Vacuum advance misfire
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- Posts: 8
- Joined: Fri Sep 20, 2019 8:03 am
- Forename: Andy
- Surname: Billett
- Location: Tunbridge Wells
Vacuum advance misfire
Hi to all,
I own a 72 bgt which has developed an irregular misfire which gets worse at higher revs. What makes it particularly interesting is that by removing the vacuum advance pipe and plugging at both ends will remove the misfire completely. But I don’t want to run without the vacuum advance, and obviously something isn’t right.
The car is pretty much standard 1800, twin SU’s etc though I have an electronic module in
Place of points with absolutely no problems. I’ve checked the vacuum advance which appears to function perfectly, rotating the distributor base plate. I’ve tried
adjusting the timing but the misfire persists. Carburettors are pretty well set up.
So runs fine with no vacuum, but misfires with, even though everything is doing what it should.. or so it appears !?
The distributor electrodes show some possible arcing though this could be where I’ve been trying timing adjustments.
I can’t seem to solve this one, can anybody help
Thanks
Andy
I own a 72 bgt which has developed an irregular misfire which gets worse at higher revs. What makes it particularly interesting is that by removing the vacuum advance pipe and plugging at both ends will remove the misfire completely. But I don’t want to run without the vacuum advance, and obviously something isn’t right.
The car is pretty much standard 1800, twin SU’s etc though I have an electronic module in
Place of points with absolutely no problems. I’ve checked the vacuum advance which appears to function perfectly, rotating the distributor base plate. I’ve tried
adjusting the timing but the misfire persists. Carburettors are pretty well set up.
So runs fine with no vacuum, but misfires with, even though everything is doing what it should.. or so it appears !?
The distributor electrodes show some possible arcing though this could be where I’ve been trying timing adjustments.
I can’t seem to solve this one, can anybody help
Thanks
Andy
Re: Vacuum advance misfire
My approach would be to substitute all of the electronic gubbins with good old fashioned points/ condenser etc., connect up the vacuum advance and see how it runs.
Original equipment in good condition is so much easier to deal with and quite adequate for most folk.
Ian F
Original equipment in good condition is so much easier to deal with and quite adequate for most folk.
Ian F
MGCC member
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
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- Forename: Vic
- Surname: Butler
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Re: Vacuum advance misfire
With the vacuum advance disconnected then the base plate isn't moving so I reckon the fault is related. It could be pulling a wire which then makes intermittent contact.
This nearly happened to me. When I removed the magic eye from the distributor I found that the wires inside the grommet in the distributor body had lost their insulation. This was because when I clipped the wires in the distributor I hadn't allowed enough free play to allow for the movement of the base plate when the vacuum advance was operating.
I suggest you check the LT wires in the distributor. If they are frayed and the ignition system is Lumenition give them a call and they'll supply a replacement magic eye.
This nearly happened to me. When I removed the magic eye from the distributor I found that the wires inside the grommet in the distributor body had lost their insulation. This was because when I clipped the wires in the distributor I hadn't allowed enough free play to allow for the movement of the base plate when the vacuum advance was operating.
I suggest you check the LT wires in the distributor. If they are frayed and the ignition system is Lumenition give them a call and they'll supply a replacement magic eye.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Re: Vacuum advance misfire
Vic - just a quick question - don't the weights move the base plate as the distributor speed increases?
Ian F
Ian F
MGCC member
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
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- Posts: 788
- Joined: Wed Dec 21, 2016 6:07 pm
- Forename: Vic
- Surname: Butler
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- MGOC Member: Yes
- Location: North West Hampshire
Re: Vacuum advance misfire
Yes, it does but if disconnecting the vacuum advance solves the problem then it has to be linked to the base plate moving. It's still worth checking the LT wires in the distributor. I don't know but the vacuum advance could move the base plate more than the centrifugal. It's certainly worth checking the wiring first as it's the easiest option.
I wonder if the engine was run at high enough revs to trigger the centrifugal advance when the vacuum advance was disconnected.
I wonder if the engine was run at high enough revs to trigger the centrifugal advance when the vacuum advance was disconnected.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
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- Joined: Fri Sep 20, 2019 8:03 am
- Forename: Andy
- Surname: Billett
- Location: Tunbridge Wells
Re: Vacuum advance misfire
I wondered about going back to points. I think it’s got to be worth a try so I’ll get into it next week. I’ve had another look at the LT wires and all appears fine.
My understanding is that the Centrifugal advance kicks in with more revs and the vacuum drops out as the throttle is opened. This all appears to be ok.
Changing to points should hopefully give me some answers...thanks
My understanding is that the Centrifugal advance kicks in with more revs and the vacuum drops out as the throttle is opened. This all appears to be ok.
Changing to points should hopefully give me some answers...thanks
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Re: Vacuum advance misfire
The vacuum advance only operates on a light cruising throttle whereas the centrifugal kicks in at higher revs. However rubber bumper B's have the vacuum advance take off on the inlet manifold which gives advance on tickover where it's certainly not needed.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
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- Posts: 788
- Joined: Wed Dec 21, 2016 6:07 pm
- Forename: Vic
- Surname: Butler
- MGCC Member: Yes
- MGOC Member: Yes
- Location: North West Hampshire
Re: Vacuum advance misfire
Another thought. Is there an earth wire in the distributor? Not all electronic ignition systems need one but it's worth checking if there is one.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Re: Vacuum advance misfire
Vic Butler wrote:
> Yes, it does but if disconnecting the vacuum advance solves the problem
> then it has to be linked to the base plate moving. It's still worth
> checking the LT wires in the distributor. I don't know but the vacuum
> advance could move the base plate more than the centrifugal. It's certainly
> worth checking the wiring first as it's the easiest option.
> I wonder if the engine was run at high enough revs to trigger the
> centrifugal advance when the vacuum advance was disconnected.
Thanks Vic
Ian F
> Yes, it does but if disconnecting the vacuum advance solves the problem
> then it has to be linked to the base plate moving. It's still worth
> checking the LT wires in the distributor. I don't know but the vacuum
> advance could move the base plate more than the centrifugal. It's certainly
> worth checking the wiring first as it's the easiest option.
> I wonder if the engine was run at high enough revs to trigger the
> centrifugal advance when the vacuum advance was disconnected.
Thanks Vic
Ian F
MGCC member
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
- Charles Farran
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- Joined: Fri Oct 31, 2014 11:08 am
- Forename: Charles
- Surname: Farran
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- Location: Warwickshire
Re: Vacuum advance misfire
Andy,
On a different tack,have you checked all the plug gaps are correct? An oversized gap on one plug will cause a missfire intermittently at high revs/speed.. I know because it happened to me last year. I was sure i had checked & double checked them,& swopped all the leadx,tried a new distributor cap & rotor arm , but still the missfire occurred. ( I have a 123 distributor but otherwise the car is as came out the factory engine wise ). A neighbour checked the plug gaps & found one to be excessive. Once corrected, the engine has not missfired since!
Cheers, Charles
On a different tack,have you checked all the plug gaps are correct? An oversized gap on one plug will cause a missfire intermittently at high revs/speed.. I know because it happened to me last year. I was sure i had checked & double checked them,& swopped all the leadx,tried a new distributor cap & rotor arm , but still the missfire occurred. ( I have a 123 distributor but otherwise the car is as came out the factory engine wise ). A neighbour checked the plug gaps & found one to be excessive. Once corrected, the engine has not missfired since!
Cheers, Charles
1980 Roadster