Adjust mixture on a BT7

[sebastianwendt]

Hi,

does anybody have an idea / a hint how to adjust the fuel mixture on the triple SU carburettors when they are built-in ?
Worst of all my car has a fan-type exhaust pipe - is there any special tool to better reach the carburettor's adjustment screw on the bottom side of the carburettor ?

Regards,
Sebastian

 

[Randy Forbes]

A novice can render a pair of SUs on an engine almost undrivable; three (3) SUs increases those chances by 50%.

The best recommendation is to find a local specialist that's capable of getting the tune spot on.

Tuning a Healey is not an exact science, and theoretically, anyone can do it; the reality is that it takes experience to recognize when the adjustments don't yield the desired response, and figuring out why. Air leaking past the shafts, rendering repeatable idle settings nearly impossible, is a good, and frequent example.

Don't think that a prospective tuner is trying to gouge you when he tells you that you need a complete tune up. The carbs cannot be satisfactorily adjusted unless it is verified that the valve settings are correct, the points & condenser are in good order__and properly gapped__along with the ignition timing correctly set, and a good, clean set of plugs, distributor cap, rotor and wires. Only when all of the above is in good nick can you even contemplate to mess about with the carbs.

So to directly answer your questions, yes, there are a number of us here capable of tuning a trio of SUs, but beyond using an actual SU wrench, there's not much else you need to access the jet nuts__most of the time, if they're clean and in good order, they can be turned without any tools. I am however, confused by your terminology; specifically "built in" and "fan type"__what do you mean by that?

If I walked up to your car, and was allowed ten (10) minutes to tune the carbs, and you swore that everything else was as it should be, the very first thing I'd do is to verify that the fuel level in all the float bowls came to the same height__better that they're all even, than to quibble about the level being off a millimeter either way! Next step would be to remove all three (3) dashpots to verify the pistons can rise and fall smoothly and evenly, plus to examine the jet needle for wear__too common, and might as well stop right there! Verify the pistons fall evenly into the jet well, bring all the jets to the same height (about 1-3/4 turns down from being flush with the bridge) and then have a go to see how well the linkage is synchronised.

Okay, so that might take eleven (11) minutes!

 

[dougie]

A novice can render a pair of SUs on an engine almost undrivable; three (3) SUs increases those chances by 50%.

The best recommendation is to find a local specialist that's capable of getting the tune spot on.

Tuning a Healey is not an exact science, and theoretically, anyone can do it; the reality is that it takes experience to recognize when the adjustments don't yield the desired response, and figuring out why. Air leaking past the shafts, rendering repeatable idle settings nearly impossible, is a good, and frequent example.

Don't think that a prospective tuner is trying to gouge you when he tells you that you need a complete tune up. The carbs cannot be satisfactorily adjusted unless it is verified that the valve settings are correct, the points & condenser are in good order__and properly gapped__along with the ignition timing correctly set, and a good, clean set of plugs, distributor cap, rotor and wires. Only when all of the above is in good nick can you even contemplate to mess about with the carbs.

So to directly answer your questions, yes, there are a number of us here capable of tuning a trio of SUs, but beyond using an actual SU wrench, there's not much else you need to access the jet nuts__most of the time, if they're clean and in good order, they can be turned without any tools. I am however, confused by your terminology; specifically "built in" and "fan type"__what do you mean by that?

If I walked up to your car, and was allowed ten (10) minutes to tune the carbs, and you swore that everything else was as it should be, the very first thing I'd do is to verify that the fuel level in all the float bowls came to the same height__better that they're all even, than to quibble about the level being off a millimeter either way! Next step would be to remove all three (3) dashpots to verify the pistons can rise and fall smoothly and evenly, plus to examine the jet needle for wear__too common, and might as well stop right there! Verify the pistons fall evenly into the jet well, bring all the jets to the same height (about 1-3/4 turns down from being flush with the bridge) and then have a go to see how well the linkage is synchronised.

Okay, so that might take eleven (11) minutes!

I think Randy said it all...

 

[bob hughes]

I've just used pair of colour tunes on my BJ7 as it was set up rich - too rich IMO. by my local specialist. ( fuel was being eaten up at a huge rate) Used them on number 2 and 5 cylinders and took them from rich to just hitting the blue with what looks to be very good results. I also found that the rubber fuel link between the carbs had started to leak as it had become porous at the clips, this would definitely not helped the fuel situation - strange 'cause this was replaced with new 3 months ago. I have replaced it with more of the same and have ordered up some new material.



Bob

 

[Randy Forbes]

I've just used pair of colour tunes on my BJ7 as it was set up rich - too rich IMO. by my local specialist. ( fuel was being eaten up at a huge rate) Used them on number 2 and 5 cylinders and took them from rich to just hitting the blue with what looks to be very good results. I also found that the rubber fuel link between the carbs had started to leak as it had become porous at the clips, this would definitely not helped the fuel situation - strange 'cause this was replaced with new 3 months ago. I have replaced it with more of the same and have ordered up some new material.



Bob

Overtightening of the clamps? Given the fuel pressure__around 2-1/2 PSI__these clamps don't need to be very tight, just enough to keep the hose from sliding over the beaded end of the atachment tube.

Sometimes I see those clamps so tight that 1) the spiral is stripped and/or 2) the rubber is partially sheared and squeezing through the slots. That's beyond too tight.

 

[bob hughes]

Randy
Thanks for the tip but the new rubber has arrived and there is a world of difference between the old and the new in terms of flexibility. I have stored it out of the sunlight and in about three months time just before the replacement starts to collapse, I will put the new on.

Just checked the plugs and we have a nice dark brown on them (instead of the sooty black deposit) so we are slightly rich of optimum ( nut brown). That colour tune sure works.



Bob

 

[sebastianwendt]

Thanks a lot for your support, gentlemen.
My Healey is a bit of a"fast road car" type so it has a "hotter cam", a special exhaust manifold(fan-type), a lighter flywheel, different valve return springs, special air filters...
I also have a 123ignition installed (so fully electronic distributor and ignition), valves have just been adjusted, and ignition timing if absolutely fine, ...
So i have already done the basics and now I am at the carburettors - I have ad a colortune spark plug installed and it is indicated as well that the engine runs too rich (but you could also smell it ;-)

I read that the SUs are very sensitive regarding the fuel pressure and as I have an uprated fuel pump, it probably is the root cause so I bought a pressure regulator for it and will try it as well.

I raised my initial question how you adjust the car mixture because it is very hard for me to reach the jet adjustment screws when the the carburettors are installed. There is very few room and of course the hot exhaust manifold makes it even worse. So is there a special tool for turning the screws when the SUs are installed or can you give me a hint how to get the adjustment screw turned when the carburettors are installed.

 

[Randy Forbes]

Notice item "C" on this page: https://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=37197

This is a link to the instructions on balancing a pair of carbs, just think in terms of three (3) and it's an identical process: https://www.mossmotors.com/graphics/products/instructions/386-300_386-298_INST.pdf

Item "B" the balancing kit includes "C" and a facsimile of "D" All should be considered essential steps, along with verifying your fuel pressure (I think SUs might be as low as 1-1/2 PSI, but admittedly, been a long time since I've worked on them; seems like I'm <4-1/2 PSI for the Webers).

 

[sebastianwendt]

Thanks, that's what I was looking for !

 

[sebastianwendt]

Gents, by using 3 colortune spark plugs I could adjust and verify the mixture (nice blue/bunsen flame)
Thanks for your support !

Regards,
Sebastian