Gear Gaiter Surround

Finally got round to doing the gearstick surround! I bought a polished stainless round surround from Europa spares and it looked completely stupid all shiny and polished as it didn't match the rest of the car so decided to paint it. The Dax tunnel isn't flat so it took a bit of work to bend it to fit the shape, just a vice, large diameter round steel bar and I got something half decent.

For the paint, I didn't really know what to do. I knew that polished shiny stainless was daft so had a look in the cupboard to see what other options I had. I found an old rattle can of grey car body paint and gave the stainless gear trim a couple of very light coats. Amazingly, because it was polished it came out a nice grey mottled effect, rather like the leather of the gaiter itself.

Four holes drilled, rivnuts in and job done.

Edelbrock Thunder AVS 650 Idle Channel Restrictor

I decided to perform a popular mod to my carb that I have read a lot about and that is increasing the size of the idle channel restrictor.

It is well documented that the Eddy carbs flow barely enough fuel in the idle/transition circuit to cope with big cams. I've experienced this through the various low speed running problems. I can see from the AFR that there simply isn't enough fuel at low RPM and there is a lean surge problem.

Closing the throttles and drilling holes in the throttle blades to make the transition circuit more active helped a little bit but the trouble is that the transition circuit is fed through the idle channel restrictor so this needs to be enlarged. In Holley type carbs, Quick Fuel, Demons etc, the top-end carbs have all sorts of changeable air bleeds and jets all over the place which can be easily swapped. On the Edelbrock however its a drilled channel so the only solution is surgery. It would be possible to drill and tap and replace with something adjustable by my approach is to make tiny changes until I notice a change and leave it at that. The risk of course is going too far then changes aren't reversible if you are drilling holes.

Here is the offending area of the carb as per the Eddy manual (note 5) :

It can be accessed by unscrewing the air horn and then removing the booster assembly that sits in the idle well. You will have to remove the throttle cable and choke assembly as well.

The channel restrictor is hiding behind the pressed-in brass jobby, labelled "idle air bleed" in the manual. Removing this is easy-peasy, get a self tapping screw and wind it in to the brass restrictor then just pull it out with some pliers, in fact one of them came out by hand.

You can now see the small hole that was behind the brass restrictor. This is the ICR that we are after.

Just so we are absolutely sure what it is we're drilling, here's a close-up with it circled in red:

It's now possible to get in with a drill and open up the orifice. I don't know what size it is to begin with but I ran a 1.5mm drill through which opened it up a small amount, it didn't remove much material. Its probably best to use numbered drill bits to 1) accurately measure what it is to begin with and 2) be sure of what size you are going to and make SMALL changes - I know its a bit hypocritical saying this when I just picked up a small bit and ran it through but the point still stands, when enlarging very small holes you can make massive differences in terms of % from what seems only a small increase in diameter.

Don't forget to do the other booster before you reassemble!

Anyway - the result, I'm pleased to say I can actually feel it and can see it on the AFR gauge. Without changing the idle screw settings, the idle AFR had richened up a little bit from 13.8:1 to about 13.5:1. This means that the idle circuit as a whole is now getting more fuel. 

When driving, the biggest difference was when plodding along under 2000rpm when the idle circuit is doing all the work. Before this mod I was creeping over 14:1 which was just about ok for constant throttle but if I made very small throttle adjustments of just a few mm on the pedal would send it into lean surge as it was clear the idle/transition circuit wasn't fat enough to give any extra fuel for small modulations on the throttle. These are pedal movements which are too small to get a fuel from the accelerator pump, just a couple of mm slow throttle movement.

Now that I've drilled the channel, the AFR doesn't spike lean and surge when making small throttle movements at low rpm and the general low speed manners have increased noticeably. I can give it a couple of mm throttle just to very lightly pick up speed and the AFR remains below 14:1 and lean surge is avoided so it was definitely worth doing - the way I see it in my head is that the transition circuit now has more "headroom" to cope with the many small changes in fuel demand that occur when you are cruising along gently at low rpm when the main circuit isn't fully active.

Missing Gasket!

The engine seems to have got a bit greedy and chomped a bit of carb gasket! More investigation required!

Update 27/03/15 - After some good thorough carb cleaning the damaged gasket was a false alarm, no lasting damage. Phew!

Edelbrock Thunder AVS 650 Secondary Air Flap Tuning

I have struggled to find information on this so I did some experimentation and decided to try and write my own tuning guide in case the weary internet wanderer should stumble on this dusty corner of the web in search of carb tuning tips.

Just to summarise and recap - engine is a 383 SBC, formerly with an Edelbrock Performer 750cfm carb (part no. 1407) and now sporting an Edelbrock Thunder 650cfm carb (part no. 1805).

The Edelbrock Performer and Thunder carbs have mechanical secondaries that open up from roughly 60% throttle opening but they have a flap above them to control when the air starts to flow. On the performer, the door has some weights on it so it opens for a certain air flow (i.e. throttle demand). The secondaries will only be active when the engine needs them. The only way to adjust this is to add or remove weight.

On the Thunder series, the air flap is spring loaded so by altering the tension of the spring, you can tune when the flap opens and the secondaries become active. I've seen the standard setting reported variously between 2 and 2.5 turns of spring tension. I've learned enough so far to never assume anything is set up correctly out of the box so the first thing I did was to unwind the tension and set it myself to 2 turns. At this point, some pictures will probably help.

Here's the carb on the engine with the air filter removed.

Flap closed:

Flap open:

Adjustment - The spring tension adjuster (flat blade screwdriver required) is clamped down with a torx fixing. Loosen the torx screw and you can then adjust the tension. Note that you need to have the flat blade in and hold it firmly before you loosen the torx otherwise the spring will just unwind and you will lose whatever setting the spring was at.

So what effect does adjustment one way or the other have? If you loosen the spring, the flap opens sooner so the secondaries become active quicker. "Great!", I hear you say. "Lets have it on the loosest setting and get more power!"

Sadly this is not the case. Fundamentally, air flow in a carburettor always reacts quicker than the fuel. Changes in fuel flow will always lag behind the air and that won't change. That's why there are things like accelerator pumps to cover the "hole" in fuel supply when you stomp on the throttle pedal. The air rushes in but the fuel flow takes a moment to catch up. The Eddy carb has a primary accelerator pump but no secondary pump (like a Holley double pumper). That's where the air flap comes in.

At the stock setting of 2 turns on the tension spring, if I press the throttle sharply I get a momentary bog or stumble. The engine briefly dies for half a second then picks up. Looking at my AFR gauge, I can see its a lean condition. This is because the engine tries to suck in a load of air/fuel and the spring allows the air flap to open, the air rushes in immediately but the fuel isn't flowing yet to keep the AFR sensible and there is a lean spike, then engine falls flat on its face until the fuel starts flowing, the AFR returns to where it should be, and the engine comes back to life and accelerates happily. If I press the pedal slowly and steadily, the air flap opens slowly, the fuel has time to be pulled to the boosters and the problem goes away, but obviously this is rubbish to drive, having to nanny the throttle pedal all the time.

No amount of fiddling with the primary accelerator pump shot can solve this as it doesn't affect the air/fuel in the secondaries. See my previous posts for pump shot nozzles, actuator linkage adjustment and pump spring upgrades. None of this worked - this is what the adjustable air flap is for. By tightening the spring, the door takes longer to open and while it does so momentarily chokes the secondaries, making the mixture richer. This gives time for the air flow to establish itself and all the clever passages that meter the fuel to start working properly. The lean spike is therefore eliminated, or at least greatly reduced. I like to think of it as the opposite of the accelerator pump. The pump shot gives you more fuel, conversely the air flap reduces the amount of air, but both methods are aiming to stop the mixture from going lean when the secondaries open quickly. The end result should be seamless transition as the secondaries come on. Anyone who says they can feel the secondaries kicking in are probably experiencing some sort of bog or stumble and need to do some tuning work. You should barely notice it.

That's my simplified explanation anyway. 

As far as results go, with the stock setting of 2 turns of tension, I was getting spikes up to 18:1 on the AFR gauge before settling down to 12.7:1. Obviously this is not good, but the one crumb of good news is that my mixture is ok at WOT once things have settled down. I started winding on the tension and ended up at 3.5 turns which brought the lean spike down to 15:1. Cruise seems to be anywhere between 13.5:1 and 14:1 depending on rpm so a momentary blip to 15:1 is barely noticeable but is still there. The engine doesn't die but you can still feel it hang momentarily after changing gear.

The only problem I have now noticed is that if you floor it from below 2000rpm in 5th gear, the engine can bog rich. I think this is because the air flow is not enough to overcome the increased spring tension on the air flap and it stays in the "choked" position. The engine wants more air but it can't get it until the rpm rises, the air flow increases and then the door opens. In lower gears this doesn't happen because the revs rise so quickly. On the open road when you're going along at 50 in 5th and want to overtake someone, its extremely annoying because I have to either roll gently into the throttle or change down a gear and this completely ruins the car. Whats the point of having a massive V8 with mountains of torque down low if you have to keep changing gear all the time?! I backed off the air flap tension and this problem goes away, but then the lean spike on flooring it and changing gear returns.

Conclusion and lessons learned

Since doing this investigation and later on moving to a Holley carb, I have learned a few useful things. Part of the reason this Edelbrock carb didn't really work for me was my engine spec and the nature of my car. 

I am not saying Edelbrock carbs are bad. What I am saying is they are better suited for a particular application. Both my Performer and Thunder carbs were very good for cruising around once I had got them set up properly. I got good fuel economy at around 18-20mpg and after my tweaks to the idle channel restrictor I had very good low speed manners and a sharp part-throttle response. They would be ideally suited to a heavy car with long gears, or automatic transmission, stock or mild cam and dual plane intake designed to give high torque straight off idle 

What the Eddy carbs are perhaps not best suited for is a very light car with short gears, manual transmission, a big cam and single plane intake designed for higher rpm. A solution exists for cars like mine and its called a Holley Double Pumper.

Carb Accelerator Pump

After doing lots of research I found some interesting information about Edelbrock carbs and the lean bog that they seem to suffer from when mashing down full throttle. My car suffers from this, if I press the throttle gently then there's no bog or stumble but if you stamp on the throttle it momentarily dies and takes a moment to pick up. The problem is more pronounced when flooring it from lower revs - something I haven't been able to do until I solved my poor low speed running problems by changing to a smaller carb.

So now I needed to find a way of covering the "hole" in fuel delivery between low speed cruise and WOT. The WOT mixture is good at around 12.7:1 but there is a nasty spike up to 16:1 or even 17:1 where it stumbles and dies before picking up again if I floor it sharply. First port of call was the accelerator pump. On the Eddy carbs you can adjust the linkage to give more or less pump volume. The picture below shows the linkage attached to the plunger in black with 3 holes to choose from. Stock, the linkage is in the middle hole. For more pump shot move to the top hole, for less, the bottom hole.

This didn't have much effect so I started messing around with shooter sizes. Edelbrock do an accelerator pump nozzle kit part no. 1475 which has a variety of sizes. You can just see the nozzles below above the venturi throat with "31" stamped on. This is the nozzle size in thousandths of an inch. The nozzle kit has several sizes up to 0.043" (43 stamped on top).

I swapped to the largest nozzle size which also failed to solve the stumble. The extra fuel just doesn't get flowing soon enough. What I think I need is a more active pump shot. I noticed that I have to move the throttle quite a bit to get anything out of the pump nozzles. The spring on the pump plunger is quite weak so this gets compressed first before pushing fuel out. Luckily, Edelbrock do an uprated accelerator pump plunger with a much stiffer spring, part no. 1468.

With the new plunger fitted, the nozzles squirt fuel with the tiniest movement of the throttle so hopefully that might improve things. Won't get a chance to drive the car until next weekend to find out though.

Update - this did nothing to cure the lean spike which I have now identified is to do with the secondary throttles opening quickly. Thankfully there is a way to tune this out but information is a bit lacking in this respect. See the next post for the solution.