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Car runs rough and backfires; petrol leak at the carb......


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Hello everyone - I just got my 4A back from a full service and a day later - on starting - it ran very rough, backfiring frequently. I have an electronic dist and all the plugs are tight and unbroken. However, I tried it again today with the same problem - running rough and backfiring - but now with a small petrol leak at the union of the first float and the petrol pipe in from the pump. It's the only pipe are with a crimp clamp vs a screw one. So a steady slow leak. Could this be caused by a broken - or jammed - float in the first carb? (forcing some petrol backout through an ill fitting clamp??). Somehow there is improper flow to both or one carb = rough running. Don't know about the backfires, though. It runs so rough this is clearly now a self-repair first to get it back to the service shop in Acton. HS6 SU carbs on the car - rebuilt about two years ago.

 

Thoughts welcome!

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Firing order (1,3,4,2), broken dizzy cap, vacuum advance or float/needle jammed on one carb with fuel flooding the cylinders. Check those first but my guess is the last one. Black smoke ?

Edited by Geko
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Yes, I'd check the carbs first especially if the garage cleaned out the petrol pump filter as part of the service......only takes a minute bit of sediment to work its way up the fuel pipe and jam the float needle

Also, ask the garage if they tinkered with the carbs....balance/mixture adjustment.....could be running too lean if she backfires

Don't forget the condensor, they can just pack up.....even a new one if the garage fitted a replacement as part of the service

Cheers, Nick

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I've got an electronic distributor so no condensor and I was told timing as spot on. However the fuel filter (inline) was replaced. I know the air filters for the carbs were cleaned and re-oiled. I pulled the last plug (brand new) and it was black and slightly wet. That said no real black smoke.

 

Have not taken the float apart for the chamber/float needle etc. I imagine that is my next stop when I have some time to play with it.

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It is running rough. It maintains idle but roughly; any acceleration causes a backfire and it does not run any smoother. The distributor is an electronic 1-2-3 which has never given me a problem.

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Check that the jets are returning from the enrichment position, i.e. fully pushed up underneath both carbs, they get lowered when using the 'choke' and can stick in the down position.

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Thanks - a good "to do" list when I get to the car this weekend. I'm pretty sure the tappets were adjusted; timing was "spot on" I have been told. I have also been told the petrol leak is the "overflow" from the jammed float. (I will check as well). Report soon - and yes I do need to check for the oil. Lastly, both carbs seem ok in that the choke works as promised.....

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It is running rough. It maintains idle but roughly; any acceleration causes a backfire and it does not run any smoother. The distributor is an electronic 1-2-3 which has never given me a problem.

It might be timing but I suspect this is ignition and not carbs unless they're spitting back, rather than exhaust bangs.

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Ron,

 

I would have a very close look at the distributor cap, looking for a crack on the internal surfaces. You could also have a look at the running engine in the dark to see if there are any sparks where they shoudn't be.

 

Cheers

 

Graeme

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Ron,

 

I would call said garage that completed the work and tell them of your problems. Any garage worth there salt will send someone out to investigate.

 

They may not if you have been tinkering yourself though

 

Tom

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Thanks for that - I've spoken to them (garage) over the phone and they have suggested I look at the float myself. I'm ok to do that, as they will send out a length of pipe and a new clamp as a backup. If I have no success I'll look at other options.

 

Re the distributor - it's a pretty new 123 electronic one. I haven't noticed any problem with cap or rotor, but will re-examine everything once I check on the float. I also need to see if there are clouds of blue smoke when started, which I haven't noticed before but will check for again.

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Ok, pulled the floats apart and the lines - blew through them and still had a rough runner with petrol leaking out or the first float. The garage guy came out on his bike and we tried again - this time no leak! We found one really fouled plug, cleaned it and ran it up slowly - less backfires. Next step is to take it out on the road...!

 

**** - err....dirt in the first needle jet is the thought right now....It will get a good run Saturday.

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Ron

Have you replaced any of the fuel pipe which leads from the fuel pump to the carbs?

I did this and when I pushed home the pipe into the first float chamber inlet, really tiny fragments of rubber pipe kept finding themselves into the float chamber, blocking the jet causing flooding

The car runs well at the moment but I keep expecting the same problem to reoccur

Any fragments of crud coming from the fuel tank?

I also put a fresh set of plugs in and it made a big difference to the starting and running

Hope this helps, I'm sure it will be something simple!

 

Nick

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Thanks Nick - the fuel lines are pretty new and the inline fuel filter brand new - the old one was pretty clear, too. The plugs were brand new a month ago but fouled at the moment. I'm going to clean them then run the car for a while. Seems to be solved but only a good run will tell that!

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Ok, I nearly stalled on the flyover out of London today, lugging and backfiring. I gave it half choke and made it to the garage. Here we: 1) replaced the rotor on the 1-2-3 which was missing the interior metal clip, causing more play on the spindle; 2) replaced the jet in the front float as a precaution, and 3) replaced all plugs (2-3-4 very fouled). It runs OK now and I made it back over the "flyover". So, no real solution but clearly one of the above was the issue. I honestly think dirt in one of the jets followed fouled plugs. But, I'm getting an extra rotor in case!

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If the car was backfiring then fuel was getting into the cyl but the firing was incorrect. No fuel = no back fire.

 

I would guess that the rotor was at fault.

 

Cheers

 

Graeme

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