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MG MGB Technical - Intermittent Misfire/Backfire

The engine starts fine and runs fine most of the time around town, although it seems that when travelling faster for longer periods the there is the occasional misfire and sometimes severe backfire from the exhaust. Just as soon as it has started misbehaving it will be okay again for a while. The other day it was so bad I thought it was going to pack up and sounding like it was only running on two cylinders and backfiring very loudly – I stopped for 5 minutes and when I restarted all was okay again.

I have replaced points, condenser, rotor arm, distributor cap, coil, plugs and leads.

I thought I had found the problem previously when I discovered the needle in carb was loose and tightened the retaining screw, this seemed to fix it but the misfire started again. I’m using twin SU HS4s which are of indeterminable age and have never been reconditioned – could the this fault be due to the carbs, is it worth getting them re-conditioned?

Any ideas gratefully received! Thanks in advance.

Cheers
Chris
Chris

It sure sounds like a carb problem. I don't have SUs so I won't comment on the need for a rebuild but I'm sure that would be a consideration. Are they adjusted to rich? Also, have you checked the timing, fuel filter and pump?
Lewis

Hi.

The backfiring is unburned fuel igniting in the exhaust.
My first thought would be an ignition problem, which it still might be, as fuel won't often get unburned through a hot engine with a spark at the plug unless it is very weak (won't burn) or very rich (fouls plugs).
The coil would be prime suspect in view of the symptoms.
Have you got the correct coil and ballast resistor (if appropriate) ?.

An air leak in the inlet manifold area is a possibility, as is a sticking carb piston.

A valve problem would be unlikely to give normal running at other times unless it was sticking open when hot.

Don
Don

Is the tach flicking with the misfire? If so it will be somewhere in the ignition LT circuit, although you have replaced most of the likely culprits. Two possibilities are a broken or missing distributor ground wire which connects the points plate to the distributor body to maintain a good connection as the points plate twists under changing vacuum. Similarly the coil wire could be broken inside the insulation where it passes through the distributor body, this is also being continually bent back and fore as the vacuum changes. If the tach is steady it could still be HT. If you can reproduce it when stationary clip a timing light to the coil lead and each plug lead and see if there is any change in the flashes when the misfire occurs. If only on the move then I'd be tempted to try rigging up a timing light in the cabin so you can se it when driving. If the HT is steady during the missfire then I would suspect carbs, but think these less likely to cause explosions in the exhaust with the misfire. Have you done a full check of valve clearances, points gap/dwell, timing, plug gaps, and the operation of the centrifugal and vacuum advance mechanisms? Done a full setup of the carbs? Were any of these difficult or impossible to get right? What was the condition of the old points and plugs? A RB coil (6v) on a CB car can cause points burning, misfiring and backfiring as the RB coil needs a ballast resistor. A CB coil (12v) on an RB car will cause weak sparking, which in itself should really be very noticeable under most conditions. You can run a CB coil in an RB car but have to bypass the loom ballast resistance. There is no good reason for doing this, and in fact it can make starting difficult under some conditionsa. If you are not sure what coil you have measure its primary resistance, a CB coil should measure about 3 ohms, a RB coil about 1.5 ohms.
Paul Hunt

Thanks everyone for all the info and suggestions. I just replaced the fuel filter with a nice chrome/glass one from MGOC and in a quick 5 mile test all seems well.

I maybe speaking too soon, but I have plenty more to try now if it isn't that ;)
Chris

Hmmm, Ive heard from the USA to avoid those as they can come apart. Let's hope they are a different design or manufacturer.
Paul Hunt

Interesting thread this as I am experiencing the back fire but mainly under load in acceleration at 4000rpm plus in 3rd and 4th. An old friend suggested that backfire, as Don says, is the unburnt fuel in the exhaust. I made some recent changes, mainly a repalcement set of HIFs on my 1980GT, which is 1860cc, stage 2 head, K&Ns and 714 cam. I have also changed the coil and alternator some time back after first getting back on the road, and fitted BP7ES plugs which I was advised to do as these run slightly cooler.

The previous set of carbs were as original and had a misfire problem under heavy right foot at 3500rpm, but it all seemed fine with the replacement HIF set properly set up (also with a HS butterflies fitted).

I have done 5k miles since the engine was set up and I think I should check the spark plug gaps although they do look nice and lean, and also check the points gap and timing (silly me - should have done this a while ago!). I had noted it was starting to pink very slightly under load recently.

Any other comments gratefully received.

TIA

Martin
Martin ZT

Martin,

I believe that kind of backfire is not under load, but when you go off the throttle.

However, misfire under load or backfiring through the carbs is well documented here. In addition, I just went through the problem and solved it all thanks to this BBS.

Frankly, it was driving me nuts. My timing was spot on, I had a new vacuum advance unit and my plugs were perfectly gapped. My points were dwelling at 60 degrees , my coil was hot and my carbs were just rebuilt and personally adjusted by Joe Curto himself. The car idled beautifully but just couldn't pull beyond 3500rpm without sounding like the Marlboro Man.

I was just almost ready to bring it to Joe, but I really wanted to figure this out. It wasn't making sense! Then, I saw the light here, or rather was led to Paul Tegler's site. Skeptical, I decided to look at my distributor a little more exactly. Sure enough, it was a 25d4...but a 41401. Something built for an Austin Marina or similar. My old advance had been busted, so I guess the dizzy worked. But with the new vacuum advance, things were just wrong.

Luckily, I still had a 41288 with proper vacuum advance in the garage. A half hour later, things were changed 180 degrees, or well, maybe just 5 at speed.

What's the lesson? Your car will only run properly if the advance curve suits the engine exactly.

cheers
philip

http://www.teglerizer.com/mgstuff/a_stumble_at_cruise.htm
Philip

Philip
Thanks for the input. I suspected it might be something along the lines of this. I am about to install a Ford EDIS (distributorless set-up) as per the Megajolt design and this will give me the opportunity to set the advance up on the lap-top to perfectly match my engine.

However, I will check the dizzy and set points etc and see if that brings any improvement.

Thanks
Martin ZT

This thread was discussed between 16/05/2005 and 20/05/2005

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