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Intake Manifold Drain Pipes

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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)
 
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vette said:
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.
Click to expand...

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).
 
RAC68 said:
....
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....
Click to expand...

The earlier HD6 cars use that concentric spring.
 
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.
 
Bob_Spidell said:
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.
Click to expand...

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.
 
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 everyoneā€™s contribution on this one. I will let you know how I come out.
Lin
 
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.
 
Well, I have driven the car several times this week. I cannot replicate the high-REVs or combustion ā€œpop.ā€ So, I guess I will just continue to monitor the situation. I donā€™t like leaving it like this, but I guess I will and see what happens. Thanks for the suggestions and thoughts.
Lin
 
Bob_Spidell said:
If not having them works, good. I believe they are installed due to a phenomenon called 'wall wetting;' i.e., when an enrichment system--choke, or whatever you want to call SU's system--is on an extra rich mixture is produced. Some of the fuel in the extra rich mixture will condense on the cold manifold walls, and puddle at the bottom of the manifold, hence 'wall wetting' (which is partly why you need an extra rich mixture for a cold engine to run, also due to the fact that as the fuel vaporizes it cools and condenses). Most of the time, presumably, the puddled fuel gets vaporized as the manifold warms up and is not problem, but if a large puddle develops bad things could happen (severe manifold backfire, for one and, conceivably but not likely, hydro lock). Note that all intake manifolds for non-fuel-injected cars that I've seen have a method--either circulated coolant, or in a Healeys case, proximity to the exhaust manifold with only some flimsy gaskets to separate them--to offset the effects of vaporization and wall wetting.

I think American V8s don't have drains, well, because the manifold sits between and above the two cylinder banks, and fuel will drain downward into the cylinders rather than puddling (and how would you route the tubes?). V8 carburettors are warmed with coolant flow around their bases to offset the cooling effect of vaporization and, for example, my '55 T-Bird has a valve just below the exhaust manifold that gates hot exhaust gasses to the carburettor base when the engine is cold (of course, if/when it sticks open you have a problem). This is similar to carburetted piston engines which usually have a pilot-controlled mechanism called 'carb heat,' which gates air heated by the muffler to the carburettor to prevent/alleviate 'carb icing' due to vaporization (carb icing is more likely to occur in warm, humid conditions, not usually in cold dry conditions, which you might expect). Modern fuel-injected cars don't need such extra 'features' because the ECM can accurately control the mixture for all conditions and in common port-injected engines the fuel bypasses the manifold (same for DI).

ps. Float needles have been known to stick open, causing a phenomenon known as 'flooding.' Where do you think the fuel goes when the intake manifold gets flooded?
Click to expand...
OK, my M100 BN6 didnā€™t have the drain pipe installed but the drain port has the screw in plug. But with that opening being about 3/16ā€ diameter this caused a massive vacuum leak. Both of my HD8s have drain lines. But with the huge vacuum leak the engine ran rough and balancing the twin carbs was impossible because the engine was always running too lean. In an effort to remove the vacuum leak Iā€™m going to plug the forward drain.

I havenā€™t seen a drain on the rear of the manifold but will check. The front carb always seems to run lean no matter what I do. So plugging the manifold drain hopefully will solve my problems.

Does anyone disagree with me doing this? You feedback would be greatly appreciated.
 
Welcome to the forum bookertmg.

First, the carburetor drain lines are from the float chambers in case the floats stick or otherwise overfill with fuel. You definitely want drains there so fuel doesn't spill down onto the exhaust manifold and there is no manifold vacuum to worry about.

The manifold drains are, as others have said, there to drain off excess fuel from inside the manifold which can collect from over rich mixtures and condensation from over 'choking' during cold start up. Or, a stuck float needle could cause fuel from the float chambers to flood the main jet and run into the manifold.

You can find lots of debate about the need for the manifold drains - I have a tri-carb set up and they have no manifold drains - seems to be only the log manifolds that have them and there were several SU carbureted engines back in the day that did not have manifold drains. I had twin HD8s for many years before the tri-carbs and always plugged the drain holes because I don't really think they are necessary (just my opinion).

You are right that without the fittings and very small bore tubing, there will be a large vacuum leak - the theory I've read is that the drains tubes are supposed to bent into a lazy 'S' shape and go down to the level of the top of the sump and this length, together with the small diameter of the tubing and the bends, greatly reduces any vacuum leak - to the point of being inconsequential. So, if you want them, I suggest copying the factory tubing shape and length.

Dave
 
Thanks for your input. Once I plug the drain port thus eliminating the vacuum leak I Iā€™ll see if there are any ill effects. If not then Iā€™ll leave the drain plugged.
 
Bob, You are correct. My BT7, which I have owned since it was 3 years old has the clips attached to the sump bolts and the drain pipes go through those clips. The rubber ferrules rotted always years ago.
 
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