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MG MGB Technical - Diagnostic help needed please send help

Thanks for coming to help, the beer is in the fridge help yourself, when you finish maybe you can give me some tips.

This car has to run smother at Idle than it is.

Completed work and specs.

1977 MGB 69000 miles Stock/ less smog pump
petronix ignitor (old)
new plugs/ wires/ cleaned cap
Have not cleaned distr. or oiled, initial tune up showed mech. advance and vacuum adv. working. Don't remember the figures.
Carb rebuild kit installed per Manuel.
Timing set to specs
New Alternator and all wiring conections cleaned and refited.
Wiring to Ignitor is questionable because we did not have instructions (BI passed the resistor)?

Symptoms:

Runs good and strong on the road, starts good. Driven about 200 miles since bringing her out of hibernation.

Checked spark plugs every few days with the same result each time.
# 3 is always dry and black the rest look fine but might be on the lean side if anything.
When my son pulls in the drive though it makes me want to scream because I can hear the miss of # 3 at Idle. ( I think) I can also feel a slight pulse (suction) in the exhaust but not enough to slap a dollar bill.

I've played with the fine and coarse Idle mixture setting and don't see any change in the plugs. "1/4 turn at a time " I have thought all along that I had a problem with carburetor or vacuum leak because the coarse Idle and fine Idle screw have very little effect. It will run a bit rough when both are turned in but it will not kill the engine. I have checked the hoses for leaks but have not checked the throttle shaft for leaks.

Having troubles holding same Idle speed. (+ or - 100 rpm} I let my son read the instructions while we assembled the carburetor and I'm hoping he didn't leave anything out. (He likes to speed read and that don't work with instructions)

My guess at this point is a bad intake valve and I plan to check compression this weekend after I find a tester.

If compression test are good I'm clueless.
Any Ideas on what else to check?

Thanks,
Kevin
Kevin Pearce

Kevin. I would expect cylinders number two and three to be a little lean--they share an exhaust port and tend to run slighly leaner than the numbers one and four cylinders.

The number three cylinder seems to tend to burn or crack its exhaust valve before any of the others exhibit a problem. Thus, a compression check is a good idea.

My rememberance of the Petronix ignitor system is that the "ballast resistor" was indeed by passed. There is a resistor wire in the circuit going to the coil. What most people refer to as the "ballast resistor" was used in the circuit to the Lucas 45DE4 power module, not the coil. The one Petronix module I helped install was not, in any way, wired to the flat, finned resistor near the coil.

If the number three cylinder is not firing properly, there should be no change in the idle speed when you pull the boot off the spark plug. If there is a change, the plug is firing.

Need more information to make a complete analysis. Les
Les Bengtson

Kevin,
Before you pull the head change the plug in number three. Could be you got a bad spark plug. At least swap it with another cylinder and see if the problem goes with the plug if it does throw the plug as far as you can.
Roy
Roy Dougherty

Very dry beer!
Check compression.
You didn't say if you adjusted valves. I just did a midget that had clearances of .001 to .006 - idled terrible.
Adjust valves and recheck compression. Check comp. with throttle open.
If #3 is dry, then it is firing. It would be wet otherwise.
Let us know and get some wetter beer!
FRM
http://www.usachoice.net/gofanu
FR Millmore

Compression test completed:

Cold engine Hot engine Plug color
#1 135 130 White
#2 140 130 White
#3 140 130 Black/Brown
#4 140 130 Tan

Plug color was after my son drove the car to his first Football game and included highway speeds.
# 3 plug is firing and was a bit cleaner after driving on the highway. Most of the driving during the past checks of plugs were short trips city driving to and from school.
The car sounds better at Idle when cold only sounds lumpy when hot. (hope you all know what lumpy sounds are)LOL

Now what?


Kevin Pearce

Kevin. Either drive it and see what happens or have it put on an engine analyzer with an experienced operator to see what can be found.

If you are not using the NGK BP6ES plugs, you might try a set. Again, I would expect the plugs for cylinders one and four to show signs of being slightly richer than the plugs two and three due to cylinder head design. Could you have an air leak around the carbs or manifold? Spray some carb cleaner around those areas while the engine is warm and idling and see if the sound changes. Les
Les Bengtson

One other thing I did notice and forgot to mention was the servo adapter on the intake manifold. It is not functioning as it will allow air flow both ways. I figured since there is another valve at the servo that this might not be causing any problems. It looks as if it should work like a pcv valve but I'm not sure if that is correct. I will try the leak test after I buy some spray cleaner. That is one product I never have on my shelf.
Kevin Pearce

Round of Beer for everyone, but Les gets the Keg.
You know I've been thinking of getting some Carb cleaner and checking for leaks because I had read that tip before. Oh well lesson learned.

"FRONT INTAKE MANIFOLD GASKET LEAKING."

I hope this solves all the problems if it don't
"I'll be back"

Thanks,
Kevin
Kevin Pearce

Kevin. When we stop learning, we are dead--functionally, if not actually. You have just learned something which should make your day. It will also be something your son has learned and he will benefit from such experiences the rest of his life. One of the reasons to teach our kids, not just raise them. Good job. Les
Les Bengtson

The reason for the leak in the intake manifold gasket was the forward stud bolt had snaped off in the head. I told my son he was real lucky his dad was so smart and was able to use an easy out to remove the broken stud, without removing the intake and exhaust manifolds. His reply; I think I'm lucky it wasn't rusted in. LOL

Thanks,

Kevin
Kevin Pearce

This thread was discussed between 04/09/2004 and 06/09/2004

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