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MG Midget and Sprite Technical - Ballast Resistor...Why??

Hello all,

After permenantly fixing short (Strange electrics post...thank you everyone), I need to turn my attention to the newest "issue". I can rev the engine up smooth as silk in the driveway, goes up and comes down to perfect idle without even a burble....BUT it has a habit of hesitating and slight popping thru the carb from about 2500 rpm to about 3500 under heavy load. Very slight accel is smooth, but can also get you killed trying to get on the highway. Above 3500 or so, it wil cruise withour complaint up to the max I cruise which is about 4200-4500 revs. I've been runnung a 12 volt coil without a ballast resistor (what does it do?) for the last ten years without a problem. Weber DGV is adjusted fine, timing is 2 degrees ADC as per spec for a US 1500, valves are right, wires, rotor and cap are good. Vacuum advance plugged years ago, but mech. advance seems to be working fine... Coil breaking up perhaps?? Thanks, in advance!!!

Al

ah harad

Your problem is nothing to do with a ballast resistor it is in fact due to a weak mixture.

Ballast resistor coils were developed to aid starting. Typically when we crank an engine to start it the battery voltage will drop from a normal 12 to about 9 or 10 so a 12 volt coil will therefore only see this reduced voltage and will have a reduced spark

With a ballasted coil wired in the coil only needs to see about 8 volts to perform well and this is produced by the ballast resistor dropping the voltage to the coil in normal running to about 8 volts. Now when the engine is cranked during start up the ballast resistor is by passed and the normal cranking voltage 9 or 10 volts is applied directly to the coil. Thus it performs at peak power during start up..

Hope this is all clear.
Bob (robert) Midget Turbo

Bob,

Ahhhh...I always wondered what it was for. Now I've got to solve the "weak mixture" problem. I tend to agree with you. The problem is how to fix. Adjusting the mixture on the Weber produced little difference. I suppose I could re-jet, but i'm not sure. Did I also mention that my mileage has dropped to a whopping 15mpg?? May be I should trade it in for an SUV.

Al
ah harad

Al,

IIRC a fixed venturi carb like a weber only allows the idle mixture to be adjusted with the mixture screw, higher range adjustment requires changing of the jets to suit the set-up. On a SU when you alter the jet height you alter the mixture throughout the range but the needle profile still has to be correct for the set-up to get the mixture right throughout the range. It sounds like you need to learn about weber set-up or take the car to a rolling road and have it jetted correctly.

15mpg would seem to indicate that it is running rich somewhere or you have a fuel leak.
David Billington

Bon,
Close but no cigar. You did a yeoman's job of describing the case for eternally-ballasted coils, but that is only half the picture. Various physical laws governing electromagnetism decree that as the primary voltage goes up, the amount of time required to properly saturate the coil also goes up. As a result, in order to get nice hot sparks at high rpm, it is good to keep the actual working voltage of the coil down around 9VDC. This wasn't a problem with the old 6-volt electrical systems, but 12-volt sytems had issues. All coils used on 12-volt contact-point ignitions are ballasted. Those on our LBCs are internally ballasted. Chrysler always used to use a big old ceramic external resistor. Chevy tended to use a high-resistance wire. As Bob indicated, the point of an external ballast was that you could bypass it to get a nice hot spark at cranking speed. An ohm-meter can usually tell whether the coil is internally-ballasted; if you get about 3 ohms across the primary circuit, it is internally ballasted. If you only get about 1.5 ohms, it expects to see an external resistor. Likewise, if you turn on the ignition with the coil in circuit and measure full battery voltage across the primary, the car expects an internally-ballasted coil. If you see about 10 volts, the circuit has a resistor in it and the car wants the externally-ballasted coil.
David "doubly pedantic" Lieb
David Lieb

David/David,

I'm the first to admit that I'm no Weber expert! This very same setup has worked flawlessly for at least ten years. I bolted the manifold onto newly re-built engine and adjusted idle mixture to spec. It ran fine during inital break-in. Why would a jet "go bad". Other than being blocked (it isn't) it just a "fuel portal".
Accelerator pump and diaphram also look good. If the mixture is weak, then why the terible mileage? If it's rich, then at least I would have some performance. There is no "black smoke",no fuel smell, and no leaks. That's why I'm leaning toward some other source...not trying to be difficult, just trying to figure it all out:)

Al
ah harad

Hmmm that's interesting David not sure I can come to terms with the theory so I must take your word for it even though to get a High (hot) voltage spark on the secondry a higher input voltage ought to be desirable but electricity is a strange force indeed.
Certainly being able to bypass the ballast resistor has helped starting for many many years and is a brilliant by-product!! :-))

Al, as tother David says a weber is adjusted by changing the jets and air correctors and such. The mixture as you describe it, simply adjusts the idle mixture. Without access to the car I can only tell what is the usual case. If an engine revs well but stutters slightly when "cruising" then it is 99 percent always a weak mixture. The weber struggles more than other carbs at the cruising point because it is at the change over point for the weber between 2 jets, the pilot and the main, and it is this point that requires fine tuning mostly on a rolling road but can be dialled out by trial and error if you have the parts.
15MPG is very low and suggests something is very wrong perhaps not even the carb but if the carb was at fault I would be looking at the float level, if this was not set correctly it would be allowing fuel straight into the engine at an alarming rate.
Bob (robert) Midget Turbo

Bob,
Yes, higher voltage in the primary will yield a higher voltage in the secondary. The issue here is that the ability to saturate that coil 400 times per second (6000rpm in a V8) is also affected. lowering the primary voltage allows for higher rpm before the spark voltage dies off. This is also the reason for dual-point systems to lengthen the dwell to improve high-rpm sparks as well as systems like the old DKWs that used 3 coils.
David "wealth of useless information" Lieb
David Lieb

Al. A couple of thoughts on the Weber. Robert is correct that, normally, a weak mixture will cause a stumble on acceleration because the mixture does not burn efficiently. That is why the Weber, and others like it, have an accelerator pump and a power valve.

The power valve is a device to add additional fuel when the vacuum in the intake manifold drops. It is the spring and plunger item that is located on the underside of the portion of the carb top which covers the reservoir. The brass fitting below it, with a plunger sticking upwards, is the valve itself. Vacuum operates on a diaphragm which holds the spring loaded plunger off of the power valve. When the diaphragm ages and looses its ability to keep the plunger out of contact with the power valve, significant amounts of extra fuel are fed into the carb. This results in low fuel mileage and a constantly, excessively rich mixture. When the accelerator pump injects even more fuel into the system, you have a similar effect to a weak mixture--a mixture which does not contain the proper portion of fuel/air to burn correctly.

The new Weber rebuild kits, made by EMPI, do contain the parts to rebuild the power valve system. Earlier kits, with Weber brand on the package, do not. It also contains the replacement parts to rebuild the accelerator pump system which can well be bad after ten years.

Another problem with the Weber DGV is the brass tubes which go across the upper part of the venturi. These tubes can back out of the fitting slightly and expose the open end of the tube to the vacuum generated within the venturi. These tubes have a small opening, midway, in the tube with meters fuel into the system. If the tube backs out of the holder a little, the open end allows significantly more fuel to enter the system than the system was designed for, causing an excessively rich mixture to be present.

I have seen one bad power valve system and two bad fuel supply tubes back out. If you do have an excessively rich mixture, excellent idea to change the oil and replace the filter to prevent engine damage due to any unburned fuel leaking down the cylinders. Do not know if this is a problem with more modern cars, but it has been a common practice with older cars running an excessively rich mixture.

The Weber DGV is similar to the design of the Holley carbs and any mechanic familiar with the Holley can work on the Weber. In point of fact, the Holley 5200 carb is a licensed version of the DGV and was used on several American cars in the 1970s.

As to the coil, they are all, today, marked 12V because everyone uses a 12V system. The information David has provided is correct, along with what Robert has said. A 12V coil marked "for use with an external resistor" is designed to operate at a constant 6-8 volts input and shows a resistance across the primary terminals of about 1.5 ohms. The original wiring had two connections to the positive side of the coil, one coming off the starter, providing a full 12V input and only active when the starter is engaged, and a second wire, with reduced voltage, that comes off the white wire circuit of the ignition switch when in the "run" position. It would be of use to know how your current system has been wired as many have changed over the years.

As to the distributor, if possible, check the mechanical advance curve across the various engine speeds to see how closely it approximates the specification listed in the factory workshop manual.

Les
Les Bengtson

Les,
Thank you for the detailed response. I'm going to check the tubes above the venturi as you suggest. The accel pump and diagram are fine. I checked the oil, and it is clean and "feels" right, but a change after I solve the problem sounds worthwhile. What I did notice, however, is that the car runs beter when the choke is pulled out almost all the way, enriching the mixture. This would lead me to think there may be a vacuum leak. However, when I cover the carb with my hand, it immedeately shuts off without revving up first (the classic vacuum leak indication). So I think if anything, I'm running way too lean..still doesn't explain the terible mileage though, unless I'm missing something.

Al
ah harad

Well, I decided (if anyone cares) that since "necessity is a real mother...so I'll invent something"...or something like that. Opened the Weber again, took out the primary jet (140), put it in a padded vice, took a twist drill and made the sucker bigger!...That's right, folks, I made my own size. I figured if it didn't work, then I haven't lost anything. I could always re-build. But whattayaknow, it worked! The cars runs superlatively! Drove around and topped off the tank and I'm getting in the high 20's (city) again. Go Know!

Al
ah harad

Al,
Now you have to tell us which number drill you used for the sake of the archives.
David "inquiring minds" Lieb
David Lieb

David,

The drill size was 1/16". Just for the record. Regards!

Al
ah harad

Al. Was it the primary jet or the jet for the secondary venturi you modified? I have been running the 140 jet, without modification, in the primary venturi and a 145 jet in the secondary on my Bs.

It is still worth checking that the power valve activation mechanism is in good working order. If, when you look down the primary venturi, at idle, you see fuel dribbling into the venturi there is a problem. It should be such a fine mist that you do not see the actual fuel. You will see some squirt into the primary venturi when you "goose" the throttle as the accelerator pump enrichens the mixture. The power valve, however, cannot be tested without removing the carb cover plate and using a vacuum pump to observe its operation. This is a two person job, demonstrated to me by am American car mechanic who, as a friend, sets my Webers prior to emissions testing.

Les
Les Bengtson

Les,

It was the primary. New issue, though...the car won't idle. Runs through the rev range well, but I have to keep it idling with the throttle. When "re-jetting" the primary, I unscrewed the jets from the top to check them. I believe these are part of the idle circuit, because I may not have seated them properly. That's when the problem started. BTW, as usual, I took your advice, fuel flow looks as you described...except now without an idle. Car simply stalls out.

Al
ah harad

BTW, this morning, in response to Tarquin's inquiry about starter solenoids, I realized that the 1978-on midgets, as well as the rubber-bumper MGBs use an externally-ballasted coil with a WLG lead from the solenoid to bypass the ballast while cranking. I realize that this is no longer germaine on this thread, but I thought I would mention it for the sake of the archives. It means that earlier solenoids should not be used in the later cars and that the coils are not interchangeable.
David "wired funny" Lieb
David Lieb

This thread was discussed between 09/03/2008 and 22/03/2008

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