Results 1 to 6 of 6

Thread: Air Vent Cut-Off Diaphragm Operation?

  1. #1
    DX User
    Join Date
    Dec 2010
    Posts
    3

    Air Vent Cut-Off Diaphragm Operation?

    On an '88 Accord DX with auto tranny:

    Anyone know what the full operation of the air vent cut-off diaphragm is? It is the diaphragm built into the side of the carburator thermovalve which is on the upper front drivers side of the carb. Mine has a bunch of holes from being so close to the fuel (it looks like its only a couple inches away from the internal float resevoir.) I can't interpret the factory vacuum diagram for this part to see what it does exactly. I know when the car is running vacuum is provided to the diaphragm (line 8) pulling a plunger to seal off the carb thermovalve inlet from the canister. When the car is off vacuum is not there and the plunger is pushed via a spring in the other direction to block off a different passage in the carb.

    My diaphragm had lots of holes so I plugged line 8 going to it with a ball bearing to stop the vacuum leak. My question is this: does the air-vent cutoff plunger need to be open when running or does it not make any difference to the engine operation. I could theoretically plug the thermovalve inlet line instead and remove the plunger to open the passage inside the carb. Should I?

    BTW, I also found my canister purge control valve diaphragm doesn't hold vacuum, so I plugged that line as well (line 19) at the canister. So I assume the canister never gets purged now. Is that a correct assumption? Does it eventually build up and vent out the bottom hose or something?

    Also my anti-afterburn control solonoid valve wouldn't hold a vacuum so I plugged that line as well. That should be fine, right?

    Lastly, my hot air door actuator is shot so I temporarily plugged that line while I look for a replacement.

    I've been hunting for a cause for the high idle on this car. I thought for sure all these leaky diaphragms were the cause. I also found and repaired a couple vac hose leaks. Still idling high even when warm. Lowest I can get it is around 1300 RPM. And it likes to climb.

    Any ideas, anyone?

    Thanks. Sorry about the long post.



  2. #2
    DX User
    Join Date
    Dec 2010
    Posts
    3

    Re: Air Vent Cut-Off Diaphragm Operation?

    Bump.

    Anyone out there know about this actuator. I need to know if I can disable it or if I should source a new one.

    Thanks.

  3. #3

    Dr_Snooz's Avatar
    Join Date
    Jun 2007
    Vehicle
    1989 Accord LX-i
    Location
    Fresno, California
    Posts
    10,638

    Re: Air Vent Cut-Off Diaphragm Operation?

    I was going to let someone else with more carb experience take this one, but I guess not.

    If you haven't already, download the manual and read up on the functions of the various controls and actuators on the carb in chapter 11. Each affects the operation of the carb in a specific way and compromising them will likewise affect carb function. If you want factory performance, unfortunately, you're going to have to fix the factory controls properly. If you have dash pots with holes in the membranes, someone on here tried RTV as a patch with success, though I'm not sure how the repair held up over the long term. The manual should help you narrow your search for the control(s) specifically impacting idle. I'm not seeing Air Vent Cutoff Diaphragm listed, so maybe there's a different name for it in the manual. I think most of our carb experts have moved on and everyone else seems to think the Weber swap is a better way to go (if you don't have to pass smog tests). You are having a lot of carb issues, which for me points to a major overhaul in the near future. Those are tricky to get right, so the Weber swap and a major vacuum line removal tends to be the solution of choice for everyone.

    I'm not seeing the canister purge control valve either, but I'm assuming you don't mean the PCV. Generally, if you don't vent the charcoal canister, it will eventually become saturated with gas and the car will always smell like fresh gas. That won't affect anything serious though.

    I don't think the afterburn valve is a problem, but might cause some hiccups during shifts or deceleration.

    For the hot air door, we had a guy wire open his hot air door and he saw a 20% improvement in gas mileage. I'd like to see that figure replicated by someone else.

    Hope that helps.
    Dr_Snooz

    "I like to take hammers, and just break stuff, just break stuff." - Beavis


    1989 Honda Accord LX-i Coupe, 240k miles, MT swap, rear disc swap

    Shop manual downloads available here: CLICK TO VIEW

  4. #4
    LXi User Buzo's Avatar
    Join Date
    Dec 2010
    Vehicle
    88 Honda Accord EX - Single Port EFI, 4 doors Aut.
    Location
    North of Mexico
    Posts
    836

    Re: Air Vent Cut-Off Diaphragm Operation?

    The Air vent cut off diagram is there to control the little amount of fuel contained into the float chamber:
    1.-When cold - to avoid the release of fuel vapors into the atmosphere in order to reduce contamination, fuel odor in the engine bay, etc.
    2.-When hot - to avoid the fuel vapors to be released into the combustion chamber and avoid a hard hot-start condition.
    Any excess of fuel vapors are directly sent to the canister.


    Power is nothing without control

  5. #5
    LXi User Buzo's Avatar
    Join Date
    Dec 2010
    Vehicle
    88 Honda Accord EX - Single Port EFI, 4 doors Aut.
    Location
    North of Mexico
    Posts
    836

    Re: Air Vent Cut-Off Diaphragm Operation?

    For the hot air door, we had a guy wire open his hot air door and he saw a 20% improvement in gas mileage. I'd like to see that figure replicated by someone else.
    This is true for winter & when the air is taken from the outside (passengers fender). When the air is taken from the engine bay it can get to ~35 C in winter and ~55 C in summer in a full warm engine. Assuming your outside temperature is 0 C, is doesn't take a rocket science to see the difference in the air density (and therefore the required fuel) at 0 degrees C compared to 55 degrees C

    This air door keeps the incoming air controlled to a certain temperature, but opens during hard acceleration to let the colder air give us more power when required.
    That's why the door is vacuum controlled, to somehow "sense" the position of the pedal and the requested power from the driver to the engine.


    Power is nothing without control

  6. #6
    LXi User Buzo's Avatar
    Join Date
    Dec 2010
    Vehicle
    88 Honda Accord EX - Single Port EFI, 4 doors Aut.
    Location
    North of Mexico
    Posts
    836

    Re: Air Vent Cut-Off Diaphragm Operation?

    Certainly every actuator in this carb is there for a reason, so plugging off the hoses may not the best solution.
    What I think we need to do is to find a good material that is not permeable & fuel resistant to replace those shot diaphragms in all the vacuum actuators.
    I repaired several actuators using glue, an old bicycle camera and my dremel, but that material is not fuel resistant so they got shot again with the time.

    We need to think in this carb as if it was a stand alone fuel management system, the difference is that the carb is a mechanic device and not electronic, but we already have all the functions that will need to be programed if someone decides to go DYI with any fuel injection system in the market.


    Power is nothing without control

Similar Threads

  1. Vacuum Advance Diaphragm
    By russiankid in forum 3geez Accords
    Replies: 11
    Last Post: 03-28-2010, 08:02 AM
  2. vent controls in the vent, and near the drivers knee
    By newaccorddriver in forum Classic Honda Community Chat
    Replies: 14
    Last Post: 01-09-2010, 10:08 AM
  3. Engine fan/cooling system operation
    By 88Accord-DX in forum 3geez Accords
    Replies: 6
    Last Post: 04-09-2006, 01:31 PM
  4. Throttle Controller Diaphragm
    By '89AccordLX(Rus) in forum Carburetor Tech
    Replies: 2
    Last Post: 02-08-2005, 06:10 PM
  5. Accelerator pump diaphragm
    By perlov in forum Carburetor Tech
    Replies: 0
    Last Post: 10-17-2003, 03:12 PM

Bookmarks

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
This website uses cookies
We use cookies to store session information to facilitate remembering your login information, to allow you to save website preferences, to personalise content and ads, to provide social media features and to analyse our traffic. We also share information about your use of our site with our social media, advertising and analytics partners.
     
VigLink badge