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Adjusting The Hs4 Carby


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#1 maystro

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Posted 11 August 2020 - 08:16 AM

Hi Guys,

 

I have just put in a new HS4 carby and trialing some new needles.  I don't have a colour tune or a dyno in my tool box so I'm just doing this by ear and the colour of the plugs.  

 

The doc's I've read say to lift the piston up 1mm to listen for idle changes but how do I know how much the piston is going up by using the spring loaded lifting pin?  I'm pretty sure 1mm of the lifting pin does not equate to 1mm of piston lift.  

It seems to work if I just push the lifting pin all the way in to get a result otherwise there is no notable changes?  

Is this the correct way?

 

Thanks Brad



#2 GraemeC

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Posted 11 August 2020 - 08:55 AM

I think the 1mm dimension is a bit of a red herring - I would go with a full press of the pin.

 

However - the lift of the piston is directly related to the lift of the pin, there are no levers involved.  But the pin may not be in permanent contact with the piston so there may be an amount of free movement before it has any effect. 

You could look down the carb inlet and see (measure?) how much the piston moves for a full press of the pin



#3 Chris1275gt

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Posted 11 August 2020 - 10:19 AM

Get a colour tune plug you can get a really good mixture setting in about 5 minutes and save all the farting about.

#4 maystro

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Posted 11 August 2020 - 10:41 AM

Are the colourtunes really that good. I will if that is what you guys recommend?

#5 Chris1275gt

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Posted 11 August 2020 - 11:36 AM

For me personally I'd say yes. I tried messing about with the lifting pin and listening to the engine but with no experience of engines and no idea what I'm listening for I struggled. Bought a colour tune fitted it and while adjusting the mixture you can watch the flame in the bore go from a bluish white, lean, to rich blue, about right, then to yellow,rich, then back to the rich blue and your done. I was planning taking it to get it tuned properly on a rolling road when funds allowed but after 1200 miles it's sweet as a nut still so I probably won't bother.

#6 sonscar

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Posted 11 August 2020 - 07:22 PM

Lifting the pin and using the colour tune only set the idle mixture.It is assumed all things being standard and good it will be mostly OK everywhere else.If anything is changed or worn and even unleaded blended fuel this may not apply once off idle.Be careful.Steve

#7 timmy850

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Posted 12 August 2020 - 06:46 AM

As above, the lift pin only tells you the idle mixture is ok. It assumes you have the correct needle for your car

If your needle needs to be swapped or reprofiled to match a tuned engine you’ll need a dyno or air fuel ratio gauge.

#8 sonscar

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Posted 12 August 2020 - 10:33 AM

Even using an AFR gauge is not simple,yes you can measure the mixture,but where on the needle does this correspond to?high load?low load?what mixture would the engine like?Minefield.Steve..

#9 timmy850

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Posted 12 August 2020 - 10:10 PM

If you really want to get into SU tuning in depth there are a few helpful books out there. Eg

Attached Files



#10 maystro

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Posted 13 August 2020 - 11:02 AM

Thanks Guy's

 

I was thinking the jet adjusting screw on the bottom of the carby would impact the fuel flow on the the whole rev range not just for idle.   After all this little pool of fuel is where the needle goes through and if there is more in the pool to displace by a fatter needle then less fuel is available, alternately a thinner needle will displace less and that is where the whole profile of the needle comes into effect?   EG:   a AAU is fatter than a AAM  so less fuel is spilling out of the pool? 

Maybe I'm wrong,  but that is how I look at it.  After all a SU carby only has one jet to cater for all rev ranges unlike other carbies.

Cheers Brad. 



#11 GraemeC

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Posted 13 August 2020 - 12:05 PM

Yes. And No.

 

The jet does affect fuelling across the entire range.  The fuelling is a combination of both jet position and needle diameter at any given piston lift.

 

What you are doing is getting the optimum jet position for the needle diameter at idle (small piston lift) to give the fueling needed at that point.

 

However when the piston is lifted to another position (say 50% max lift), the optimum relationship between jet position and needle diameter will be different. But the jet position is fixed so the needle diameter needs to be changed.

That's why there are hundreds of different needle profiles and why people file needles to tune cars.

 

Effectively you are only setting the fuelling at one 'station' of the needle.  If the engine needs more (or less) fuel at a different station, you can't change that by  the method you are using.



#12 Chris1275gt

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Posted 13 August 2020 - 01:18 PM

Hi Maystro
I'll expand on my previous post on my bog standard 1275gt it has a refurbed engine and gearbox and a refurbed carburettor it's my daily driver, being retired it's shopping and a 60 mile drive every week because it makes me feel good
For the novice like myself the colour tune plug will allow you to set a pretty good idling mixture. Once set With a perfect set up the needle should be ok across all positions of the throttle. With short busts on the throttle with the plug left in you can still see the colour of the combustion flame across all positions of the throttle. As I had only 1 plug I placed it in all 4 Pistons, 3 we're fine but on idle the no 1 bor had a slightly yellow flame but on higher revs all 4 gave a pretty good blue flame picture so I put it down to anomalies in the intake flow to no1 bore. If I'd of had the funds I would of taken it to an expert on a rolling road to check it was operating correctly at all points of the needle. As it turned out it is running fine for me under all revs and loads, it's not using oil and the engine temp is fine with a pretty smooth acceleration and no hesitation. As I said previously I've done 1200 miles, I checked the plugs every 100 or so and they all look the perfect colour and exhaust is looking ok.
So set your idle with the plug and drive it down the road and see what it feels like, if not right or you're not sure get it on a rolling road. Should I come into some disposable income I will still probable get it fine tuned on a rolling road.




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