Setting twin SUs

[Ian Vincent]

Those who have read my post on 'Old Petrol' will know I have been fiddling with the tuning of my car, (TR3a).

 

It has twin HS6 carbs on a TR4a manifold

 

I started by setting both the carb jets .035" down, started the engine and got it thoroughly warmed up by taking it for a run of about 5 miles. When I checked the plugs, the front ones were about right, maybe a shade rich; the back ones were very black and sooty. Then using first a pair of colourtunes and then the little button on the side I adjusted the mixture until both carbs were properly adjusted and took the car for another run to confirm this. The result was that both front an rear plugs are now a sandy colour and the engine runs very sweetly - albeit it fluffs a little at low revs, but the jet on the rear carb is only about .015" down from the bridge whereas the front one is .032" down.

 

So my question is - Is this variation in the jet heights normal or does this indicate that something else is amiss with the carbs?

 

Rgds Ian

[RogerH]

Hi Ian,

are the needles new or old

Are the needles the same number.

 

The jets (holes) could be a slightly worn.

 

Roger

[rogerguzzi]

Hello Ian

As Roger says but I would start by checking the float heights.

 

If the jets are old replace them and needles (you could measure then at the same points) say top(thick end) to half way

 

You could check the jets if you something the right diameter.

 

Or of course it could just be an air leak?

 

Roger

[peejay4A]

If they're not biased needles are they centred and both at the right level in the pistons?

[boxofbits]

Hi Ian

 

You might also check things like compression, tapper clearances, air leaks as these could have an impact on the efficiency of each combustion/ ignition cycle and how well the fuel is burned. Also are the dampers topped up, and with the same oil, and filters both okay? Other than that as per Roger/ Roger and Peejay.

[ctc77965o]

Ian,

The problem is most likely airflow balance.

The manifold has this balance tube...at idle this can allow all four cylinders to be fed by one carb...away from idle this doesnt work but it confuses setup.

 

triple-check the airflow through both carbs, adjust the linkage to synchronise. Use the hiss method, the airflow meter method, or even the piston-lift method.

 

Then set your carbs back to two turns down from the bridge. Once warmed-up adjust them in unison use the lifting pins to check the mixture. Only once its 96% correct consider individual carb adjustments...even then don't allow the 2 carbs to vary more than a few flats of the mixture nut.

 

[Peter Cobbold]

+1 for air flows.

Air flow sets the piston lift and that sets the operative needle position.

I always set the hisses equal first of all.

Peter

[Ian Vincent]

Thanks for all the replies; so the message is the jets levels should be pretty similar.

 

I have set the balance with a balance tube, I iterated between setting the mixture and setting the balance until both were as they should be. I have also checked the piston lifts and they are pretty much synchronised as well. Both pistons drop with a nice snap when lifted so I assume that the needles must be centralised properly and both jets and needles are newish (SM), so I guess that only leaves the float levels as being the variable.

 

I will give that a go when I get a moment. Once again thks for the comments.

 

Rgds Ian

[Lebro]

the jet on the rear carb is only about .015" down from the bridge whereas the front one is .032" down.

 

So my question is - Is this variation in the jet heights normal or does this indicate that something else is amiss with the carbs?

 

Rgds Ian

 

+1 for checking float heights, also you could have a slight air leak on front carb.

 

With HS6's the front float chamber is in front of the carb, The rear one is behind.

The engine does not sit horizontally, it is higher at the front than the back, therefor, with the float heights set as per book, i.e. both set the same the level of fuel in the jet of the front carb will be higher than the level in the back carb.

This does not explain your observation as it is the wrong way round, but may be of interest.

I have set my float heights up to give the same level of fuel in each carb.

 

Bob.

[Peter Cobbold]

The fuel level in the jet is not critical, providing it is a mm or so below the mouth so that fuel cannot flow out without 'suction' present. This is because the 'constant depression' is around 0.15psi* and the 'suction' needed to lift fuel 10mm is a mere 0.01psi. So if fuel in one jet is 2mm below mouth and the other 7mm down the difference in mixture will be 3 to 4%. We wont see that on the plugs or colortune.

Peter

* if someone will weigh and measure a piston it can be calculated)