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Intermittent misfire


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Just prior to our honeymoon we did a run with the Cumbria Group up to St Mary's Loch. On the way home the TR4 (rebuilt to 4A spec engine) started to misfire and needed a bit of revving to ensure we got home. It had run faultlessly in my ownership until then. I tested it before our honeymoon and it was running well. A few days faultless running was rudely interrupted by the return of the misfire. I cleaned up the rotor arm and it seemed better but returned on our journey home. It's a horrible feeling when you don't think you're going to make it back.

In the meantime, I bought a DD rotor arm to eliminate that and I changed all my carb hoses with ethanol resistant ones. Before anyone asks, there is an in-line filter which looks clear! I then had a leak from the rear HS6 SU float chamber. I tried swapping out some used bits Float, needle valve, lid etc. In the end I replaced the needle valve with a new Viton tipped one and set off into the Lakes last Friday. After about 75 miles perfect running (with a 3 hour break in the middle), just as I was congratulating myself,  the misfire returned and the old fear about not making it home returned.

I have a Powerspark distributor and Accuspark coil fitted by the previous owner which have done about 7.5k. I know that these are not the best but quality ones are very expensive if that proves not to be the issue. The cap seems OK and likewise the leads. I will check the float chamber cap again for rubber blockages (from the original tube between the filter and the cap  though I thought it was nice and clear.

Next weekend we are doing a Lakes trip with the club and I don't want to struggle on "the Struggle" (KIrkstone Pass). It is not the sort of place you want it to cut out.

Does anyone have specific thoughts or do I just have to replace components one by one?

Andy

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What do the sparkplugs look like? Could it be fouling the plugs?? 

If the accuspark is failing I would expect it to fail instantly...meaning the misfire would appear in a moment...if it's more fuelling and carbs it could be a more gradual degradation of performance.

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Thanks Dave. The sparkplugs seem fine though I haven't checked them this time. I know that if I start it up today it will be fine. That's the problem with intermittent faults

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Sounds to me like heat build up is causing something to break down. This can happen with coils, and maybe when you cleaned the rotor arm it made an easier path for the spark and took some load off the coil, or simply gave the coil chance to cool down a bit, likewise when you put in a new rotor arm. Coils are not too expensive and even if it does not cure it you can then carry a spare (I do).

Likewise spark plugs can break down if they get too hot because the mixture is too weak, this tends to happen after a longish run at a steady speed. Stop start and around town gives the plug chance to cool down.

Being bolted to the engine is not the best place for a coil in the first place, and are better located on the inner wing, less vibration and heat.

Good luck

Ralph

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5 minutes ago, Ralph Whitaker said:

Sounds to me like heat build up is causing something to break down. This can happen with coils, and maybe when you cleaned the rotor arm it made an easier path for the spark and took some load off the coil, or simply gave the coil chance to cool down a bit, likewise when you put in a new rotor arm. Coils are not too expensive and even if it does not cure it you can then carry a spare (I do).

Likewise spark plugs can break down if they get too hot because the mixture is too weak, this tends to happen after a longish run at a steady speed. Stop start and around town gives the plug chance to cool down.

Being bolted to the engine is not the best place for a coil in the first place, and are better located on the inner wing, less vibration and heat.

Good luck

Ralph

Is there any way the coil can be tested for a fault or will it only happen after a while? The weather is pretty cool up North so it shouldn't have overheated

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Coils will heat up anyway just from having electrickery running through them. Leaving the ignition on without the engine running is a sure way to overheat them (and burn out the ignition module).

I don`t think there is a way of testing other than by substitution. Usually the insulation breaks down when hot and the current will jump to earth it being the easiest path for it.

Ralph

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I would replace the ignition components, trigger, rotor, coil ...normally I blame condensors for intermittent ignition faults...but this time its starting to sound like the trigger.... the new one should have some heatsink paste in the packet..make sure that paste makes a good contact to the dizzy plate...otherwise these triggers will overheat.

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Have you checked for debris in the fuel tank? This could either something really simple like the foil seal off a fuel additive bottle which floats around in the petrol till it goes past the fuel outlet and gets sucked onto it causing a partial blockage - when the pump stops ,the vacuum is released and the seal moves off.

It could also happen with flaking from inside the tank collecting around the outlet during a reasonable length journey - largish flakes can do this without a lot of fine particle evidence in your filters - is it an original tank?

It doesn’t sound like your fuel pump because I don’t think it would show up as an intermittent problem if it was defective

cheers Rich

 

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Hi 

It could be a multitude of things plugs, leads, distributor cap, electronic trigger in the distributor, coil  (it may be fuel but I would expect some more symptoms) I think its a case of substituting components starting with the cheapest first my bet would be coil

Chris  

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7 hours ago, rcreweread said:

Have you checked for debris in the fuel tank? This could either something really simple like the foil seal off a fuel additive bottle which floats around in the petrol till it goes past the fuel outlet and gets sucked onto it causing a partial blockage - when the pump stops ,the vacuum is released and the seal moves off.

It could also happen with flaking from inside the tank collecting around the outlet during a reasonable length journey - largish flakes can do this without a lot of fine particle evidence in your filters - is it an original tank?

It doesn’t sound like your fuel pump because I don’t think it would show up as an intermittent problem if it was defective

cheers Rich

 

thanks Rich. I don't know if there are any flaking issue. The fuel flows freely, particularly when leaking at the overflow. Can't rule it out though.

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7 hours ago, trchris said:

Hi 

It could be a multitude of things plugs, leads, distributor cap, electronic trigger in the distributor, coil  (it may be fuel but I would expect some more symptoms) I think its a case of substituting components starting with the cheapest first my bet would be coil

Chris  

I'm inclined to go tor a coil first. If it isn't that at least I'll have a spare

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Hi Andy,

I have a 1968 TR5. I suffered exactly the same sort of misfire on a recent run out with friends. Just before our lunch stop the car was misfiring horribly under load. I checked the car over but couldn't see anything wrong. After lunch everything had cooled down and the misfire disappeared but started to return as we neared home. I checked out the petrol injectors and all was fine so changed the coil as the Lucas unit that was fitted was dated 1999. I have not been on a long run out again since but so far the car is running perfectly. So, I suggest you go for the coil. By the way, with the TR5 the coil should be mounted on the engine but a past owner moved it to the NS inner wing where it remains...

Regards,

Denis

Edited by DenisMc
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Hi all

Have been doing some detective work and according to Powerspark my distributor (Powerspark electronic) should be matched to an appropriate coil such as a Lucas DLB198 (0.8 ohm) or a Viper dry coil (0.8 ohm). What I have is an Accuspark (3 ohm) red sports coil. This could well be the source of the intermittent nature of the misfire.

Andy

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5 minutes ago, Lebro said:

Those 1st two should read 8.0 ohm not 0.8.   A 0.8 ohm coil would destroy the unit instantly !

The 3 ohm one could well have damaged the accuspark electronics

Bob

Thanks Bob

I've just asked the guy at Powerspark if it is likely to have caused damage to his module and am awaiting his response. However the car runs very well until the intermittent issue returns (probably due to heat after about 60 miles (I don't drive hard these days) so maybe I'll get lucky. Funny that both coils on the website are stated as 0.8 ohm

Andy

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I happen to know from a friends E type, fitted with Opus ignition, that there were some serious ballast resistors used with it, the coil actually ran on considerably less than 12 volts, so I don't think this one would be suitable

Bob.

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The two that I mentioned are not suitable for use with a ballast resistor. The Viper VCE was designed for Powerspark high energy ignition kits and distributors so seems the obvious choice though a little dearer than the Lucas

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Interesting response from Powerspark when I placed an order for a new coil. They feel that the issue is unlikely to be heat related as this should reveal itself relatively soon, not after 70 odd miles. They say the original coil will not have caused any damage to the ignition module but the replacement will give a more powerful spark. I will keep the Accuspark as a spare. The good news is that the ignition module doesn't need replacing. The bad news is that it may not resolve the issue. If not, I will have to look at the carbs again. The joys of intermittent problems.

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  • 2 weeks later...

I have a TR3A and used to have hot starting / running problems.

  1. Initially the local engine rebuild company had failed to assemble the liners accurately to the correct height. This was the well known engine company in Brighton. Darryl Uprichard at Racetorations rebuilt it and  solved the temperature when running.
  2. Later I had a starting problem when the engine was hot. It turned out to be the condenser which lives in the Distributor. The regular units have a short length of membrane internally. Distributor Doctor , in Somerset ,  identified the manufacturing defects and he  has his own made to a proper specification. I keep one of his in the spares department, i.e. the Boot. But the one on the car in use for many years is a special condenser made for racing mini-minors. It is too big to live in the distributor and is attached to the  engine under a fixing bolt for the coil. It was expensive as well, but there is no choice between £30.00 and breaking down

My overheating problems have gone away and the starting is much more reliable. I hope that this may help you I have also upgraded the RB106 by fitting a solid state upgraded board inside the RB106 casing.

Good luck Richard & B

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