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MG MGB Technical - Not MG but need input on a miss

I am trying to finish a refresh on a 79 spitfire for a friend. Car has 55,000 miles and sat for 10 years. Initially, we could not get the car to stop running rich. This is with the single ZS carb. Even the british car specialty shop could not get it to run right.

We changed it over to a SU HIF4 carb. It no longer runs rich, but we found it has a consistent miss that is affecting the ability to get it to pass smog (California car).

Car has an Allison 700 (crane) electronic ignition.

This is what we have done.

New plugs, cap, rotor, wires. Have adjusted the Allison so it fires on the leading edge of the optical sensor.

Cap is not showing any issues, plugs are brown gray in color. Compression is 128 to 134.

We did find that there is only 9.6 volts coming in to the Bosch blue coil when we measure it with the key on. On the negative side, we are seeing 8.2 volts. So I am assuming it is a resister coil. Planning to jump from the battery directly to the coil and see if that impacts the miss.



Wiring is original and I did not find anything out of the ordinary when I pulled the dash and looked at the wiring.

So what is changing the voltage from the battery through the ignition switch to the coil? I have pretty much exhausted the minds of the triumph folks. Since the 79 spitfire 1500 uses similar parts as the 79 MG, I thought to give this group a try.
Bruce Cunha

9.6V is high if it is a ballasted system which usually allows for 6 to 7 V. Check the resistance of the coil primary windings by putting a meter from + to neg, with all connections removed. A coil intended to run off a ballasted supply will read somewhere close to 1.5 ohms, a standard coil 3 ohms and a coil intended for electronic ignition around 1.0 ohms. Also check the battery output just incase that is flat.
Just one HIF 4 will not flow enough for a 1500cc engine. UK 1500 and 1275cc midgets had twin HS 4's. You might get away with just one HIF6 at a pinch. The miss could just be the engine is being starved at medium to high revs.
Also check the fuel pump delivery, 1 litre a minute or thereabouts.

Allan Reeling

Allan

1275 Midgets had twin HS2s.
Dave O'Neill 2

Thanks Dave.........typo/brain!!! B's were 4's of course!!
Allan Reeling

A 79 Spitfire was almost certainly ballasted, so to repeat some of the previous that should have a 6v coil and a ballast resistance (in the harness on an MGB) of the same resistance as the coil, i.e. about 1.5 ohms in each case.

What voltage you see on the coil +ve depends on the coil resistance, the ballast resistance (if there is one), and the resistance through the points or trigger to earth.

The points have to be closed, but electronic triggers may or may not provide a path to earth depending on what type they are. Even with points if they are in poor condition, or any of the circuit from the coil -ve, you will end up seeing more voltage on the coil +ve than you should.

So the first thing to do is measure the coil primary resistance i.e. across the spades with the wiring removed.

The connect the ignition feed wire to the coil +ve, connect an earth to the coil -ve, turn on the ignition, and measure the voltage on the coil +ve.

Foe a ballasted system you should see about 6v, which shows that half the voltage is being dropped across the coil and half across the ballast resistance as it should be (for running, not cranking). If you see around 9v that implies you have a 12v coil plus the ballast resistance, which reduces the current through the coil by 25% and ups the coil voltage in this test to ... 9v.

But if you are seeing 8.2v on the coil -ve at the same time as seeing 9.6v on the coil +ve, then the coil only has 1.4 volts across it instead of 6v for a ballasted system or 12v for an unballasted - very wrong. But if that were the case I doubt it would even run, so I suspect your tests aren't telling you the true story for some reason.
paulh4

Thanks all. I will do the coil checks and let you know what i find.

As I understand it, this car would have had the lucas electronic ignition and the in-line ballast. What does that ballast look like and where did they normally put it.

I do note a finned aluminum item with white wires attached to the alternator bracket near the coil. Is that the ballast?

The Allison says it does require a ballast. "The XR700 requires ballast resistance to limit coil current"

As for carbs, all the 79/80 MG's and triumps had a single ZS carb. I would convert this in a heartbeat, but in California, you are not able to modify the car. In this case, we are taking a chance switching to the su, but most of the people working in the smog check shops don't know a ZS from an SU>
Bruce Cunha

Bruce.
On a MGB the "ballast" is a length of resistance wire in the loom, pink and white, but confusingly it's a different colour at each end. White and light green at the coil + and white at the fuse box. There will also be another wire on the coil+, probably white and blue. This goes back to the starter solenoid to provide full battery voltage when cranking. Assuming BL, in their "rationalisation mode", did the same with Triumph it may be the same. As for your "finned" item, follow the wiring. If someone has put a ballast resistor in a circuit which is already ballasted, it could explain the "odd" voltages, the misfire and the inability to get the emissions right as there will be incomplete combustion.
Allan Reeling

"finned aluminum item with white wires "

That is probably the early 45DE4 electronic ignition system as used on MGBs. That does have a discrete resistor to feed the distributor electronics, as well as the harness ballast for the coil.

On an 4-cylinder MGB it is white/light-green at the coil +ve and the solenloid boost contact, it is the V8 that has white/light-blue.

As every component bar one (that I can think of) on the MGB has different coloured wires each side of it, it would be odd if the harness ballast had the same colours both sides.

The Crane instructions are very confusing if not incorrect when they say "The XR700 requires ballast resistance to limit coil current". It only does so if you are using a 6v coil i.e. one that measures about 1.5 ohms between the spades. A 12v coil which measures 3 ohms between the spades is also correct and does not need a ballast resistance. The instructions acknowledge this, albeit in a very confusing way, when they talk about coils with an internal ballast resistance and indicating they are also suitable. If a coil contains an internal ballast resistance it will measure about 3 ohms between the spades and will be a 12v coil. You won't know if a 12v, 3 ohm coil contains a 1.5 ohm internal ballast resistance plus a 1.5 coil winding, or whether it contains a 3 ohm coil winding and no ballast. The effect as far as connecting and using the coil is exactly the same, internal ballast is irrelevant and simply shouldn't be mentioned. It simply boils down to one thing: If your coil measures about 1.5 ohms it needs an external ballast resistance of some kind. If it measures about 3 ohms then it doesn't.

The Crane instructions are also incorrect when they say "All vehicles with OE breaker points are factory equipped with ballast resistance. This can be external resistance in the form of a ceramic ballast resistor or a resistance wire between the ignition key and coil + terminal". As we know, chrome bumper MGBs (for example) do not, and I'd lay odds that all BL vehicle prior to the early 70s don't either, as well as other manufacturers.
paulh4

V8's, the only MGB's I have Paul, just assumed (silly me) that 1800 B's would thus be marginally less confusing!!
Allan Reeling

Is the mis-fire a regular miss or intermittent?
Maybe a burnt exhaust valve? Also check for an air leak at the manifold by squirting some carb/brake cleaner along its length while iding. No change in engine note should be noticed.
I was just trying to think of things other than ignition.

Colin
c caygill

plan to spend most of tomorrow checking out the instructions provided.

Colin. Mis-fire is regular and across the acceleration range You can hear it in the exhaust, but the plugs look all the same. Used a timing light to look at how each plug was firing, but cant really see any one that is different.

I have put a lot of time into checking for vacuum leaks. None that I can find on the manifold to engine, or carb to manifold. Also checked all the vacuum lines.

It appears the miss has been there even prior to changing the ZS to SU.

I have set the valves twice and feel comfortable they are set right. Compression range from 128 to 134. Have not done a leak down. That may be next if I don't find a coil or voltage issue.
Bruce Cunha

Right...well I'm going for burnt valve. If as you say its a regular miss throught the rev range.
See what your leakdown test reveals.
Colin.
c caygill

128 to 134 i.e. 5% lowest to highest is well within the 10% max that is usually given for compression range. You could well have a burnt exhaust valve, which you may well hear in the exhaust, but it would be so slight I'm surprised it would affect an emissions test as combustion must be almost complete. Unless perhaps they are very marginal on emissions at best.
paulh4

Sorry for the length of this post. As it was stated. Diagnosis via the internet is tough.

Did a lot of checking on the electrical system. Here is what I found.

With all wires off the Bosch coil (blue) Resistance across the + to - is 3.6 ohms.

Power wire (White and Yellow)runs from the starter solenoid to the coil. Going to the same terminal on the solenoid is the Ballast resister wire. (White and Red). This comes off of the starter switch and also goes through the drive resister (white wire)

The wiring for the drive resister is white and gray from the - of the coil. This then ran to the drive resister and to the distributor. The system for the lucas electronic ignition was removed from the dist but the wiring harness plug is still present

With the ignition on, I get 12 V on the white yellow wire (power to the coil) from + to ground.

With the Allison ignition hooked to the coil, the voltage on the + side to ground is 9 volts with the ignition on.

When the car is running, the voltage at the + to ground is 11 volts and 4 volts from the - to ground. (Tack comes off the - terminal)

Crane says

Quick Test for Ballast Resistance:

You can determine if your vehicle has external ballast resistance with this simple test. Disconnect any wires going to COIL-. Reconnect battery. Turn the ignition key on but do not start the engine. Use a volt meter as shown in Figure 23 and read voltage between COIL+ terminal and ground. It should be about 12 volts. Then momentarily jumper the COIL- terminal to ground. If voltage at COIL+ terminal drops below 8 volts, there is ballast
resistance between the ignition key and COIL+ terminal. To determine if your coil has internal ballast resistance, use an ohm meter as shown in Figure 24. Coils with internal resistance
will read 3 to 4 ohms from COIL- to COIL+ terminals.


So by this, the input voltage of 9 volts with the XR700 attached to the coil is low. The coil resistance would indicate it does have internal resistance .

Would the 9v with the XR700 attached, but 12 v with it off the coil indicate an internal issue with the Allison?

I have adjusted the optical trigger per the manual and have the carb set so I have light tan plugs. and still have a miss from idle through high rev.

Rechecked for a vacuum leak with carb cleaner and cannot find any.

I also did a leakdown and did not find any one cylinder that leaked down faster that any other.

Going to borrow a standard distributor and different coil from a friend and try that. It should at least tell me if it is the ignition system.

Thoughts?


Bruce Cunha

Here is the wiring diagram for this car.




Bruce Cunha

It looks like you have the wrong coil and you are also feeding the Allison ignition via the ballast wire, assuming that the Allison ignition is powered from the coil.

Possibly the simplest solution would be to remove the white/yellow wire from the coil completely and run a new 12v feed from the white terminal at the fuse box to the coil +.
Dave O'Neill 2

3.6 ohms is a little high for a 12v coil, but should be OK without ballast.

You connected the Allison for your voltage tests, you should have done what Crane (and I for that matter) said and that is connect an earth to the coil -ve, leaving the Allison disconnected altogether. If you see 12v, there is no ballast and your coil is correct. If you see closer to 6v, then you have ballast in circuit and hence the wrong coil. Your diagram does show ballast resistance in the wiring.

You can't test using the Allison connected unless you know what voltages you are supposed to get from it. 9v with the engine stopped is too high by my reckoning, as is 4v on the coil -ve. Likewise 11v running is a bit too high, whereas 4v is too low. But it all depends on what dwell the Allison is supposed to use, which is why you can really only tell whether it is right or not by substitution. If it runs right with points and a condenser for example, then the Allison is faulty.

Your diagram is too low a resolution to make out the wire colours though - at least for me! This http://www.triumphspitfire.com/images/wiring/78diagram.jpg looks the same.

For running, power comes from the ignition switch on the white, and branches to the distributor and drive resistor for the electronics, and also to the pink/white ballast wire.

The pink/white goes to the bypass contact on the solenoid, where there is also a white/yellow.

The white/yellow feeds ballasted power to the coil +ve.

When cranking the solenoid connects battery voltage to the white/yellow and pink/white. The former does northing, but the latter goes to the coil +ve to boost the coil voltage when cranking. In practice this should go from about 6v with ignition on (assuming the points for example are closed), up to about 10v with a good battery and connections when cranking, then drop to about 9v when running.

But as Dave says the simplest solution is to connect a white to the coil +ve. If you remove the white/yellow from the coil, insulate it so it can't short to anything.



paulh4

Incidentally white/red is nothing to do with the ignition directly, that is the wire that operates the solenoid when you crank.
paulh4

Thanks Paul.

I did run a jumper from the pos of the battery, directly to the + on the coil and then removed the white/yellow. It did not affect the miss.

Next step is to put in a point distributor and see what it does.

I have been told that any spitfire distributor will work in the 79. It was also suggested that an MGB dist may work. I have a spare dist with points for my 67 B. Anyone know if they are interchangeable?

If not, I have a neighbor that is rebuilding a 74 spitfire and I may be able to borrow his coil and dist.
Bruce Cunha

I'd say that pretty-well any 25D4 or 45D4 distributor will work, the only difference is in the curves. That means you may well have to set the timing slightly different to book to avoid pinking, and get different levels of performance and economy from one distributor to another, but they should all run the engine 'normally'.
paulh4

Thanks Paul. At this point, this would be a test to see if the miss is being caused by the electronic ignition. If I can put my B Distributor in it, it will be a lot easier than borrowing a point distributor from my neighbor.

If it shows that the electronic is the issue, I will probably just remove the allison and add a pertronic.

Let you know how it goes.
Bruce Cunha

Well, rule out the coil. I used my lucas coil from the B and hooked it up using a wire from the 12 v positive side of the battery to the coil. Car started right up, but still has the miss. Have a spare 25D4 distributor and will put new points and condenser in it tomorrow. Hoping that will fit and I can try it to see if it is the Alison Electronic unit.
Bruce Cunha

I want to thank everyone for their assistance in what has been a very time consuming issue.

I replaced the spitfire distributor with my spare 25D that I have for my 67 B. It is set up with a full plate/point/rotor from Advance Distributors. After installing it, the 1500 runs perfectly. Must have been a fault in the Allison electronic ignition, or some other issue with the original distributor. (no play in the shaft I can detect).

Now I need to know if I can install a set of points in the spitfire distributor. I note there are no lobes on the upper shaft.

Rather not give up my spare 25D. Looks like I can pick up a new 45D for around $60 on ebay. Will also check the for sale area here.

So I am hoping with this issue resolved, the car will pass smog and I can get it out of my garage and get back to my B and TD. No more British cars newer than 1974 for me.
Bruce Cunha

If the original 45DE4 distributor (which is what the diagrams indicate) was modified with the Allison then it probably won't have lobes as it would have been factory electronic with a special rotor to trigger the pickup.

Another demerit for electronic, and that they can only be diagnosed by substitution.
paulh4

Got an e-mail from Jeff at Advance Distributors. Probably one of the best Dist. Shops for british cars in the US.

Jeff stated that he would stick with the 25D. He also said the 45d that came from the factory with electronic ignition cannot be changed to a points unit.

So, I think I will just add the cost of a 25D to the refresh bill for the spitfire.

I have noted there is still the slightest of miss. It appears to get less when I tweak the HIF4. I went back and rechecked the spark plug gaps. (all plugs are a very light brown).

I used up a can of carb cleaner checking for a vacuum leak. Could not find anything except when I spray around the EGR valve pin (pin that lifts the valve. It is very slight.

Going to go back over the valves and I also need to check the exhaust. The connection between the exhaust manifold and the exhaust pipe was disconnected when I got the vehicle, so I am hoping for a slight leak.

If this was not CA. I would drive the car as is, but as I need to pass smog, I have to try and get the car to run absolutely perfectly.

Carb is at .7 below the bridge and the car runs/idles very smooth at 900 rpm. Timing is set at 10 DBTDC. Which the label on the hood says is correct for this car.
Ponts are set at .15, plugs to .25

Any other areas I should look at?

Bruce Cunha

Unless he has told you of a specific 25D4 distributor that will be better with today's fuels, you can equally well use a points-type 45D4. Having said that the 25D4 has come back into its own as the vernier timing adjustment can be used to cope with different fuels.

There are very many curves in both 25D4 and 45D4 - see here http://www.starchak.ca/tech/pdfs/lucas.pdf, and you can end up with something very unsuitable for your engine etc. Aim for something close to your original.

Oh yes, and watch out for places who tell you theirs is better than your original, it's probably the one they have available :o)
paulh4

Jeff said he uses 25D's in most of his own vehicles. I don't know a lot about distributors to know why one is different than another, but it does work.

I ran the car up to 2000 rpm *smog test rpm) and still note a slight miss. Just an occasional hiccup in the exhaust.

I have pretty much run out of things to check.

I think this car has the most perfect timing, dwell, plug gap, and valve lash of any car I have worked on.

Very smooth, Could set a glass on the valve cover at 2000 rpm with bairly a wiggle. accelerates very well. Good power. I would drive it like this if I could.

So what causes a miss?

I have new wires, new points, rotor, cap (all advance dist parts). New plugs, good compression, no vacuum leaks I can detect.

I cant check the bottom side of the intake manifold as the exhaust manifold blocks the underside.

Unless anyone has more suggestions, I may need to pull the manifold and replace the gasket to eliminate the potential of a leak on the lower side.
Bruce Cunha

Bruce,

Here's an added "Twist" to your 25-D questions.

www.youtube.com/watch?v=3IFluuwJ1S0

Regards,

Larry C.

PS; sorry about the link, you will need to copy and paste.
Larry C '74 B/GT

"still note a slight miss"

Could be anything. But as it is only occasional I would suspect ignition or fuel rather than anything like a manifold leak. You can check for those by spraying propane or carb cleaner around the joint, and seeing if it changes the engine note. But you may have already done that.

You can look for hiccups with a timing light, or Colortune, but that gets harder as the revs rise.

You don't say anything about carb adjustment method or needle and spring selection. Try variously weakening and richening the mixture and see if that changes anything, it sounds more likely to be weak to me. Setting the jet at a certain point below the bridge is fine initially to get it running, but then it needs to be set dynamically to get the correct mixture for your installation. However even that only sets it correctly at idle, off idle i.e. as the piston starts lifting the needle out of the jet is a different matter. With a non-standard carb you may have to experiment with richer or weaker needles.
paulh4

I think fuel is the issue. This is one of those frankenstein projects. Original ZS was rebuilt three times, once by a british car shop, and neither of us could get it to stop running rich.

We ended up replacing the ZS with an SU HIF4. Stayed as a single carb, because of the smog issue. Technically, you cannot change out any parts for different parts, but the smog shop I use does not know an ZS from an SU.

The SU came off of another triumph. Adjusting the needle height does have some impact on the miss. I can get it to almost no miss at idle, but at 2000 rpm (the rpm they use for smogging), I still get a slight miss.

I have checked the float, but probably need to rechedk it again.

I am going to go back to the smog shop (got to know the owner pretty well, so a few dollars and I can get the car on the tester and play with it a bit.

Larry. Thanks for the link. I have been in contact with Jeff at Advance Distributors and he recommends not using the vacuum advance on the 25D. Feels it is not needed I find that interesting also.
Bruce Cunha

Today, I looked for a vacuum leak again. Figured I would tighten the manifold and low and behold, I found that the PO left off a holder and nut from the lower attachments. Other one was loose.

If you have never taken off a manifold on a 79 spitfire, you have missed a lot of frustrating work. Probably one of the harder jobs to do on this car.

You cannot get a socket under the intake manifold to tighten the two lower bolts. You have to take an end wrench and put it straight on the nut and then turn it with a screwdriver in the box end of the wrench. That gives you about 1 flat of turn. I can see why the PO did not fit both back on.

I am going to build a new tool that I can use to tighten the nuts.

Manifold gasket shows definite signs of leaking. SO, a new gasket and holder are on order. Here is hoping that is the issue.
Bruce Cunha

This thread was discussed between 27/09/2016 and 23/10/2016

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