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MG MGB Technical - Curious Misfire

Hi Folks. I wonder if anybody can provide any pointers:

Symptom: Engine misfires when throttle is opened from cruising RPM, or to maintain speed on an incline. What's interesting is that it only seems to do this after I've been running for about 20-30 Minutes. Up till then it runs beautifully.

How long it's been happening for: About a month, though I've had the car 15 years and not come across this previously.

Setup: Dynamic timing per manual. Standard (Non ballast coil), Accuspark ignition. std twin SUs, std cam, Engine temp seems normal.

What I've changed since the problem occurred: Plugs, Leads, cap, coil, rotor arm, fuel filter & re-set valve clearances.

I'm a bit baffled by it. I can't quite make out why this takes 20-30 minutes to manifest itself. The thermostat is in and operating correctly. I'm assuming (and I know this is dangerous) its not fuel vapourisation, on the basis that the heat shield has not been changed, moved or damaged. I've never had a problem with this before.

I'm wondering if it's something right out on the periphery that is warming up and causing trouble? As I type this, I think I'm going to check the Vacuum Advance pipework at lunch time.

Does anybody have any bright ideas?

Regards... Phil
P Clark

What year? Is the tach jumping about?

Could be fuel starvation, have you done a fuel delivery check? Disconnect a fuel pipe from a carb (if the engine has recently been on you should get a spurt, and you should be doing this with everything at normal running temperature), direct it into a container, and turn on the ignition. You should get a minimum of one pint per minute and in practice double that, in a continuous series of pulses with negligible bubbles.

Are the carb dampers doing their stuff? Unscrew them, lift them up, press them back down, and you should feel the resistance of the damper in the oil before the cap reaches the piston cover.

What plug gap are you running?

The chances of it being vapourisation are negligible to non-existent. As I've said many times these cars run in desert states without that problem, let alone in the UK at this time of year.
Paul Hunt

All of Paul's above. When you say "standard coil" I assume you mean 12v (3+ohms primary resistance)? Hesitation from trailing throttle could mean lack of oil in dampers, faulty diaphragm in vacuum unit, split vacuum connectors. Conversely, if the pistons are sticking when hot then the engine will "bog down" on throttle increase.
I suppose if you are running on the borderline of too weak a mixture, as everything warms up and the fuel air temp rises and density decreases, then the mixture could become too weak. This is not likely with HIF carbs as they have a fuel temp compensator built in, assuming you start with the correct mixture.
If you have a servo, check the vacuum hose. Also the crankcase ventilation hoses.
Have you got K & N's?
Allan Reeling

you don't have details of your car in a vehicle profile or say what year, model year, etc. your car is

first thing to look for is if the rev counter twitches with the misfire, if so consider low tension side of ignition

you set the tappets first then check set plugs, then timing, then carb mixture

setting by manual is a good starting place but your car is no longer new and you have different components on it now, the Accuspark at least so you may have to vary the setting by ear and test driving or gun and figures if you prefer

the wires off igniter heads tend to be delicate and can get scuffed and damaged inside the dissy and outside

it's a good thing to replace things like HT leads, cap, rotor but it does depend on their quality - if they were part of an Accuspark kit then I'd carefully inspect the new HT lead set

if the HT leads, cap and rotor arm weren't part of a kit then I'd suggest:
HT leads - check all 10 connection ends and that they seat properly, if the rubber end covers are tight they can lift prevent proper connect of the lead terminals
rotor arm - if it's not red rotor from Accuspark or Distributor Doctor then some newly made ones can cause problems

best to rule out electrics before moving on
Nigel Atkins

Hi Guys,

Thanks for all the help.

Sorry. The car is a 72 B roadster (1800) It has had K&Ns on for about three years.

I've always been a bit skeptical of the HT leads, and have yet to find a set I'd be happy with, i was lead to believe (and paid a premium for) "OEM Quality" leads the last set I got, but they assured me they were better. I'm not convinced.

I'll have a look over what you've suggested over the weekend, and report back.

P Clark

(apologies to Paul and Allan I started my post before seeing yours and then had a break from computer)

did you change the needles with the K&Ns and have you got any stubs stacks fitted to the K&Ns?

for HT leads I'm sure if the present ones re not too old they'll be fine but I can recommend a manufacturer of very good quality and made HT lead sets (I think they also make for others to put their name on) they have been tested and found to improve performance by a very sceptical B-series engine builder and tuning specialist who sometimes frequents these boards - Peter Burgess
- 'performanceleads' sales by phone -

for Peter's report on them look in the 'midget and Sprite Technical' forum 'Archives' for a thread called 'Performance Leads Test (at last)'

Nigel Atkins

I developed that same symptom on our TD and after much flopping and twitching trying to find the problem, I removed vacuum chambers from the carburetors (one at a time) and used a solvent soaked rag, cleaned the inside of the chambers and the edge of the corresponding piston. Since doing that, I have not had the problem reoccur. Cheers - Dave
David DuBois

When I got my V8 I replaced the leads, and was shocked how expensive the OEM i.e. carbon string were. I've had problems with those in the past when they were factory provided, they go high-resistance and affect the quality of the spark over time. In the 70s I fitted silicone-cored and never had a problem with them, and I've used those on the roadster and the V8 as well. The V8 ones (Hotwires) have now done almost 100k in 20 years. I did replace the roadster ones, not because of a problem with the leads themselves, but the brass connector on the king-lead developed a weird coating that acted as an insulator. I just couldn't shift it, so replace the set quite a few years ago now and no further problems so far.

"the wires off igniter heads tend to be delicate"

A good point (no pun intended). If you have vacuum advance the 'points' or trigger plate is continually being twisted back and fore as you move the throttle, flexing the wires going to the points or trigger module. The original wires (there needs to be an earth wire as well as the points wire) on the 25D4 are cloth-covered braided tinsel which are very flexible indeed. The points wire on the 45D goes through a hole in the body and is also noticeably more flexible than standard wiring. If you have an under-cap electronic module that has its own two wires, and also needs the original earth wire, and how flexible those module wires are I don't know. Other ignition systems with an under-cap trigger, but an external electronics module, don't need the earth wire, but their two wires also need to be just as flexible.
Paul Hunt

Hi Chaps,

Having spent ages poking around the ignition side of things, I focused my attention on the fuel side.
Here, I noticed that the fuel filter had run dry - though the pump is still pumping. The arrangement of the fuel filter (inline) is such that an airlock can form.

I have re-primed the system, excluding all the air but haven't had a chance to get it out for a good run yet.

The question is, will more air get into the filter next time I take it out?

I can't quite make my mind up how it is that air is getting in on the pressurised side. Logic would dictate that it should be fuel getting out, unless it's somehow drawing air in from the tank side of the pump (which I've yet to investigate).

I've never been overly happy about those cheap and nasty inline filters. They're scarily fragile looking, and on my car, lives right above the exhaust manifold, which I recon is asking for trouble one day.

I work in the marine industry, and the answer has been staring me in the face. A nice neat little bulkhead mounted spin off cartridge filter that you might use on an outboard motor. It'll annoy the purists, but mines a driver that I don't want bursting into flames!

I'll keep everybody posted, and again, thanks for all the tips.

Regards... Phil.
P Clark

hi phil.
i had problem with my bgt for the first 3 yrs i had it.
it had k n filters already fitted on when i bought it. found out later that k n filters have to have richer needles fitted. done that and car run superb for ever after.
regards bob.
bob taylor

Fuel filters work as well "half full"! Provided the fuel in the float chamber is at the correct level, the level in the filter doesn't matter. Your marine solution sounds neat, tho I'm afraid I just fit the nasty 2 jobby every couple of years. Did you do the fuel delivery test?

You can replicate "richer needles" by pulling the choke out -only a little- nothing like as much as when cold, but more that just the portion that raises the idle speed. It's not a solution, but it might eliminate /indicate the solution. Odd tho if it's run ok for 3 years.
Michael Beswick

Nothing wrong with the spin on marine fuel filter system. Usually, they incorporate a double filtering element, the inner one designed to trap water and the out one to trap foreign particles. It's very common to see an inline fuel filter half full. It doesn't mean that fuel isn't flowing through it though. I have a supercharged '67 B, with the see through filter mounted pretty close to the exhaust manifold and after twelve years I've yet to encounter a problem with it. RAY
rjm RAY

you'd best have a look at PaulH's excellent site mgb-stuff before deciding about the in-line filter

I use plastic disposable in-line filters (changed annually) on my present daily drive Midget and previously on my daily drive Spridget

I also used them on my daily drive '73 BGT without issue

often they can appear empty and other times quite full but it makes no difference to starting or running

did you say if the rev counter needle twitches with the misfire?

there are loads and loads of threads like this in the Archives, sometimes a rarity cause and solution pops up but more often it's the standard faults or mistakes
Nigel Atkins

Quick update chaps.

Thanks for all the info and advice.

So. I took it out for a run last week and smelled petrol. when I stopped and opened the bonnet, there was fuel leaking from the float bowls (HIF) via the jet adjuster.

Carbs off, soda blasted, and through the dishwasher along with the K&Ns (whilst the management was out) and then a full rebuild with one of the SU kits.

I've not done this job before, and learned a lot doing it - which is good. What i also learned was that the choke was set up completely wrong. The spindle wasn't engaged in the counterbored nuts on the enrichment shafts. Goodness knows how I missed this in my 15 years of ownership!

I also learned that the "OEM" leads I was sold recently are of infinitely poorer quality than the ten year old items I pulled from my "come in handy box" and fitted.

Anyway. To cut a long story short, the car now runs perfectly, and is cold starting better than it ever has. I have also have a better understanding of how these elegantly simple carbs work, and how to set them up properly, so good all round.

Thanks again, folks. Case closed!

Regards... Phil
P Clark

glad you got it sorted and thank you for reporting back to complete the thread

when you want to improve on those 10 year old HT leads - phone 'performanceleads' sales by phone -

I'm not sure about the K&Ns in a dishwaher, might be fine I've no idea, but for others the K&N cleaning procedure -

Nigel Atkins

You brought back an old memory.
1954 MG-TF On the way to a GOF in south carolina we had to use a section of I-95. Car lost power but would maintain about 45 MPH. Every time a large truck would start to pass the TF would pick up speed and run fine. The moment the passing truck would get 150 Feet ahead the car would lose power. On arrival at Jeckle Island I removed the vacuum chambers and one piston was solidly stuck. cleaned pistons and runup was perfect.
ss sanders

Phil, et al: Went to a GOF this past w/e 70 mile trip. Car ran great. Coming home it ran great until about 15 miles to go. Stalling at idle. Tach was OK.

Cleaned out the Weber DGV today, changed out the in-line fuel filter (all gunked). Ran great again. Went to re-install the K&N filter element with the engine idling @ 1000 RPM and noticed the revs dropped to 800 RPM.

Thinking back, I've never replaced the element and only cleaned it a few times in the 12 years I owned it. A replacement filter element has been ordered.

To substantiate your finding, Phil. I believe I had pretty much the same issue as you.

Thanks for posting.


79 MGB
gary hansen

the whole point of having a K&N air filter is that you clean it when required and not replace it!

IIRC they used to say million-mile use clean at 100,000 miles (in normal driving conditions) now they say clean at 50,000 miles but many owners clean more frequently than this to maintain performance levels (usually on much more powerful cars than MGBs)

I put before a link to the K&N cleaning procedure, using K&N Air Filter Cleaner and K&N Air Filter Oil for re-oiling -

Nigel Atkins

Nigel. I've had my 79 MG for 12 years and this is the filter that came with the car. I cleaned over the years, but this time it has not been successful, as it still clogged. It is also distorted in shape. I suppose time has a way of deteriorating them.

I was able to get one through work for $10. So I ordered 2 of them.



79 MGB
gary hansen

fair enough Gary, I'm surprised it only lasted 12 years if you were cleaning it correctly, many don't know about the correct cleaning - obviously in some more adverse conditions the filter needs cleaning
Nigel Atkins

The new filter arrived today from California. Placed on the carburetor and took the car out for a ride. A major difference in the acceleration, idle and hopefully the tailpipe will lose its pitch black color after a good open road drive.

Nigel. The old filter was on the car 12 years ago when I bought the car. Who knows how old it really is. The old filter was in a twisted distorted state as well brittle. It also shrank from its original size. I wished I took a pix of it to share with you. I think you would have made the same choice I did.

Where we lived before in upstate, New York, the winters were long and had sub-zero (F) temperatures, so the car only went about 3000 miles a year. I'm sure that freezing weather took its toll on the filter as well.



79 MGB
gary hansen

don't get me wrong I'd have binned it the first time it didn't clean up properly and I'd have cleaned it soon after getting the car as I'd not trust the previous owner(s) to have necessarily looked after and cleaned it properly

if it was twisted and distorted again I'd bin it, pity you didn't have a link or picture of it as I'm not sure which type it might be

if your previous filter was clogged then just taking it off would help the car to run better

filters went from oil bath to disposable paper element because owners didn't bother cleaning the filters enough so regularly change your paper filter to keep things in good condition, at least once a year
Nigel Atkins

This thread was discussed between 04/04/2014 and 22/04/2014

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