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Misfire then cutting out


Baxter

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Grad a timing light and when you get a misfire check each HT lead for constant rhythmical spark. If something is wrong, do the same for the king lead. 

 

It's a simple test and gives you something to work from when the misfire is happening. You'll be able to pinpoint whether it's a spark or fuel issue.

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Paul, thanks for your ongoing support - I'll be sure to let you know the outcome. Pretty certain a ballast has not been added. Worth checking.

Rob, my pump is a Huco which is highly recommended by someone on this forum, wasn't cheap and seems well made. I did only manage 2 runs on this pump before the problem started; but the first was a good hour long and 35 miles each way (in August). I wonder if my noisy old facet pump had the power to overcome deficiencies in my fuel line routing? I don't want to refit it. I'll persist with improving routing. Is the 800ml/min my flow rate test measured from the Huco sufficient? I've not yet investigated tank to filter for issues/restrictions. Tank is newish, but fuel line is old I think.

Peter, I've been wanting a timing light for a while...thanks for the excuse

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Quoted from RobPearce-
Quoted from PaulB-

On the Spitfire and I think it’s the same for a Vitesse, if you have a ballast ignition system,

The Vitesse never had a ballast ignition system from the factory. Of course, someone may have modified it but normally people take the ballast system off...

Thanks for clarifying Rob as I wasn’t sure about the Vitesse. 

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Baxter - it sounds like things might be moving in the right direction, but only slightly!

What model Huco is it? Sure you've got the right item but there's one that is designed for mounting near the tank away from heat sources, which might somehow not tolerate high temps...?? Clutching at straws maybe. They are meant to be good pumps by all accounts; was going to buy one myself but then a mate had a spare SU pump which did the job ok.

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Pete, great idea...all these straws need checking and I hadn't thought of that one. Will have to check tomorrow if I get over to the workshop. Certainly intended to buy the suction one.

I'm wondering whether to replace fuel line from tank to pump, just in case. Concerned about getting 800 ml/min when the pump is rated at 2.1l/min. I'll finish improving the current filter-pump-carb layout first in case that helps. Trying to make and test changes one at a time.

I've also been advised locally to measure coil voltage next time it happens and consider running a direct live from battery to coil to see if that fixes it.

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never dispel the ideas of the dreaded rubber slivers nasty little sods get cut off the hose bore when inserting metal pipes , they randomly float about and block the back of the float needle valves can drive you mad 

need to remove the valve and check behing where they hide and also pump a little fuel into a jar see if you catch some 

also check the hose to the top fuel tank outlet , good place to suck air but not show a leak due to no head of fuel present when parked up.

Pete

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Pete L, Guppy, thanks for your suggestions. Never heard of these dreaded slivers before. I am familiar with cleaning out SUs - I seem to have had to do it too often. I did service these 3 last summer and replaced needle valves and reset float levels. I also renewed the rubber hoses at the same time. They've been on and off a few times since, so maybe there are a few slivers cut off? I'll check this next.

Pete S, I checked the Huco pump model number and it is the correct suction model (PN 133010). I was happy to find I'd fitted the right one, then suddenly disappointed that I hadn't just discovered an easy-to-fix mistake!

I was reading Practical Classics Thursday evening and saw the following (on a list of advice for dealing with a car struggling up hills):

I was already concerned about measuring only 800ml/min to carb #1 so decided to proceed immediately with replacing the fuel line from tank to pump. Did that this morning and flow rate went up to 940ml/min. I then measured at the pump outlet and got 1,250ml/min, so I'm now halfway through replacing the pipe round the front of the engine. Hopefully I'll be test driving in the morning.

 

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Quoted from Pete Lewis-

also check the hose to the top fuel tank outlet , good place to suck air but not show a leak due to no head of fuel present when parked up.

Pete, this is an interesting one. I'll have to look at mine again. I have a rubber hose coming down through the boot floor into which I fitted and clamped my new fuel pipe, just in front of the drain plug. Not sure what happens inside the boot behind the tank but am thinking this might not be standard. I also don't seem to have the lever on the top to turn the feed pipe round to reach the reserve fuel supply

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It seems that all else has failed, so how about rigging a temporary fuel tank, a 2gallon plastic container would do, gravity fed bypassing the fuel tank. I've known a bit of rubbish in the fuel tank to temporarily  block the outlet before today.. At the very least you'll know its not the problem !

Tony.

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Quoted from nang-

 I've known a bit of rubbish in the fuel tank to temporarily  block the outlet before today..

Or in my case, as I mentioned a little earlier, the POR-15 "liner" peeling off and doing so. It only needs a very small bit of crud to block the tank outlet enough to stop fuel getting through at the rate you need. Could be a flake of rust or a chip of paint.

This is more of a problem on those tanks where the outlet is on the bottom - early Spitfire, GT6, Herald estate - then those with a top pick-up pipe like the Vitesse, but even then it can happen.

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Update:

So, I completed the fitting of a new fuel line from tank to carbs and headed out for my test drive, eager to see what difference I'd made with the higher flow rate. Have I cured some of the 'lack of power' on hills that has long bothered me? It certainly seemed promising. Then, after just 15 miles and 40 mins (including a long spell behind a tractor), the misfire appeared and developed to stalling within a few hundred metres. Leaving me stranded, as I'd expected either to be trouble-free, or at least have 17/18 miles range plus another 1 or 2 limping home. On the upside, I was stuck in a beautiful Cotswold village and got to see about 50 lovely classics drive by while waiting for a lift: https://www.cotswoldclassicrun.co.uk/

I changed the coil, just to definitively rule this out. Car started and revved cleanly, but misfired as soon as I pulled away. I got a lift home and returned 5 hours later with towing necessities. Tried running in case a little cooling down had temporarily fixed things, but it ran very roughly just revving in neutral. Then I swapped out the fuel filter - which seemed to have had too much air penetrate it - for a piece of straight hose and tried again. This time it revved sweetly and I set off with my wife following in the tow car. I made it 5 miles back to my workshop; waited for the final straight to test it accelerating hard, and even that went smoothly. Has this whole nightmare been caused by a fuel filter? Certainly the pattern seems to fit with vaporisation, and the filters are said to be a weak spot for this. I'll need to wait till the weekend to be able to do a long run to test this.

As for what's coming out the tank along with the fuel, there were already quite a few small black specks in the new filter after just 15 miles, but nothing more sinister. Likewise with the filter that had been on the last year or so.

Thanks for all the support and suggestions. I'll let you know if it's sorted. I hope so, because I've had my fill of breaking down the last few weeks!

 

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Quoted from Baxter-

...Has this whole nightmare been caused by a fuel filter?

Baxter, in the second post in this thread you considered removing the filter to see if it's having any effect!

5 hours cooling time though, and it still ran rough in neutral? That is surprising. 

Quoted from Baxter-

Then I swapped out the fuel filter - which seemed to have had too much air penetrate it - for a piece of straight hose and tried again.

When you say "had too much air penetrate it" do you mean you could see the filter wasn't full of fuel? Any gas in there would be petrol vapour rather than air. Looking again at your engine bay photo on the previous page, if vapour is visible in the filter then it would also be in the fuel line above. And that would seriously restrict flow, to zero. 

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Quoted from PeteStupps-

Looking again at your engine bay photo on the previous page, if vapour is visible in the filter then it would also be in the fuel line above. And that would seriously restrict flow, to zero. 

Not necessarily! Somebody on the other club's forum did some investigations on this as his filter was never more than a third full but didn't seem to cause any problems. It turns out that the cavity inside the paper element can be completely full of liquid fuel while the outside is nearly all air. It's a bizarre result but seems to be correct. Our best guess at an explanation is that surface tension means it's easier to suck liquid through a saturated permeable membrane than to break the film and draw air through.

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Quoted from RobPearce-

Not necessarily! Somebody on the other club's forum did some investigations on this as his filter was never more than a third full but didn't seem to cause any problems. It turns out that the cavity inside the paper element can be completely full of liquid fuel while the outside is nearly all air. It's a bizarre result but seems to be correct. Our best guess at an explanation is that surface tension means it's easier to suck liquid through a saturated permeable membrane than to break the film and draw air through.

Interesting Rob, I can see that being the case if it's orientated horizontally but Baxter's filter is (or was) vertical, with a further upward loop of fuel pipe. Any vapour obviously collects at the highest point it can. But of course this is all speculation from me!

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The investigation on that other forum was with a vertical filter too. I agree it's easier to imagine it with a horizontal filter, although I could also see it working the opposite way round. Fluid behaviour around membranes can be unexpected.

I'm not at all sure whether the loop of hose is significant. I could almost see the high point above the pump inlet being more of a problem than the filter!

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Yes, indeed.

The presence of a high point just before the pump inlet allows air (or vapour) to get trapped, which in turn allows the pump to become "dry", in which condition it suffers reduced efficiency. It then has to pull liquid fuel up to a higher level than the inlet using a partial vacuum in the gas bubble, which is hard.

The presence of a filter - even one vertically mounted - somewhere below the pump inlet appears not to produce gas bubbles at the pump. Even when initially dry, the pump isn't having to pull the liquid as high, and once primed it's got proper "incompressible" fluid down to the level of the filter.

So I can see that there may - based on a limited understanding of fluid dynamics and a physics degree some 35 years ago - be a mechanism for the big loop of hose being more problematic than a vertically mounted filter, especially if the filter is below the pump.

The arrangement I ended up with on my Spitfire does have the filter slightly above the pump but since both are in the boot and both are below the fuel tank it's a very different scenario!

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Guys - thanks for all the comments!

Fluid dynamics is taking things beyond anything I recall from my physics A-level! Pete S, you are on the ball with your '2nd post' comment! I kick myself now - as to why I wouldn't have swapped the filter out on a subsequent breakdown...I even added spare hose and clips to my onboard spares box. What a fool. You also recall that my filter 'was' vertical. Here's my new layout:

No more high points, and all lines shorter (especially round the other side between carbs 1 and 2). In the second photo you can see the filter is not completely full of fuel. This is how it typically looked in its former position for the previous 2 years, and during most of that time it worked fine. What struck me on Sunday was that the fuel level was below the top of the filter on both sides. Now I'm onto layout #3, sans filter.

I really hope this was the problem. I'll crack open that cold beer in the driver's seat in the garage once I've completed a long test drive. (Of course, the weather has now changed for the cooler and wetter so I'll need to go twice as far before I'm happy. BTW is there a post on here regarding rain water collecting in the Vitesse boot? Mine is as efficient as any water butt. That'll be the last thing I need to fix before it's ready for daily use.)

As for the filter, I only added it to the layout I inherited as I found the 'thimble' filters at the inlet to each fuel bowl to have lots of small brown/black deposits. I'll happily clean these out every few months. (Oh, question from Pete L, I think, re filter I just removed - it still has rubber hose attached to each end, so I can't say for sure whether either end had been crushed.)

B

 

 

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Update: I did a longer drive at the weekend, only 24 miles/50 mins; but no sign of trouble. I'm fairly confident it's fixed now.

And no longer kicking myself for not taking my own advice and removing the fuel filter earlier - taking the slow route to the solution led to the discovery of restricted fuel flow. Since changing the whole line from tank to carbs the car can now go up some local hills without losing speed. Happy days.

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  • 8 months later...

Hey @Baxter I just imported this thread from the old forum and so pleased I did! My GT6 is suffering from pretty much exactly the same problem. I've been though quite a few of the troubleshooting steps above, but not replaced any fuel-lines yet. Did the problem stay fixed since your last post? And if so, did you pin it down to a specific thing, or was it just some restriction somewhere in the lines? 

Thanks, and thanks for everyone's advice on this thread. It's gold. 

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Ben, thanks for importing and finding it helpful. I feel a bit stressed reading the thread again! Support from forum members has been fantastic throughout, and the new website seems to have greatly increased activity.

As for the fix - a lot of wet weather and covid restrictions have limited outings since my previous post. No problems yet, and two longish runs albeit in not such warm weather as when I suffered the breakdowns.

Too many variables for me to say for sure one change fixed things. However, I didn't have problems untill I put the Huco pump in in place of the facet. Ive persisted with the Huco and I think replacing the fuel line tank to carbs (50% better flow) and removing the filter (vaporisation suspect) have  even the two most influential factors.

I'll keep you posted after a couple of long runs on hot days. Needs proving before RBRR!

B

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