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Fuel mixture - colortune vs spark plug colour


daveg
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Herald 1360

Carb mixture set and checked using colortune but spark plug colour is black suggesting a rich mixture. Plugs are NGK BP6E..

Which should I trust?

Note that car is seldom used for short journeys.

Thanks,

Dave

 

 

 

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if you have run on choke it takes a good few hot miles to clean it off so checking plugs after a short run will be inconclusive and probably still sooty 

i prefer my ears to colour tune,  turn the jet adjuster to give the best running then reset the idle and repeat if any adjustment ups the idle speed.

what you air filter condition , colour tune at idle can be miles out when running if there is a filter fault 

Pete

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A Rover/triumph dealership trained mechanic friend of mine says the plugs should be chocolate brown in colour.

I have adjusted my carbs to acheive this, car runs well with good economy.  I adjusted the mixture nuts very slightly on every run untill I got the correct colour.

Hope this helps

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dave with the mods you say you think you have  does the car have a   standard twin filter air filter box or some orrible KN or panacake filters 

   what ever ,  make sure no gasket or whatever manages to cover the small holes on the filter faces of the carbs 

Pete

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Thanks for the replies.

I have the standard single carb (not sure about spelling carburettor!)

 "Adjusting the mixture nut slightly"  - a quarter turn at a time?

I'll give in and take the carb off, give it a good clean and check the gaskets.  Carb was fully overhauled a couple of years ago but I better obtain a new gasket set as some are tear - any supplier not recommended. No! I'll rephrase that to "any supplier recommended"

Dave

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Unless my carb is miles out I go 1 flat at a time - helps me keep track of what I've changed.

With the colour tune is the flame colour right at tick over, on throttle or both? I've found on SU carbs I struggled to get colour tune to look right all the way through the rev range.  (even with a decent needle selection to try) These days I tune by performance, sound and plug colour. Checking with colour tune shows it "wrong" at tick over, but right further up the rev range.

 

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Remember too that you're dealing with carbs/mechanical distributor/40 years of bodging and wear and tare so 'close' or 'good-enough' migth be as good as it gets.  You MAY have the carbs set correctly at idle, but modifications/wear/someone fitting random needles and springs in the past means you don't have the correct mixture at higher RPM.

If you're just trying to check the Colortune mixture is correct check your carbs for lifting pins.  If fitted use these to lift the piston a little bit and listen to the engine note.  Up + stays up = rich.  Down + stays down = lean.  Up but drops back = correct.  The amount you're meant to lift is ~1/4" and the first time you do this it's best done with the air filters off, so you can see there's some free movement of the pin before it starts lifting the piston/how much push is required to lift it 1/4".  After that it's best done with the air filters on, as they do restrict airflow slightly and so affect mixture.

If you're still not happy you have things right you need someone with a CO2 meter and to get measurements at a few points.  For a road car I'd say one at idle, one around 2,500RPM, and one around 3,500/4,000RPM, as that's the sort of range most of us actually do our driving in.  If you're someone who regularly goes nearer the red line than that then add a fourth measure at higher RPM.  One tip here is that if you have a twin exhause make sure you block one while taking the measurements.  Doesn't need to be air tight, just enough to stop one pipe robbing from the other - on my car we found a huge difference between the two exits (of a 4-2-1-2 system) down at idle, fixed by someone putting their size 9 boot over the end of one pipe.  From that you'll be able to see where across the RPM range you're lean/correct/rich and start playing with different needles to get it correct across the board.

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more ramble..    taking the filters off upsets the mixture and you get a false result  so really  ....... its best done in a running condition 

yes lifting pins are a very touchy feely thing you do not just yank the things up just get to touch and then a few MM will give you a clue as said .

but whatever   your ears will tell you are making headway to smooth idles 

Pete

 

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Took carburettor off and thoroughly cleaned everything.

All well until connecting the throttle cable and due to the inner cable splaying at the end it was near impossible to connect up without omitting some of the inner strands. The last time I ended up fitting a new cable.

Does anyone produce a cable where this splaying doesn't occur or is there a trick.  I thought on using an electrical connector which would have a bigger hole.

I know the carbs are different but will the accelerator cable for a 1200 fit my 1360?

Thanks, Dave   

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Dave, it used to be that you could solder the cut end of the cable to stop it fraying.  These days a lot of cables are stainless steel which doesn't rust - it doesn't solder either! (Well, not without VERY specialist solder/flux) So now-a-days you can put a blob of superglue on the end which does the same thing.

Cheers, Richard

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On 04/06/2020 at 17:47, Pete Lewis said:

but whatever   your ears will tell you are making headway to smooth idles 

Always gone off lifting pins and plug colour, though always had a slight uneven idle on my Vitesse (which could be down to lot's of variables I guess and has always been pretty much the same, even when my timing and carbs have been pretty way off, as I was learning this stuff in the last few years ) which maybe normal? (especially for some wear on engine, dizzy, carbs)  and not really bothered me

Maybe worth to try a fine tune with the ears on the mixture which I will do, though interested on any thoughts?.

Dave  

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