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Engine Power Fluctuating

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Engine Power Fluctuating
  This topic is about my 1972 MG MGB GT
airjordan13 Steven J
Auckland, Auckland, New Zealand   NZL
1972 MG MGB GT "Mugsy"

Firstly - I'm only a young fella with little/no experience in classics so forgive me if the answer to this is obvious!

While on a drive last night, my engine starting having fluctuations in power - at first it just seemed like the tach wire was a little loose as it was bouncing up and down, but it then became apparent that it was accurate. It was most noticeable in 4th gear, but towards the end of the drive (maybe 15 minutes later) it was happening in all gears, with the engine really spluttering under throttle. When I got home I noticed that one of the rear seat belts had managed to slip down into the battery box - could this have done damage if the metal end connected with the terminals?

When I took it out today it was noticeably worse and power was seriously cutting in and out, including almost dying at one point before bouncing back. I looked to see if the distributor was loose but it didn't seem to be, and I can't see any melted wires or loose connections anywhere. I checked the battery and from a visual check it looks OK - it is under a year old too. The carbs have recently been rebuilt, and it starts up a treat.

Has anyone experienced this? Does anyone know where I could start to diagnose the problem, or at least how I could rule things out?


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late 64 Terry O
Milton, Ont, Canada   CAN
rebuilt carbs, how long ago........? how old is the gas in the tank, do not let anything get down on the battery terminals. could be disastrous....was it running fine before..... can you remember doing any changes that could have affect this? you say it starts fine, cold or hot....? does your carbs need the oil in the dashpot.......?? lots of questions to help you out, keep at it.......thumbs up

airjordan13 Steven J
Auckland, Auckland, New Zealand   NZL
1972 MG MGB GT "Mugsy"
The carbs were rebuilt under a year ago - towards the tail end of 2016 - and the gas is fresh as I just filled it up a few days ago. It was running beautifully before so I'm starting to think it must have to do with the seatbelt end touching the battery... how disastrous are we talking? I'm praying it hasn't ruined the whole wiring harness or anything!

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late 64 Terry O
Milton, Ont, Canada   CAN
the seat belt would only be a problem if it was grounded and touched the positive side of the battery, this would have blown this off or welded together....... none of the electrics would have been hurt down the car as this would have been contained at the battery..... but i doubt your seat belt "buckle" would have been able to do this unless the belt has conductivity in it....... so i would say you have water in your fuel....... as they have said here before. pull the fuel line off at the carbs and run some into a jar and see if you can see a line near the bottom where you may have water in it....... if so drain tank and carbs and refresh........thumbs up

Gokart Avatar
Gokart Silver Member Rob Illingworth
Palmerston North, Manawatu, New Zealand   NZL
If the metal end of the belt has hit the positive terminal and the body of the car at the same time it could have shorted and robbed you of some power I am guessing. It certainly wouldn't have done the seat belt any good.

I would stick with the seat belt running interference for the moment and ensure it doesn't happen again. With the seat belt safely tucked away from the battery does the car now run as it was before the problems?

Cheers Rob

airjordan13 Steven J
Auckland, Auckland, New Zealand   NZL
1972 MG MGB GT "Mugsy"
I removed the seatbelt last night and made sure the covering plate was screwed down before the drive today, which was unfortunate as I was hoping that would solve the issue. I'm hoping its the water in the fuel issue, or at least I can use that as a first step of diagnosis.

Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, OH, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
The seat belt and electrics could be a red herring.

- Pull the fuel supply off the carbs, rig a catch bottle and turn on the pump. The pump should fill 1/2 to 1 liter in a few seconds.
- When was the last time you tuned up the car?

If it were me I would do the easy stuff first

- RIg up a longish fuel hose from the firewall into a 5 gallon gas tank and and use the car's pump to pump 5 gallons of gas out - let the car and fuel tank set over niight prior to pumping - water will settle to the bottom over time and you have the best chance of pumping water out.
- Change the fuel filter just as a matter of course - cheap
- Verify that the tank is venting. Either a vented cap or by blowing through the hard line vent going to the charcoal can and verifying no restriction

You've now verified fuel supply is clean and adequate. Next...

- Start the car and set the idle screws to 3,000 RPM - check total timing is at around 30-32* BTDC.

WIth fuel and timing out of the way the only thing left is electrical breakdown. Coil, condonser points (if fitted) rotor, cap, wires and plugs

Dioes it run badly immediately after start and a quick worm up or do you lose power later when the car is fully warmed up and getting hot?

The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty -
Bart -

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Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
Checked the obvious things like coolant and oil levels?

Sinewave Avatar
Sinewave T. Keith Vezina
Kenner, LA, USA   USA
It sounds electrical to me. I would bet that the points have closed up or a dirty connection on one of coil wires. You can eliminate the possibility of the ignition switch by running a jumper from the fuse box to the coil terminal that has the white wire on it.

T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I

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dickmoritz Avatar
dickmoritz Platinum Member Dick Moritz
Philly 'burbs, PA, USA   USA
Oil in the carb dashpots?


Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

mdubash Avatar
mdubash Silver Member Manek Dubash
Lewes, East Sussex, UK   GBR
1968 MG MGB "Odysseus"
Check the ignition circuit first. Is the coil leaking? Are the leads to the distributor connected firmly? Is the HT lead firmly connected? Are the points properly adjusted?

- If duct tape doesn't fix it - you haven't used enough duct tape

RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
Sounds like an open circuit in the distributor low tension system. Check the small ground wire running from the body of the distributor to the point plate. RAY

airjordan13 Steven J
Auckland, Auckland, New Zealand   NZL
1972 MG MGB GT "Mugsy"
Thank you for the advice thus far - I have done a couple of checks and have a couple more symptoms that will hopefully help identify the fault, otherwise I'll check the fuel this weekend.

I have checked all the wiring, distributor (I have electronic ignition, no points), coil but nothing seemed amiss other than a quite frayed wire leading to the coil (photo below showing this). A new rotor arm was fitted in September last year, and at the same time the mechanic also noted that the fuel pump and coil seemed in good condition.

I ran the car last night and it was idling OK, but it was when I put it into gear that it seemed to start acting up a little. Driving down the street it seemed fine going one way, but then when I turned around, it started the 'bucking bronco' act again. This was driving only around 1km.

The car was also getting extremely hot, with the temp gauge showing almost as hot as it could go. I have an electric fan and I've never had any issues with cooling, but then fan wasn't kicking on so this seemed very unusual. A check of the radiator showed only coolant with no water visible - I've attached a photo, as maybe it's the coolant that doesn't need water? (this was put in before my ownership)

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Johnnyp1510 Avatar
Johnnyp1510 John Paidas
Warren, OH, USA   USA
1979 MG MGB "Maggie"
When the seat belt buckle hit the battery, did you get a backfire? Sounds like maybe you have a vacuum line off or leak. I had a backfire earlier this year that blew a cap off of sealed emissions port in the intake manifold in my 79 B and you would have thought the timing belt had jumped a tooth. That's how bad she was running. 50 cent plug later and all is fine.

steve12955 Steve B
1970 MG MGB "Emma"
Coolant color looks fine, I'm sure it's mixed with water. Check that the heater control valve isn't dripping onto the distributor. It is bolted to the right side of the head and has a hose that runs back to the heater core on the firewall. The internal seal leaks with age and when the cooling system is pressurized it may be dripping a little onto the distributor cap which will short the spark. Run your fingers under the valve, if it's wet replace it.

As a stopgap, you can put a small Ziploc bag around the valve and wire tie it to the valve. It will also show you if the valve is weeping.

Otherwise, I'd suspect the condenser.

Edited 2 time(s). Last edit at 2017-07-21 06:53 AM by steve12955.

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