Hrald 13/60 Misfires When Hot

[cbjroms]

Really grateful for all the advice and it encouraged me to go back out this morning and check everything again. What I found is that the bolt through the bulkhead was quite loose and so I undid it, replaced the bolt/nut (the nyloc insert had burnt), rebuilt the connection with a couple of shiny washers and tightened it up. I also removed the in/out feeds from solenoid, cleaned the terminals, replaced nuts/washers and tightened. Then did the same to the starter motor postive post. This has resulted in perfect cranking and so I am dead chuffed!

I still have the misfire as the engine gets warm and made a recording - Misfire

It still seems to me that the misfire is not specific to one cyclinder as removing plug leads one by one makes little difference. But the more I test the less sure I become! I tested the compression on each cylinder and the results were fairly consistent (110/105/105/115psi).

Does the recording suggest anything other than coil or distributor cap or rotor arm?

[glang]

Good we're making some progress then! I hope the metal was nice and clean where you bolted the cable lug and you could put a bit of grease over the whole connection to stop moisture/air ingress.

With the recording it seems more like big end bearing rattle to me but that could be just down to the sound quality.....

If youre certain its misfiring all you can do is first work through all the settings if youre not 100% certain about them:

tappet clearances,

carb mixture adjustment and operation (when reving the engine you should see the piston inside the carb entrance (air filter removed) rising and falling smoothly),

ignition timing and operation of the distributor - when using a strobe to check the timing you should see it advancing as you rev the engine which confirms the centrifugal advance/retard is working correctly,

After if its still misfiring then try swopping components: spark plugs, coil, leads, distributor cap and rotor arm.

 

[RobPearce]

Excellent news on the starter!

I don't think I agree with glang on the recording. It does sound loud and "thumpy", so I see where he's coming from, but it sounds to me like some cylinders not firing and pumping combustible mixture straight into a hot exhaust manifold, sometimes known as "eight stroking". Was it reluctant to rev as you were doing that? You said it's a hot problem so I'm guessing you'd warmed it up before taking that recording?

There are two common causes of the sort of severe misfire I'm thinking of. One is a very weak spark - works OK on a light throttle but not when you open it - normally because the coil is about to die. The other is a very weak mixture, which could be as simple as no dashpot oil if it only happens on revving. I don't think 13/60s have a closed breather system (as I recall, the hose runs from the rocker cover to the air filter), otherwise I'd say leaks in that. The 2500S engine does it if you don't fit the air box but the Herald's filter is not so disruptive to air flow.

[Rosbif]

Quoted from RobPearce-

I don't think 13/60s have a closed breather system (as I recall, the hose runs from the rocker cover to the air filter), 

You're correct there

[cbjroms]

Yes, the engine was warm when I did the recording. It starts and revs alright for a few minutes but then starts misfiring when I rev it.

At the moment I am running with just an open end on the rocker cover breather - no hose fitting so just venting to atmosphere. It is loud and thumpy but it is defintely not big-end; far more like a firing problem.

[cbjroms]

This afternoon I have adjusted all the valve clearances and redone the compression test. The results are the same as before BUT it has struck me that I should be getting 150psi per cylinder and yet I am only getting between 105 and 115psi! Just spoken to the guy who rebuilt my carb about adjusting the mixture. He pointed out that the carb will not operate properly at this level of compression, that I need to strip the head, check the exhaust valves and only adjust the carb mixture as a last resort. Now it could be that my compression tester is reading low - the consistency of the results  could suggest this? - but I do think the engine is running very rich and it doesnt even need any choke at all to get it running on the coldest of days. Grateful for your thoughts.

[glang]

The mixture adjustment on your carb should be very simple so theres no need to worry about fiddling with it. You need to remove the airfilter housing and underneath the carb you'll see a brass adjuster which has a slot in its underside. If you screw this in (needs a chunky but short screwdriver) the mixture gets weaker and out richer. Now look in the mouth of the carb and lift the large piston in there upwards to reveal its needle and the brass 'jet' that it slides in to. You need to screw the underside adjuster in or out until the top of the jet is flat with the top of the brass housing in which it fits. Count the number of turns of the screwdriver needed to do this so you can always get the jet back to its original position. Once you have the jet flush undo the adjuster two full rotation so the jet goes lower in its housing. This should be a good initial setting to run the engine correctly and then any fine tuning to get the mixture perfect with the air filter in place can be done later.

If thats the true compression its not good but try to get everything else correct first. Then if youre sure of the readings you could try putting some oil in each bore to see if they improve so indicating that the problem is with the head gasket or valves....

[RobPearce]

I don't actually think those compression readings are all that far wrong. There are so many variables in how you take compression readings, even before the vagueries of hardly-precision instruments, so for a relatively low compression engine like the 13/60 I'd say 105 to 115 is well within tolerance.

Your carburettor man is trying to scare you. The carb will not care how inefficient the engine is - it regulates fuel by air flow with little regard for how the mixture gets used downstream. Adjust the carb first, undertake major engine surgery as a last resort!

[cbjroms]

Back out on the car this afternoon - first chance since the Christmas break.

Conscious that my compression and misfire problems are unlikely to be connected, and before purchasing any new components, I decided to replace the female connectors on the end of the wires that go to the coil. Defintely 'clutching at straws'!!

Ran the engine and the same misfire started occuring. But then I suddenly noticed a spark refelcted in the shiny vacuum unit which is attached to the distributor. First guess was that I have a crack in the distributor cap but, given I didnt have a spare to hand, I decided to elimenate the Dysnamo first. Unplugged the two female connectors from the Dynamo, ran the engine and misfire has disappeared. Engine sounds sweet under both low and high revs - albeit I am just running under no load in the garage.

I had given the Dynamo a full test before putting it onto the engine a few months ago. So my first thought is that there must be something wrong with the wiring or the Control Box. But can anyone suggest how the Dynamo could be the root cause for a misfire under load? Could this be the cause of the weak spark leading to the "eight stroking" suggested earlier?

[RobPearce]

Possibly a silly question - you mention putting the dynamo on recently, which I don't think you'd mentioned before - did the misfire start shortly after you replaced the dynamo?

Possibly more helpful question - when you replaced the dynamo, are you sure you polarised it correctly? As you hint, it's hard to see how a dynamo would cause a misfire unless it's reverse polarised and actually flattening the battery instead of charging it. Then, the harder the dynamo works, the lower the voltage and the weaker the spark will be.

[glang]

How about just the fact that running without the dynamo will result in a lower system voltage which in turn gives a lower HT? This reduced spark voltage is still enough to run the engine but not enough to track in the distributor cap....

[cbjroms]

I mentioned fitting the dynamo a few months ago but it was actually the wiring that I was replacing. In fact, up to that point the engine was revving without any misfire BUT the Dynamo was not connected.

When I first connected the Dynamo the Ignition Light was not working properly and I found that the wiring into the Control Box was incorrect and so I swapped the connections around to get the Ignition Light working.

I didn't polarise the Dynamo at all and so it could well be incorrect. Looks like my first port of call needs to be the Dynamo wiring into the Control Box and then checking the polarisation of the Dynamo.

[glang]

Dont think it's necessary to go in too deep with the dynamo yet. Measure your battery volts, start the engine with the dynamo correctly connected and check the battery voltage is higher. It might take a few minutes for the battery to recharge after the start but it's voltage should end up somewhere near 14volts. If it stays lower than the initial voltage something is wrong!

[cbjroms]

Ran the engine with the Dyanmo disconnected again this afternoon and no sign, whatsover, of a misfire. Then I put a jumped between the D & F terminals of the dynamo, put a voltmeter across D and earth and with the engine running measured 1.5 volts whereas I expected 14 volts? So then decided (having checked that D on Dynamo connected to D on Control Box and F on Dynamo connects to F on Control Box) to put the leads back on the Dynamo and run the engine again with voltmeter across the battery. With the engine running I was getting 14+ Volts across the battery but arcing in the back end of the Dynamo was very pronounced yet there was no sign of the previous misfire!

So, in summary, on Thursday the misfire disappeared completely when I pulled the D and F leads off the back end of the Dynamo. When I ran the engine (for about 30 minutes) today there was no sign of a misfire, even after I can reconnected the D & F leads. With the Dynamo connected and engine at fast idle I am getting 14+ volts across the battery terminals but as I rev the engine harder there is significant arcing visible in the back end of the Dynamo.

Does this make any sense?

[glang]

The arcing is not good and will damage the dynamo commutator. It sounds like the brushes are not making good contact either because they are stuck in position or (more likely) worn too short. It's an easy job to remove the back bearing plate to access the brushes. Hopefully the commutator is still smooth and not burnt.....

[cbjroms]

Took the back plate off the Dymamo this afternoon and commutator looks fine. Both brushes seemed to be making good contact with the commutator but the lead to one of the brushes seems to have started to burn-out/fray. Cant understand why this should have happened but I guess that this would likely cause the arcing.

Is there any reason why I should not be able to get away with just fitting a new pair of brushes?

 

[glang]

Yes your commutator looks fine so just new brushes should do the job. However Im replacing the brushes on my dynamo at the moment as the 1967 originals are too short and Ive now bought two sets of currently available replacements (blue sheaths) only to find that both have much weaker looking leads! Ive got no confidence that theyre up to the job but dont know where Im going to find anything better....

Also replacing front roller bearing as its rough and a new one is very cheap (1.29 incl del).

[nang]

Clean up the commutator and lube the shaft !

Tony.

[thescrapman]

That commutator does not look very good to me, but needs an expert to come along.

i am sure it should be shines copper, 

the brushes and wire look like they have overheated

[RobPearce]

I don't think the commutator looks bad per se, but it really needs cleaning before making any judgement. Once clean, it should be smooth (if not shiny) copper with clean, straight gaps.

[cbjroms]

I have cleaned commutator, fitted new brushes and polarised. But arcing from back-end of the Dynamo is as bad as ever. Just checked the wiring to the Control Box (Large D to D, F to F) and checked that Ignition light is correctly on before start and off once engine running.

Actually, when polarising the Dynamo I understood that it would only arc the first time the positive lead touched F. On mine it arcs every time.

So either the Dynamo and/or Control Box are duff. If I change to an alternator then this should solve both problems. What is the best alternator to get (ie that will fit lower mounts) and is their a modified upper adjusting arm available? Will a Spitfire alternator and adjustable top bracket fit straight in?

[RobPearce]

The Spitfire engine is indistinguishable from the Herald one and the bonnet is lower, so yes, a (MkIV) Spitfire alternator on (MkIV) Spitfire brackets (you need both top and bottom) will drop straight on. Actually, you can use only the lower Spitfire mounting and retain the long (dynamo) upper one, with a spot of ingenuity, as that's what my Spitfire has. It will need a different fan belt in at least one of those combinations. I don't know of a cheap alternator that fits the long lower mount. If you need a lower mount block, though, I'm pretty sure I'm not the only person with a spare.

[glang]

I would expect current flow everytime from the field winding. However the sparking from the commutator isnt correct and if not from poor or contaminated brush contact then could be one or more damaged rotor windings. If you want to check this you need to measure the resistance between opposite commutator segments ie put the probes on the two segments the brushes would touch. Once you have a reading move both probes round one segment and you should get a very similar resistance and so on...

[PeteStupps]

Where are you Chris? I've got a couple of spare dynamos and control boxes lying around, if you wanted to try swapping components to establish exactly where problem is before you spend money. However I'm trapped in central London with no cars working at the moment so can't offer to deliver I'm afraid!

It does sound like your dynamo charging circuit is the culprit though. 

[PeteStupps]

If it turned out to be the armature, replacements can be had for £25 and are straightforward to change. https://www.autoelectricalspares.co.uk/c40-dynamo-armature-12v-22a-replaces-lucas-227271-3770-p.asp

(if you did go this route, a herald/spit hub puller can be used to press the old armature out of the bearing, making it an absolute doddle of a job).