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Zenith carb question-diagram?

Discussion in '"Stock" Alpine' started by leigh, Jul 8, 2020.

  1. leigh

    leigh Donation Time

    Does anyone have a diagram for the zenith carbs on SII? I'm used to finding pretty much everything on Google but my searches turned up nada.

    My 61 alpine has the original zenith carbs, which were rebuilt at some point by the PO. Lucky for me because I have carb-o-phobia.

    Have recently had some backfiring thru the carbs on first gear acceleration so the time has come to attempt some adjustments. Trouble is, I don't know enough about where things are on the carb to follow instructions so need a diagram to guide me
  2. beamdream

    beamdream Gold Level Sponsor

    Carbs, if original, are Zenith WIA or WIP, plenty of info here and the net, have a look for the WSM on this site, its got everything you need.

    You don't mention checking the timing ? With a backfire under acceleration that's where I would first start, nothing wrong with the assumption that carbs might need attention but before pulling things apart it's pretty easy to eliminate timing.

    If you get into the carbs, backfires indicate lean mixture, so in order order of potential issues I would be looking for:

    with engine not running, air cleaner/s off, work the accelerator linkage and look for good solid stream of fuel into both carb throats

    next, carb manifold bolts tight no air leaks around the carb base or manifold

    next, fuel pump output

    next, remove float bowl covers and look for foreign matter which could block jets, then check float levels are set right

    If you need to go beyond these checks, it's probably strip down, cleaning, blowing, throttle shaft bushing wear etc.

    And finally.... good luck.

    Oh yes, presuming of course that prior to this, the engine was running ok and other stuff hasn't been going on in the background?
    leigh likes this.
  3. bernd_st

    bernd_st Bronze Level Sponsor

    Backfire through the Zeniths can usually be addressed to wrong timing. Would check the dizzy , ignition components, centrifugal and vacuum advance at first. Needless to stress the firing order has to be correct...
  4. Gordon Holsinger

    Gordon Holsinger Diamond Level Sponsor

    Check your ignition timing and points clearance
  5. husky drvr

    husky drvr Gold Level Sponsor

  6. leigh

    leigh Donation Time

    Beamdream, thanks for the great advice. I could see I have an issue at your very first point-with engine off one carb has a super strong stream and the other just dribbles. What is the usual cause of that?
  7. beamdream

    beamdream Gold Level Sponsor

    Ok so the carb that "dribbles" seems to have a problem with the accelerator pump not providing sufficient fuel.

    First thing is to take off the top (float bowl) cover and see that you have fuel in the float bowl, if there's no fuel perhaps you have a problem with the needle valve or float, or even a blockage to that carb.

    If you have sufficient fuel (about 15mm deep) then push down on the accelerator pump piston (round brass piece approx 10mm dia.), there should be a good solid stream of fuel into the carb throat.

    If you don't get decent flow then the accelerator pump jet needs to come out for inspection (care when you remove this as a tiny steel ball 3mm) hides under here and will drop out when you tip the carb upside down (if it's not there that is a likely cause of your problem.

    Next possibility is a faulty check valve in the bottom of the float bowl, if this fails fuel won't be pumped up to the accelerator jet. Look for bubbles in the bottom of the float bowl when you push down the pump piston.

    With all of the above you will be needing the work shop manual and illustrations.

    Not too scary ? Good luck, let us know the outcome.
    leigh and windlessdog like this.
  8. bernd_st

    bernd_st Bronze Level Sponsor

    Almost perfect answer however and before taking everything apart it makes sense to check whether the accelerator pump lever of the "dribbling" carb is moving when you press the throttle pedal. It should move synchronously with the OK carb. If not the coupling between the two carbs could be loose...
    leigh likes this.
  9. leigh

    leigh Donation Time

    Thank you both!! Very helpful!! With 7 old cars it was past time for me to learn about carbs. My project for the day, after I buy my kids back to school supplies, clean the garage, talk with my mom....will keep you posted.
  10. leigh

    leigh Donation Time

    I finally found a little time to take a look. After some bolt/screw tightening there were good squirts in both carbs when moving the throttle linkage. So progress on that. Both carbs had gas in the float bowls and both floats seemed to move ok.

    But at idle and at steady higher rpms I could see the gas streaming in on the rear carb from what the manual calls the main discharge jet and the front carb had zippo, no gas seemed to be getting in at all. So I'm worried that something's maybe blocking the metering jet or the main jet. Does that seem like a logical conclusion on my part or may I be missing something?

    Is taking it apart the only way to fix/check? Dumb question but how to remove the float? The manual unhelpfully just says 'pull it out' but I'm worried of damaging something by yanking on it.
  11. beamdream

    beamdream Gold Level Sponsor

    20200924_094226.jpg Could be a few thing's going on here, fuel streaming out at idle isn't normal, higher rpm yes. Possibly here you have a float/needle valve that's allowing the float bowl to overfill, black sooty spark plugs on cylinders 3 & 4 would confirm this.

    No fuel from the front carb may well mean a blockage somewhere in the carb.

    So you haven't mentioned whether the carbs are in sync, that is, are both throttle valves opening the same amount when you push the accelerator; a basic check on this can be done by using a piece of hose to your ear and listening to the sucking noise in the carb throat(s) at idle, they should both have the same sound. If the carbs aren't in sync then you'll have different fuel flows between front and rear cylinders. Getting these balanced can be quite a chore, so we can walk you through that later if required.

    As to getting a float out, with the top cover removed you'll see a little bridging piece that the float pivots off in the wall of the float bowl, you may need to pull up on that (carefully) with a pair of needle nose pliers, then the float will come out.

    Whilst you mentioned that there's fuel in the float bowl(s) is the fuel at the correct level? check the WSM I think it's supposed to be 16mm and of course bowls need to be at the same depth.

    There's a few more things to do, but start here and see how you go.
    Last edited: Sep 24, 2020
  12. bernd_st

    bernd_st Bronze Level Sponsor

    Follow the description on the suspect carb as mentioned by beamdream above. Care to be taken when removing the pump jet that nothing falls into the inlet manifold. The little steel ball needs to be there. If not you need to find a proper replacement...
    Last edited: Sep 23, 2020

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