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MG MGB Technical - Spit or Cough When accelerating

Just out for my first extended drive through the countryside yesterday. When accelerating, and sometimes at steady speed, there was a noticeable "cough" or "spit" (not backfire).
I used a fuel stabilizer before the winter rest. Could there still be some water in the fuel? I'm thinking of adding some dry gas.
Any recommendations welcome,

Pat
'73BGT/HIF4s
Patrick Callan

Does the tach dip? If so ignition LT. If not could be HT or fuel.
PaulH Solihull

I used to have something I called the occassionaly hic-cup when simply cruising along. Not the hic-cup associated with a bad spark plug or a bad wire.

I narrowed it down to two things, and of course I did both at the same time so who knows which it was.

1. ignition wire at tach was a loose connection that aroused my suspicion.

2. carbs may have been a bit too lean.

Once in a great while, once or twice a year, it returns, but nothing like it used to be, as it was a regular anticipated hic-cup during every drive.

Hope that helps.
R.W Anderson

Don't see any dip in the tach, although it happens so fast that I might not be noticing it.
Will check all connections and mixture.
Doesn't happen consistently. Makes it tough to diagnose.
Plugs look good, so I don't think it is from too lean a mixture.
Also plan on filling with fresh fuel and maybe adding some dry gas.
Patrick Callan

I too never noticed the tach drop, but the other telling hint was that occassionally I'd go to start the car and it would only crank, and the tach was at 0.
R.W Anderson

I have a similar problem but also when I switch off there is a bit of pre igntion - any ideas?
E McGee

Doesn't seem to be connected to either starting or switching off issues.
Hoping to get out this weekend with some dry gas, to see if that makes any difference.
All connections look good. Cleaned and checked the distributor cap and rotor (have pertronix Ignitor instead of points). Also added a dab of oil to the center of the distributor.
Patrick Callan

Update:
Looking more like fuel issues. After dry gas and about a half tank of fresh gas, and cleaning as noted earlier, the run this afternoon was much better. Still a few chugs for a while, but acceleration and up-hill climbs MUCH better.
Will probably add a little more stabilizer or dry gas at next fill up.
Patrick Callan

"it would only crank, and the tach was at 0."

Other than trembling from mechanical vibration neither of mine give a reading during cranking.

Pre-ignition on switch-off or running-on/Dieselling to give it its correct name is completely different, and is caused by hot-spots in high-compression engines, as is pinking. Some era's of MGB seem particularly prone, like early to mid-70s, and unleaded fuel has made it worse in a lot of cases. I've had to fit an anti-runon system to my 73 as it got so bad on unleaded. Some tuning manuals recommend you grind back the promontary that sticks into the combustion chamber between the valves very slightly, which actually has the effect of reducing the compression ratio very slightly. Many moons ago excessive carbon would cause it, but I've not seen carbonised cylinders for probably 40 years given the fuel that has been available in the UK at least.
PaulH Solihull

Another Update:
Thought I was on the right track, but took it out this AM and the chugging continues. Seems to be ok at low speeds (<40) and idle, but chugs at higher speeds and especially accelerating up hills. It is not a constant or even consistent chug, and there does not seem to be any impact observed on the tach. Very random, and very annoying.
Back to the hunt for a cause.

Pat
Patrick Callan

With modern fuels, correct mixture is not that easy to judge by plug colour. I'd be tempted to wind in the mixture screws 1/2 a turn as an experiment.
"Running on" is not the same as pinking, nor is it Dieseling. Running on or pre-ignition is the result of the mixture being ignited from a source other than the plug, as Paul said usually a hot part of the combustion cahamber, a sharp edge, burr, over-heated exhaust valve or the pointed "promontary" . But pinking is the uncontrolled explosion of the "end gas" by a combination of heat and pressure, hence "dieseling", usually caused by low octane fuel or too advanced ignition or too high a compression ratio. Too weak a mixture doesn't help either, the cold fuel going in to the combustion chamber helps to cool things down a touch.
Allan Reeling

Getting frustrating:
New distributor cap and rotor, checked timing. Seems to be getting worse.
Can a Pertronix Ignitor cause similar symptoms if it starts to go bad?
Thinking of replacing points and condenser to see if that makes any difference.
Interesting side observation - balancing the carbs usually set at about 14 in order to get an idle speed of 800-900. Right now, the setting has to be up to about 20-25 in order to achieve that idle.
Patrick Callan

Have been to the pub, so might be intellectually challanged, but what is "balancing the carbs at 14 or 20-25?
Allan Reeling

Alan,
I have a synchrometer that measures air flow. The settings on each carb are adjusted for equal flow. The units are cubic decimeters per minute.
I have always been able to achieve a nice balance for idle at about 14. Not sure what could have changed that reading for proper idle speed.
Patrick Callan

Electronics can fail in all sorts of ways. If you can replace the Ignitor with points and condenser then do so, it will eliminate that.

Sounds like the throttle is having to be open nearly double to obtain a given idle speed, which means the engine is grossly inefficient to what it should be. Could be timing way retarded, or mixture way off, or possibly weak sparking. Get this idling situation fixed first before dealing with the cough or spit when accelerating.
PaulH Solihull

Would you believe:
Removed the Pertronix and replaced with an old standard points and condenser that I had saved. Took a while to get the dwell correct (it has been a long time since I last did that evolution). After the change, the carbs were able to set back to a normal range of air flow balance.
Took it out for a drive - no spits/coughs.
Not sure I understand what was happening, but the Pertronix must have been in a gradual state of failure, which was affecting other things.
Guess I should invest in a new set of points and condenser.
Hopefully that resolves the problem. Will drive for a few days to claim a victory, but the first drive was a MAJOR improvement.
Thanks for all the comments and recommendations,

Pat
Patrick Callan

The electronic module, if it's Hall effect, can be thrown by several things, odd sparks, arcs and shorts caused by cracked dizzy cap, failing rotor arm, un-surpressed leads, badly fitted leads, cracked insulator etc, The use, on the later cars (with the electronic dizzy), of resistor plugs was to combat such disruption to the electronics, as well as give better radio. Check things out before you bin the Petronix.
Allan Reeling

Allan,
Thanks for the info and I will check all of those areas - haven't thrown the Petronix out yet, but I would suspect any of those issues to give similar symptoms even with points (?).
At any rate, the Petronix has lasted well over 10 years, so I have certainly gotten my money's worth out of it. Hate to return to more frequent tuneups though.
Do you know if there is a way to check the Petronix when not attached to the distributor?
Patrick Callan

If points and condenser doesn't give the problem but the Pertronix does, as seems to be the case, then you have proved it to the Pertronix or its wires - nothing more to be done with it. *Except* ... the Pertronix like points is on the points plate which continually twists back and fore as you open and close the throttle. Standard ignition systems use very flexible wires between that and the body of the distributor or the hole through it. I don't know what type of wire Pertronix uses but the conductors could be fractured inside the insulation and giving erratic ignition, although that would normally show on the tach as I first said.
PaulH Solihull

Paul,
No sign of external wear on the wires, but as you said, damage may be inside the insulation.
I'm still a little confused about:
1. How gradually worse the symptoms became - I always heard that when on of those units goes, it goes fast.
2. Why it seemed to affect my ability to adjust carb air flow to the proper balance settings.
At any rate, I will drive it for a while on the points and see if any of the symptoms return.

When I first installed the Pertronix unit, it seemed to me that it should fit a little tighter in place (screwed down only with the small screw that usually holds the condenser), but it lasted a long time in that configuration.
Patrick Callan

After a couple of nice long drives, running much better on points/condenser.
While the wires on the Pertronix look fine, 3 of the magnets in the collar unit that fits over the shaft were loose. They actually fell out when I removed the collar. I did recover all of them (scared me a bit when I noticed them missing) but they were very loose. Not sure if that would/could induce enough wobble to affect the electronic pickup.
At any rate, thanks again for all the recommendations and info.

Pat
Patrick Callan

If the unit is on the way out it may be weak sparking which could well need the idle screws turned in more to keep the engine running at an idle, which opens the butterflies and pulls more air. OTOH it's quite common for electronic triggers to have a different trigger point to points, which is why you should always retime the ignition after changing, and even after changing points. If the Pertronix made it idle faster than the points, even if you didn't alter the idle screws, then again that will pull more air per minute through the carbs.
PaulH Solihull

This thread was discussed between 20/03/2012 and 10/04/2012

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