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MG MGB Technical - Distributor Advance problems

I have just started noticing acceleration problems with my ’70 MGB. The car has been in perfect tune and running fine, carbs working great. Just the other day while out hiway driving I noticed a loss of power and very ruff acceleration. While parked in the driveway the car starts and idles easily. I can even gun the engine but at around 3000 rpm the engine runs ruff and will not rev over 4000rpm. When I drive the car and put any demand on the engine, fast acceleration or going up a hill, the engine loses power and runs real ruff. I have replaced the spark plugs, coil, points, condenser and the distributor cap. Wires are good. Still easy to start and idle but under a load no power. Can this be an advance problem with either the vacuum or centrifugal advance? How can I tell?
Banderso

Hi.

If you suck on the vacuum pipe you should see the distributor baseplate moving.
The advance mechanism can be checked by manually turning the rotor arm.
Insulation breakdown in the HT circuits can give this symptom.

I would suspect fuel starvation, possibly due to blocked tank ventilation.
Does removing the filler cap solve the problem with the car on the drive?.

An air leak in the manifold area or a cylinder-to-cylinder head gasket leak are other possibilities.

What colour are the plugs ?.

HTH.. Don


Don

I have sucked on the vacuum hose but only saw minimum movement - 1/8 of an inch counter clock rotation. I can force it to rotate further by hand but my suction did not make move very far. Placing my finger over the vacuum hole on the carb, I can feel some suction when I rev engine in park. Any other means to check distributor advance? I've also checked the fuel flow going into the carbs with fuel pump on - plenty.
banderso

Not an advance problem, and please don't force the vac movement =it is hard on old diaphragms! What you see is about right, you would have a little more if you connect a vauum pump to it, and the movement is clockwise. CW movement is very slow, so you see the CCW snap back when you release it. Since you replaced all those parts, and IF the points gap/ dwell is right, this is probably a fuel problem.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Your right I think the movement was clockwise. If this is a fuel starvation problem and fuel pump seems to be delivering a sufficient amount of gas to the bowls, where should I be looking?
banderso

The problem could be anywhere from the fuel tank to the carbs. If you have a fuel filter, replace it. Tune your carbs and adjust your engine timing as needed. If you can afford it, have your carbs professionally rebuilt, if necessary. Replace all rubber fuel lines and flush out metal ones. Make certain none of the metal lines are kinked, and replace as needed.
Jeff Schlemmer

Go to my website, www.custompistols.com/ and click on the MG section, then articles. David DuBois has a tech article on the SU fuel pump and lists the proper fuel volume and pressure specifications (and, he has cross checked these with Burlen to make sure they reflect modern production). If the fuel pump shows proper pressure and volume, both the pump and the system up to the carbs is working correctly.

Rough running can be caused by several different things and you need to troubleshoot the problem in a systematic manner to see what is working correctly and what is not working correctly.

You might watch the tachometer while reving up the engine to see what it shows. If it dips down towards zero, it might indicate a fault in the low tension circuit of the ignition system. There have been some bad Lucas condensors in the parts systems. But, check the fuel pump and system, check the carbs (are they set up the same and syncronized), check for air leaks, then go back to the ignition system. Post what you find if you do not discover the problem. Les
Les Bengtson

As FR and others suggest, it is possibly a fuel problem. One of them can be water in the fuel. Water from condensation, water that finds its way into stored fuel at your gas station or tank farm. It often manifests itself in a manner such as you describe, tho not always. Sometimes the simplest things are the easiest to overlook. Gas line "drier" simply allows the water to mix with an alcohol so that part of that mixture burns rather than your engine conking out on you altogether.
Bob Muenchausen

I have thoroughly checked out the fuel pump ( AC Delco Ep42s) and by all indications , It's working fine. Delivers fuel at the cards with plenty of pressure and flow. And I understand that with electronic fuel pumps they either work as advertised or they are completely shot. When I look directly into the throat of the Carbs at 3K -4K rpm I can see lots of gas being sucked into the intake.
Could the loss of power and rough running at 3K rmp and above be due to an air leak between the carbs and the intake manifold or somewhere else?
banderso

Have you tried the gas drier as Bob suggested? It's easiest and may work. Attempt 2 is: Remove air cleaners, get the engine up to 3000 or so, and block one carb with your hand, til it nearly stops, repeat a couple of times on each carb. If it is a small amount of water this will suck it through.
I have had cars act like this due to water or rust in the float bowl. It just sits there at idle, but at higher speeds it sucks into the jet and blocks it. YES, that's right BLOCKS. Water forms a big goober which fills the space between needle and jet, which keeps gas from passing. When demand drops, it falls down to the bottom, and all is well until it gets sucked up again. Fine silt or rust will do the same. Remove the float tops and floats and have a look. It takes about 1/4 teaspoon of H2O to do this. Soak up the fuel with a rag, and get it clean. I remove the suction chambers and carb pistons and blow through the jet to clear it all. Had a Jag with a rusty tank that did this about every 1000 miles til I took the tank out and fixed it.
FRM
http://www.usachoice.net/gofanu
FR Millmore

I had a similar problem. The car would do 45mph just fine but sputtered if I tried going faster. New fuel pump fixed it. When you say your fuel pump has plenty of flow, did you actually time it and measure the amount of fuel per the spec. Remember, there are a frightening number of comments these days regarding NEW parts that are defective, right out of the box. I've had a BRAND spankin NEW bad coil, dist rotor and speedo cable.


Also, here are 2 comments from the archives:

The problem could be a bad connection between the points and condenser

I second the condenser theory. My car would start and idle beautifully, but miss the faster it went, new condenser fixed it right up.





Fred H

I 've just added the fuel drier to the gas tank. Will start car in the morning. I have replaced the rotor, dist. cap, points, spark plugs and coil. All new parts perform the same as before this problem started except now the car starts easier than before. Idles beautifully. The fuel pump filled half a mason jar in 3 seconds. I can see plenty of fuel being sucked up equally from each jet at 3k rpm but still running very rough and no power under a load.
banderso

Yes but at 3K no load you are not pulling fuel at the rate you would be on the road. Or have you fastened yourself on the fender whilst driving? It is also worth removing and carefully cleaning the suction chambers/pistons from the carbs. One piston sticking due to a speck of dirt, or a lot of build up deposits, can do this very nicely. Good to do this while checking the float bowls for garbage.
PS, the fuel drier needs to get into the carbs to soak up water there, so don't expect instant cure.
FRM
http://www.usachoice.net/gofanu
FR Millmore

I've just uncovered a very interesting aspect of this problem: while the engine is running rough at 3K in park, if I pull out the choke almost all the way, the engine smooths right out. Does this tell us there's an air leak? Where should I look? Gasket between cabs and manifold?
banderso

It indicates that it is probably running lean for some reason. An air leak from a bad gasket would likely have a greater effect at idle than higher up. Problems caused by leaks of this sort are due to high vacuum and small airflow through the engine at closed throttle; at open throttle the vacuum drops and the flow increases, so that small air leaks have little effect.
You most likely have a fuel problem as I've been saying; Do the checks listed. The dirt/rust in jets will behave in this manner; pulling the choke drops the jet down so that the garbage doesn't block the fuel flow. Eventually it will either block up completely, or (rarely) clear itself. The hand on carb trick may work here.

If you check the float bowls, blow out the jets, and clean the suction chambers/pistons without finding a problem, then you may want to go back to all those new bits in the IGN. At this speed on the road, there is starting to be a good load on the engine, and bad ign components may start showing up. The richer mixtures are a bit easier to fire and can aid a shaky IGN.

There are lots of more esoteric things, but no point in discussing it till you check what we have given you so far.
FRM
http://www.usachoice.net/gofanu
FR Millmore

I've done all the checks suggested; float bowls check out fine- no blockage due to dirt or rust and no water evident, blew out the the jet lines back to the bowls with the jet/piston removed. Suction chambers & piston working fine. Fuel drier added. tank toped off with high test gas. Drove car to gas station and still the same rough running and no power at 3500 rpm and above.

Timing looks good for 10% btdc advance. Have noticed difficulty getting the left hand carb to give me the same slight idle increase when trying to match the mixture to the right hand carb using the piston lifter. Rpm will not stay constant at 3500- floats up and down.

banderso

This thread was discussed between 30/05/2004 and 05/06/2004

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