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MG MGB GT V8 Factory Originals Technical - Misfire


Does anyone recognise the constant heavy misfire I am getting between 2500 and 3000 rpm, but only when I accelerate hard (eg to overtake). If I build the revs up gradually, there is no problem.

Car is a standard factory 3.5 I have a nearly new distributor, coil and leads, SU's with K&N filters & tubular manifolds. I think the fuel pump is ok - it clicks when the car is at idle, but I seem to recall from previous cars this is pretty normal.

Any ideas?
Don Bryson

Cylinder pressures increase during accelleration, which makes sparking at the plugs harder, which raises peak HT voltage, which can break-down any marginal HT components. You don't mention having changed the plugs. You *could* try getting someone else to rev the engine in gear slipping the clutch against the brakes in the dark for brief periods while you look for electrical discharge - standing at the side, not in front!

Yes, the standard SU pump does tick at idle, but the important thing is 'how often?'. If the pump is leaking back it will be ticking faster than normal and there may not be enough pressure at full throttle to supply the engine. You can check this by leaving the ignition on for a while with the engine offr. The standard pump should not tick more than once every 30 secs.

In theory, if it was the pump, I would expect a short delay between flooring the throttle and the misfire while the float chamber level reduced sufficiently to cause the misfire. You may be able to check this by putting a manual switch in the fuel pump circuit (there is a handy bullet connector in front of the pedal box). Switch off the pump when you get the misfire, and if the engine *immediately* cuts or gets worse you know you have insufficient fuel in the carbs. If the engine continues to run for a few seconds, albeit misfiring, it probably isn't the pump. Just a theory, you understand.

Paul Hunt

I was going to suggest you're running a bit lean. I know only enough about SUs to be dangerous, but since they don't have an accelerator pump I would guess that what may be happening is the carbs are having trouble transitioning to that higher-airflow/lower vacuum state when you ask the car to accelerate hard. An American 4-bbl carb would "sense" the fairly sudden press on the accel cable and spritz the primaries with a good dose of gas (and the oil companies would thank them) -- anticipating the fact that you'll basically lose your vacuum signal for a couple of seconds. Long and short of it is, the motor needs to be substantially enriched during periods of acceleration (although not too enriched because it will bog). My $.02.

No offence intended, just my two-penn'orth, but in my experience insufficient enrichening on accelleration is characterised by 'bogging-down' for a while rather than a misfire - you can check this by removing the dashpot dampers. The factory (at least) V8 seems to run tolerably well on a huge range of mixtures and timings (far more so than the 4-cyl), the only time I have ever had misfiring on mine was when the points gap was way too big (and gave some spectacular back-firing on accelleration into the bargain) and when my bottom hose was split and spraying water onto the plugs. I have to run mine lean occasionally to be sure of getting it through the annual emissions test. I've got a gunsons but it reads about 2% low, so I adjust it to that to be sure, then adjust it back in compenation after the test when I have the testing stations print-out in my hand.

Paul Hunt

Don - Paul's advice about the different effect of fuel starvation is spot on - a sudden attack of constipation is quite different from missing. It's difficult to troubleshoot at long range but first off I would try another coil first then if you are using a modern dizzy,the electrical connection between your amplifier and the dizzy which reverses the advance if you have connected it back to front - then check the vacuum tube and finally if they all checkout - look for a short either in the main HT cable or somewhere in the LP circuits feeding the coil/dizzy (if you still have breakers).
Roger Walker

Interesting; more than one way to look at it I guess. Usually IME bogging -- giving the carb more gas than it knows what to do with -- is augmented a puff of black smoke out the tailpipe. Is this occurring? If I understand the SUs' architecture at all -- which I may not -- I don't really see how bogging could occur in the first place and it seems to me that the more likely eventuality would be the opposite.

In case it is the carburetion rather than the ignition, you might want to avail yourself of one of those inexpensive air/fuel meters and see what's happening to the mixture on hard acceleration. They aren't very useful at all at telling you _how_ rich or lean you are at any given moment, but they'll at least tell you whether you are north or south of "stoich" or 14.7/1 air/fuel. For example if you nail it with a decently-tuned 4-bbl, you will instantly see the air/fuel ratio decrease _substantially_.

(Now that I wrote this, just out of curiosity, does an SU simply have two circuits (idle + primary) or is something more subtle going on?)

The short answer is the fuel is controlled by the needle and the air is controlled by the butterfly during steady state driving. When accelerating, the lag of the dashpot enrichens the mixture by limiting the volume of air to the engine.
George B.

I have had a similar experience (Ford 351 v8 engine). After replaceing or adjusting everything I finally as a last resort replaced my relativly new (6 month old) plug wires. That cured the

As Paul mentioned increased High Tension voltage causes faults in the system to manifest themselves. You might want to try replacing your wires.

Larry Diede

Thanks to everyone for their advice.

Problem cured by replacing fouled plug on no.4. Had also apparently been running a bit rich - adjusted tickover very slightly & fuel consumption is noticeably better. Car now goes like a rocket!
Don Bryson

This thread was discussed between 01/07/2001 and 14/07/2001

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