MGB Need help diagnosing MGB engine issues

[Tim_Creger]

I've got a 1972 MGB with a stage 2 professionally built 1800 by Ivor Searles Engines of England. I just bought this car this summer, and have been struggling with keeping it running since August. I have reset the Weber DGES 38/38 to factory specs, and believe it is not the cause of the problem. I am now down to suspecting either the coil or distributor system. Here are the symptoms:

Starts well cold (imagine that!), runs out smooth and strong when cold (another amazing thing for a LBC). As the engine warms up, probably 10-15 minutes worth of driving, it starts to cut out under excelleration, then as it goes on it is doing so in all gears, all loads, and all rpm, even idle. The electronic tach is bouncing up and down this whole time, but every so often, especially when I romp on it, it will take off with little noticeable problems, but once leveling off starts to buck and cut out again.

Any ideas? I am suspecting worn points/rotor, cracked cap (which has been inspected and I can't see any visible cracks or spark tracks), a worn coil, or a worn distributor shaft in combination with any or all of the previous three.

Tim C.

 

[Marvin Gruber]

Check basics first. Make sure you have good ground, then check the electrical connectors for tightness or corrosion.

Marv

 

[PAUL161]

Starts well cold (imagine that!), runs out smooth and strong when cold (another amazing thing for a LBC). As the engine warms up, probably 10-15 minutes worth of driving, it starts to cut out under excelleration, then as it goes on it is doing so in all gears, all loads, and all rpm, even idle.

I am suspecting worn points/rotor, cracked cap (which has been inspected and I can't see any visible cracks or spark tracks), a worn coil, or a worn distributor shaft in combination with any or all of the previous three.

Tim C.

If everything in your diagnosis works properly when cold, with the exception of the coil, there should be no change when it's hot. There have been instances when a condenser will act funny also which is a cheap and easy fix. Coils however, can sometimes be the culprit of poor engine performance when hot. Not saying you haven't already done this, but make sure the coil is wired properly. Make sure it's the proper <span style="font-weight: bold">type</span> of coil for your application. I could be very wrong, but if it was mine, I'd give the coil a hard look first before digging deeper. JMHO. PJ

 

[davester]

The first thing I thought of was the condenser, which Paul has also mentioned. Given how cheap they are, I'd replace the condenser, points, rotor and cap first of all. If that doesn't fix it, start thinking about the coil and/or HT leads.

 

[Tim_Creger]

Thanks folks, I agree that replacing the points/condensor/rotor/cap is the chealest and easiest trouble shooting to do. I'll give that a try and see what happens. Maybe I can get this done before the snow starts to fly!

Tim C.

 

[Grantura_MKI]

Condensers either work or they dont...ie hot or cold. switch the ignition on and see if the coil warms up? Be sure that the petrol pump is not charge through the white/ ignition side of the switch. This will interupt power to the petrol pump. This curcuit also feeds the coil/tacho.

 

[Grantura_MKI]

Soory, forgot to mention...bin the carb and go for a DCOE.

 

[davester]

Condensers either work or they dont...ie hot or cold.

That is not my experience. I had a bad condenser that worked only intermittently when warmed up ....a hellish thing to troubleshoot.

 

[Stewart]

The carbs fine leave it alone. Clean the grounds first and check the grounding wire in the distributor itself as I have had one fail before. Change one part at a time. Are you sure the distributor is hooked up right? If it's backwards it will run fine but kill the condensors fairly quick (don't ask how I know). If your worried about the distributor being worn out contact jeff at advance distributors https://www.advanceddistributors.com/ He does very good work at reasonable prices.

 

[Grantura_MKI]

Never had a con work intermittent. The dizzy must be wired correctly in order to run at tickover.
Just meant that i am not fond of down draught carbs.

 

[glemon]

I have had wonky condensors too, hardest ignition problem I ever diagnosed, gave spark, but weak, worse when hot, would idle OK, not accelarate.

Tim I have ot a number of old coils sitting around and not too far away if you want to try swapping out sometime.

Greg L.

 

[Tim_Creger]

Hi Greg,

Thanks for the coil offer. I'd like to try that sometime, just to say I've eliminated that possible element. I'll be around for the next couple of days, then gone until next week. Give me a call. I've also got the "affordable" basics on order and will replace them anyway, even if the coil turns out to be the issue. This engine was rebuilt about 9 years ago, has roughly 8,000 miles on it, but the distributor has no marks on it other than Lucas Made in England and a four digit code number. All the sites I've seen say the model number should be 5 digits. Based on the length of the cap clips, I can tell it's a 45D model, so that is what I used to order a new cap, rotor, points and condenser.

Does anyone know if a bad cap to coil wire would cause this sort of behavior? As for the the coil, it does get very hot when the switch is on for more than a minute or two. The fuel pump is direct wired into the coil, not the fused circuit. Should that be changed - based on Grandtura's comment?

And Stewart,can you help me understand what you meant by hooking up the distributor backwards? Wrong spark plug wires in the wrong order? High/low voltage wires reversed to the points/condenser?

Tim

 

[fogliner]

Hi Tim
I took a browse through University motors tech advice page and came up with this on the coil wiring. There is some other stuff in there you might find interesting.


Cut n pasted from a john twist answer

"The coil is powered with twin WHITE/GREEN wires. One of these comes from the starter motor and gives battery voltage only at startup (about 10 volts). The other comes from a resistor wire, in the loom, and provides a lower voltage to the ignition coil (usually 6-8 volts WHILE THE CAR IS RUNNING, but can read lower or higher depending if the points are open or closed). The other side of the coil carries a twin WHITE/BLACK - one goes to the distributor, one goes to the tachometer. There is also a "ballast resistor" next to the coil, this is powered by a WHITE and feeds a WHITE/BLUE."

Lots of good info on his site

https://universitymotorsltd.com/qa.php/

I also watch his video's on youtube for motivational therapy too..hope this helps a bit.. Mark

 

[Tim_Creger]

Well, as usual, I am "two wires short".

I've only got one wire on each side of the coil, and they don't have the same color coding as the ones in Twist's notes. I know this whole engine and wiring setup is cursed by numerous DPOs, so it may be easier to replace one piece at a time to see if any one of them will correct the problem. I got my ignition parts in the mail last night, so will see what happens in the next couple of days. I'll start with the condenser, since that is the part with the most votes right now, then move on to points, cap and rotor, in that order. I will report back to this thread so all of you can join me in the Ah-Ha moment!

Tim C.

 

[DNK]

Tim I think it is different for different years.
But I'm a TR guy

 

[DrEntropy]

Condensor first, Tim. Then the rotor! Just fer grins, can you get movement of the rotor "fore-aft" more than what the camshaft travels? Have you gone into the dizzy far enuff to be certain the mechanical advance and the plate it is attachet to are in good nick?

 

[Bruce Bowker]

Long shot but I went through the same thing. Went over every inch of the ignition which was obviously the problem only to find out it was the fuel draw tube in the tank plugging up (slowly) as the car ran.

 

[twigworker]

Go to Advance or AutoZone and buy a new coil, probably an internally resisted coil.

Bring it home and swap the wiring over from the old coil before you wrap it in a towel and let it sit on the fender apron where it won't fall through to the pavement.

Then go thorugh you usual routine of starting and running the car.

If the symptoms disappears, make a permanent swap.

If the symptom remains, disconnect the wiring and put it back where it came from. Then take the coil back ofr a refund. They will take it back without a question, but should they ask, just say that it was the wrong one for the car.

Others certainly may have had bad capacitors from time to time but I have never had one go bad in 45+ years.

BTW: If your present coil is the OEM unit you might have a small bit of trouble getting the new coil to fit inside the clamp. Just remove the cinch screw and nut and spread the clamp just enough to accept the new coil. Then mount the clamp. 99 % of the time the tightening of the two cap screws will draw the clamp in enough to hold the coil securely.

Jack

 

[Grantura_MKI]

Also be sure that no wires are earthing..ie, in side their selected areas? In my experience, con's work or they do not. A hot coil is a bad sign of a faulty iggy bit.

 

[glemon]

Tim, I will bring a spare coil with me to work tommorrow, am free for lunch will try to swing it by and drop off, only concern I am not absolutely sure if it is internal or external balast type, you electrical gurus any way to tell?

I thought I had 3-4 coils lying around, only found 2, but maybe I didn't look in enough boxes....