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Help needed with a misfire


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Hi guys, looking for some help with my 1966 TR4a, which has developed a misfire when hot and under load.

The car underwent a rebuild about 6 years ago, including a full engine rebuild. All the water ways are like new, twin SUs, K&Ns, unleaded, 4-2-1 exhaust, thin belt conversion, lightened and balanced, Kenlowe fan, slightly bigger bore and Pistons, and electronic ignition.

All ran fine for the first 5000 miles, but over the last year, she has started to misfire when hot. Hot being above normal operating temperature, but below the temperature for the Kenlowe to cut in.

The misfire only happens when under load, either pulling away or going uphill, she will run fine on the flat at a steady speed.

I originally though it was fuel vaporisation, but have since ruled that out as we are operating below the Kenlowe kick in temperature.

I'm coming around to thinking its electrics, and possibly the electronic ignition.

Before I rip it all out and go back to points, does anybody else have any experience or suggestions as to the cause and possible rectification.

Cheers

Bully

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Hi Bully

 

I haven't got electronic ignition on my 4a, but I had a similar problem a couple of years back.

 

It is likely to be electrical. On mine, it finally gave up the ghost on the MOT ramp of all places, and as a result failed!

 

It turned out to be a dead ignition coil. If you're able to run it at night and look under the bonnet to see if there is any stray sparks or arcing, as it could still be the dizzy cap cracked, faulty leads or rotor arm. If you haven't changed any of these for some time it would'nt hurt to change all these just as a belt and braces measure including plugs, as they are all service items and none them are particularly expensive.

 

The above is only my best guesses, as it could be within the electronic ignition unit, so no way of knowing without substitution as with most electrical parts!

 

Hope this helps.

 

Regards

 

Kevin

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My first thoughts are HT leads - plugs - coil.

All worth checking / cleaning.

 

Bob.

 

Just seen above post which came up while I was typing !

Edited by Lebro
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What cap and more importantly what rotor arm did you fit? The rotor arm especially can be of very suspect quality! The red ones from a known source are the way to go. Moss, Rimmers, or Distributor Doctor.

Electronic ignition is usually very reliable, more so the better quality ones.

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I 2nd what Daven has said re quality of parts.

 

In fact I have just remembered that when I replaced the coil, bought from a local motor factors, it would not start and just let out a huge bang from the exhaust when turned over. It took me a couple of hours and much head scratching to come to the conclusion the new coil may also be faulty!

 

I was lucky they had another in stock, but shows that even new coils are not as good as they used to be.

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Hi Alan, yes, ease off the throttle when under load, and the misfire goes away.

 

OK, also check all the obvious things you have and others have suggested, but thats a classic symptom of a carb engine going over rich at a wide throttle openings. I have spent far too many hours with AFR logging not to spot the symptoms, and try cure them.

 

If I'm right then your problem is that either the piston is lifting to quickly, or the filters are starving the air supply.

 

SU's and Strombergs both have a potential to go over rich if the piston lifts too quickly, and if the AFR dips much below 11 you end up in a vicious spiral that is particularly pronounced on hills, below 11 the mixture is too rich to burn correctly and power reduces, but your on a hill trying to accelerate and the power is dropping so you open the throttle more and the AFR worsens, the lift allows the piston to drop a bit, AFR improves and bingo the power comes back. I have a hill close by that purely by changing the oil in the dash pots I can either WOT and missfire my way to the top, or can WOT and accelerate all the way up. NB I can also create the same effect purely by changing between different air filters.

 

OK so two simple trials, when did you last thouroughly clean and reoil your K&N filters. Recently about when the problem started, or you havent?

 

When did you last check the oil in the dash pots? What did you fill them with?

 

Are you running SU's or Strombergs?

 

Alan

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Misfiring under continuous load would not point me to the SUs, if in normal tune.

But low HT can cause that, the less compressed mixture is easier to ignite. Perhaps low supply voltage from the switch, or coil, or the electrickery playing up.

 

Spark timing has been checked??

Is the misfire load dependent over a range of rpm ( can we exclude centrifugal advance gremlins?)

 

Peter

Edited by Peter Cobbold
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Many thanks guys, this is all great stuff.

Misfire under load, and when hot, happens at all rev ranges. I can change gear, but problem persists.

Coil, rotor and cap all from distributor doctor, so don't see any issues there.

K&Ns were cleaned last year, not sure when was the last time I checked the dash pots.

Think tomorrow I will clean and check all the simple stuff, plugs, HT leads, cap, rotor, coil, dash pots, K&Ns, timing etc.

If that fails, then it may be change back to points.

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All ran fine for the first 5000 miles, but over the last year, she has started to misfire when hot.

The misfire only happens when under load, either pulling away or going uphill, she will run fine on the flat at a steady speed.

 

 

K&Ns were cleaned last year, not sure when was the last time I checked the dash pots.

 

 

OK guys, so we have an engine that's running fine until a year ago, it develops a misfire under load, so we naturally assume that its every possible electrical component failing (although I still would try a clean supply to the ignition module), instead of asking what changed a year ago.

 

Misfiring under continuous load would not point me to the SUs, if in normal tune.

 

 

From the opening spec, it appears to be slightly more than a cooking road engine. And why not elliminate the simple mechanical before looking for a complex electrical. Most carbs I see are woefully maintained, way out of balance, and it doesn't matter most of the time as the car will start, will poodle about, or cruise happily as most driving is done at tiny piston lifts and it doesn't matter if its all a bit rich or a bit lean, or a bit rich on the front two and a bit lean on the rear two. It's only when you hit the peddle hard that it realy become apparent.

 

 

Alan

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OK guys, so we have an engine that's running fine until a year ago, it develops a misfire under load, so we naturally assume that its every possible electrical component failing (although I still would try a clean supply to the ignition module), instead of asking what changed a year ago.

 

 

From the opening spec, it appears to be slightly more than a cooking road engine. And why not elliminate the simple mechanical before looking for a complex electrical. Most carbs I see are woefully maintained, way out of balance, and it doesn't matter most of the time as the car will start, will poodle about, or cruise happily as most driving is done at tiny piston lifts and it doesn't matter if its all a bit rich or a bit lean, or a bit rich on the front two and a bit lean on the rear two. It's only when you hit the peddle hard that it realy become apparent.

 

 

Alan

Alan, My experience with SUs is the reverse of yours - I find they can be awkward for starting and at low piston lifts but smoothe out when the butterflies are well open.

But if the throttle linkage is only opening one buttlerfly on Bully's engine that could produce a bad misfire especially under load.

Peter

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Don't forget to check the glass bowl of the fuel pump - if full of s**t, this will be forced upwards under heavy load and start to block the gauze filter and can cause the engine to stall. Having stalled, it is possible to restart the engine and run at tick-over - but then the performance will be repeated if one ventures onto the road.

If, having cleaned the bowl, more s**t is carried from the fuel tank, then it's a Sloshing job (see the Technicalities CD)!

Ian Cornish

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It was the electronic ignition. Just replaced the Pertronix sensor with a known good one, and all is well again.

Thanks everybody for your help and suggestions. After scouring the Internet it would appear there can be numerous caused and remedies on all sorts of makes and models, so I'm just glad it didn't take me too long to solve mine

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Well that diagnosis was wrong. Went on a 100 mile tour with the guys from Glavon today and the problem was back, so I guess we can rule out the electronic ignition.

However, we swapped the cap, ht leads and rotor, and that seemed to cure it.

Will need a little more investigation.

When I swapped the distributor cap last year, I didn't cut off the last end of each HT lead, so when fitted to the cap, a second hole was made to form a connection. I wonder if the old hole(s) are therefore shorting out and causing a misfire?

Going to re-terminate each lead and see if that was the cause.

Watch this space.........

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