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Thread: Intake manifold drain pipes

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  1. #21
    Obi Wan RAC68's Avatar
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    Re: Intake manifold drain pipes

    Hi All,

    An occasional stuck throttle is not uncommon. Given the "Rube Goldberg" linkage from the accelerator peddle to the carburetors with it's incorporation of the overdrive control linkage, it is somewhat amazing that a sticking accelerator is not more prevalent then it is. Add to the complexity of the linkage is the return spring setup. A lever clamped to the end of each carburetor shaft secures the spring on one end with the other hooked to a bracket on the intake manifold. Since the rotating lever is meant to maintain a constant spring distance to produce an even force through the full rotational travel of the carb shaft, any miss-adjustment or the lever or age-developed weakening of the throttle springs, could result in the throttle plates not closing. As a result and in short, occasionally the RPMs do not diminish as they should when releasing the accelerator peddle.

    Will replacing with a stronger springs help?
    After rebuilding my carburetors and remounting them on the engine, I found that my carbs would not shut down as previously expected and sometimes RPMs would even increase. To address the issue, I temporarily replaced my original throttle springs with stronger units. Although the throttle plates now closed completely and consistently, the added resistance increased initiating peddle pressure and, after passing the initiating point, the added foot pressure would cause the throttle to jump forward. Some may have found a good way of dealing with this issue but I went back to the original springs and now have ordered new versions.

    I noticed that some Jaguars and TRs use a coiled shaft return spring set around the accelerator shaft and located right on the carburetor. This could be a better approach then the Healey setup and one that is less mechanically involved. This approach may provide a less complex approach that I may consider if the newly ordered springs do not resolve the issue.

    Just my thoughts,
    Ray(64BJ8P1)
    Last edited by RAC68; 10-05-2019 at 01:04 PM.

  2. #22
    Yoda
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    Re: Intake manifold drain pipes

    Quote Originally Posted by vette View Post
    My carburetion and linkage is stock except that I am running HD8 carbs. Even tho HD8 carbs are by design suppose to idle with the throttles closed and run on slow run screws in reality my throttles are cracked open a tech at idle. All other carbs naturally have throttles open some at idle.
    Agree and me too. I never liked the idea of the throttles being completely closed, I know it's unlikely but possible, I think, for them to wear a groove in the carb bore over may miles. Plus, I could never get an idle over 600RPM or so with just the slow run valves, and my engine idles best around 750 (and you have some wiggle room when the engine loads up, say, on a hot day in traffic, and above 700 the generator is charging; the downside being run-on).

    An unexplained fast idle is probably due to an air or mixture leak. Too much advance is remotely possible, but setting the advance per the book usually results in the best/fastest idle and unless the distributor clamp is very loose not likely. I've forgotten to tighten the clamp after setting timing before and the distributor didn't move for a few miles before I realized my mistake. What are the chances the distributor would move back to a proper setting and stay there?

    My SWAG is something in the linkage. The Rube Goldberg throttle linkage is pretty wonky, hence why some replace with 'modern' cable linkage. If the original bushings are all still in there I'd replace them; the cross-shaft--the one under the transmission tunnel that is nigh impossible to get to--has brass/rubber bushings and if original they are almost certainly worn, and the rubber long since gone--and could bind (aftermarket teflon/nylon are available from Moss; I replaced mine when I noticed the shaft was bent, probably from a ham-fisted previous engine install).

  3. #23
    Yoda steveg's Avatar
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    Re: Intake manifold drain pipes

    Bob - I've been battling run-on for years. My engine currently idles at 750-800 (on slow-run screw only). Do you believe a slower idle may prevent run-on?
    Steve Gerow
    Altadena, CA, USA
    Maker of most complete Big Healey rear disc kit
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  4. #24
    Yoda steveg's Avatar
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    Re: Intake manifold drain pipes

    Quote Originally Posted by RAC68 View Post
    ....
    I noticed that some Jaguars and TRs use a coiled shaft return spring set around the accelerator shaft and located right on the carburetor. This could be a better approach then the Healey setup and one that is less mechanically involved....
    The earlier HD6 cars use that concentric spring.
    Steve Gerow
    Altadena, CA, USA
    Maker of most complete Big Healey rear disc kit
    Check out my galleries:
    http://www.pbase.com/stevegerow


  5. #25
    Yoda
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    Re: Intake manifold drain pipes

    I do. But, I prefer a slightly higher one for the reasons I mentioned (newer, computer-controlled cars seem to prefer an idle between 700-750 as well). I think higher octane fuel--I run 'premium' but, as you know, the best we can generally get here is 91-octane--might help alleviate run-on. Note it's not a Healey-only problem, some other cars, MGs I think for one, have an 'anti-dieseling' valve to positively shut off fuel flow when the key is turned off (but, from what I hear it's problematic). My rebuilt engine has pretty good compression, 170-180PSI on all 6 last I checked, which is a contributing factor. For now, I use the 'put it in gear and dump the clutch' method, which I don't like at all but run-on, like oil leaks, has no definitive cure that I know of.

  6. #26
    Yoda steveg's Avatar
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    Re: Intake manifold drain pipes

    Quote Originally Posted by Bob_Spidell View Post
    I do. But, I prefer a slightly higher one for the reasons I mentioned (newer, computer-controlled cars seem to prefer an idle between 700-750 as well). I think higher octane fuel--I run 'premium' but, as you know, the best we can generally get here is 91-octane--might help alleviate run-on. Note it's not a Healey-only problem, some other cars, MGs I think for one, have an 'anti-dieseling' valve to positively shut off fuel flow when the key is turned off (but, from what I hear it's problematic). My rebuilt engine has pretty good compression, 170-180PSI on all 6 last I checked, which is a contributing factor. For now, I use the 'put it in gear and dump the clutch' method, which I don't like at all but run-on, like oil leaks, has no definitive cure that I know of.
    Ditto all the above. I also have to do in-gear-clutch-dump shutoffs, too. Would like to be able to do plug cuts, but no such luck.
    Steve Gerow
    Altadena, CA, USA
    Maker of most complete Big Healey rear disc kit
    Check out my galleries:
    http://www.pbase.com/stevegerow


  7. #27
    Jedi Knight Lin's Avatar
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    Re: Intake manifold drain pipes

    I am not able to work on the car this weekend. I will the first of the week. As I have thought more about my issues and after factoring in comments here, I am going to focus initially on throttle components first. I think it is most likely that my problems lie there. The high reving (up to about 3,0000) does suggest too much air in the system. I am still curious about the impact of closing off the rear drain in the intake manifold, but I will address that only if I need to do so. Also, much easier to access throttle than to get to that manifold drain! Although, I am wishing that I had just left well enough alone.
    Thanks very much for everyone’s contribution on this one. I will let you know how I come out.
    Lin
    1959 AN5 Bugeye - now with my son 😀
    1960 BT7 3000 MKI
    1987 Alfa Romeo Spider Quadrifoglio

  8. #28
    Yoda
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    Re: Intake manifold drain pipes

    I'm tempted to cut the idle back a little--using the slow-run valves, another advantage to leaving some 'wiggle room'--but, before I do I'm taking an overnighter up to the mountains next week and I'll see if the less-dense air, and subsequent slower idle, mitigates the run-on. I'd like to blame ethanol but, as best I can recall, my BJ8 has always had run-on in the 35 years I've owned it. My BN2 runs on, too, but not as bad (compression 160 +/-), but I have to keep the idle up else it gets close to stalling ... it's a fine line.

    Edit: ps. Run-on isn't just annoying, but can conceivably cause damage. I think some machinery--timing, distributor and oil pump gears for example--would prefer to not be run backwards.

  9. #29
    Yoda John Turney's Avatar
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    Re: Intake manifold drain pipes

    The last time I had a 3,000 RPM idle, it was a small bit of gasket caught in the throttle plate.
    John, BN4

  10. #30
    Jedi Knight Lin's Avatar
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    Re: Intake manifold drain pipes

    Well, I have driven the car several times this week. I cannot replicate the high-REVs or combustion “pop.” So, I guess I will just continue to monitor the situation. I don’t like leaving it like this, but I guess I will and see what happens. Thanks for the suggestions and thoughts.
    Lin
    1959 AN5 Bugeye - now with my son 😀
    1960 BT7 3000 MKI
    1987 Alfa Romeo Spider Quadrifoglio

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