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setting fuel mixture


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I'm a little puzzled, having eventually got my pride and joy started after what seems like a long winter I thought it's time to sort out the carbs.

Firstly I took of the air filters and tried the lift the piston method, took quite a few "flats" to achieve the slight increase in revs the book advises, happy with my efforts I thought I would just check using the colour tune this is where I got puzzled the colour tune indicated I had set the mixture too rich.

Question is which system is the more accurate?

I'd be interested to hear the collected wisdom of members.

Thankyou David C

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It's my experience that ColorTune is very difficult to use.     I'd rely on the old fashioned methods and how the car feels to drive!

Adjust then test drive - what better excuse to use your TR?

John

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That's a plan, i'll stick to the lift the piston settings and try it on the road, (now it's a little warmer)

Neither method is very time consuming, so if necessary .................

Thank you John

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I have a colour tune that I've had for years.  I set my twin SU's on my TR4A very easily with it. Watching the colour of the flame change as you adjust the mixture is an excellent way to balance them.  Great  device.

Rich

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The limitation is that the colourtune only tells you what is happening at tickover. Likewise lifting the pins. Good for a base tune

The performance on the road is perhaps more indicative of how it is running under load.  Rolling road tune may well be the optmum.

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You can rev the engine with a Colourtune Andy.

SU used to sell a kit for balancing twin carbs, with a couple of wire flags that pushed into the piston dashpots so you could check they both rose and fell together. No use fo setting the mixture of course, but the airflow needs to be balanced first.

Pete

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I had another look at this issue today, having made up my mind to follow Johns advise..............however I found that to achieve that small increase in tick over one of the carbs adjusting nuts was actually up against the carb body, so that was no good, despite taking the car for a short test drive and it running quite well albeit smelling "a bit Petroly"

I have moved on to option two, setting with the Colour Tune, this seems to have given me a decent result though I haven't been able to go for a decent run yet.

I am puzzled though as to why with option one (adjusting via the lift the piston method) one carb could only achieve the desired result with the nut screwed fully up. could I need new needles or something?

 

Thanks all David

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6 hours ago, david c said:

I am puzzled though as to why with option one (adjusting via the lift the piston method) one carb could only achieve the desired result with the nut screwed fully up. could I need new needles or something?

Hey David,

Thats certainly odd...it can happen if the butterflies/thottles are out-of-sync...because theres a 'balance pipe' between both carbs, you can get (at idle especially) a situation where almost all the air is flowing through one carb and the other is doing very little. As others have said, start by getting airflow balanced (butterflies synced)...you can do that by slackening the clamp between the throttle rods and then use the idle screw on each carb to adjust the butterfly:

  1. Hiss method (your ears are surprisingly accurate)
  2. The airflow meter thingumy that you place over the carb throat (need to remove aircleaners)
  3. The 'piston lift' method - as above insert a piece of wire into each dashpot piston and measure the height of the piston when the engine is running, . This is how you do it on an  RR shadow... bits of wire can be replaced with dial gauges if you wanna go posh. (Piston lift corresponds to airflow thru the carb)

Once both carbs are the same airflow, you can re-tighten the clamp: Thottles synchronised!

Once airflow is correct, reset jet height to 6 flats down from the bridge - take off the dashpots to first set the jets at bridge height. At same time examine needles for wear and ensure both are fitted with the step flush to the piston body.

Now start it up on choke and commence your tuning once it's fully warmed-up & choke pushed fully-in, use piston lift or colourtune ....importantly adjust both carbs by same amount and keep notes on what you have changed...only when its nearly perfect consider individual tweaks to each carb...

If you still find the need for wildly different settings then there must be a problem with one carb at least...maybe an air leak, wrong needle, bad float chamber etc.

Edited by ctc77965o
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David,

 

An additional method on the road; find a smooth straight road, speed up to about 3000rpm in 3rd gear, shift into neutral, turn off the ignition, stop the car, unscrew the spark plugs and look at the color. These should be about the same... and good in color, not too dark and certainly not too light...

Marcel.

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Thanks both.

Dave, method three is one I haven't tried yet, but if its good enough for RR then its certainly good enough for me, I'll let you know how I get on, but I have a feeling i'm going to have to do a bit of stripping down and have a good look at the needles.

Marcel,  not sure when the last time was you drove on British roads, we don't have many roads that are smooth, straight and quiet enough to try this method, but I will be happy when I take the spark plugs out after any sort of drive if they are not black!

Thank you for your time and help David

 

 

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Never overlook the obvious.

Having had a good fiddle a bit of a think I went back to basics.

It all started with its reluctance to start in the cold weather, battery was good, spark was good so it had to be a fuel problem, not rocket science.

Now in the summer (basically the only time I really drive it, I know i'm a wimp) it's no problem start perhaps not on the button but reasonably well.

That made me think, well perhaps the choke isn't working properly, I could start in cold weather but only after a dozen or more attempts and some judicious throttle jiggling, not the sort of thing you want to be seen doing in Tesco's car park let alone outside the pub on club night.

So looking really hard at the choke gubbins I noticed the choke itself was really hard to pull out, a quick check  on this forum put me straight, it's no good trying to reroute the cable so it looks tidy and doesn't rub the underside of the bonnet it's go to be as smooth a run as possible, and if it doesn't foul the bonnet that's  a bonus.

Further fiddling revealed the choke cable was actually never really operating the butterflies, neither were they actually closing either, which made me think, again! Perhaps  the choke never really closing might have lead to the rich running and my inability to set the carbs properly, could it also cause the over run on switching off?

So having simply routed the choke cable properly and oiled the linkages a bit...............it started second time even in this chilly weather! if is sorted out the over run time will tell.

Thank you all for your help and advise .

David.

 

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