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6 cylinder running rough but....


AidanT
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I thought I would start a new thread on this as I'm not sure I yet have the solution

So symptoms

When the car (GT6 mk1) is up to temperature the car miss-fires when pulling away and is a little jumpy until it gets to over 40 or at least until 3rd gear It then runs well. 

Changing the plugs seemed to fix the problem but now it's returned

I don't think it's fuel evaporation unless fuel composition has changed this year making it evaporate more

The car is perfect when starting from cold and is happy to accelerate smoothly through all gears

I have not changed anything apart from plugs ( and fuel obvoiusly) from last year when it had no issues at all

I've not replaced any fuel piping 

So what could the cause be?????

Aidan

 

FURTHER DETAILS

Electronc ignition  - H&H (2016)

Quality Leads - Lumenition blue (2016)

NGK plugs (Club Shop)  - New set on order anyway 

Coil  - Lucas Sports Coil (2017)

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Aiden

So (new trendy way to start every sentence), when you have misfired and jumped your way to 40mph and it settles down, if you then let the speed drop and then accelerate again, does the problem reoccur ?...if so then something doesn't like being on load. Does it idle OK ?

Clearly it's either fuel or ignition as long as nothing else has changed.

On fuel side I would check that:-

  • tank outlet is clear (floaters in tank blocking outlet)
  • pump filter and any inline filters are clean
  • breather is clear (not sure there is one on the Mk1/2 as I think it vents through the cap
  • carburettor diaphragms are OK
  • carbs are synced

On electrics side (I assume Delco distributor):-

  • No experience of H&H Ignition (sorry)
  • Have you tried putting points and known condenser back in (I did this when my ancient Mobelec E/I failed and it runs really quite well with a Shacktune external motorsports condenser)
  • Rotor arm (Accuspark do a red jobbie for Delco)
  • Distributor cap (TD Fitchetts have original Delco caps)
  • Try a coil swop

PS others will be along soon.

Ian F

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Hi

So thinking about it, yes if does re-occur when slowing down and then accelerating again  so that's one reason why I was excluding fuel as a problem. Filters, one in the glass pump and one just before the carbs are clean and full of fuel so evaporation will be unlikely in my mind. Carbs were proffessionally rebuilt three years ago and balanced on a rolling road two years ago. 

I will check the plugs again but they were always sooty around the rim and light brown on the tip will add a picture later

Aidan

Yes it's a delco d200

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do check diaphragm for small perforations give it a stretch  if they are baggy wash in petrol reforms them back to size  

that sooty could be the opposite of where we are thinking its a bit rich, the hot weather will exasperate this and make for lumpy

pete

 

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My GT6 (a Mk3) has very similar symptoms. One difference is that I don't think the problem reappears if I slow down unless I stay at low revs for a while. What I mean is, when it's running smooth, I can drop it to 1500RPM in overdrive and it pulls smooth. If I have to idle for ten seconds, it will stutter and stumble as I pull away until I get it over 2000-2500RPM, but then be fine changing up through the gears.

Like Aidan's, mine improved a lot after an ignition service (I think the item that made the difference was the disi cap, in my case) but the problem reappeared shortly after.

I don't think it's a fuel supply issue (floaters in the tank, etc.) as I'd expect that to give problems at sustained high speed/load, not initial pull-away. Vapourisation, I could believe, but something about the way it clears up feels wrong.

Aidan, when yours clears, is it sudden or gentle? Mine feels like one or two cylinders have suddenly switched back on.

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I think it just goes away There does not seem to be a power surge though If I rev the engine on take off then the problem is less noticeable, and the car never stalls tick over is fine.

If it's running too rich Pete these are on the bottom of my carbs and they o my move anti clockwise as in they undo so what do I need to turn to weaken ?

Or could it just be that I'm burning a bit of oil and that's why the plug find are black?

Aidan

 

20180712_164822.jpg

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Aiden

That's the diaphragm out of the Smiths breather valve (looks OK by the way)

I actually meant the carburettors diaphragms, which you will need to get at by unscrewing the tops of the carbs. Should be fine if only three years old since rebuild, but you never know. (watch out for spills of dashpot oil and be careful not to bend the needles-diaphragm will only fit in one position on reassembly)

Another one to add to the list is dashpot oil, as this is partly controlling mixture (richening) during acceleration

It doesn't sound like fuel vaporisation to me. My experience of FV is the engine dying when it gets hot in traffic or not restarting when left after hot running. I my case when it happened, taking the top off the pump I could see the fuel boiling the base of pump. Reassembling the pump and bleeding out the vapour at the carbs allowed normal service to resume. (now finally 'cured' with a Huco electric pump - I hope)

I think I would check these fuel items and then concentrate on the ignition bits as above ie:- cap, rotor arm and coil. Do try it with your emergency set of points and condenser, it will be good practice in any case (I had to do it once in anger after 'failing to proceed' off the ferry at Portsmouth, which was annoying as we had been all the way to Florence and back without any problems). Also check the timing as well with a strobe and that it is advancing when you rev it, as balance weights in the distributor can stick.

Hope this helps, please report back as you work through the list...and don't change everything at once otherwise when you fix it you will never know what the problem was.

Ian F

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Ian

I will check those diaphrams out tomorrow. The dash pot oil did seem a little low so I have just topped that up. Another note, it did seem to use a lot of fuel - about half a tank on an 80 mile drive yesterday so possibly another symptom 

Aidan

 

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I had an intermittent problem like this which took ages, and much bad language, and going through all the above checks before curing.  Just before Classic Le Mans stripped the carbs and found debris, not hose slivers, behind the needle valve on the rear carb evidenced by a needle valve that did not consistently drop freely.  Changed valves in both carbs, re-centred jets, set the mixture and she dealt with the hot weather last weekend no problem.  On a US site found something similar and the chap got his valves moving freely by squirting carb cleaner directly into the inlet where the hose attaches.

Dick

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Aidan  your carbs are simple  cd or cds  the jet adjuster is the lower screw with the coin slot in it

Screw it upwards into the carb raises the jet and leans ,, unscrewing lowers the jet and richens

Do any changes 1/4  to 1/2  a turn make small adjustments , when its up to temp

Dont go mad , but if the idles starts to increase youre going the right way , if it starts to faulter its a step too far

Count what you do so you can return from whence you came 

Pete

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Aidan, 

14 hours ago, AidanT said:

carb cleaner down the fuel intakes on the carbs?

Yes, that's the ones; assume he did it to clear any 'gunge' that had dried on the valves.  I would follow Pete's advice first and, if it helps to avoid burnt knuckles, I use a piece of angled metal rather than a screwdriver to adjust the mixture.  Does not take much physical effort to adjust and something like that is almost certainly required if you fit heat shields to the carbs (recommended) .

Dick

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Aiden

The list of checks is certainly growing and it looks as though you are going to have a busy weekend, but it pays to be methodical and thorough.

In my experience with float valves, if there is problem it will usually result in the carbs flooding due to not being able to shut off the fuel, in which case fuel will be flowing out of the breather hole. It is possible to remove the valves for cleaning or replacement with the carbs in-situ. You will need to remove the float bowls (make sure to undo all of the screws) pulling it down to get clear of the jet bushing screw (the big brass bit). Be careful not to twist it to much as you might damage the float. The float needs to be released from the pivot and the float valve can then be removed. Clean in carb solution and then check that it closing properly by blowing through it whilst touching the centre pin. Before refitting I would pump a bit of fuel through with the priming lever on the pump which should flush out any debris in the lines if present (collect it in a container and have a look). Refitting is reverse of the above ideally with new float chamber gaskets.

Regarding mixture there is no reason why the mixture setting should have changed since last year when you reported it running OK, so I would leave that alone for now. There is a test described in the WSM involving slight lifting of the air valve that should be used to determine whether any adjustment is required. If the car is not running right at present, the plugs will be a poor indicator. (luckily you have the carbs with the easily adjustable jet. I have similar but in CD175 flavour)

Is there anyone in your local area that can offer practical or moral support.

Further reports awaited with interest.

Ian F

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Thanks Ian

So one step at a time. The diaphrams look in excellent condition (new) 

 

Sorry not the best shot but I gave them a clean with carb cleaner anyway

I am not that comfortable doing a further strip down but will keep the carb cleaner down the fuel intakes as an option

Aidan

 

20180713_095158.jpg

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How is your timing? Check it and make sure you're at the correct setting for both the MK1 engine, and unleaded fuel. I set mine to handbook spec back in March and it ran terribly, yet setting it beyond the recommended setting - about 15 / 16 degrees BTDC - has it running perfectly ever since.

Incidentally I had my Delco dizzy rebuilt by H&H including their electronic ignition, an excellent job and it's running well.

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Hi Colin

Yes its near the top of the list  - Just purchased a Bosch Coil,  £22 off ebay so not bad, I still think its electrical.

Pete Do you remember at the first tinker day, my timing was all over the place?  I fixed it by cleaning the plugs?  Anything possibly to do with it?

Rob, what are your plugs like?

Aidan

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10 hours ago, AidanT said:

Rob, what are your plugs like?

I haven't checked them recently. If I get time this weekend (between trying to put the Spitfire together enough to take it for an MOT) I'll check and report. My suspicion is that it's probably running a little rich - being a Mk3 it has the horrid CD3 carbs with the special tool you have to poke down the dashpot for tuning.

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Rob make sure the temperature compensators are really  closed by tighening the small nut inside, and pop the air piston out , use a normal long allen key to set the needles delrin washer level with the base of the air piston   job done , no special tools  refit and it should be set  ok..

Pete

 

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