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MG MGB Technical - 77 MGB Running poorly
| My 77 B runs ok for the first 10 miles then I get this popping out of the exhaust and starts running poorly.If I come to a stop it will stall out and I have to pump the gas petal while cranking to get it started, Then I have to run on half choke to get back home.This started about a month ago when the weather got into the 90's.I drained the fuel tank today and filled with ethanol free gas, I took the car out today in 70 degree weather same problem.|
Other things I change or checked:
Fuel pressure 2.5 PSI, Fuel flow 3/4 quart per min.
Took the weber DVG apart cleaned and set float.
Compression 150 150 142 142
Vacuum around 20 going down the road.
New plugs,plug wires,coil and fuel filter.
Ran with gas cap removed.
Made a heat shield and put between the intake and exhaust manifolds.
I have a crane ignition and a 25d distributor that I have not touched yet.
I owned this car for the last 15 years and has always ran very well. Any comments and suggestions would be depreciated.
|Ed - 3/4 pints per minute is way too low for your car, it should be a minimum of 2 pints per minute. Your pump is either week or there is a clog somewhere in the system. See the article, Fuel Delivery Troubleshooting Guide in the SU fuel pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ for tips on isolating your problem. Cheers - Dave|
|Agree, look for a pinched pipe 'tween tank and carbs first.|
|I checked my fuel flow again this morning and got a full quart or 2 pints in 55 sec.I am checking flow at the fuel filter using a clear hose there were NO air bubbles.Then when I was attaching the hose back to the filter I noticed a kink in the hose where is routes around below the brake servo.Rerouted a new hose along the brake lines to weber, I will take it for a test ride later,we have rain coming in now.|
Thanks for your suggestions.
| Just got back from a test ride, Same problem, ran good for six miles then it started popping out the exhaust, ran on half choke to smooth it out.Then it died when I came to a stop sign, Took the gas cap off, waited a few minutes and had to pump the gas petal while cranking finally it started.I rode back home on half choke and no gas cap then it stalled out again by my house.|
|Does sound like a fuel blockage. Are those first few miles, when it runs well, on the choke? I would almost be tempted to rig up and auxillary fuel tank, and run a new line to the carbs see what would happen? Of course there is risk. If you have a flake of rust, or sediment somewhere in the line you may need a new line anyway. Just another thought, when you grind to a halt, after your few good miles take the float chamber lids off, see how much fuel there is in the chambers? Mike|
The first six to ten miles when running good the choke is off, Then I start to hear that popping noise out of the exhaust.
I have the lid off the weber now and checked the float level 41mm & 51mm.
Tomorrow I am going to take it for a ride and when it starts acting up I will pull into a parking lot and when it stalls turn the key off and pull the lid on the weber and check the level of fuel.
|Aaah, a weber carb on the car. I was thinking it was a pair of Su's!I did not pay full attention to your specs. Now webers have an idle circuit and an off idle jet then emulsion tubes and main jet, is there a blockage in one of these circuits? But check the level anyway after the run, if it is ok, or up to spec then suspect some dirt somewhere it the carb system. Mike|
|On the same lines, is/was the bottom of the float chamber clean?|
| Yes the float chamber is clean.I just ordered a weber rebuilt kit and power valve so it will be a few days till I get back together to run.The rubber on the power valve was in bad shape, Probably from the ethanol in the gas. We have 4 gas stations in town that sell non ethanol gas and that's what i will be using now.|
To find non ethanol stations in the USA go to
| I rebuilt the weber yesterday, Ran it at idle for 5 minutes then took it for a ride. It ran great for the first 5 miles then the same problem, Popping out the exhaust and stalling out. When I got home I started looking for a different source causing my problem and noticed the coil was real hot.Last night I went on PaulH Solihull web site and read about Ignition Coils. I found out that I have ballast wire and using a 12 volt coil. I changed this coil out a few months ago when I first started having problems with the car, The original coil that was in service since owned the car (15 yr.) was also a 12v coil (3.5 ohm). I disconnected the ballast wire and the wire coming off the starter from the + terminal on the coil then ran a wire from the fuse block to the + terminal. Took it out for a ride went about 20 miles with no problems.|
I may change the coil to a 6 volt coil at another time.
|A long shot but check the vacum advance thing on the distributor - I have just noticed on mine that the heater valve is leaking down onto the dis and the vacum unit is getting a greeny look to it from the coolant - it would not surprise me that some residue is working its way down the membrane inside - I too have a weber and runs terribly at the moment...another job to do very soon|
|If you're running a 25D distributor, a 12 volt coil should work just fine as long as its operating voltage is restricted to 9 volts once the engine is running. Otherwise, the points will "flash" over and cause a reduction in the voltage that reaches the coil. This will cause the engine's power to fall off dramatically. RAY|
| The vacuum advance is working and no coolant leaks.|
I have a crane xr700 that the previous owner installed.The red +12v wire is connected to + terminal on the coil which is only 9 volts with the ballast wire connected to coil. I am going to order a crane PS40 coil(1.4 ohm)and use the ballast wire and connect the red +12v wire to the white/brown connection on the fuse box.
What should the spark plug be with is setup? I have them set at .035 now.
Thanks for your suggestions.
|If your distributor has a set of points (non electronic, pertronix etc.) check your condensor. When cold, the condensor works fine until it gets hot (in about 10 miles out) At that point it begins to fail thus creating the symptoms you described. Also, check to see if your coil wire is fully seated in the coil and distributor cap.|
Keep us posted
|A 12v coil operated in series with a ballast will run *cooler* than either a straight 12v coil, or a 6v coil with the ballast, as the current flowing is significantly lower, giving a weak spark. However if the dwell is too high i.e. points gap too small the coil will get hotter than it should. As the points are usually closed when the engine stops normally, there is constant current flowing through the coil when the ignition is turned back on, and that *will* overheat the coil. Bypassing the ballast seeming to cure the problem would tend to indicate that there is an intermittent connection in the ballast circuit, but that would be revealed by the tach jumping up and down when the engine started misbehaving. If the tach is steady then the ignition primary circuit is fine, with the possible exception of the condenser. |
"a 12 volt coil should work just fine as long as its operating voltage is restricted to 9 volts once the engine is running"
Very much incorrect. 25D4 distributors run 12v coils at 14v on all chrome bumper cars. It's only 6v coils that need the running voltage reduced once the engine has started, and that is performed by the ballast resistance being put into the circuit. The distributor used is irrelevant, although the distributor changed from 25D4 to 45D4 when the ballasted system was introduced for rubber bumper cars. The points don't 'flash over', but they are carrying double the current so tend to burn out. Ballasted coils running at 9v is also a misconception, the ballast is about the same resistance as the coil, so when the points are closed the coil is operating at 6v. When the points are open, and the coil isn't drawing any current, there is 12v on the +ve terminal. If you monitor this point on a running engine because the points are open about 50% of the time you *will* see about 9v with an averaging meter such as an analogue instrument, i.e. mid-way between 6v and 12v or more correctly 7v and 14v, but that doesn't make it a 9v coil or a 9v system.
Ran the car yesterday afternoon for a few hours, made a few stops at some stores, It started right back up with no problems.
1.8 ohm is the resistance of my ballast wire, My tach gives a steady reading. I dug up some info on crane xr700 ignition that I have in my 25d distributor.
The XR700 unit is designed for ballasted ignition system, a needing 1.6-ohm coil and 1.6-ohm ballast resistor. The electronic unit normally runs on 6-7 volts and tolerates a momentary 12 volt input during crank starting (same as the ballasted ignition coil).
Looks like I need to keep the xr700 red power wire on the + side of the coil rather than the fuse box and change to a 1.6 ohm coil.
|The coil has a spec tolerance of 1.43 to 1.58 ohms and I'd expect the ballast to be of the same order. 1.8 is a smidgen high but may include a couple of connections, but shouldn't cause any issues.|
You need to be vary careful what you read on the web. For instance this site http://www.nonlintec.com/sprite/ignition/ says the XR700 "does require a ballast resistor in the primary circuit; if your car does not have one, or lacks an internally ballasted coil, you will have to add one. As it happens, my coil is internally ballasted. This can be determined by measuring its primary resistance, approximately 4 ohms." http://www.vclassics.com/archive/cranprn.htm says the same thing
An external ballast has a completely different effect on the ignition module to a ballasted coil. With an external ballast the module is connected between it and the coil, and hence does see 6v - but only when the coil is energised, it will see 12v when the trigger generates the spark, so the statement on the MGA site that "the coil and electronic unit run continuously on 6-7 volts" is not correct. For a ballasted coil the module is connected to a full 12v all the time. The concept of an internally ballasted coil causes great confusion. A coil (for an MGB) is either a 6v coil of about 1.5 ohms or a 12v coil of about 3 ohms. Whether the 12v coil is made up of a 1.5 ohms winding plus a 1.5 ohms resistance inside the case making 3 ohms in total, or 3 ohms of winding is irrelevant, it is still a 3 ohms 12v coil and an electronic trigger connected to it will be running at 12v (actually 14v) all the time and is what at least one person has done with no mentioned problems. It would be interesting to see what the original information said, I suspect reference to a different coil and ballast is just a marketing ploy.
Good explanation about a external ballast connection vs a coil with a internal ballast.
I connected the ignition and coil back on the ballast wire as it was originally and changed the plugs from .035 to.25 gap.Took it out for a long ride without any problems. When I shut the car off and tried to restart I had to crank for about 10 seconds to get it to fire back up. I am going to order a crane PS40 1.4 ohm coil,It comes with a ballast resistor that I will use instead of the original ballast wire.
This thread was discussed between 19/06/2011 and 27/06/2011
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