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MG MGB Technical - Sometimes fires on 3

Folks, I have a real odd ball problem with a 1977 MGB. We did a run last weekend without any trouble going round the Northumberland hills. On the way back along the motorway I started to get problems.
I can be driving along without trouble at 55 to 65 with no trouble but if I accelerate or come to a slight hill it starts to stutter and sounds like it is firing on 3. If I slow right down say for a bend or a roundabout and drop down a couple of gears when I start to accelerate out and go up through the gears it is fine even if I rev it above what would be the 55/60 speed revs in top/overdrive. It will then sit there quite happily until we get the next hill or I try to get above 55/60. The car is fitted with electronic ignition. I took it to my local garage who tell me that the timing is fine. They are waiting for the retired former owner to come with his knowledge of old engines.
Does anyone have an idea what is the problem please
Regards
Mick
M I Rookes

Sounds like a fuel pump or filter problem. When climbing or accelerating you need more fuel .
DARNOC31

You may also have a faulty distributor cap or rotor that is failing when the engine is put under extreme loads. The replacement items, available today, are notorious for poor quality and short lives. RAY
rjm RAY

I'm with RAY on this. Something in the electrical system is borderline under "pressure" or load. Time to check the distributor cap, rotor, wires, plugs and coil. If that doesn't fix it, then maybe the vacuum advance unit.
JM Morris

Thank gents,
I was thinking plug leads followed by distributor cap. It has electronic ignition so that is OK and it has a new capacitor on it.There was a new fuel pump on it this year and the fuel filter is clear. It also has all new fuel lines after a minute bit of rubber got stuck in a jet.
Regards
Mick
M I Rookes

Bit of an update gents.
I called at the garage on my way home, the plugs, leads, coil, etc are all OK. They are now looking at the carbs and the problem seems to be with the front carb. When it is stationary you can rev it right up and there is not a problem. If you lift the inside of the rear carb no problem but do the same on the front carb and it all but stalls. The mechanic had it out on a couple of test runs and it seems to run OK until the fuel in the carb/filter/lines is used up. It then starts firing on 3 or even 2. Pull over and let it tick over and it settles down back onto 4. When you set off again the same thing happens, its as though it gets through the fuel it 'stored' on tick over and when that is used up it starts running on 3 or 2 pots. A lot of head scratching on this one. Monday they are stripping the carbs, fingers crossed.

Regards
Mick
M I Rookes

Mick - keep us updated on this problem, it would be useful to know what's causing this.
Brian Shaw

Before stripping the carbs down they should check the fuel delivery first, about 2 pints a minute. If not change the filter and see if that makes a difference.
If thats ok check the pistons rise and fall the same on each carb, when lifted about 5mm they should fall with a light clunk.
Check the linkage betweeen the carbs, make sure the nuts are all tight and it operates on both carbs simultaneously, a common mistake is to adjust the carbs without undoing these.
Listen to the induction noise on each carb with a length of hose,they should sound the same.
Take the dashpots off and check for scoring and dirt, also check the pistons for the same, check the needles are not bent or ridged and that they are same type
While these are off you will be able to see the jets, if they look oval they are worn, the shoulders on the jets should be the same level on both carbs.

All of this is fairly simple and can be done yourself in about an hour. Beyond this with the HIF4 the carbs will have to come off to look at the float chamber.
c cummins

One thing at a time. Easy stuff first


Replace fuel filter. Simple, easy job - then check by going for a spin.

If above does not work, replace all spark plugs - easy and cheap - check by going for a spin

If above does not work, check delivery of fuel pump - easy job. Disconnet pipe downstream of fuel filter and pump into a jam jar. Pulsing flow is good, a piddling trickle means pump is u/s. Replace it.

My gut feel is fuel pump if it is struggling uphill but OK on flat.
Mark O

.... If you lift the inside of the rear carb no problem but do the same on the front carb and it all but stalls...

So, what has this mechanic done about it - surely not left it alone and took it out on a test run..?!

Lifting the pin with result that engine stalls = mixture in that carb is too lean. Adjust it so that when you lift the pin, the engine revs rise then settle back to normal. Do this on both carbs and THEN look at all the other things.

And btw, do not be lulled into a false sense of security because a capacitor is new. Some are of very poor quality and break down
Hal Adams

Folks,
It turns out that there was a longish sliver from the inside of the rubber fuel pipe that blocked the jet when it had to work hard on the front carb.
The offending piece came from the section after the filter and before the fuel pipe that goes round to the carbs. It is a relatively new piece and they think that a section must have been 'sliced' off when the pipe was replaced.
Thanks to all who made suggestions it is much appreciated. Hopefully the weather will be good this weekend for a final run before it comes off the road for the winter.

Thanks Again
Mick
M I Rookes

This thread was discussed between 10/09/2012 and 19/09/2012

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