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Mgb revs and dies.. but why

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Mgb revs and dies.. but why
#1
playground Joe W
Saint Helena, CA, USA   USA
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So my ‘73 ran when I parked it a couple months ago.

Now when I start it, it revs to about 2500 and then dies. Pump the gas pedal doesn’t help.

The fuel pump is working, I have spark and fuel is going to the carbs.

I want to attach a video but not matter how much I edit it, the file is still too large.



Edited 1 time(s). Last edit at 2024-04-15 07:08 PM by playground.

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Re: Mgb revs and dies.. but why
#2
lancaster44 Avatar
lancaster44 Silver Member Matt G
Portland, OR, USA   USA
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1977 MG MGB
1991 Rover Mini
How long does it take to die?

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Re: Mgb revs and dies.. but why
#3
playground Joe W
Saint Helena, CA, USA   USA
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Literally seconds. Video timed it at about 4 seconds

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Re: Mgb revs and dies.. but why
#4
playground Joe W
Saint Helena, CA, USA   USA
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Also for some more data.

After it dies, I can just restart it and it does it again. And I can literally start it over and over and over and over again. And it does the same thing each time.

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Re: Mgb revs and dies.. but why
#5
Ken Lessig Avatar
Ken Lessig Gold Member
Nevada, Tx., USA   USA
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Does it sputter before it dies, or quit suddenly and completely? Sputter may indicate fuel issue, sudden and complete may indicate electrical.
What do you have for emission gear on the car? Thinking the overflow tubes on the carbs may be plugged, not allowing air into the float bowls and starving the engine.

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Re: Mgb revs and dies.. but why
#6
ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
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1971 MG MGB
Sounds to me like your ballast resistor may have failed. If it runs while cranking, but dies as soon as you release the key, it is likely the ballast resistor (which in the case of the MGB is a resistance wire in the harness).


Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Re: Mgb revs and dies.. but why
#7
allanreeling Avatar
allanreeling Allan R
Randlay, Telford, Shropshire, UK   GBR
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1970 MG MGB V8 Conversion
1973 MG MGB GT V8
Pumping the "Gas" pedal doesn't achieve anything if you have SU carbs. There is no accelerator pump!!
As said previously, instant death after revving, with the Tachometer similarly instantly plunging to Zero, is a Low Tension issue, i.e., supply to the coil/distributor.
A slower coughing death, with the tachometer showing engine speed as it dies, is likely fuel related,

You don't say whether your distributor has points, or is electronic, or whether you still have a 1.5ohm coil and a ballast supply. (Two white/green wires on coil+).
Knowing helps.


soonerv8
Do it right, do it once!

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Re: Mgb revs and dies.. but why
#8
StoffelB Stoffel B
Nijkerk, Gld., Netherlands   NLD
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You usually won't find a ballast resistor on a '73 and I'm afraid the rev counter doesn't provide much information within 4 seconds.
What you need is fuel, air and a spark.
Once you have established a good spark, look in the float chambers (HS4?). If there is enough fuel in the float chambers immediately after it dies and there is no mouse nest in the air filter, I would use the multimeter.
But in any case, provide more information about the car. Did something change to the ignation? Different carburetors?

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Re: Mgb revs and dies.. but why
#9
B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
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1950 Willys Jeep Pickup "Ratrod"
1964 Land Rover Series IIA
1971 MG MGB
Sounds like a huge vacuum leak or lean condition. It could be caused by no dashpot oil? Watch the carb pistons when you start it.

Anti run-on valve stuck open?


jeff@advanceddistributors.com



Edited 1 time(s). Last edit at 2024-04-16 08:30 AM by B-racer.

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Re: Mgb revs and dies.. but why
#10
ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
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1971 MG MGB
Quote: You usually won't find a ballast resistor on a '73

He's right - my bad - the ballast resistor was added in 1975 (maybe 1974 1/2).


Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Re: Mgb revs and dies.. but why
#11
playground Joe W
Saint Helena, CA, USA   USA
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Thank for all the help.
Answer some question here's more data:

It seems sputters, on start up rpm gauge immediately goes to up to 2K and then back down, it's quick. again wish I can post a video, but here's a link

- no resistor
- it starts up without continual key engagement
- no smog equipment on it, removed by the previous owner. I've owned and driven it for about 3 yrs with no issue, everything is properly plugged
- It has HS4 SU carbs on it rebuilt about a year ago with a new intake manifold
- it has dashpot oil and appear to be the correct level according to the one book I have, if anyone can confirm how to check the appropriate level that would be great
- it has points system - new distributor about a year ago
- I checked and float chambers are full after stalling
- air filters are off currently so I can inspect what is going on (I reconnected the choke)

I'll look at the dash pot positions again. I attached a pic of the set-up


Attachments:
IMG_5195.jpg    55.8 KB
IMG_5195.jpg

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Re: Mgb revs and dies.. but why
#12
allanreeling Avatar
allanreeling Allan R
Randlay, Telford, Shropshire, UK   GBR
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1970 MG MGB V8 Conversion
1973 MG MGB GT V8
My 1973 Factory V8 had a ballasted system, thought the 1800s did too...........live and learn!!


soonerv8
Do it right, do it once!

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Re: Mgb revs and dies.. but why
#13
Ken Lessig Avatar
Ken Lessig Gold Member
Nevada, Tx., USA   USA
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Try disconnecting the overflow pipes (the ones on the float bowls that aren't the fuel inputs) temporarily. I've had dirt daubers plug them with their nests in the past, and that would result in your exact symptoms.

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Re: Mgb revs and dies.. but why
#14
Watauga Avatar
Watauga William B
Richmond, IN, USA   USA
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1974 MG MGB
Joe, have you changed the the PCV system and blocked off the feed to the carburetor air chambers recently? You seem to have no crankcase ventilation from what is shown in the picture?



Edited 2 time(s). Last edit at 2024-04-16 03:00 PM by Watauga.

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Re: Mgb revs and dies.. but why
#15
B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
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1950 Willys Jeep Pickup "Ratrod"
1964 Land Rover Series IIA
1971 MG MGB
I can't tell by the tach but my first assumption would be that the coil loses power when you let go of the key.
Second guess would be the carb overflows are plugged, which will act like a kill switch.
Third is still an end plug missing in the intake manifold - can't see front or back so check!
Fourth - are the intake and exhaust manifolds the same thickness? If not you'll have a huge leak there. It sounds like you have a massive exhaust leak in the video.


jeff@advanceddistributors.com

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