Mk 6 Special engine power loss

[Mark Taxis]

I am having problems with my Mk6 special.
I have just driven for 2 hours, the car has gone up and down hill, along the highway for 15mins at 60 mph ,a couple of stops and restarts at the shops and no problem.
Just as I was approaching home up a fairly steep and twisty road it started to lose power, cough and splutter, easing off on the accelerator made it run smoother but of course slower! Putting my foot down again resulted in more missing, once the hill was climbed then of it went no problems running smoothly at speeds up to 60 mph, but after slowing down to turn into the street which has a slight uphill gradient it starts missing again, almost sounds as if it has lost power to a couple of cylinders, again once the flat is reach no problem and plenty of power.
The car did not get above 85 degrees c (on the gauge) it was not boiling when I stopped the engine.
This is now the third time that this has happened, after the last time I cleaned all the plugs, reset and cleaned the points, checked all the HT leads and all seemed to be fine.
What I do not understand is why it runs so well for an hour or two before it does this.
I have noted that the engine bay does get very hot and that the coil feels very hot to touch, but I am not sure if it is the coil heating up or whether it is just the ambient temperature in the engine bay.
Any thoughts or advice gratefully received, this problem is driving me nuts!! 
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Mark Taxis
Sunshine Coast
Queensland 4560

[John Robins]

Hot coil might be worth addressing first, either swap it or relocate into some air flow. However fuel evaporation seems more likely, especially as it takes a while to happen, but will be harder to sort out. Are the fuel pumps keeping the flow up, have they been serviced recently and how about the filter?

Good luck with the hunt!

[John Murch]

Might be worthwhile draining the fuel tank in case there are flakes of something blocking the fuel take up pipe.
John

[Mark Taxis]

Thank you for the feedback.
The car does not have the original SU fuel pumps this was changed out many years ago for a single electric pump not sure of the specs for this pump, does anyone have the pressure and flow rate required? maybe this pump is on the way out.
The new style fuel filter looks clean.
The tank has just been put back in the car having been recently drained and removed for the respray of the body.

Might be best to replace the coils, one is about 30 years old the other is 10 years old
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Mark Taxis
Sunshine Coast
Queensland 4560

[Christopher Carnley]

Is there something sloshing around in the tank blocking fuel pick up?

Is the exhaust clear or have you got a partial internal collapse?

If the above fails, are you able to run the car without the bonnet, to check for overheating of the electrics?

[Mark Taxis]

Christopher
Now you have me thinking.
There has been this rattle from the exhaust which is getting worse, only happens when the car is hot, I wonder if one of the baffles in the silencer has let go???
Will get to the exhaust place and see what they think .
I can take the side panels off the bonnet and see if that cures the problem, but I think I might get the exhaust checked first
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Mark Taxis
Sunshine Coast
Queensland 4560

[Mark Taxis]

I have checked the fuel pump; it is delivering 85 litres /hour. From what I can find out the original SU pumps were rated to 136 litres / hour, so the fuel is not being delivered fast enough.
My pump is just above the delivery rate if only one SU was operating.
Has anyone driven one of these cars with only one pump in use, if so did the car run ok at high speed or was it starved of fuel?
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Mark Taxis
Sunshine Coast
Queensland 4560

[John Robins]

R-type driver's handbook page 46: -

"Duplicate pumps are provided primarily to ensure reliability. They also ensure that there shall be no starvation of fuel at maximum engine demands"


Maybe your lower capacity pump, plus under bonnet temps, are permitting vapour locks. It certainly looks worth investigating.

Extra thought: -
If I remember correctly, didn't the Derbies have an A, B or both switch for the fuel pumps, maybe a four and a quarter driver knows the answer.

Extra extra thought: -
There's an under bonnet picture of an AC 16/80 on page 34 of "The Automobile" for June. Although only two litres, it's got three SU carbs and two SU fuel pumps.

[Malcolm Gordon]

My Special went off song at Scottish Whit. Firstly on the way there when there was a misfire whilst driving on the level which quickly became worse and then died completely. I put in a spare rotor arm and cured instantly.

I did have a further misfire similar to you when the car was under load after getting hot after about 45mins of driving. It was missing when pulling away from a junction or under load going up a hill but was perfect the rest of the time. I did eventually have a mishap at a junction when the engine died and would not restart. The coil was exceptionally hot (I could barely hold it) It was swapped for a spare coil I was carrying and the engine fired up instantly and has good for the rest of the day as well as whole of the return trip.

Another car at the weekend had the same problem and a new coil sorted it out.

Fuel vaporisation was also mentioned by some so might still be a possibility.

[Mark Taxis]

Another update to this saga!
I have replaced the coil, condenser and the points. No change to the status, runs perfectly for about 40 minutes, even up the steepest hills then it starts coughing , missing and popping, in fact if anything it has got worse, barely got up the driveway, had to resort to 1st gear and it only just made it. And when I did switch off there was a very loud back fire from the exhaust.
I have ordered a new pump and will fit it next week.
The rattle from the exhaust is getting worse, so I will get that checked next week.
In the meantime work on the car stops as I have to go to work for a few days.
Thank you to everyone for their advice - I will get there in the end, it is just so frustrating
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Mark Taxis
Sunshine Coast
Queensland 4560

[Stephen Blakey]

Over at the RREC, Morgan Whelan had very similar symptoms on his Silver Dawn.

He is a nuclear scientist and kept meticulous notes of all the tests he did and the fact that nothing seemed to make any difference.

In his case, the fault was eventually cured by changing the distributor shaft bush.

He read his notes some time later and told me it read like the journal of someone descending into madness.

Have you got a spare dizzy to try? It could be as daft as a crack in the cap or, as Malcom says, a dodgy rotor arm.

Have you confidence in the new coil? Tony Tomlinson recently put a brand new coil on his MKVI special and it wouldn't start. He wasted a lot of time looking at everything but the coil. Having eliminated everything else, he put a 40 year old coil on it and it's fine.

Can you rig a fuel pressure gauge?

We feel your pain.

[John Robins]

Is the vent on the fuel tank clear? It could be that a partial vacuum is building up in the tank and working against the pump. Immediately after one of these episodes have you whipped a spark plug out to see what colour it is? I'm always prepared to be proved wrong, but spitting back and backfiring sound like weak mixture fuel problems more than electrics to me, or maybe even a valve sticking at worst.

The distributor bush could have been allowing variable ignition timing, but if you have fitted new points I would have thought that the requisite play would have been apparent when balancing the two sets.

Hoping that the barrel is now well and truly scraped!

[Mark Taxis]

John
I am thinking that the problem is in the ignition, It has been pointed out to me on the RR forum that this is the only area I have touched and the symptoms are now worse than before.
The distributor was remodelled some time ago to single points, I think I need to take this to a good auto tune or Elec to get the ignition system thoroughly checked, I am not convinced the dwell angle is correct and the engine just does not sound “right “when it is timed with a strobe it sounds like it is too far retarded.
I think it will be some time before I can get back to the car, work and a trip overseas coming up.
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Mark Taxis
Sunshine Coast
Queensland 4560

[David Cocking]

Mark, I had exactly the same symptoms a while back, albeit in a modern car. After much fiddling, replacement, unnecessary expense and gnashing of teeth it turned out to be debris in the fuel tank. In your case it is also interesting that the tank had just been removed and refitted prior to this fault, activity that could have knocked something loose inside or inadvertently introduced debris.

[Mark Taxis]

David
I have checked the fuel filter and whilst the tank was out of the car and empty there was no sign of debris, also cleaned out the carby bowls, a small amount of debris but nothing significant
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Mark Taxis
Sunshine Coast
Queensland 4560

[Stephen Blakey]

It would be good to fit an AFR gauge to see if the mixture suddenly goes lean before the misfiring starts. You would have to get your exhaust place to weld on an EGO nut but it's pretty straightforward otherwise.

Fingers crosed that you find the fault before such measures are called for.

[David Cocking]

FYI my fuel filter was also spotless, but "flakes" of something or other were blocking the entry to the fuel line in the tank. Took a good while for them to build up and block on each trip and then finally showed up when under load. Easy to check - be ready with a temporary fuel hose that you can switch to next time it happens, and take suction directly from a fuel can. Good luck.

[Chris Card]

A while ago, I was chatting to another chap, who was having a similar problem to yours - turned out to be a dead wasp in the fuel tank that kept getting sucked into the end of the fuel pipe and temporarily blocking it.

Chris

[Mark Taxis]

I have just replaced the fuel pump, the new pump sounds a lot healthier , (quieter), fingers crossed that this was the problem, so far so good ,no reoccurance
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Mark Taxis
Sunshine Coast
Queensland 4560