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MG MGB Technical - Power lacking accelerating in 3rd and 4th

Hello,

I have a 1980 MGB GT (UK spec) that has only covered about 2900 miles from new (2200 when I got it late last year). That is a bit of a story, but the car is in excellent condition and is just about as it left the factory....unrestored and having had only minor changes made.

That said, it starts perfectly from cold or hot; idles perfectly; sounds great; and runs well. The plugs are a nice grey colour (maybe they should be slightly brown....mixture off a tad????).

In first and second, acceleration into the 3-4K range is good. In third and fourth, it is lacking and takes some time to get up to speed.

I have noticed this since I got it back last year, but have not driven it enough or had the time to look into it. This past weekend, while taking it on a spirited club drive up and down some twisties, I finally decided to look into it.

I thought it might be timing. Maybe the advance mechanism. I thought perhaps---due to age--the diaphragm in the vacuum advance might be deteriorated. But, I used a vacuum gauge on it tonight, and it held vacuum fine. The vacuum pipe from the dizzy to the manifold is also clear.

Any thoughts....just timing perhaps??? I'd like to look into it next week during my last week of vacation for the year, so welcome any input!

Thanks,
Julian
Julian Dufour

Does it have overdrive? Is that permanently switched on? The switch should be on the gear knob. The O/D will be locked out in 1st and 2nd gear by the switch on the gearbox. If it is on, when you change to third it will come in and the effect will be as if you went straight to 4th gear.
Mike Howlett

Acceleration will always take longer in higher gears. The manual gear lever switch was always iffy, the wires can short and it's unfused from the factory which will burn wiring, always a good idea to add a fuse particularly on this type. Also the fuel pump, both mine and several I have worked on have all had pump shorts.

The easiest way to check if OD is stuck on is when changing from 2nd to 3rd - if you do a quick change the OD should take a moment to come in so you should see and feel the sudden drop in revs as it does so.

Alternatively compare revs with speed - 4th should give 18mph per 1000 rpm and OD 4th 22mph per 1000rpm.

The advance mechanisms should be the same in each gear for a UK spec as it doesn't know what gear you are in (USA spec restricted vacuum advance to 4th gear only). A 1980 should have the inlet manifold advance source and if you disconnect that at idle (plugging the source) then the revs should drop. Other than that you can check the function of both centrifugal and vacuum advance with a dial-back timing light.

If it seems to be struggling in higher gears does pulling the choke help? If so could be weak mixture, the plug colours in the attached work well for me.

Is there enough oil in the carb pistons for the dampers? If you unscrew the caps, lift them up, press them back down and can feel the resistance of the damper before the cap reaches the neck then you have enough.



paulh4

Thanks guys for the quick reply.

Yes, it has overdrive, but it is definitely not stuck on...works perfectly!

The plug colour you show in the image is one that I would agree is right on the money. The mixture could be possibly slightly off, as I did note that I thought it should have slight brown tinge on the plugs, which it does not (although the PO had it professionally tuned in the UK).

I will try disconnecting the vacuum source at the manifold (you are correct, this is where it is on mine). Also, I do have a digital timing light with rpms, so will see if it shows the advance working properly.

Good thought on the oil in the dashpots...forgot that one!

I'll check all of these next week when I'm off....any other comments feel free to post them!

Cheers,
Julian
Julian Dufour

I think I have corrected the problem.

-I checked the dashpot oil.....was already topped up.

-I checked the timing, and it was pretty much set to the workshop manual specs.

-I then checked the carbs, and they were balanced (first time using my unisyn synchronizer).

-I then adjusted the mixture equally and by about a 1/4 turn on both carbs.

Test drove the car, and it seemed to pull well in 3rd and 4th gear. The plugs looked a good colour too. I might try richening them just a tad more, but I THINK it might be solved!

Thanks,
Julian
Julian Dufour

Makes one wonder what altitude you're living and driving at. I've never fully understood richening and leaning carbs at altitude but I know it's something that airplane drivers do. Could high altitude/less dense air have a bearing on your issue? Just asking.

Jud
J K Chapin

Julian.
Are you using the factory supplied needle or has the car been set up for your higher altitude? Simply adjusting the idle mixture does nothing about the mixture when off idle. You need a needle that has a leaner mixture throughout the entire range of piston travel.

About the ignition timing. I seem to remember from when I was living at such elevations, that the common practice was to set a little more ignition advance than is standard at lower altitude. John Twist commonly recommends setting the timing at 32 deg BTDC at about 4,000 rpm with the vacuum advance disconnected and plugged. Statements such as "the timing is close to the factory setting" does not provide any real information. The exact timing that you have noted through testing is of more use than vague statements.

Is your Unisyn device the one that has a float ball riding inside a glass tube and is placed over the mouth of the carb? If so, it is rather useless for properly testing of carb syncronization. It can tell you if both carbs are drawing the same amount of air at idle, but cannot tell you whether the carbs are properly balanced off idle and on acceleration.

What octane fuel are you running? I was in Colorado last week and the week before that and noticed that your octane ratings run two points lower than the fuel ratings in New Mexico and Nebraska, states that border Colorado.

Things such as the octane rating of your fuel, the exact timing curve of your distributor, and what needles you are running in the carb would be useful in figuring out what is actually happening.

Les
Les Bengtson

This thread was discussed between 15/10/2024 and 24/10/2024

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