The exciting world of HIF6 carbs (or how I learned to love the Skinner Brothers)

sdibbers

Well-Known Member
Well, after a few years of having converted my 2000TC to a 2200TC including switching from HS8's to HIF6s I thought I'd share my experiences. As mentioned in the conversion thread they do give much better mid-range performance and a much smoother idle compared to the HS8 2" carbs. However, I did find them a little more finikny to set up. But, once set they seem to hold a tune better. With the proviso that the carbs and engine are in good condition.

Here's a few observations from my (US) based car:

[1] Engine needs to be correctly set up (true for all carbs):

[a] So, valve clearances should be within spec. Not too tight, with proper sealing when closed.
Spark plugs: gaps should be correct (0.025" for conventional points and balasted 1.5ohm coil, 0.032" on non-ballasted sports coil with electronic ignition),

If using suppressed plugs they should all have resistance within spec (Normally around 40-60k ohms). (I've had Champion plugs with 9000k ohms before and that will definitely affect running).

[c] Ignition timing should be correct at idle speed, along with dwell angle if using points still. If you have a problem with ignition scatter (timing marks jump around under timing strobe) You may need to have your dissy rebuilt. Don't use an off the shelf new dissy if you can help it. They have a generic timing advance curve that's not going to give you the best performance. Distributor Doctor in the UK and Advanced Distributors in the US both offer re-curving services. They will even do custom curves to your build spec and accounting for modern fuels. Its fairly affordable and pays in the long run.

[d] Check for any vacuum leaks in the intake side of the engine.

So that means:
Intake manifolds,
Giant O-rings in the carb mounts,
Vacuum lines to distributor,
Vacuum line to brake servo(s),
Vacuum lines to thermo valve on air filter box (if fitted),
Vacuum line to windscreen wiper switch (if fitted).

[2] OK, done all that? Now you can play with your carbs!
The carbs should be in good condition.

[a] This means the butterfly valve spindles should be smooth and play free. The bushes they ride in shouldn't be worn, and the rubber seals on the spindles should be soft and seated against the body to form a good seal. You can test this easily; have the engine idling, and using a can of carb cleaner spray each area where the spindle enters the carb body. If the engine revs change (up or down) you have a leak there and you should fix that before trying to get a good tune. If you're lucky you can get away with
new seals and a spindle. Otherwise you may have to send off the carbs to have re-bushed at Burlen in the UK or Joe Curto in the US.

If you have access to a machine shop you can try making delrin ones like mine. Check near the bottom of this page to see drawings and method I used to do the work. The delrin bushes will last longer than the originals, seal better and have lower friction for a nicer throttle feel.

If you feel the carb is tired you might want to get a rebuild kit and replace the internal seals. Both Burlen and Joe Curto sell them.
Don't be tempted by cheap NOS kits on ebay! Most likely the rubber in those isn't ethanol resistant and will fail in short order and dump fuel over the exhaust manifold (Ask me how I know!). While in the carb inspect the floats for cracks or fuel inside the floats. The HIF floats are notorious for
leaking and sinking, thereby flooding the carb.

[3] Setting them up - mixture:
The mixture is the amount of fuel relative to air going into the engine. A rich mixture means more fuel is going in than it should, a weak mixture means less fuel is going in than it should.

[a] If you haven't already consider fitting an inline fuel filter. The cheap clear plastic jobbies like this are perfect. I fitted mine between the pump and
the front carb on my car, its very easy to see if its full or if there's crud in there.

Set the jet height(s). I prefer my method (of course he does): Remove the vacuum bells and pistons (3x screws each carb)
Lift off the bells with the pistons intact so as not to lose the springs and set safely aside -
mark each one for front and rear if you have a TC.

[c] Using a digital or vernier caliper use its depth gauge (the tail that comes out the bottom) to measure the distance from the carb bridge to the top of the jet. Like this:
su.JPG
I find a starting point of 1.65mm seems to work well. Wind in the mixture screw to enrichen the mixture, wind out to weaken
(I always think indoors rich man/outdoors poor man) - I know, I know......


[d] Refit the pistons and vacuum bells.

[e] Start the car and get up to running temp, don't worry if it feels lumpy or rich, just get it up to temp first.

[f] I find most methods of getting the mixture right are very personal. My method is to listen to the car, smell the exhaust and other generally petrolhead fetish stuff.

[g] With the car idling at roughly 650-700 rpm it should be pretty smooth. If you hear a slight popping at the exhaust (like a slight misfire) richen up each carb by 1/6th of a turn in (always do the same to both carbs on a TC, that's why we set both jets to the same height). If it's not popping, try winding both carbs in 1/6th of a turn, the idle will probably rise a little. Keep doing that incrementally until you hear the popping or the idle drops, then go back up a little to smooth it out.

[Note!] If you have the engine idling for more than 2 minutes you will want to rev it to 2,000 rpm for 20 seconds to just cool everything down. Then continue with the above.

Congratulations! You have set the mixture.
 
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Continued:

[4] Setting them up - balancing:
This is known as balancing or sync'ing the carbs. What it really means is making sure they both suck in as much air as each other. If you have an SC you don't have this problem.

[a] Loosen the clamps on the spindle connector bars for both the choke and the throttle. This will allow both carbs to be tuned individually for balance.
There are two 5/16" nuts on each bar. You know they are loose enough when you can open the throttle on one carb and the other stays still.
Same goes for the choke control.

You are going to need a carb balancing device of some sort (I know people say they can hear the difference between two carbs using a length of hose pipe in the intake, but I've seen the results and they are normally a way off when measured).
You can use something like this unisyn or this Gunson carbalancer and it saves an awful lot of guess work. For my money the unisyn is a more reliable tool.

[c] Using the tool measure the amount of air being sucked into each carb. If you're idling too fast loosen the locknut on the idle screw of the carb sucking hardest and
loosen the screw a little. Remember: The value will go down on both carbs so check the slower carb frequently. When both carbs have the same amount of 'suck' Blip both throttles a few times and wait for them to settle.

[d] Measure again to be sure, if they are still sucking the same you can set the idle speed. Adjust both idle screws the same amount now.
When you are at around 650-700 rpm you're good. You can lock off the idle screws and check with the gauge one last time (I'm sort
of obsessive about this I know).

[e] You now want to tighten up each clamp on the top balance bar. Make sure its finger is in its slot of the spindle end of both carbs. The idle is that both butterflies open at exactly the same time when you touch the pedal. If one opens first it will feel rough each time. You'll also lose power for take off because one carb will be doing the heavy lifting for the first bit.

[f] Turn off the engine.

[5] Setting them up - Choke:

[a] We now want to do the same thing with the choke balance bar. So make sure the fingers of the clamp are sitting in their respective slots on the and of the choke valve like we did on the throttle. Same idea; to have both open at the same time.

Start the engine again. And pull the choke out just enough that the fast idle screws are just on their respective cams. See pic below:
choke.JPG3: Cam, 4: Fast idle screw

[c] Adjust both fast idle screws to get an idle speed of around 1,200 RPM. Push in the choke.

That's it! You're done!

Things I've found while working on these that might help you:

If the float chamber is overflowing it could be one of the following things:

[1] Leaky or sinking float (replace with new)
[2] Float needle valve has dirt stopping it seal (remove, clean and reinstall)
[3] Damaged float needle valve (Unlikely unless old and worn)
[4] If you have an area that only has 5 or 10% ethanol fuel and higher temps (say 85F/30C or higher) you may need to add extra heat shielding. I found that ethanol drastically reduces the boiling point of petrol. It caused the fuel to boil in the float chamber making the float sink and it to dump fuel onto the exhaust manifold. I used this stuff on both sides of the standard heat shield and its been fine ever since.

If you find it keeps losing tune after you've done everything above:

I found the bi-metal assembly that adjusts the jet on mine had failed. It would move one way but not the other each time I used the car. Making it progressively richer. After replacing it the car has been perfect. The part is no. 17 on this exploded diagram

I hope this is useful to some others, and if you have been, thank you for reading.
 
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Continued:

[4] Setting them up - balancing:
This is known as balancing or sync'ing the carbs. What it really means is making sure they both suck in as much air as each other. If you have an SC you don't have this problem.

[a] Loosen the clamps on the spindle connector bars for both the choke and the throttle. This will allow both carbs to be tuned individually for balance.
There are two 5/16" nuts on each bar. You know they are loose enough when you can open the throttle on one carb and the other stays still.
Same goes for the choke control.

You are going to need a carb balancing device of some sort (I know people say they can hear the difference between two carbs using a length of hose pipe in the intake, but I've seen the results and they are normally a way off when measured).
You can use something like this unisyn or this Gunson carbalancer and it saves an awful lot of guess work. For my money the unisyn is a more reliable tool.

[c] Using the tool measure the amount of air being sucked into each carb. If you're idling too fast loosen the locknut on the idle screw of the carb sucking hardest and
loosen the screw a little. Remember: The value will go down on both carbs so check the slower carb frequently. When both carbs have the same amount of 'suck' Blip both throttles a few times and wait for them to settle.

[d] Measure again to be sure, if they are still sucking the same you can set the idle speed. Adjust both idle screws the same amount now.
When you are at around 650-700 rpm you're good. You can lock off the idle screws and check with the gauge one last time (I'm sort
of obsessive about this I know).

[e] You now want to tighten up each clamp on the top balance bar. Make sure its finger is in its slot of the spindle end of both carbs. The idle is that both butterflies open at exactly the same time when you touch the pedal. If one opens first it will feel rough each time. You'll also lose power for take off because one carb will be doing the heavy lifting for the first bit.

[f] Turn off the engine.

[5] Setting them up - Choke:

[a] We now want to do the same thing with the choke balance bar. So make sure the fingers of the clamp are sitting in their respective slots on the and of the choke valve like we did on the throttle. Same idea; to have both open at the same time.

Start the engine again. And pull the choke out just enough that the fast idle screws are just on their respective cams. See pic below:
View attachment 158873: Cam, 4: Fast idle screw

[c] Adjust both fast idle screws to get an idle speed of around 1,200 RPM. Push in the choke.

That's it! You're done!

Things I've found while working on these that might help you:

If the float chamber is overflowing it could be one of the following things:

[1] Leaky or sinking float (replace with new)
[2] Float needle valve has dirt stopping it seal (remove, clean and reinstall)
[3] Damaged float needle valve (Unlikely unless old and worn)
[4] If you have an area that only has 5 or 10% ethanol fuel and higher temps (say 85F/30C or higher) you may need to add extra heat shielding. I found that ethanol drastically reduces the boiling point of petrol. It caused the fuel to boil in the float chamber making the float sink and it to dump fuel onto the exhaust manifold. I used this stuff on both sides of the standard heat shield and its been fine ever since.

If you find it keeps losing tune after you've done everything above:

I found the bi-metal assembly that adjusts the jet on mine had failed. It would move one way but not the other each time I used the car. Making it progressively richer. After replacing it the car has been perfect. The part is no. 17 on this exploded diagram

I hope this is useful to some others, and if you have been, thank you for reading.
That’s excellent clear and comprehensive. Well done. Can I just point out that the nuts on the balance bar are ba2. The 5/16 on the top are captive and should not be turned.
 
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