Hrald 13/60 Misfires When Hot

[cbjroms]

My Triumph Herald 13/60 is fitted with an Accuspark distributor and a Stromberg CD150 which has been professionally rebuilt.

It has been running sweetly for some time but has just started to misfire when warm. When started from cold it runs fire and drives well until the engine gets warm and then it starts to misfire. Stopping the engine and allowing it to stand for 10 minutes results in it ticking over smoothly for a few minutes and then the misfire starts again.

I have replaced all 4 plugs but this has made no difference. The coil does not feel hot and I have checked the timing. This does not seem to me to be a problem with a partcular cylinder as removing any one of the 4 spark plug leads when the engine is running has the same impact on the misfire. I have also run the engine with the fuel cap off in case there is an airlock but this has made no difference. Looks to be plenty of nice clean fuel going through the fuel filter.

I would be grateful for any advice that you guys could give me.

[nang]

I had a similar problem with my S when I rebuilt the engine. After going through everything I finally swapped distributors with another that I had. Both had Pertronix units which I believe are the same as Accuspark.

This cured my problem.

Tony.

[Clive]

Accuspark is a relatively budget brand, and I know of one local who had an accuspark distributor that was hopeless out of the box and had to be returned. Think he just used his old one.

Petronix is rebadged as Aldon, and more of a premium brand.

I think an alternative distributor, ideally straight from another running car, would be my first move.

[RobPearce]

Hot misfires are a common symptom of poorly designed (i.e. cheap) electronic ignition modules, so I'm with Tony and Clive.

[glang]

you could have a look at the distributor as although it probably came fully assembled its worth checking the set up.

Theres the gap of the rotor to pickup which is adjusted by loosening the pickup backplate fixing screws and should be small but obviously not allow contact between the two.

Also while checking this you could confirm that thermal paste has been used when fixing the pickup to its backplate and I even like to put it between backplate and distributor base plate to maximize the cooling of the pickup....

[cbjroms]

I have replaced the electronic ignition unit with a new one and the problem is exactly the same. As the engine becomes warm the misfire starts and gets worse as throttle is increased. So plugs and ignition module have both been changed, leads are all brand new as is the coil (tested on a multimeter and looks ok). Could this be some sort of mechanical problem?

[glang]

Its difficult to see it being a mechanical problem as not much should change with engine temperature. The valve clearances do close up slightly so if incorrectly adjusted I suppose one could be held open but you say the misfire is not related to any one cylinder. And your carb doesnt have any temperature compensator does it so it shouldnt be a mixture problem either.

The first thing on my list would be a replacement coil because although youve tested yours they can break down electrically when warmed. Its always useful to have a spare and theyre cheap (make sure you get the correct type). Also, with care, you could trying running the engine in the dark to look for any spark tracking....

[RobPearce]

You don't say whether you've replaced the rotor arm or disi cap. It's not so common for them to give temperature-related misbehaviour but it's not unknown.

[cbjroms]

Thanks for the repsonses. I will do some more tests this afternoon.

By the way, a couple of other things I have noticed. First the misfire seems to be getting worse over time. When it first started happening it was, by no means, as pronounced as it is now. Second, engine started quite easily from cold but having run for 10 minutes (and got warm) I switched it off. When I tried to start it again the battery could barely crank it over and starter solenoid positive post got very hot, very quickly.

[RobPearce]

Check that the solenoid connections are clean and tight. Terminals get hot because they are resistive, which normally means loose or dirty. As engines can get tight when they're hot, and the cranking load is higher than when cold, this could explain your "new" symptom.

The original problem getting progressively worse is to be expected if it's a component "on its way out".

[glang]

yes sounds like youve either got a starter motor problem, so it cant develop enough power, or a high resistance in the solenoid. As Rob says the latter could have excess resistance either on the cable connection (remove, clean and tighten) or internally which would require its replacement. 

If theres still a problem then the starter motor will need removal so that its brushes/commutator can be checked and winding resistances measured.

[cbjroms]

I did some testing after dark and noticed that, when cranking, the negative lead from the battery to the bulkhead is sparking at the bulkhead end. The lead is bolted through the bulkhead with a tight and clean nut/bolt from which the spark appears to be coming. This, I presumed, was causing the slow cranking. What I cant understand is that putting a jump lead between the nagtive battery post and the gearbox, in addition to the negative lead, made no difference. Should this not overcome any poor negtive connection?

[Dannyb]

The bulkhead needs a good earth as all the electrics use the body for the earth.

You need a good connection from -ve terminal to both engine and body.

 

 

Danny

[glang]

Are they good quality and condition leads? Some are awful from new as they look the business but the crimp connection from cable to clamp is poor (usually hidden under a plastic cover) so that when the starting current tries to pass heat is produced. This is then a downward spiral because the connection degenerates and the resistance goes up so more heating.....

Also, in addition to the battery earth cable issue, you could still have a starter motor or solenoid problem.

[cbjroms]

Thanks guys, does the sparking (where the negative lead bolts to the bulkhead) indicate that the power cannot flow freely enough through the lead into the bulkhead? Why would the jump lead (negative attery post to gearbox) not provide a sufficiently low resistance path ?

[glang]

I guess it does but it must either be a really poor connection or youre drawing a greater than normal current! If its the second then either the engine is excessively difficult to turn or the starter motor has a problem...

[PeteStupps]

I have to say I'm surprised you're getting arcing at a fixed connection point, that's not something I would expect. If your earth connection is poor at that point then I would also have expected a jump lead to alleviate the problem. How confident are you the jump lead was making a good clean connection on the gearbox? 

On the misfire, I agree it's most likely ignition but don't suppose you'd done anything with the fuel lines before it started happening? Know it's the wrong time of year now but fuel vapour lock has given me a lot of similar grief recently. 

[cbjroms]

Thanks again guys. The jump lead was definitely making a good connection to the gearbox flange and battery earth posts. Also, I fee that this slow cranking and sparking at the fixed connection point is something that has started recently and seems to be getting worse. So noting, but setting aside for now the possibility of fuel starvation, leaves me with the suggestion that the starter motor has a problem, or the engine has become excessively difficult to turn or the solenoid is introducing a huge resistance for some reason.

 

[RobPearce]

Just a minute...

Last time I looked at a 13/60 it had a similar earth lead arrangment to the cars I have in my fleet, where the big braided cable runs from the battery to the engine with only a brief side-visit to the body. That earth to body fixing is not involved in the circuit for the starter motor! If it's sparking when cranking then there's something wrong with your wiring. Look for a possible short to the body near the solenoid. Adding a jump lead from battery to engine won't make any difference to that sort of fault.

[glang]

No I think Chris has the same set up as my Vitesse with a short run of braided lead going from the battery -ve terminal onto a bolted connection through the angle plate at the side of the battery that is welded to the top and front of the bulkhead.The engine/gearbox should have its own braided cable(s) to connect it to the bodywork....

[RobPearce]

That sounds wrong to me. The braided cable should run direct from the battery to the engine. It is bolted to the plate as you describe but using a side-crimp and without breaking the continuous cable run. That's how my Vitesse, Spitfire and GT6 all are, and I still have the cable from the 13/60 that was the same. Don't rely on the bodywork for the starter motor circuit, it leads to unreliability and (even more) problems with loose bolts and paint.

[cbjroms]

These are my leads. The positive goes from battery post to solenoid. The negative goes from battery post and is bolted (through the angle plate) to the bulkhead. I also have a braided cable bolted to sump and to chassis.

So, without the jump lead, the only earth to body connection is through the black lead bolted to the bulkhead. The starter motor is then getting its earth from the gearbox/engine which is getting its earth from the braided cable connecting chassis to engine. So surely the jump lead (battery terminal to gearbox) should have elimanated an earth issue?

The cranking has always been a little slow except when battery has just been fully charged (so the solenoid may well be causing a problem) but the situation has recently deteriorated and I have only just noticed the sparking at the bolted earth connection.

 

[glang]

A direct connection isnt a bad idea but theres many cars that use the body as a return for the starter motor current and it doesnt cause any problem. My Vitesse definitely has the original battery -ve lead and it just bolts to the side plate along with a few other smaller earth wires. 

[glang]

With the ignition off you can always bypass the solenoid to see if its causing a volt drop by connecting a jump lead to the actual starter motor terminal and then putting the other end to the battery positive (do it in that order to avoid shorting the battery accidentally). If theres an improvement in spin speed theres a high resistance somewhere....

[PeteStupps]

+1 to Glang's suggestion of bypassing the solenoid, maybe it's got scorched contacts or something.

Negative earth on my Spit is same as yours Chris, short lead from battery terminal onto bulkhead. 

However I'm still mystified as to why you would be getting sparking (or arcing*) at the bolted earth connection. Is there anything close to it that would be at 12V which might be causing this? If it's just a poor earth connection I would expect it to get hot but not arc. For arcing you need an insulated gap and high enough voltage between two conductors - you can create those conditions when you break a circuit that's carrying reasonable current, but that shouldn't be the case for a bolt through the bulkhead. Unless it's rattling about while you operate the starter motor!

 

*Sorry for being a pedant but by 'arcing' I mean a visible electric current path through the air or over an insulating surface. Sparks can be pieces of glowing hot material ejected from something