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Misfiring

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nrw34260 Nigel Webb
Xabia, Alicante, Spain   ESP
Hi, I have just purchased a 77 BGT 1800 with twin SU carbs. I am having a real problem with misfiring when hot. According to the MOT's (UK) the car has only been driven by the previous owner less than 50 miles last year in the UK so not sure she would have had this problem, the car is now in Spain and it is a lot hotter.

The car starts fine and runs well but after about 20 minutes it starts to misfire until it eventually will cut out, if I turn the car off for 5 minutes it will start fine again. I have tried leaving the car on my drive running for 45 minutes to get it hot and then I take it for a drive but it drives fine, again until it has been driving for about 20 minutes.

The last service was done in March and had new points (Intermotor INT 22580), condenser (Intermotor INT33720), fuel filter, plugs (Bosch Super4 - WR78) and carb tuning. The previous service 600 miles before it had a new coil, rotor arm (Black with Rivet), distributor cap and leads (High Temperature Double Silicone ISO3808).

At first I thought it may be vapour lock, when I removed the petrol cap it felt as though there was a vacuum so I put fresh fuel in and drove with the petrol cap off and still had the same problem. There is also a heat shield on the carbs so do not think it is a carb problem. It feels more electrical. Can anyone help shed some light on this problem please?

The engine is a replacement lead free fitted in 1999 by Abingdon Car Restorations with only 6000 miles on it, it is not an MG engine so how do I know what points and condenser I need?

Thank you for any advice.



Edited 2 time(s). Last edit at 2018-06-13 05:28 AM by nrw34260.

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tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
What engine is in it would be a help. You say not an MGB, but is it still a B series engine with SU carbs, Lucas distributor etc? All cars work the same, but there are many details that it could be. For instance, if the rotor is black or has a rivet in it, they can short to the shaft. You want the red one from Distributor Doctor. New wires. But what are they? What do the plugs look like. What are they, what gap? HS or HIF carbs? What condenser ( many bad ones) , what coil? Do you have a solid 14.2 Volts from the alternator when hot? It has points, but does it have any additional electronic spark box? Is the miss random or one cylinder? Is it different at idle or under load?

A picture of the engine would help to identify it.

I would put a timing light on it when it is miss-firing and look to see if the timing was solid, no dropouts and no scatter. That would help identify if ignition or fuel



Cogito ergo sum periculoso

nrw34260 Nigel Webb
Xabia, Alicante, Spain   ESP
Thank you for your feedback, I have edited the post with further info.

I have a solid 14.2 when running at 4000rpm

Not sure on the make of distributor (Fitted by Fisher Services) or coil, this was done before I bought it and there is nothing on the invoice to say what make and there is no marking that I can see. The garage has written on the invoice "Unable to fit electronic ignition kit".

I have uploaded some photos which I hope help.

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nrw34260 Nigel Webb
Xabia, Alicante, Spain   ESP
OK, just ran the car for 1/2 an hour on the road. It seems tat the misfiring occurs under 2000rpm and only when under load, when ticking over no problem and when just sitting and revving it no problem.

It does feel as though the overdrive is kicking in in 2nd and 3rd as I will be driving along and the revs will drop and the car speeds up. Strange. It is like driving an automatic car and the gearbox keeps changing gear.

nrw34260 Nigel Webb
Xabia, Alicante, Spain   ESP
Also, sometimes the charging light comes on and then goes off, checked the charge and even with the light on still getting a charge to the battery.



Edited 1 time(s). Last edit at 2018-06-13 06:48 AM by nrw34260.

B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
The charging light is a bad alternator, which may be causing your ignition issues.
The points distributor looks original to the car, and while it has new points installed, it may need a rebuild.
Go with standard copper core NGK spark plugs. It will run better. Your ignition system does not support a multiple-electrode plug.
Don't drive your car under load at 2000 rpm. Its designed to be driven under load above 2000, up to 4500 rpm on a regular basis. "Lugging" the engine at low rpms is not good for it.



jeff@advanceddistributors.com

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Looks like a regular B series engine. Plate is from some re-builder. Standard Lucas 24D dizzy. Good. Get rid of the "magic" plugs and fit some standard solid core, preferably NGK plugs gaped at .025 for a start. Whoever put them in does not know anything about old engines, so all work would be suspect. Get rid of that rotor. See the rivet in it? It will short out. Order one from the Distributor Doctor in England. ( We use Advance Distributors over here) . While at it, as it is an unknown, get a set of low DC resistance helical core suppression wires. NGK, Bosch, Cobalt, Pertronix etc. Under 1K Ohm per foot. Coil looks old, so maybe that was not screwed up. Quality of ignition parts is a big issue. There is a supplier in England who still has decent condensers for the Lucas dizzy. If you search the forum or WEB, you'll get a lot of history on the problem.

That will probably fix you up. Under load at low RPM is the toughest thing for ignition, so if it is not 100%, it will fail.

Sounds like the wiring is a bit boggered if the OD kicks in in second. For diagnosing the miss-fire, turn it off for now. Fewer variables.

I suspect a poor connection somewhere. Your alternator light should go out and stay out after starting unless your RPM drops so low, it is no longer working.



Cogito ergo sum periculoso

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B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Actually, that's a 45D Lucas distributor. Used in the NA market in 1975 only, across the rest of the world 1975 - 1980. Only we in the NA market were subjected to the horrible electronic ignition.

There's no such thing as a 24D. A 25D is the predecessor used 1962 -1974.

Your plug wires are likely fine, but you can test them to verify they are under 2000 Ohms resistance for the highest wire. If they are over 5000 Ohms each, they are no good. I suspect your bigger problem is the 5000 Ohm resistor in your spark plugs. As I posted earlier, in stall a copper core non-resistor plug set and gap them at factory settings.



jeff@advanceddistributors.com

nrw34260 Nigel Webb
Xabia, Alicante, Spain   ESP
Hi, Thanks for your input. The distributor has been replaced . I will change the plugs, these were put in on the last service. What is the optimal idle speed? Although the alternator light is coming on, it is still showing it is charging properly. Could it be just a wiring issue?

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nrw34260 Nigel Webb
Xabia, Alicante, Spain   ESP
Thank you. Yes the engine was fitted by Abingdon Car Restorers in the UK in 1999, who are MG specialists. It has only done 6000 since then.

The coil was supposedly fitted 600 miles ago.

I will change the plugs, wires and rotor.

nrw34260 Nigel Webb
Xabia, Alicante, Spain   ESP
In reply to # 3754840 by B-racer Your plug wires are likely fine, but you can test them to verify they are under 2000 Ohms resistance for the highest wire. If they are over 5000 Ohms each, they are no good.


Just tested the #1 lead using te plug cap and distributor cap which is 12" and it is 4.41 kOhms

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Dome shaped cap, I thought meant a 25D. Weren't the 45s more squared off? 4, 5 typo. That blue cap we can just see looked more squared off.

4K wires are pushing it, but we agree, it is probably the plugs and rotor. I would go for around 1K .

I disagree on "horrible" as the CEI is a superb ignition. It just needs to have the correct coil which MG did not bother to fit. OPUS, well I'll give you that!



Cogito ergo sum periculoso

nrw34260 Nigel Webb
Xabia, Alicante, Spain   ESP
Hi,

Just found this on another post :

The White wire circuit that powers the OD on/off is the same circuit as the ignition and fuel pump

Wondering if there is a wiring problem as the misfiring occurs when the O/D starts playing up.

Blueosprey90 Avatar
Blueosprey90 Jeff Sienkiewicz
New Milford, CT, USA   USA
Coming from the MGA world and chasing multiple high speed misfires for about 5 years on my race car, I suspect both the coil and the condenser. After driving for your 20 minutes, feel the coil. If it is blazing hot, the coil is bad. Otherwise just try swapping in a known good coil. New condensers are also known to fail promptly.

Ultimately, my underlying misfire was that I was running the car too rich. But that was discovered long after I had all those other coil and condenser problems.

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
At least for the 25D you can get a good condenser.



Cogito ergo sum periculoso

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