Running Lean???

[40phil41]

Another issue with my '40 this past summer was that it seemed to always be running lean. When hot and idling, if you place your hand partially over the carburetor the engine actually speeds up; like it was getting too much air before. Still can't get the idle adjustment down below about 800 R.P.M. and keep it running. I do plan on removing the intake/exhaust manifold this winter and check the surfaces for tru and replacing the gaskets. Also, the heat riser is just flapping in the breeze (I think the spring is broken) so it will get some attention. The carburetor is (was) a N.O.S. unit (Stromberg) that I installed a very long time ago when the car was under construction. Any thoughts?

Phil

[Brad Lustig]

Somebody correct me if I'm wrong, but I think just by placing your hand over the carb, you effectively lower the amount of air AND fuel since the fuel is added based on the amount of air going through the carb (sucked into the throat by the venturi). I may be thinking about it incorrectly. How do your plugs look? I would bet that your engine is speeding up because of a dashpot squirting extra fuel into the carb. Have you tested it for vacuum leaks?

[Plymouthy Adams]

Brad..when you place your hand over the carb the restriction actually are now drawing fuel through both the idle and the choke passage..

[40phil41]

It's been a while but I did take a vacuum reading and it was steady at about 8 in. Hg (I think). I also took some starting fluid, based upon someone elses suggestion, and squirted it around the perifery of the intake manifold and it had no effect. Later also tried the same trick using WD-40. Same result. It was an old time mechanic who said that the engine was running lean based upon the hand test. Doesn't mean he is right. I don't really know. The only thing certain is that I cannot get the idle down to factory spec. (450 r.p.m.?) without stalling. The last time I looked at the plugs they were a brownish colour.

P.S. I knew that this question would elicit many opinions - slow computer day :-)

Phil

[Guest rockabillybassman]

Gee, 450 seems very low. Perhaps I'm used to modern engines idling at 650.

[40phil41]

Just going by what is stated on the main P15_D24 Web site under tune up specifications. Nevertheless, it really doesn't want to run at 650 r.p.m. either.

Phil

[Ed Griffin]

The way I was taught to check for a vacume leak other than whats been mentioned was to simply put your hand over the carb and seal it where no air gets in. If it ran then you were getting air somewhere else, if it died no air. Dont know if this holds true to these cars but it worked well for a straight 6 300 I piddled with for 12 yrs and probably a couple other engines that had carb's. With your hand over you'll also increase the sound of where the air is leaking thru sometimes.

[40phil41]

Thanks Ed. I'll try that the next time I have it running.

Phil

[bob_amos]

8 inches of vacuum is real low. It should be about twice that reading at warm idle.There are a number of things that could be causing that. Retarded timing, both ignition wise as well as cam timing. Also, a vacuum leak somewhere would cause a low reading. The hand over the carb throat is correct to a point. If it is running lean it will increase and rich will cause it to drop in RPM. But a partial covering is all that is used. Remember that fuel flow into the venturi area is a result of air flowing past creating a vacuum that draws fuel through the ports and into the carb. (Outside air pressure is now higher than within the carb and that higher pressure is actually pushing the fuel into the carb.) Low fuel levels will cause poor performance too. To test for a vacuum leak try a squirt bottle full of water. Now, after fully warmed try spraying the water in areas where leaks may occure. Such as the carb base, throttle shaft, intake manifold to cylinder block and vacuum lines. If a change results when you apply water to a certain area take more time to investigate that area. Once you find the leak reseal it and redo your vacuum test for proper readings and proper idle control. When the idle is set to the optimum 14.7 to 1 ration you will find a slight increase in idle speed when you try the hand over a portion of the carb inlet opening.

[40phil41]

Bob,

I also thought that 8 in. hg was too low but I can't find a spec. for this measurement. I would have thought that water would quickly evaporate before having any effect on the vacuum leak. As mentioned above I did previously try different fluids/oils. I'll give the water a try and see what happens.

Thanks,

Phil

Phil

8 inches of vacuum is real low. It should be about twice that reading at warm idle.There are a number of things that could be causing that. Retarded timing, both ignition wise as well as cam timing. Also, a vacuum leak somewhere would cause a low reading. The hand over the carb throat is correct to a point. If it is running lean it will increase and rich will cause it to drop in RPM. But a partial covering is all that is used. Remember that fuel flow into the venturi area is a result of air flowing past creating a vacuum that draws fuel through the ports and into the carb. (Outside air pressure is now higher than within the carb and that higher pressure is actually pushing the fuel into the carb.) Low fuel levels will cause poor performance too. To test for a vacuum leak try a squirt bottle full of water. Now, after fully warmed try spraying the water in areas where leaks may occure. Such as the carb base, throttle shaft, intake manifold to cylinder block and vacuum lines. If a change results when you apply water to a certain area take more time to investigate that area. Once you find the leak reseal it and redo your vacuum test for proper readings and proper idle control. When the idle is set to the optimum 14.7 to 1 ration you will find a slight increase in idle speed when you try the hand over a portion of the carb inlet opening.

[greg g]

normal vacuum at idle with a single carb should be 18 to 20 inches steady.

Where ae you conecting your vacuum gauge?? Should be on the intake manifold, vacuum port for the windshield wipers is the easiest and probably most accurate source.

[bob_amos]

The water is a diagnostic aid we have used in the industry for years. I was taught to use water way back in '67 and it had been used years before that. You are not misting it you are using a steady and strong stream of water. It will be pulled into a vacuum leak and help to temporarily seal it allowing the idle to change. As to placing the vacuum gauge hose. It needs to register manifold vacuum and not a source above the throttle plate. If it is from the wrong source such as vacuum advance the reading will be useless.

[Ed Griffin]

Bob, using the half open method over the carb the way you described it sounds like a good method for checking if it's lean or not but does not sound like it is for checking the pull of the vacume of the carb and covering it completly has always shown me that if it wants to shut down then it is showing that it has no leak. The half cover also works like a choke and this is why you get the rich variation in the carb when done that way. I usually try to take my hand off before the vehicle actually dies but I've used this method with known vacume leaks and once sealed you would kill the engine by using a full cover over the carb.

I used various ways to spray for vacume leaks over time and starter fluid always seemed to show a leak quick. You just cant go crazy with it because it is so flammable. WD-40, Windshield wiper fluid etc all do the same thing once they find the leak. If you have a very clean engine and have to worry abt keeping it that way you dont want to use any petrolium based chemicals but if you dont have to worry abt it then any type spray does the same job. Bad thing abt using water if the manifold gets too hot is cracking it with the hot cold effect. But thats if your using too much water. This is just my 2 cents from a shade tree wannabe

[40phil41]

FYI, my '40 has electric windshield wipers; from the factory. So, there isn't a vacuum connection for the WW's. I did the vacuum test at the carb where the vacuum advance is connected. From the above, this isn't a good spot. However, there isn't any other connection to use that I can see. Suggestions?

Phil

[Ed Griffin]

Even without vacume wipers what about the plug on the back top of the intake? There is usually one in that location as Greg pointed out where you hook into the vacume for your reading. This is the spot Greg and Bob were talking about getting your reading. If you have one you cant miss it because it's the only one on top of the intake and if memory serves me correctly it is a square instead of octagon.

I re-read your first post and you said you had a NOS Stromberg for your 40 engine. Which Stromberg is it and is it the correct one for that motor? I'm just wondering if your reason for not being able to ge it to idle down is that you may have the wrong size carb/ jets etc. Whenever I would get a vacume leak the vehicle would idle erratically, stumbling, shuttering.

Two vehicles I've had that wouldnt idle down far enough had carb problems. One of these was a early 80's Plymouth champ with the mitsubishi 4cyl. I could never get it down to spec but it needed a new carb and they wanted way too much for me at that time and I just drove it with a mildly high idle. not saying you should only maybe it is the carb and not a leak of vacume.

[40phil41]

Ed,

Very good question about the carb. Actually, the engine in my 1940 Dodge is a D24C (1946-48, Cdn. 25 in. block) but the carb. is the 1940 Stromberg (BXV-3). Could this be the problem? What carb. make/no. came with this D24 engine?

P.S. I'll have a look for that vacuum plug.

Phil

[Guest bizzycoupe]

If you are not running fluid drive, you have the right carb, If it's fluid drive it shouls be (bvxd-3)

[greg g]

The carb vacuum reading is not what your are looking for. You need to tap into the intake manifold to get a proper indicative reading.

[bob_amos]

About the water cracking the manifold. Well, I just use room temp water and have done this for over 40 years. It was taught to me by an old guy that was using the method for years before that. I have never, or even heard, of cracking the manifold from doing this... Look at it this way... when driving on a rainy day, does the COLD water that splashes up into the engine bay ever crack a manifold? And believe me, that rain water or snow slush is much colder and in much higher doses than the water that is sprayed on the manifold. Plus, and this is a big plus, water is always safer than any of the alternatives that people use. So.. don't be conserned about damage from the water. As a professional auto mechanic of 40 plus years this is the safest way to test with liquids.

I agree with you about the total cover up of the carb showing the ability to pull a vacuum. I have always used a vacuum gauge to get the most accurate reading. But if one is not avail. I too do the carb total cover thing. It would take a pretty big leak to allow the engine to continue running this way though as well as the fact that once you stop air flow past the venturi you also stop the fuel flow... Fuel flows due to the vacuum effect created by air flowing past the restriction of the venturi within the carb throat.

It's good to see that you guys are really putting some thought into this subject as it is one of the biggest misunderstood things about the basic operation of an automobiles internal combustion engine. Even some professional mechanic don't really understand how this operates. I had been an automotive instructor for many years when I worked at Chrysler dealerships and had many students never really grasp the vacuum operations and fuel flow withing the carb and engine.

[Jerry Roberts]

Sometimes an unlit propane torch is used to find a vacuum leak . Listen for an increase of RPM s .

[grey beard]

I've found vacuum leaks successfully by spraying WD40 or starting fluid around the suspedted leak area. Engine will pick up speed when it pulls this stuff into the manifold. Water sprayed into the carb has traditionally been our fix around here for removing carbon from combstion chambers. Sure does a good job when head gaskets leak, not?

Also, I believe if you check that you will find that every healthy flathead will speed up from hot idle speed when you partially hold your hand over the air horn. You're enrichening the afr and the engine should respond to this treatment. I've also noticed that this is more pronoumced or noiceable when the timing is retaded too far towards BTDC. JMHO:)

[grey beard]

I've found vacuum leaks successfully by spraying WD40 or starting fluid around the suspected leak area. Engine will pick up speed when it pulls this stuff into the manifold. Water sprayed into the carb has traditionally been our fix around here for removing carbon from combstion chambers. Sure does a good job when head gaskets leak, not?

Also, I believe if you check, you will find that every healthy flathead will speed up from hot idle speed when you hold your hand partially over the air horn. You're enrichening the afr and the engine should respond to this treatment. I've also noticed that this is more pronoumced or noiceable when the timing is retaded too far towards BTDC. JMHO:)

[40phil41]

It does not have fluid drive.

Phil

If you are not running fluid drive, you have the right carb, If it's fluid drive it shouls be (bvxd-3)

[bob_amos]

Yes, the propane enrichment is also a good way to test for leaks. At Chrysler in the '70s and '80s we used this method quite often to set idle enrichment. Of coarse it was being used generally for different a reason but we would cheat and use the propane for vacuum leak tests as well. But do be careful as it is very flamable, but does a good job though.

Water is great for cleaning out some of the carbon in an engine. I still use this on lower compresson engines.

While we are on the subject of covering the carb throat. We use to perform a thing called "The 98 cents tune up". What we would do is rev the engine up to a high RPM, one the engine could safely handle, then we would plug off the carb throat until it almost died. We would repeat this as many times as needed until we got a better idle. What was happeneing was the high vacuum was pulling large amounts of fuel through all of the carbs passages and with it would come any build up inside these carb passages. If it was not to terribly plugged up the car would now idle and perform much better. Of coarse, if you don't remover your hand in time the car would stall. Restarting would be a little tougher as all of that extra fuel was now flooding out the cylinders. Eventually it would clear and restart and you would continue doing the process until all was as clean as it was going to get.

[40phil41]

Just had a look at my intake manifold and it does not have the little plug shown in your photo. The plot continues. Where did my intake manifold come from? Who knows!

Phil

Manifold plug on my '51 by the #5