FIXED !!!!! - Xflow - misfiring/uneven running when hot (still happening)

[HendrixsWhiteStratReturns]

Evening...

Irritating problem with the car, interested in anyone who's had similar experience.

My 1700 Supersprint had developed a bad misfire/poor running a while back that was eventually traced (thanks to a site member) to dodgy choke mechanism messing up the Webers. I had them looked at at the time and all was fine and I don't use the choke now (at all).

However - car has developed a persistent misfire when running hot. Have changed the leads, checked and regapped the plugs (which were nearly new anyway) and replaced the coil with Bosch blue. The throttle cable was sticking recently so I replaced that and operation is smooth and silky.

Symptoms:

- car runs and starts perfectly from cold with an even tickover

- drives normally for 10-20 mins at all speeds, pulls well, smooth acceleration

- usually after 20 mins it starts to misfire at speed, eases off when I drop down to 50mph (and often disappears) and then as soon as I try and accelerate it coughs and splutters and would grind to a halt if I didn't ease off. Lots of popping and bangs through the exhaust.

Any ideas ? The only thing that hasn't been touched is the dizzie (an Aldon) I thought it was electrical  but thoughts now switching to but Fuel (pump) although does seem heat related issue ? The Fuel tank was replaced with new last year (and was swilled out and cleaned before being put in) so I've ruled out crud in the tank etc.

Any thoughts appreciated !

 

HWS

 

 

 

 

 

 

 

[Jonathan Kay]

It's not quite right, because of that total recovery on slowing, but how about fuel starvation from marginally inadequate breathing into the tank?

Can you reproduce it by stopping just after it happens then revving at rest?

Usual test is to see what happens when you remove the filler cap: if affected you'll hear the rush of air in and it will then run normally.

Jonathan

[HendrixsWhiteStratReturns]

Thanks Jonathan - will give it a try !

[Colin Cooper]

I had a similar problem, eventually traced to a dodgy fuel pump (by James Whiting by putting a sensor on it). 

Worth checking out, and only a few quid to replace.  

Col

[elie boone]

Check your needle valve, do you have a electric fuel pump ? if so put ign on and wait for a few moments if 1 carb starts to drip ( can take a few minutes ) then your needle valve needs replacing and your float level needs to be checked.

[john g]

Had a similar problem and it turned out to be the plugs, and like yourself they were pretty new, 12 months old. For a few quid or if you have some old ones in the garage might be worth changing them and trying it.

[Terry Field]

Sounds more like fuel starvation than heat related. Air starvation In the tank is possible, but unusual on a x flow - they tend to leak everywhere!

I would look at the fuel pump. When you cleaned the tank out, did you change or clean the fuel supply pipe to the pump? Could that be blocked?

Terry

 

 

 

[SM25T]

Does sound like no air getting in to the tank to replace the fuel consumed. When it happens, remove the filler cap and see if there is a big inrush of air. If there is ... you have the answer.

[HendrixsWhiteStratReturns]

Gents

Thank you very much for taking the time to post - appreciated. Meant to reply yesterday but waylaid by relatives ! I'm going to check everything and work through it, including the fuel pump as it looks original and not touched for a while.

Jonathan and SM25T - you could be on to something, If it does turn out to be no/not enough air getting into the tank (and is confirmed by a rush of air when I open the filler cap, what's the cure? I have one of the aero type caps fitted (came with the car when I bought it).

HWS

 

 

[Jonathan Kay]

You increase the ability to suck air in by fixing or replacing the breather. But AFAIK there are various different kinds of caps and breathers and valved tubes used on 7s and I don't know my way round them.

I was about to ask about how the best way to test fuel pumps. I'd want to check the feed and earth and the sound they make... but what comes after that?

Thanks

Jonathan

[SM25T]

Certainly the breather hose with a one way valve fitted .. to allow air in.

[elie boone]

Or take the rubber O ring out of the barrel of the aero filler cap

[HendrixsWhiteStratReturns]

Afternoon all

First chance I've had to post but just wanted to offer a collective thanks. I took the car out and sure enough after 20 minutes it started playing up. I pulled over, took the filler cap off and can't say I heard an inrush of air but the engine note immediately picked up and the car ran with no problems for the rest of the run.

I did change the fuel tank for a brand new one not that long ago and I wonder if I have pinched the breather somewhere - Boot floor out tomorrow for a look see.

Thanks again to all who posted - appreciate it !

[Jonathan Kay]

...the engine note immediately picked up and the car ran with no problems for the rest of the run.

I think that's the crucial functional test. As above I don't understand the variety of breather systems, but NB SM25T's comment about the one-way valve... if that's the wrong way round or blocked it may not be visible from the outside.

Jonathan

[Paul Deslandes]

I can't remember whether your car has one of the Newton fuel filler caps, bright aluminium rather than black, with a lift up tab to release.  If so, these need a small hole drilled in them to let the air in, always assuming that the filler neck doesn't have one of the modern one way breather valves fitted.

http://www.newton-equipment.com/aero-400-filler-cap.html

The more recent filler necks have an elaborate breather system that allows air in but stops fuel escaping in the event of an accident inverting the car.

Delighted to hear that car is going well otherwise.

Paul

[HendrixsWhiteStratReturns]

Cheers gents.

Hello Paul !! Yep I've got the 400 fitted on my car, cool - will do.

Yep she's all good thank you (touches wood) and has been out on a few runs :-)

Hope all good with you?

Best

 

Martin

[Paul Deslandes]

Hi Martin

I drilled a 2.5mm hole under the lift tab as per photo.  Just make sure that you don't drill through the plastic on the inside.

Yup, all good here thanks

Paul

[HendrixsWhiteStratReturns]

Ah ha ...that's useful, thanks for posting Paul !

 

[Paul Deslandes]

Don't drill too close to the locking barrel.  Mine could have been a bit further 'north' although it is clear of the barrel.

[HendrixsWhiteStratReturns]

Morning gents

Despite carrying out the mods for the fuel filler (worth doing anyway) and checking the tank breather is okay the problem came back with a vengeance on the way to the MOT this morning and I had to cancel the test. Broke down on a bend on a fast road, no run-off and huge lorries whizzing by - a bit hairy at times.

Recap of the problem:

- car runs and starts perfectly from cold with an even tickover

- drives normally for 10-20 mins at all speeds, pulls well, smooth acceleration

- usually after 20 mins it starts to misfire at speed, eases off when I drop down to 50mph (and often disappears) and then as soon as I try and accelerate it coughs and splutters and would grind to a halt if I didn't ease off. Lots of really loud popping and bangs through the exhaust. as it misfires.

- eventually the car coughs and grinds to a halt and although it will start it sounds dog rough on tickover.

- If I wait 10-15 mins for the car to cool it then starts and runs perfectly, accelerates, pulls etc normally....all fine....then after another 10 minutes of running the problem starts all over again.

It's recently had new plugs/leads/coil/ new fuel tank etc ....

Definitely seems a heat related problem - a couple of people on here have mentioned the fuel pump, I'm happy to change it to eliminate things but is it likely to cause the issue ? I thought that if the fuel pump goes it would just fail period whereas this is intermittent ? The only thing I haven't replaced is the fuel pump and Aldon distributor...or could it just be the Webers playing up ?

Any final thoughts much appreciated - literally pulling my hair out. I love this car but it really is being to make me just want to stick the thing in the garage and forget it ! 

Cheers

HWS

 

[Piers300]

I had a Super Sprint from 1985 to 2010. I recall issues of the in tank sealant being sucked into the fuel line and blocking it. The way to clear it, is disconnect fuel line at the tank and carbs and either use an air line or foot pump to pressurise the fuel line. What flew out of mine way interesting.

Piers

[HendrixsWhiteStratReturns]

Thanks for the post Piers. Appreciate it.

[Paul Deslandes]

Hi Martin

just finally to eliminate the tank breathing from your enquiries, I assume that you tried physically removing the fuel filler cap when this happens.

Otherwise my feeling is towards ignition failure.  If it was fuel starvation, it's likely that one carb would run out of fuel before the other and you'd run for at least a few seconds on two cylinders. 

Do you have an Aldon dizzy or just the points replacement module?  I understand that they can be a bit unreliable although the module I have in the MGA has been ok so far. 

Can you borrow another dizzy to try?  At the least the fault is very repeatable which makes it easier to locate.

If you can get the fault in your driveway, can you blow cold air from a hair drier onto the dizzy and see if it comes back more quickly.  You could also try heating it up but I'd be very careful in case you really cook something.

You will get there

Paul

[HendrixsWhiteStratReturns]

Hello there Paul - soon you'll know the history of my car better than me ! 

Yes I took the filler cap clean off this morning and tried to run without it for a few yards but no joy.

I think you could be right actually. I'm not convinced its the fuel pump and the Aldon looks pretty old and is one of the few ancillaries I haven't changed on the car. It's an Aldon dizzy with ignitor (side entry cap). When the car grinds to a halt it reminds me of a Triumph Spitfire I had once as a teen that did the same, and that was invariably down to dodgy points. I might just bite the bullet and buy a new distributor and then take it for a long blast down some quiet roads on a day off and see what happens !

Thanks for posting, hope all good with you.

Best regards,

 

Martin

[Roger King]

I second the idea of debris in the fuel pick up. Twenty five years ago we ran a Crossflow race car that would always run beautifully in the paddock, but then was guaranteed to develop a misfire about three laps into the race. As soon as the car was taken back to the paddock it would run fine again (it wasn't a breather problem).

This happened for about six events with no fault to be found until we changed the tank and the problem went away. Investigation eventually found a lump of loose sealant in the original tank; it had been getting stuck in the pick up and then dropping out again once the car was stopped. This is a slightly different scenario to the one Piers experienced, but closely related, and might explain why the problem comes and goes.

There's no guarantee that this is the problem, but it's worth investigating, along with the ignition system.