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PostPosted: Fri Feb 07, 2020 9:18 pm 
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Since I rebuilt the engine 18 months ago the car has been going well. It is completely standard with SU carbs and Delco distributor still with points. Tonight it started to misfire, but only when cruising on a steady throttle at about 60 mph. On give and take roads where you are accelerating and slowing with frequent gear changes it performs nicely. But once on the dual carriageway it runs along sweetly for a mile or so then starts to stutter and won't accelerate. Get back onto normal roads and all is well again.

It's going to be fuel or sparks, but which? Could the mechanical pump be unable to keep up with demand, or is the horrible Delco dissy more likely to be the culprit? It's a weird one. Why doesn't it misbehave when not cruising? Any ideas where I should start?

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PostPosted: Fri Feb 07, 2020 10:01 pm 
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Since I rebuilt the engine 18 months ago the car has been going well. It is completely standard with SU carbs and Delco distributor still with points. Tonight it started to misfire, but only when cruising on a steady throttle at about 60 mph. On give and take roads where you are accelerating and slowing with frequent gear changes it performs nicely. But once on the dual carriageway it runs along sweetly for a mile or so then starts to stutter and won't accelerate. Get back onto normal roads and all is well again.

It's going to be fuel or sparks, but which? Could the mechanical pump be unable to keep up with demand, or is the horrible Delco dissy more likely to be the culprit? It's a weird one. Why doesn't it misbehave when not cruising? Any ideas where I should start?
If it is OK on "give and take" roads and under short bursts of acceleration then I would rule out ignition and go for fuel starvation. Either knackered pump or partially blocked fuel line. A good test is to find a stretch of road with a long incline and put you foot down, if it gradually dies I would say fuel starvation. Ignition usually shows up with a) warm/hot engine and b) under acceleration where the mixture has richened up causing a rise in HT to get a spark, causing any weaknesses in the HT side to show up. A temporary fix for a blocked fuel line is to disconnect the fuel line at the pump and use a compressed airline to blow it back to the tank. Let it settle for a while before starting otherwise you will just suck the crud through again.. Also check any inline fuel filters, they could be clogged. The crud comes usually comes from rust in the tank. It could be the condenser on the way out on the ignition side, they nearly always give odd symptoms before they fail completely.

You could just disconnect the fuel pipe from the carbs and crank the engine and make sure you get a good fuel delivery out of the open pipe into a suitable collection "jug", that usually gives a good indication of potential fuel starvation issues.

Roger

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PostPosted: Fri Feb 07, 2020 10:13 pm 
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Hi Mike,
Been there … etc; etc; etc;
There are 4 stages to this.
First I would suspect electrics rather than fuel, but why it doesn't show the problem on normal roads I don't know. It helps that you are still running points.

The 1850 Delco dizzy is actually a masterpiece of high-voltage engineering. Look at the long high-voltage tracking distances compared with the Lucas Sprint dizzy.

1. If the rev; counter bounces around when the engine misfires then it will be points needing cleaning and adjusting.

2. Can you check the L.T. wire where it emerges from the dizzy base. It has a knot in it. It can break here.

3. Can you also check the same wire where it goes round to the points inside the dizzy. Here is a piccy.

4. On my 1850 years ago I found that changing the coil solved the problem, but that involves spending money. Solutions 1 -3 are cheap.

5. It may be a faulty capacitor. P.M. me an address and I can send you a high voltage R.S. components capacitor on croc; clips you can try out instead of the internal distributor one. Just connect from coil negative to a good earth point.

6. Now the expensive solution. O.K. you had an engine overhaul done 15,000 miles ago. Was the head done ? On MPD many years ago I had a problem where it wouldn't go up gradients at more that 25mph and was coughing and spluttering. Yet on the level it was fine. After a lot of changing bits I had the head off and found an enormous build up of carbon on the valve stems, which coupled with ( probably) weakened valve springs was causing the inlet valves to stick open.
Cured by a decoke. Brilliant car after that.
Tony.


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PostPosted: Sun Feb 09, 2020 9:21 am 
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Thanks for the ideas chaps. When the bally weather settles down I'll get some of the checks done! I must admit I didn't notice whether the rev counter flicked when the problem occured. It was dark and the instrument lighting is is on the dim side of unreadable.

Tony, I don't know where you found the idea that the engine had done 15,000 miles since the rebuild. Make that about 2,000 and you would be closer. The head was decoked at the time and everything was fine. I'll try a few things and then if I need your kind offer of the external capacitor, I'll let you know.

If it turns out to be the fuel pump, as a long-standing MG owner I would quite like to fit an electric one. Where do folk mount them when fitting? The MG ones are usually under the car close to the tank.

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PostPosted: Sun Feb 09, 2020 11:37 am 
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Hi Mike, my Dolly does exactly the same thing when the dash pot oil is low,(Strombergs) it drinks the stuff :lol: I think it must leak as it's not a daily driver. My 2500S does the same but that is on SU carbs. why they do it i dont know. But an easy fix for me.

Tony.

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PostPosted: Sun Feb 09, 2020 1:52 pm 
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Hi Mike, my Dolly does exactly the same thing when the dash pot oil is low,(Strombergs) it drinks the stuff :lol: I think it must leak as it's not a daily driver. My 2500S does the same but that is on SU carbs. why they do it i dont know. But an easy fix for me.

Tony.
Strange that. I have owned many MGs with SU carbs, plus the odd Mini, and have never needed to top up the dashpot damper regularly, if at all. In theory the oil in there has nowhere to go, so why should it need topping up? I always use the same oil as is in the sump, ie 20/50 multigrade.

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PostPosted: Thu Feb 13, 2020 2:23 pm 
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Hi Mike, my Dolly does exactly the same thing when the dash pot oil is low,(Strombergs) it drinks the stuff :lol: I think it must leak as it's not a daily driver. My 2500S does the same but that is on SU carbs. why they do it i dont know. But an easy fix for me.

Tony.
The Strombergs have a small O ring at the bottom of the dashpot. These can and do go hard and crack with age. Once they have done this oil will seep past and into the main carb. They are readily obtainable and fairly straight forward to change.

As far as I know the same issue can not occur on an SU carb unless it has been physically damaged.

Roger

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PostPosted: Thu Feb 13, 2020 3:43 pm 
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The dashpot oil in an SU carb is in a closed metal tube. There is nowhere for it to go. The diagram is of an older type, but the dashpot is the same on later versions.


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PostPosted: Thu Feb 13, 2020 4:01 pm 
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The Strombergs have a small O ring at the bottom of the dashpot. These can and do go hard and crack with age. Once they have done this oil will seep past and into the main carb. They are readily obtainable and fairly straight forward to change.

As far as I know the same issue can not occur on an SU carb unless it has been physically damaged.

Roger
[/quote]
That would explain things Roger, the Dolly has not been touched since new. It is an early 1972 so i would think that is the cause despite only doing 27k.
Dont know why the 2.5 does it Mike, but it's something i can live with :D
Thanks both,

Tony.

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PostPosted: Fri Feb 14, 2020 4:47 pm 
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Right, I had a chance to take the car out today and the misfire is still there. It is odd because it only happens when at a constant speed. I held the motor in second gear and shoved down the throttle letting it rise up to 5000 rpm and a bit more, and it didn't miss a beat. But holding it at around 50 - 60 mph in fourth gear it staggers. Driving in town at 30 mph or less it never happens. The rev counter doesn't flick at all so I am leaning towards a fuel problem, but its a weird one. I'll take the top of the pump off and check for muck, also the float chambers. And I'll check for damper oil in the SUs too. Since the car is living outside I need to wait for a dry day. Might be a bit of a wait...

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PostPosted: Fri Feb 14, 2020 5:14 pm 
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How about driving it at the same revs in another gear Mike, does it do it then?

Tony.

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PostPosted: Fri Feb 14, 2020 6:46 pm 
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How about driving it at the same revs in another gear Mike, does it do it then?

Tony.
Yes, I tried various scenarios. Holding it at constant throttle in any gear (overdrive in or out) at any speed above about 40 mph results in the engine misfiring. Backing off the throttle calms it down, but as soon as I try to build up the speed again, back the problem comes. Its a strange one.

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PostPosted: Fri Feb 14, 2020 10:50 pm 
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It is a strange one Mike, have you got a spare set of plugs to try, or a spare set of leads? I'm fairly sure it's one or the other.... If it was fuel it would go worse when you accelerate. Even if they are new? you can still have a faulty plug or lead! or cap or rotor for that matter.

Tony

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PostPosted: Sat Feb 15, 2020 1:37 am 
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Agree now that fuel supply issues look unlikely. Is the car running with Strombergs?? If so have you checked that the carb diaphragms are OK? Usually they develop a large tear and give bad running across the rev range, but a small hole or tear can give rise to odd symptoms. Quick and easy to check, just undo the four screws and lift the dashpot off. Carefully check for any holes, nicks, splits or tears in the rubber. When replacing the dashpot make sure you put it back on the same way round, there is a small notch on the edge which corresponds to a raised "lump" on the edge of the diaphragm.

Easy and quick to check.

Still not convinced it is ignition as it only occurs under constant running above a certain engine speed, but not when accelerating with effectively a wide open throttle where if it was ignition I would have expected some hint of a misfire somewhere across the range.

If the diaphragms look OK then There is also the possibility of some issue with the advance on the distributor (yes I know this is ignition:) ). Have you checked the timing with a strobe across the rev range to check the mechanical advance is working correctly and there isn't any jitter. You could also try blocking of the vacuum advance and see if that has any effect - on a partial throttle there will be some vacuum advance going on, but little or none whilst accelerating.

Roger

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PostPosted: Sat Feb 15, 2020 10:48 am 
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Its a long shot, vac advance not working?

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