Old misfire - again

[Clifford Pope]

Every year or so this misfire returns. I go through all the tests, replace components that were probably good anyway, and then it seems to go away again and I never know what really has caused it. So here we go again:

Mk 1 with a 2500 S engine, hs6 SUs, standard ignition.
Having previously been running perfectly, today it suddenly developed a slight misfire when pulling hard uphill. It passed after a few seconds, but for the next few minutes the engine had a slight tremor even when not pulling hard. I stopped shortly afterwards. It started and ran perfectly on the return journey, but again misfired uphill. In each case it only developed after about 15 minutes. It pulled normally before that, even when I tried hard acceleration uphill to try and induce it.

The condenser is nearly new Chris Witor, itself a replacement for another new CW one replaced last year as a precaution. neither made any difference. Likewise the red rotor arm, points, cap and leads. The coil was also replaced, with no change last time. I felt both coil and condensor immediately after this misfire, and both were cool to the touch.
So it really doesn't seem to be ignition-related?

What fuel problem can cause this? Why would it take 15 minutes to develop, but go away after parking for a couple of hours? Why is it induced by pulling hard, but then remains, slightly?
When I got home the engine would idle perfectly, would rev smoothly up to a point, but if I tried more robust revving the misfire came back.
After cooling, and starting again, it ran perfectly.

[Carboy0]

Accelerating uphill, puts the biggest load on the fuel delivery system. In no particular order I would check the following;

1. Dirt in the float chambers - whip the tops of the float chambers and check - add filters, blow out the hoses, clean out fuel tank.
2. A slowly failing mechanical fuel pump - either rebuild your existing one or replace with a new one.
3. Replace any filters in the system, a clogged filter will restrict fuel flow.
4. Check all the flexible fuel piping for leaks and/or replace the lot from tank the carbs

Re 4. I had a very similar problem on a classic some years ago and did all the above 1-4, what the problem actually was, the ethanol content fuel had rotted out the hose from the inside and it had become porous, allowing air to be sucked through the porous rubber, so the pump was sucking fuel and air, which under acceleration was enough to undersupply the carbs leading to the misfire. I finally found it when immediately after I had had the problem, I stopped the engine and checked in the carb float chambers and they were virtually dry.

New hoses fixed the problem. Use ethanol resistant hose to avoid the problem occurring again.

If you don’t use your car much then the fuel sits in the hoses for weeks/months and slowly dissolves the old rubber from the inside. On mine it was the first section after the fuel tank, but it could have been anywhere in the system.

[Clifford Pope]

Thanks Chris - I hadn't given much thought to the hoses, but I'll investigate now.

I've been confused by the time delay - whatever it is takes about 15 minutes to develop. There's no fuel starvation up till then.

[Carboy0]

Porous hoses really only show themselves when the engine is at full chat, that’s when the mechanical fuel pump is developing maximum suction, to pull fuel through to feed the wide open carbs.

Also if it’s happening on uphill sections, then the pump is pulling against gravity as well.

[Charles H]

I had a similar issue with an S. It would drop to three cylinders uphill and eventually clear and be fine again. Took ages to find out the cause. It was in the end a small water droplet in the dash pot of the front carb which 'rolled' over the bar to the chamber of the tube to the jet. Being more viscous than petrol, the carbs were unable to draw it through. Once going over the crest of the hill, it rolled back again and the starved carb slowly got petrol back. Just looking into the dash pot revealed a clean bowl with clear liquid in it. The water was not easily seen. May be worth a try cleaning out these bowls completely.

[Clifford Pope]

I 've just looked at the fuel hoses - they appear all right, and there is a good flow from the pump. I'd also had another look in the carburetor bowls and cleaned those out.
On starting up it ran all right, but with a slight flutter, and after a bit of revving it spluttered, wouldn't idle and then stalled. I checked the chambers - full.
I started again, and tried pulling plug leads. Pulling no. 3 made no difference to the idling, so I tried replacing the lead - no difference, and then the plug.
It seems to be all right now - at least it idles nicely and will rev up without the spluttering.

So the focus has now changed from fuel starvation on hills when warmed up to misfiring after start up owing to a faulty plug. The plugs are new Denso (said to be the best make) W20 EPR - U, done about 100 miles, the same as the HT leads, cap and rotor. Gap 30 thou.
I've found this occasionally with spark plugs and HT leads - one out of a new batch goes dud after a very small mileage, with no apparent cause.

The plug colour is perfect - no soot, no white deposits, just misty brown/grey over the clean metal.

I haven't tried the car again yet on a run, but this is looking encouraging as I appear to have found a fault, and fixed it. But I know from past experience there can be two intermittent faults, unconnected, but taking it in turns to frustrate the owner.

[Carboy0]

Good pickup on the dodgy plug - well done Mr Holmes!

Hopefully that’s cured it.

[Clifford Pope]

Yes, it has. (fingers crossed) I've just been out for a run. The idling smoothness was much better immediately, and even on hard acceleration I couldn't make it misfire or hesitate.
It's so annoying. You can work methodically through a list of potential faults, testing and replacing things as necessary or as routine periodic replacements, like plugs etc, and get the car running perfectly. Then the fault recurs, or in a slightly different form so you think its fuel rather than ignition. Then eventually it turns out to be one of the things you've already replaced, new, even from a reputable source, that has started to fail after a minute mileage. It's like going round and round chasing your own tail.

Still, it's been worthwhile to have done the check on fuel supply.
Thanks all.

[Clifford Pope]

I spoke too soon - again!

It ran perfectly for 20 minutes, pulled like a train, really smooth, and then suddenly started having the odd tremor, which quickly turned to a misfire. I discovered that if I de-clutched and revved the engine it would then be all right for about a minute, then it did it again.
After stopping for about 3 hours it started and ran perfectly until about 20 minutes again. I managed to get home - the final climb was touch and go - it would just about run fast in first gear, but then cut out at our gates. It then refused to start or show any sign of firing.

I pulled it up the drive and got it into the garage, and out of interest tried again about 20 minutes later. It started first shot and ran well, with no trace of the misfire.

So what now? I've tried all the fuel-related steps everyone has suggested, apart from replacing the fuel pump. Is it worth a gamble just buying a new pump on the off-chance?

After it had stalled I did try having a look in the distributor. Nothing out of place, but I did notice the distributor itself was too hot to touch. Is that normal? Would it it be at engine block temperature anyway?
The condensor was hot too, but then I suppose it would be in a hot distributor bolted to a hot engine?

(Incidentally the clips holding the cap on fall out very easily, only being slotted in from the side. They were so hot I dropped one on the gravel drive, but managed to find it)

The capacitor is new from Chris Witor.

The coil, normally an old suspect, was quite cool. But that's because I moved it from the engine block to the battery tray a few years ago because it was getting too hot.


I've run out of ideas now.

[Carboy0]

Cliff, following up on my earlier posts, I had this a few year’s ago on a non-Triumph. The cause was fuel evaporation in the float chambers, caused by the modern fuel boiling off due to the heat from the exhaust system and no heat shield.

The mechanical fuel pump was suffucient to overcome the evaporation with the engine screaming, but a low revs and low fuel delivery the fuel was boiling off faster than the pump could supply it. Obviously until the exhaust system and engine was fully hot, the evaporation wasn’t a problem.

I finally diagnosed it by waiting for the car to display the problem, promptly stopping the engine and whipping the tops off the SU float bowls and they were both dry.

The short term fix was to fit a good heat shield between the exhaust and carbs, although the engine would falter occasionally after a hot run, particularly in the summer. In the end, I fitted an electric pump and never had the problem again. The Stag setup would be the obvious solution, but there are lots of electric pumps about. SUs & Stroms both run at about 2-4 psi, so don’t buy a pump for a a Weber as these are usually around 6-7 psi, which is enough pressure to lift the float needle valves off their seats, flooding the carbs. I have seen fuel pouring out of SU float bowls when a pump for a Weber carb was used.