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Intake Manifold Drain Pipes

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Lin

Lin

Jedi Knight
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Hello all,
I have probably screwed up (again) and need some knowledge and some advice regarding the need for the fuel drain pipes on the stock AH3000 intake manifold.

So here is my story....

I had a crack on two of the mounting ears of my stock intake. I replaced it with another stock unit (thank you Mike Salter) that I had Jet Hot Coated. I have Phoenix headers on my car that have been in place for about eleven years. I have always been uncomfortable with how close the rear copper drain pipe was to the header pipe although I never experienced a problem with it in those eleven years.

When I put the new (used) manifold on the engine I decided, perhaps foolishly, to plug the rear drain by removing the pipe fitting and pipe with a bolt. I kept the front drain pipe in place.

I should add that during this process I had the carbs rebuilt by Thomas Bryant, who by the way, I highly recommend.

Every now and then when I restart the car after it has been running and is hot, upon start up, the engine revs go way up though I am not activating the choke or pressing the accelerator pedal. I typically cut the ignition off right away and wait for cool down. The car then starts as normal. The car acts as if the throttle is stuck open but as best I can tell there are not sticking throttle issues. I use a throttle cable - not the stock mechanical linkage.

Today after an enjoyable one hour drive, I stopped for fuel. When I attempted to start the car I experienced the engine racing again but this time it was accompanied by a loud pop. Different sound than a carb backfire.

I donā€™t know what is going on, but since this happens only when the engine is hot, and since I did remove the rear drain pipe, I am wondering if fuel is pooling in the intake manifold and then igniting explosively when the ignition is turned on and I hit the starter button - like a ready fuel enrichment source. Very odd I admit, but I am thinking this way because it seems that once that fuel is burned the car returns to normal and runs just fine.

I am really baffled by this one. Could the removal of the rear drain pipe be the cause of this problem? Can someone educate me about the necessity of the drain pipes. Is there something about the ā€œlogā€ design of the intake that makes the drains a requirement. As far as I know, the DMD manifold does not use them, I donā€™t believe the AH Spares custom intake uses them, nor do I think the Ruddspeed independent intakes use them.

You might say, ā€œwell, why donā€™t you just reinstall the rear drain pipe and see if that solves the problem?ā€ A reasonable question, but if you have tried to do this before you know that I will have to remove the air cleaners, carbs and etc. to get to the fitting. If I have to, then I have to, but I sure hate to do that if the blocked pipe fitting isnā€™t really my problem.

Sorry for all the background, but I am trying to give you as much info as possible to explain what I may have done to myself! I would appreciate your thoughts on what to do next.

Thank you.

Lin Rose
 
A couple times, over almost 150K miles of ownership, my BJ8 when parked will pee gas out of one or both of the drain tubes, usually when both car and atmosphere are warm/hot. Carbs are tuned properly and car runs well otherwise, so I chalk it up to 'just one of those things.' If fuel is allowed to puddle in the manifold, for whatever reason, I think you'd get the symptoms you described.

Also, are you sure the carbs are tuned properly (even if rebuilt by an 'expert')?
 
Yes, I think the carbs are better than they have been in the 48 years I have owned the car!
Lin
 
FWIW, for the last few years I've been running triples with no drains but prior to that for about 15 years I ran a stock log manifold with HD8s and the drains plugged and never had a problem. I believe that if the float needles are set right and the mixtures are right there should never be raw gas in the manifolds - if the floats are set too high, gas can flow out of the jets and collect in the manifold.

Like you I had questioned the need for them too - mostly because American v8s have no drains and they don't seem to have a problem either, like the examples you listed.

Not scientific but that's been my experience. Interested to hear others experiences.

Dave
 
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?
 
I recently had fuel coming out of both copper manifold drain pipes as the engine shut down. After both carburetors were rebuilt, shop found the overflow tube on one or both SU's was blocked. This had something to do with the air pressure in the bowl not allowing the needles to seat completely sending extra fuel into the manifold. Cleaned out the carb tubes and it solved the problem. Quick and easy fix, if it works for you.
 
I believe the manifold tubes are a precautionary measure for such cases where a carb float sticks or is maladjusted. Also consider that the Healey stock intake manifold is not necessarily an example of flow efficiency.
Many aftermarket V8 intake manifolds do not have provisions for heating or cooling under the carb. An ex being the Edelbrock one on my 383 stroker.
 
Why are we talking about American V8 manifolds. The configuration is nothing like the AH3000. The discussion should be on American straight 6 manifolds and other Britixh cars with straight 4 mainifolds.
Do these sesigns have fuel drain lines??
 
Hi All,

I believe Bob has hit the reason for manifold drain tubes on the button. Keep in mind that starting the Healey on a cold day calls for substantially rich mixture and the speed of condensation of the carburated fuel-mix is quite fast. Although most of the condensed fuel can be vaporized and eliminated through piston detonation in warm weather starts and/or ongoing operation, cold weather starts tend to require greater choke because of quicker condensation of the mixture. As a result, less condensed fuel is makes it to the pistons and the warmup vaporization is slow to take place.

Even though I have set my carburetors on the richer side and tend to need full choke on cold weather starts, I have only notice a very small amount of drained manifold fuel on the cardboard under the car. As such, I have never given the manifold drains much thought.

Vette's thoughts on the drains being a precaution for stuck float output seem logical. However, there are drains on HD8's float chambers that are meant to expel the majority of fuel overflow and, in combination with manifold drains addressing excessive condensation from an excessively rich mixture or even raw fuel passing through the carburetors, should be able to handle the output of the stuck float. Considering the excess of fuel present, I would expect the car wouldn't start or run quite rough.

Just my thoughts,
Ray(64BJ8P1)
 
It might have to do with the design of the log manifold. My DMD does not have them as there's no place for the fuel to puddle. The runners all angle downward to the ports. Evidently the AHspares manifold doesn't either.

Assuming the similar manifolds on the TR6 and Datsun 240Z don't have them either.
 
On hot days my BJ 7 was prone to engine detonation / pinging with the manifold drain ports plugged. Installing copper drain tubes bent to the recommended "lazy Z" pattern immediately resolved the problem but introduced another - dribbles of surplus gas on garage floor. I now remember why the ports were plugged in the first place; it was also explained to me that the drain tubes are essentially "plugged" with surplus gas which prevents substantial manifold vacuum leaks. GONZO
 
Lin, perhaps the muted pop noise emanated from the front muffler(s) where raw gas is collecting. Something worth investigating.
 
Lin said:
Every now and then when I restart the car after it has been running and is hot, upon start up, the engine revs go way up though I am not activating the choke or pressing the accelerator pedal. I typically cut the ignition off right away and wait for cool down. The car then starts as normal. The car acts as if the throttle is stuck open but as best I can tell there are not sticking throttle issues. I use a throttle cable - not the stock mechanical linkage.

Today after an enjoyable one hour drive, I stopped for fuel. When I attempted to start the car I experienced the engine racing again but this time it was accompanied by a loud pop. Different sound than a carb backfire.


Click to expand...

I've been thinking about your symptoms and, setting aside the importance or not of the manifold drains and their purpose, I don't think the idea of puddled fuel when warm explains the problem.

In order for the revs to go way up there has to be more air entering the induction tract - simply excess fuel will cause it to stumble and, if extreme enough, eventually die. For an experiment, back off the fast idle screws and run the warm engine with the choke pulled out. This would be a very rich situation without additional air...I'll bet it will stumble but not increase in revs, if anything the revs will drop.

So aside from whether you need the drains, I would be looking at how/where additional air is entering the system. You say you are cable operated and I don't have that, or any real experience with cables, but possibly throttles hanging up? Frayed inner cable?

Just some additional thoughts.

Dave
 
Thanks for the thoughts and suggestions! I will do some more exploration the first of the week. Will let you know how I come out.
Lin
 
Lin, I have had the condition of high rev for no reason happen on my Healey twice in all the years I have owned it. In Both cases the throttle was completely at itā€™s idle position and the chokes were not stuck. One time it was only a mile or two from my house just as I put the clutch pedal in coming to a stop. The other time was just after start up in my garage. I had plenty of time to investigate it but have never discovered the reason.
 
In my case it was up to around 2000 rpms. I never gave it a thought about possibly the distributor causing it. I am sure that my distributor is in good shape but lets assume that the centrifugal weights might stick in the out position this might cause it. But I do remember that I blipped the throttle many times while nosing around under the bonnet and it had no effect. You would think that if the weights had stuck out they would have released during that time. So next thought might be could the vacuum advance cause it. Well to be honest with you I have never seen the vacuum advance to much of anything on my Healey.
 
vette said:
In my case it was up to around 2000 rpms. I never gave it a thought about possibly the distributor causing it. I am sure that my distributor is in good shape but lets assume that the centrifugal weights might stick in the out position this might cause it. But I do remember that I blipped the throttle many times while nosing around under the bonnet and it had no effect. You would think that if the weights had stuck out they would have released during that time. So next thought might be could the vacuum advance cause it. Well to be honest with you I have never seen the vacuum advance to much of anything on my Healey.
Click to expand...

Just wild-a** guessing on my part. Am having a hard time with the idea that it would rev that high with the butterflies fully closed.

In Lin's case, the cable mechanism seems logically to be the most likely place to look, as that's what's changed. The rule in troubleshooting is something's either broken or it's been changed.

Vette - are you running a cable throttle?
 
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.
 
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