Hello

This ien't actually my own car - it's a mates which developed a strange missfire about a year ago. He started fault finding but due to illness hasn't looked at the car. To cheer him up I decided to have a look and get it up and running. After a bit of reassembly, unsticking the fuel pump contatcs and un jamming the brakes I was happily whizzing up and down the alley.

Then a missfire appeared. After playing for a while I've worked out that the problem is heat related. If i leave the car sat there at idle with the bonnet open - the temp gauge never reaches the N on the gauge and everything is fine. If i put the bonnet down, the temperature comes upto the the N on the gauge and the missfire starts.

If I lightly press the throttle the revs will rise without a problem. But if I accelrate quickly (off load) the car missfires like a machine gun!

I've tried running new +ve and earth feeds to the coil and the same thing happens. I have noticed that the coil gets too hot to touch. There is a spare one in the garage so I swap that over next time I go over.

Does anyone have any ideas what the cause could be?

The car is a 1970 MGB GT, it has a Peter Burgess ported head, wrapped tubular manifold and is fitted with a magnetronic ignition system instead of the points.

Cheers

Dave

Sounds like the thermostat may be faulty , can't really see why it doesn't reach normal with the bonnet up if it is anything else but it may not be the cause of your problem . Possibly worth sticking in a new set of plugs and trying it. Distributor cap, rotor arm and leads aren't too expensive and do break down with age and possibly heat. Have you stripped, cleaned and adjusted the carbs as an incorrect mixture could well cause a misfire when hot?

the carb dash pots probably need some oil as its been standing a while, doubt it will cure it but its a start

Is the heat shield fitted between the carbs and the manifold ?

Is it a 12V coil or one that needs a ballast which may be missing or bypassed. ? The coil shouldn't get that hot.

Does it still have a vacuum advance and does that work ? If you suck on the vacuum pipe, you should see the distributor base rotate.

Thanks for taking the time to respond.

I've opened the dizzy up to check the magnetronic unit was fitted, also fitted new rotor arm and leads but that made no difference. I haven't checked the vacuum advance on the dizzy.

The heat shield behind the carbs is still in place and the pipes are wrapped all the way upto the head.

I will check the dash pot levels.

The coil seems to have a small ballast resistor to the side of it. The spare coil there is a Lumenition Mega Spark 3 with a ballast resistor - just printed the wiring instructions so I will pop over in the next couple of days and swap this coil in.

Cheers

Dave

The thing to the side of the coil could be a supressor condenser. It would have one wire to it from the coil and would be bolted to the chassis to earth it. A ballast resistor, often a white ceramic block, would have two wires to it, one from the + 12V and the other to the coil.

Macbeast - that sounds about right - next to the coil there is a small round unit about 1" diameter by 1" long with a wire coming out of the end and going to the dizzy.

First chance I will swap the coil over for the other one that definitely has a ballast resitor with it.

When old fashioned coils fail under heat they usually stop the engine dead untill they cool down.

As the heat shield is intact then given the symptoms I would guess the engine has HS6 carbs of the Waxstat jet type, Waxstat jests can be spotted by the 10p sized disc at the bottom of the jet. (See ref 10b in picture below )

Wax stat jets were fitted to give better emission stability as the engine warms up but are notorious for temperature problems. istr a conversion kit to the traditional HS style jet (ref 10a in picture) was available, get your carb spec off the id tag strip on the carb and check with Burlen services.






[Edited on 2/8/11 by britishtrident]

Waxstat conversion see http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=12663

Thanks ffor the tips on the carbs - will check what they are. Would this affect only a single or couple of cylinders - see below

Had a play on monday night with a mate who used to rebuild B series engines in the Sherpa van. Now we are really puzzled!

Swapped the coil - no difference. Mate said it could be one of the carbs so we had a fiddle adjusting the idle up and down to see if one was dominate but no change. Then checked that the firing order was correct and the TDC was correct - all fine. Changed dizzy cap and rotor arm - no change.

Tried pulling off the plug caps while running - 1 and 4 had a major affect on the running, 2 had no affect and 3 had little affect. Plugs out showed that all were dry except 2 which was wet with petrol. Then moved on to do a compression test and all 4 cylinders had a perfect 12 bar that held.

So where next ?? It has been suggested to take the carbs and exhaust off and check the sealing to the head in cases there are any leaks between the inlet/outlet side of things.

Any help much appreciated.

Cheers

Dave

Check the valve clearances

I was just going to say that^^^

It would be interesting to see what the spark is like at cyl N0 2. Take the plug out, reconnect the HT lead, lay the plug on the head and spin it over. Is there a healthy spark ?
Let's not forget it seems to be heat-related, the coil is getting excessively hot and the automatic advance / retard may not be working.
I would look at the magnetic pickup in the dizzy - has anything come loose? I would also suspect the Magnetronic amplifier unit itself.
If the plugs in cyl 1 and 4 look ok - not wet, not excessively sooty or white, it's unlikely to be a carb problem.

Can't see in your postings that you have replaced the plugs leads and dizzy cap. For relatively small cost, this is the place I would start.

All of these can have temperature related failures with no visible signs.

A bit more information and it is now this starting to make sense if the idle speed is too low and the mixture settings are not correct B series engines with twin SU's have a tendancy to idle on the outer cylinders only. Get the idle speed mixture and timing right and the problems should disappear. The reason for this characteristic is the twin carbs on an engine with siamesed inlet ports, it dosen't occur on B series with a single carb.