    Now that I have the head modded/ported and skimmed, I'm on to the carbs. On a series 1, heres what I have.

    Front carb is stamped C1688F
    36 WIP2
    1046821

    Rear carb is 36 WIP3
    1800118
    C 1774F

    Obviously I have a mix and match set. The front two cylinders seemed to be running very rich based on the plug/head condition (sooty), the rear two were pretty much spot on (light tan).

    Is there any way to set these two up so they are close? I haven't taken them apart yet, but looking in from the top, the front is a 28 venturi and the back is a 30.


    Thanks
     
  2. alpine_64

    alpine_64 Donation Time

    Firstly yes mis matched carbs. The jettimg and chokes are different.

    What condition are the carbs in... Zeniths are usually worn with spindles and butterflys allowing air and fuel mixtures to be out of spec.

    If you want to stay with the Zeniths id start by sourcing a matched pair.. Ideally off a SII if you have modified the motor and do a full rebuild.

    Contact Bernd from the forum ( hes in Germany) he made new spindles iirc.
     
  3. beamdream

    beamdream Gold Level Sponsor

    As Michael has said, best to have a matched pair.

    Nevertheless if you're stuck you could try a smaller main jet on the front carb, that's assuming you have ready access to parts.

    It might be worth getting a small mirror under the carb bowl(s) and letting us know what size main jets are installed at the moment.
     
  4. bernd_st

    bernd_st Bronze Level Sponsor

    All answers already correctly given. If you want to go original find a 1688R - especially if you still run the 1500cc engine. If you have already upgraded to 1600cc and want to go for best performance find a matching 1744R. Both sets to be properly overhauled of course...
     
  5. clhiller

    clhiller Silver Level Sponsor

    Hi again.

    Well, the carbs are apart and heres what I found. Based on the WSM data, it looks like all the jets are correct, even though the carbs aren't matched.

    Front carb C1688F Rear carb is C1774F
    36 WIP2 36 WIP3
    1046821 1800118
    28mm Choke 30mm
    130 Main 142
    50 Slow 45
    50 Pump 70


    Also the small balls are missing from the non-return valve underneath the accelerator pump discharge.
    I will contact Bernd about acquiring two new throttle shafts and get replacement balls for the accelerator pump non-return valve.
    Other than that and a good clean up with new gaskets, any other suggestions? Should I try to match the jets or just stick with the factory settings?

    I'm not wanting to spend a ton on getting different carbs at this time, so hopefully they will work well enough for now.
     
  6. clhiller

    clhiller Silver Level Sponsor

    Here, this is a little easier to read


    Front carb C1688F ------Rear carb is C1774F
    36 WIP2--------------------- 36 WIP3
    1046821--------- ------------1800118
    28mm --------Choke-------- 30mm
    130------------ Main---------- 142
    50------------- Slow----------- 45
    50 ------------Pump -----------70
     
  7. beamdream

    beamdream Gold Level Sponsor

    Well on paper it looks ok, if I were to add conjecture, it would be based on progression between the carbs with different venturis.

    I would pay close attention to balancing the carbs in terms of air flow, do you have access to a carb balancer for setting the throttle stops ??
     
  8. clhiller

    clhiller Silver Level Sponsor

    Yes. I bought a uni-syn to sync the carbs.
    What impact would the absence of the balls in the non return valve have?
     
  9. alpine_64

    alpine_64 Donation Time

    At the mimimum the chokes and throats have to be the same carb to carb... Same with jets....
     
  10. beamdream

    beamdream Gold Level Sponsor

    The balls prevent a drain back after the pump has discharged, there's also a check valve in the bottom of the float bowl which should hold a charge for the next discharge, check how effective the accelerator pump(s) are working.

    The balls are 1/8 dia and most bearings suppliers sell them loose.
     
  11. bernd_st

    bernd_st Bronze Level Sponsor

    You need new steel balls otherwise the accelerator pumps won't work properly. Here are pictures of my repro spindles:

    IMG_20191219_220957.jpg IMG_20191219_221056.jpg
     
  12. bernd_st

    bernd_st Bronze Level Sponsor

    Here we are 1744 F:

    IMG_20200118_131855.jpg IMG_20200118_131745.jpg

    Can either do cast iron or nylon accelerator pump lever as mentioned...
     
  13. bernd_st

    bernd_st Bronze Level Sponsor

    P.S. Accelerator roller lever not included, but you may reuse the one you have on your 1688 ...
     
  14. clhiller

    clhiller Silver Level Sponsor

    Here are the newly rebuilt, matching 1744’s. Thanks to Bernd for the help and parts. F3A7F582-5B36-4E4F-AD5A-90C93D1984DA.jpeg 7CF6BE92-7F7A-425B-9BE1-21AF97FF14AF.jpeg
     
  15. clhiller

    clhiller Silver Level Sponsor

    Just a temporary mounting. I know there’s no gasket
     
  16. bernd_st

    bernd_st Bronze Level Sponsor

    Looks like a great matching pair now. Hope you will feel the performance improvement. On a side note and if you are not driving in cold weather leave the water pipe through the manifold disconnected. This requires that you replace the Y outlet in the water pump by single outlet ( same as the heater outlet on a spare cylinder head, plus a plug for the rear outlet ). You'll be surprised...

    P.S. Interesting engine block you have. Never seen such an angled flange on the lower block next to the Dynamo...
     
  17. clhiller

    clhiller Silver Level Sponsor

    I do plan on leaving the manifold heat disconnected, just in the process of fabricating a single outlet fitting.
    The block is from a series II Rapier, or so I am told.
     
  18. clhiller

    clhiller Silver Level Sponsor

  19. bernd_st

    bernd_st Bronze Level Sponsor

    Thanks for posting the additional picture. 2 things to be mentioned here:

    a) You are missing the fast idle lever/rod assy on the rear carb. Hope you can reuse the one from your replaced Zenith

    b) You are fortunate to have the competition early airfilters without internal wire mesh. They are good for maximized airflow but almost no filtering. Furthermore they don't fit to 142 mains. Engine may lean out on higher revs. Either you install normal early series meshes or you go for 150 mains...
     
  20. clhiller

    clhiller Silver Level Sponsor

    I looked at that. The fast idle rod on the front carb advances the throttle a small amount and with the linkage to the rear carb, it advances the rear throttle also. Is it necessary to have the rear rod installed as well? I don’t believe it was on the rear carb previously, but if required I can scavenge it off the old front carb.

    The air filters do have a mesh in them. It is a black mesh so it is hard to see. What should the early filter mesh look like? I’m at work right now, but can take a picture of the mesh when I get home.
     

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