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gt6 engine stumble at constant speed


Norgt6

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I hope someone is familiar with this. My engine (standard tune gt6+, with K&N filters, carbs are 150 CDSE) developed a stumble (studder) at constant speed. It does not matter which rpm. Acceleration is smooth and also stationary the engine runs nice. I have adjusted the float height to the correct value (16 to 17mm), which resulted in the very nice stationary and acceleration, but the studder remains. The spark plugs have a brownish colour. The only thing is that the exhaust smells, but there is no smoke.
The studder seems to get worse as the longer I drive.
I am starting to feel like Sherlock Holmes, but less successful. I was planning to take the car to England to visit the Triumph day, but as it stands now I think it will be my modern car  :'(  

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fuel filters dirty ,low fuel prssure .white deposit from modern fuel floating about in lines snoting up the jets every so often,my shed does it,I swap the in line filter top it it up with  high grade un-leaded shove some decent octane booster in it and flog the nuts off it for 20-30 miles or so and the stutter goes away for a couple of thousand miles-in fact about once a year it gets the "clear the pipes" treatment

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I have replaced the fuel filter and have fresh 98 octane in there. I have a spare fuel pump lying around, maybe I should give that a try.
I have electronic ignition installed. The fault may be there, but difficult for me to check. The advance seems to work ok. It seems weird that the problem gradually gets worse the longer I drive. To me this suggests that it is not the ignition.

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I think you are describing what I would call "lean hitching".  At cruising speed speeds, throttle openings will be quite small and the carb needle profiles will have been designed to give as lean a mixture as possible at this point for best economy.  Trouble is, if you go just a bit leaner you get this intermittent light misfire which can range from just a slightly edgy, hesitant feeling to full-on checking and surging.  The defining feature is that as soon as you get on the throttle, it clears.  Quite often you will find it goes on a slight uphill section, or even when driving into a headwind because you are using that bit more throttle.  On the GT6 /Vitesse Mk2 engine this effect occurs (on an injected engine at least) at between 16.5 and 17:1 Air/fuel ratio, which is pretty damn lean considering the vintage combustion chamber design.  On carbs it probably needs to be a bit richer.  I've spent alot of time messing with cruising mixtures on my EFI system so I'm pretty familiar with this effect.
Under bonnet temperatures and fuel temperatures will play a part and this probably explains why you find it gets worse on longer drives.  You may also find the exhaust smells sweet due to the partial misfires and the engine may run a little hotter as the effect is a little akin to running retarded ignition.

I suspect that if you richen the mixture up a little, the problem will go.  The K & N filters will make the engine run a little leaner and you may well find that with the standard needles, you need to have it set a bit on the rich side at idle to give the right results at cruise.  You may also find that disconnecting the vacuum advance pipe will cure the symptoms, but this is due to needing more throttle to overcome the torque lost due to reduced ignition advance, so is not a good long term solution because it wastes fuel.

Try richening the mixture a little and see what happens.

Cheers

Nick

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Nick, thanks for your comment. It definately sounds like this. The problem is that my carbs cannot be adjusted. These are the early 150CDSE ones. I have ordered adjustable needle ones, so hope this will work. The temperature is a bit weird. At the moment the air temperature is 6 deg C here, quite cold, so the engine is not completely on normal temperature although warm enough to run at normal.  The exhaust does smell indeed.

The engine always ran pretty lean with very light coloured spark plugs, but have not noticed it this bad. Something has changed. I checked for false air leaks. There is some play on the carb butterfly shafts. This may be the source for the extra air.  

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